The Motor Vehicle 2010 Part 16 pot

But when atorque is applied to the steering wheel to overcome a resistance to a steeringmotion of the road wheels the torsion bar is twisted and relative motionoccurs between the inner v

Fig 40.47, now has a variable pitch so that when the roller follower B is in

the central position shown, the ratio angular motion of cam : angular motion

of drop arm (C), for a very small motion, is about 21 : 1 but when the drop

arm and follower have moved about 12° away from the central position thatratio has fallen to 13 : 1 and thereafter it remains constant at that value Thevalve which controls the servo action now operates by the rotationaldisplacement of its two main components instead of by their axial displacements

as in the Ross system

The cam is carried in the casing on two angular contact ball bearings and

is coupled at the left-hand side to the torsion bar D by a cross-pin At theright-hand end the torsion bar is coupled, also by a cross-pin, to the sleeve

E which is splined to the steering-wheel shaft and which forms the innermember to the valve The left-hand end of the sleeve E is formed withsplines F which engage splines formed in the right-hand end of the cam Abut these splines are machined so as to allow 7° of freedom of rotation andare only to provide a safeguard against over-stressing of the torsion bar whenthe steering is operated without servo assistance Pipes connect the pump,which is driven by the engine, to the valve and the latter to the outer end ofthe servo cylinder; the inner end of the servo cylinder forms the casing whichhouses the cam A and follower assembly and the valve is directly connected

to that space The outer member G of the valve is coupled by the ball-endedpin H to the right-hand end of the cam A The servo piston J is integral with

a stem on which rack teeth are formed and these teeth engage teeth machined

on the end of the drop arm forging C

The principle of operation of the valve is shown by the diagrams Figs40.48(a) and (b) It is really three valves in parallel, parts relative to whichare denoted by suffixes 1, 2 and 3 – following the letters P and S in thediagrams – but the action will be described in relation to one of them Theports P are connected to the delivery of the pump and when the valve sleeve

E is centrally placed, as shown at (a), fluid flows equally to each of the

pockets S1 and S2 which are connected at their ends to the return pipe to the

pump The pressure drops across the apertures b and a are equal to those across the apertures c and d and so there is no pressure difference between

the spaces CL and CR which are connected to the ends of the servo cylinder.Hence there is no net hydraulic force acting on the servo piston But when atorque is applied to the steering wheel to overcome a resistance to a steeringmotion of the road wheels the torsion bar is twisted and relative motionoccurs between the inner valve sleeve E and the outer one G, as is shown at

(b) The passage c is thereby increased while the passage d is decreased and

so the pressure in the space CR is raised Conversely, the passage b is decreased and a is increased so that the pressure in the space CL is lowered A pressuredifference is thus established across the servo piston and the drop arm isrotated As this occurs the cam and outer sleeve of the valve rotate so as tofollow up the inner valve sleeve and bring the valve to a central position Thedrop arm having thus been rotated the required amount the servo actionceases and the system remains in equilibrium The use of three sets of portsprovides a valve in which the radial hydraulic pressures are balanced and therequired port areas are obtained with a valve only one-third the length thatwould be needed if only one set was provided

Provision is made for adjusting the mesh of the roller B with the cam A;

this is done by means of the screw L which, when turned, will move the droparm shaft C in or out and thus bring the roller into closer or looser mesh.Similarly, the mesh of the rack teeth on the servo piston stem can be adjusted

by means of the screwed plug M which bears on the underside of the stemthrough the spherically seated pad N

40.30 Electrically powered systems

Electrical is much more amenable than hydraulic power to electronic control.Consequently, many more variables can be taken into consideration for theprovision of assistance appropriate for differing conditions, such as the speed

of the vehicle, condition of the road, rate of change of speed of rotation of the steering wheel and degree of braking or acceleration, if any

The advantages of electric control therefore are as follows:

1 Increased safety at high speeds because, in these conditions, the provision

of power is independent of engine speed so assistance can be appliedmore sensitively

2 By virtue of direct mechanical application of assistance by an electricmotor and the potential for using irreversible gearing, the kickback felt

at the steering wheel, when driving off-road for example, can be evenless pronounced than with hydraulic power assistance (Fig 40.49)

3 Lower energy consumption, hence reduced emissions, and improvedacceleration of cars with small engines Claims of up to 5% reduction

in fuel consumption by comparison with engine-driven hydraulicallypowered systems have been claimed, although it seems likely that anoverall figure of about 2–3% would be more realistic

4 The elimination of hydraulics leads to compactness, fewer components,reduced weight and maintenance, no fluid to be kept topped up, noleaks, and installation costs are reduced

5 Steering assistance is maintained if the engine stalls

6 Much better performance under very cold conditions, when hydraulicswould be adversely affected by increased viscosity of the fluid

7 With electronic control, it is easier to provide failure warning, diagnosis and self-protection systems, and it is possible to design systemsthat can be easily and rapidly tuned to suit individual applications andcan be integrated with other electronic systems such as ABS and integratedvehicle stability control (IVSC) It is even practicable to design driver-selected feel into the system

self-40.31 TRW systems

In addition to producing electrohydraulic power steering systems, TRW,which now embraces Lucas Varity Electric Power Steering Systems, offersthree electric power steering arrangements: rack drive, column drive andpinion drive All three are of what the company terms integrated architecture,

in other words, they are self-contained

The benefits of a fully integrated design are as follows:

1 All components are contained as a single sub-assembly: that is, withoutharnesses between the ECU, motor and sensor

2 Such units can be fully tested by their manufacturers, ready for installation

5 Numbers of parts reduced to a minimum

6 Motor-to-sensor connections and protected circuits are unnecessary

7 The single housing forms an effective screen for obviating both radiointerference and protection against radio frequency emissions from, for example, roadside installations

8 No connections liable to be made incorrectly or damaged during vehicleassembly

9 Fault diagnosis is simplified

10 Such installations are very compact and of light weight

40.32 TRW rack drive system

The rack drive units, which weighs 16 kg, has four main components Power

is provided by a 93 mm diameter brushless reversible AC motor with rareearth permanent magnets and copper windings in the stator The rotary motion

of this motor, which is extremely compact, is converted into linear motion

by a recirculating ball-nut rotating in grooves around the rack, which iscoaxial with both these components, Fig 40.50

There are two sensors, one for input torque and the other, on the motor,indicates with high precision the position of the rotor At a rotational speed

of 360° per sec, the rack force is 7650 N The output power is 360 W at 65

A Steering feel is programmable Fuel efficiency is claimed to be betweenabout 0.3 to 1.5 miles/gal better than with hydraulic power steering At thetime of writing, units are being developed for handling rack forces of up to

Outboard housing

DX rack bushing

Preload device Wave spring Integral end-mounted ball-nut Motor bearing

Motor tube Stator shield Motor position sensor

Pinion Manually applied torque

Manually applied force Total force to steering arms

Power assistance force

Motor position Reference

signal

Ball-nut and ball screw assembly Assist torque

Reference signal Drive currents

Rack

Fig 40.51 The ECU is mounted on top of the motor and rack-and-pinion gear

drive for the rack and, at the other, the motor position sensor Outboard of thelast mentioned is the rack-and-pinion in a housing bolted on to that of themotor A torque sensor is fitted immediately above the pinion

In common with all the variants of the rack drive system, the input torquesensor is of the inductive type, Fig 40.52 This has three pick-ups from thetorsion bar, one being the median, or datum, reading and the other two theend readings Consequently, electronic noise is cancelled out because it isduplicated in a positive and a negative mode, and thus cancelled out Thereare two tracks for transmission of the signals to the ECU

Alternative sensors that might have been used are: strain gauge on torsionbar, with overtorque protection, which produces only small signals;magnetostrictive, which is entirely new technology with unknown reliability;potentiometric, with torsion bar and overtorque protection; or optical, whichrequires a stiff torsion bar with overtorque protection, Fig 40.53 However,the last mentioned is the only alternative which offers intrinsic positionsensing

40.33 The column and pinion drive variants

For the column-mounted, Fig 40.54, and pinion-mounted, Fig 40.55, variants

of the system a brushless, rare earth magnet type AC motor drives respectivelythe column or pinion through a worm-and-wheel gear, Figs 40.56 and 40.57

In both variants the motor is used in association with an optical position sensor, Fig 40.53 The motor is electronically controlled, quiet, and

torque-and-is driven by a 3-phase power bridge in the ECU It torque-and-is not only simple andwithout rotating contacts but also relatively quiet and has low inertia.Alternative power units would have been: the variable reluctance motor,but this would have been unnecessarily complex and costly; a brushless DCmotor would have been less costly but noisier and its output would have been

a square, instead of sinusoidal, wave; a brushed motor with an H-bridge andwithout a position sensor would have been simpler still to control, but itsinertia and friction would have been higher Incidentally, square waves have

the disadvantage of sending a strong ripple feedback to the steering wheel.With sinusoidal, or near sinusoidal, waves, the reversals are not only inherentlyless sharp, but also their timing can be such that, as each successive wave isascending the next is descending so, to a major degree, they tend to cancelout

Although with the pinion mounted variant, the ECU can be better protected

by mounting it remotely, in the cab or body, the simpler solution is to integrate

it with the pinion drive, as in Fig 40.55 If space around the pinion is at apremium, it may have to be mounted remotely under the bonnet, althoughthis introduces parts that are vulnerable to damage, and assembly into thevehicle costs more Moreover, particular attention has to be devoted to sealingand, since underbonnet temperatures can be high, especially in summer,costly heat resistant materials may be needed for some of the components.Because of the high level of radio frequency radiation, screening is required.Extra protection against short circuiting is needed and the electrical resistancewill be higher than that of the integrated version

40.34 ZF Servolectric system

The ZF Servolectric system is powered directly by an electronically controlledmotor Thus, it dispenses with hydraulics and the consequent complexity of

Fig 40.56 Motor and worm drive of the TRW system

control needed to regulate the supply between the pump and the hydraulicaccumulator Because it is controlled electronically, the system can be adapted

to suit precise requirements such as speed-related assistance, condition ofthe road and changes in circumstances, for instance when braking oraccelerating Damping characteristics can be programmed to suit changingconditions, including on- and off-road driving There is even a possibility oflinking the electronic control to a satellite navigation system

By virtue of the fact that energy is consumed only when the vehicle isbeing steered, the overall fuel consumption of the vehicle may be as little as0.01% more than that of its manually steered equivalent Indeed, the energyconsumption of this electrical system is claimed to approach 80% of that of

a hydraulic system, thus reducing the fuel consumption of a medium size car

by up to about 0.25 litres per 100 km

Three versions of the Servolectric unit are available: in the first, which isfor light cars with a maximum load on the steered wheels of 600 kg, theelectro-servo is incorporated in the steering column; in the second, for mid-range cars, maximum steering axle load 900 kg, it drives the pinion; in thethird, for larger cars and light commercial vehicles, the servo acts upon thesteering rack.

40.35 Honda EPS and VGR systems

Honda first applied a mechanically driven hydraulic pump type power assistedsteering rack and pinion gear to its NSX car in 1990 In 1991, to save energy,

an electrically driven hydraulic pump replaced the engine-driven one and arotary spool type valve was introduced for the system installed on the HondaCivic Their first electrically powered system was introduced in 1993, andspeed dependent power assistance in 1995 The latter system combined reducedsteering effort at low speeds with increased stability at high speeds

In 1997, Honda announced the first variable ratio, electronically controlled,electrically actuated (EPS + VGR) rack-and-pinion steering gear As can beseen from Fig 40.58, the variable ratio is obtained by pitching the centralgear teeth of the rack more closely than towards its ends Consequently,manoeuvring for parking is easier and steering during, for example lanechanges, at higher speeds can be effected more smoothly

Overall, the rack is longer than normal Near one end, the variable ratioteeth mesh with a spiral toothed pinion Mounted coaxially around the racknear its other end is the reversible electric motor By rotating a ball-nutassembly running in spiral grooves around that end of the rack, this motormoves the rack axially in a direction determined by which way the steeringwheel is turned Spherical sockets at each end of the rack receive the ballends of the steering rods

Signals indicating the torque applied by the driver to the steering wheelare transmitted, from a sensor interposed between the steering shaft and the

Centre of rack Steering column

High ratio Standard ratio High ratio

Fig 40.58 The teeth near the centre of the rack of the Honda EPS + VGR system are more closely pitched than those nearer its ends

pinion, to the electronic control unit (ECU) The differentials of these signalsrepresent the speed of rotation of the steering wheel.

Other inputs from sensors to the ECU include the torque being delivered

by the electric motor, the resistance torque from the road surface, kickbacktorque, vibration due to wheel and tyre imbalance, and self-alignment torque

A current of up to about 10 A is required by the motor but, of course, onlywhen power assistance is actually being applied The motor and ECU aremutually adjacent, to minimise losses in the cables

Wheels and tyres

Over many decades, the wheel and tyre assembly has become increasinglyregarded as an integral part of the suspension system Section 42.2 and Fig.42.1 Consequently, it is appropriate at this juncture to examine it in detail.Since wheels and tyres are largely standard components used throughout theindustry, frequent reference has to be made in this chapter to BS AU 50 Parts

1 and 2 These Standards comprise respectively about six and eight sectionsand many subsections, taking up something like 200 pages Since all aresubject periodically to revision, readers needing more detailed informationare advised to write to the British Standards Institution, 2 Park Street, LondonW1A 2BS for the latest details

First, some historical notes While wheels date back prehistoric times, thepneumatic tyre, Fig 41.1, was invented by R.W Thomson, of Edinburgh, in

1845, and produced in the Macintosh Works in Manchester It comprised aninner tube of rubberised canvas enclosed in a leather casing This was longbefore rubber of adequate durability became available for use in tyre production

It was John Boyde Dunlop, a Scottish veterinary surgeon living in Belfast,who introduced the first pneumatic tyre for cycles Prior to this, solid rubbertyres were employed for a wide variety of vehicles Indeed, it was not until

1912 that the first pneumatic tyred truck appeared, and solid rubber tyredheavy vehicles were still to be seen on the roads up to the early 1920s Even

so, it was Michelin who, in 1891, first manufactured detachable pneumatictyres for bicycles in quantity By 1895, this company was providing tyres forthe rapidly expanding motor manufacturers in France

The early tyres produced in quantity comprised an outer casing designed

to embrace, reinforce and protect an inflated air-tight inner tube, and toprovide the essential wear resistance and grip on the road Isolation of thevehicle structure from the high frequency vibrations applied to the wheel as

it rolled along the road was, and remains, the basic function of the wheel andtyre assembly Nowadays, vibrations at frequencies below 20 Hz are absorbedmainly by the suspension springs, while the tyres absorb vertical andlongitudinal vibrations up to about 400 Hz

In general, early tyres were narrow and inflated to about twice the pressurescommon today The high pressures were necessary to reduce the chafing, due

to the scissor-like movements of the warps and wefts of the woven canvasreinforcement, which tended to cause failure by overheating the carcass of

the tyre Consequently, these tyres had to be of small cross-section and werepracticable only for use on large diameter wheels Moreover, they wereunsuitable for carrying heavy loads at speeds in excess of about 20 mph (32km/h).

To overcome these shortcomings, the wefts were eliminated by producingparallel, though biased, cord fabric casing plies In 1923, Michelin producedthe first low pressure tyres for cars, and in 1926 the first low pressure tyresfor trucks appeared However, it was not until about 1932 that tyres withdimensions and pressures similar to those in use today were introduced.During the 1930s, Michelin developed a method of continuously coatingsteel wire with brass, so that rubber could be bonded on to it This led to theuse of fine steel wire in carcass construction, and the introduction of theMichelin Metallique truck tyre which made long distance high speed transportpossible The next major advance in this context was the introduction in

1947 of the first tubeless tyre by Goodridge This was followed in 1948 bythe announcement of the first radial ply tyre, the Michelin X Incidentally,the radial ply tyre with a crossply, termed bias ply in the USA, tread bandwas invented in 1913 by two Englishmen, Gray and Sloper, but they failed

to develop the idea for production

During the early 1950s, tubeless tyres increasingly gained credibility Inthese tyres, an air-tight layer is applied to the inner wall of the carcass Theremay also be soft inner layer of very high viscosity material which flows into

Rubberised canvas

Felloe

Fig 41.1 R W Thomson’s tyre, 1845, was invented when rubber of a quality capable

of withstanding the wear and tear associated with running on road surfaces was unavailable, hence the leather outer casing

minor punctures to seal them During assembly, care must be taken to ensurethat the rims, against which the beads form a virtually perfect seal, are notscored or damaged in any way Tubeless tyres, by virtue of their robustconstruction, retain the air pressure much longer than inner tubes Furthermore,with one instead of two components to be assembled to the wheel, assemblycosts are reduced Car tyres are now almost universally tubeless as also aremany commercial vehicle tyres Many heavy commercial vehicles, however,have multi-piece wheels and rims, Section 41.3, and therefore cannot providethe seal necessary for tubeless tyres.

Incidentally, for many years two different types of bead were produced,one on the continent of Europe and the other in the UK and the USA Thelatter, Fig 41.2, left, was based on an invention registered in 1890 by Welch,

an American, and the former, Fig 41.2, right, by an Englishman, WilliamBartlett, just 36 days after Welch Nowadays, of course, the high tensile wirebead reinforcement system of Welch is virtually universally employed

A further major advance was made by Dunlop in collaboration with Michelin

In the event of a significant loss of pressure, or total deflation of the tyre, thebeads were likely to be ripped from their seatings, thus causing loss ofcontrol of the vehicle To counter this danger, the Dunlop Denloc beadlocking system was developed, and was first applied to the TD tyre, Fig.41.3, in 1984 From the illustration, it can be seen that a reinforced rubberbead, extending radially inwards from the conventional bead, registers in agroove around the wheel rim This locks the bead in its seating Any sideforces on the tyre when deflated force the Denloc bead more firmly into itsgroove, so the side wall pivots around it, instead of sliding laterally out of the

Fig 41.2 These two wheel and tyre assemblies, both invented in 1890, were the forerunners of our modern designs The Bartlett rim, left, was the basis of the British and US types, remaining in use for 40 years, and the Welch rim, right, is that of the continental and, in particular, the Michelin tyres

bead seating Obviously, such a tyre will perform safely only in rims designedfor them To avoid potential mismatching of TD tyres to conventional rims,the letters TD are included with the name of the tyre on the sidewalls: forexample, Dunlop TD SP Elite or TD SP Sport.

41.1 Wheel and tyre assemblies

Design requirements for the wheel and tyre assembly are as follows:

1 Light weight is essential otherwise each successive peak of roughness

in the road surface will propel it upwards clear of the surface This willgreatly reduce its efficiency in braking, acceleration and steering

2 On the other hand the whole assembly must be extremely strong toresist both the static and dynamic forces acting upon it along all threeaxes, and to cope with shock loading

3 Ease of cleaning and, so far as possible, self-cleaning are necessary,otherwise a build-up of dirt will increase weight and could introduceout-of-balance forces

4 The whole assembly must be in both static and dynamic balance, Fig.41.4 In this illustration, the right-hand diagram illustrates the condition

in which if the bearing were frictionless, the wheel would not rotate.However, if it were to be rotated, a rotating out-of-balance couplewould be created To eliminate this couple, the balance weight wouldhave to be fitted on the opposite rim, as in the diagram on the left

5 Since tyres are liable to be damaged or punctured, the tyre and wheelmust be easy to remove, as a complete assembly, and replace with aspare, but it must be fixed so securely to the axle assembly that it willnot come off inadvertently

41.2 Wheels

Having been developed from the wheels of horse drawn carriages, automotive

Bead extension Direction of rotation of bead

as pressure falls

Denloc tyre Conventional tyre

Fig 41.3 The Dunlop TD/Safety tyre was first applied to the BL Metro car and Michelin applied the same principle to its TDX range The bead retaining groove holds the tyre on the rim This was claimed to enable the driver to run with the tyre deflated, at speeds of up to 40 mph (64 km/h), for far enough to enable him to choose

a safe place to stop and change the wheel The hatched sections represent the

underinflated situations

Denloc bead retaining groove

Balance weights

Fig 41.4 Each of these two diagrams represents a wheel and tyre with, at the bottom,

an out-of-balance weight and, near the top, a weight applied to the rim to balance the assembly That on the right illustrates the condition in which both the centrifugal forces and the couples about the wheel centre O are in balance, while that on the left demonstrates static balance only; in other words, although the forces cancel each other

out, the rotating couples M × d and W × d are additive

wheels originally had wooden spokes, soon to be followed by metal onesalthough wire braced wheels were also employed The overwhelming majority

of modern cars, however, have disc type wheels, of which there are manyvariants, the most common being that illustrated in Fig 41.5 They may have

a central, inset, or outset nave configuration, Fig 41.6 These pressed steelwheels comprise a disc, which is spot, or fillet, welded to a rim The discs are

generally termed naves and the trim clipped on to them, nave plates Large diameter trim discs are sometimes referred to simply as wheel trim plates, while annular trim fitted to rims may be called rimbellishers.

Wheels are defined dimensionally by their nominal diameter This ismeasured beween the two diametrically opposite points on the circle formed

by the intersection of the bead seats and the inner face of the vertical walls

of the rim flange The rim profile is represented by letters such as J, JK, K,

C, etc., as defined in BS AU 50: Part 2 Tyres and Wheels Similarly, the beadseat type is represented by letters such as SL, H1, H2, etc., as defined by thesame British Standard For example, a specification might be 13 × 4J SL,where the figure 13 is the nominal diameter, 4 is the distance between thevertical inner faces of the flanges: the dimensions are in inches Metricdimensions can also be used

Among the other types are the cast light alloy wheels, which are widelyemployed for sports cars, and offered as options for some saloon and convertiblecars For cars, these are usually cast in one piece, although some compositewheels are produced with cast alloy centres and steel rims Wired bracedwheels, in which the rim is carried by a set of wire spokes secured bynipples, are suitable only if tyres with inner tubes are employed

The following definitions are based on those given in BS Au 50: Part 2:

Section 0 Each wheel has a rim and a nave, the latter being the disc transmitting

the loads from the rim to the hub Most wheel rim sections for cars have a

or seam welds

Well Tyre bead seats Flanges

Nominal diameter

Nominal width

of rim

Pitch circle dia.

Flange height

Fig 41.5 Section through a typical car wheel The top half shows a section in which are studs over which the lipped rim of a nave plate or other trim can be sprung The nominal diameter is sometimes called the fitting diameter

well between the flanges Figure 41.5 shows some of the options For removal

of a tyre, its two beads are pushed inwards until they drop into the well Thisenables the whole of the tyre to be displaced radially until it is eccentricrelative to the rim, over which the beads can then be pulled and the tyre taken

away This type is termed the well-base or one-piece wheel.

For commercial vehicles, wheels with multi-piece rims are generallyemployed to facilitate the mounting on them of the stiff heavy duty tyresrequired Two-piece wheels, Fig 41.7, are fitted mainly to special purposevehicles The bead seat is generally set at an angle of 5° to locate the tyreradially This type of wheel is unsuitable for tubeless tyres, unless specialprovision is made, perhaps O-rings, for sealing the flanges

41.3 Rims

A wide variety of rims is in use, so many in fact that they cannot all beincluded here Readers needing to know more are therefore advised to consult

Fig 41.6 Wheel disc configurations: left, inset nave; centre, central nave; right,

outset nave

Attachment face Outset

Fig 41.7 A two-piece wheel for a relatively light vehicle

BS Au 50: Part 2 Where tyres are to be fitted with inner tubes, the holes forthe inflation valves are usually slotted but for the valves used with tubelesstyres round holes are pierced in the rims.

From the illustrations, it can be seen that a well, or drop centre, is common

to vertually all rims for cars and some of the lighter commercial vehicles.This, of course, is because these tyres are removed by dropping the beadsinto the well so that they can be displaced radially and their beads leveredover the rim Wells are usually offset away from the centre to bring themclose to one flange to facilitate assembly of the tyre on to the wheel Beadseating surfaces for cars are usually inclined 5° upwards towards the rimflanges to help in forming an air-tight seal and to reduce to a minimumrelative movement between the bead and its seating In some instances, theinboard ends of the bead seatings are humped to help to retain the beads inposition if the tyre pressure falls

In general, the rim section illustrated in Fig 41.8 are typical of thoseemployed for commercial vehicles They include wheels with two-piece,

Fig 41.8 A selection of wheel rim sections for commercial vehicles Not shown are the wide-base rims, which are similar to the flat-base types, but may have a spring lock ring of a section similar to that of the three-piece semi-drop centre type shown in the centre of the illustration Whereas the latter has a 5 ° ramp immediately inboard of each of its flanges, the one-piece rims for tubeless tyres have 15 ° ramps to effect an air-tight seal with the tyre beads The spring lock rings and spring flanges resemble huge circlips, although, of course, the flanges are of more complex section

Two-piece flat base rim Three piece flat base rim

Detachable endless flange Spring lock ring

Four piece flat base rim

Detachable

endless flanges

Detachable spring flange Spring lock ring

5 °

Three-piece semi-drop centre rim

Detachable endless flange Spring lock ring

three-piece, and four-piece rims of the flat base and drop centre and drop centre types and well-base rims The last mentioned are for tubelesstyres.

semi-Another type is the heavy duty flat-base divided rim wheel This has two

parts bolted together similar to that illustrated in Fig 41.7, but of heaviersection and, of course, with a flat base These are used mainly for somespecial purpose vehicles having tyres that would be extremely difficult, oreven impossible, to fit to and remove from one-piece or even multi-piecewheels Also for special purpose applications, there are adjustable wheels inwhich the offset of the nave relative to the rim is adjustable Other varietiesinclude reversible wheels, Fig 41.9 These have deeply dished naves, eitherface of which can be mounted on the brake drum, to provide respectivelyeither an inset or an outset layout giving a narrow or wide track For axleswith two wheels at each end, there are dual, or twin, wheels Fig 41.10.Tyres for heavy commercial vehicles, however, are so heavy and stiff thatthey could be neither removed nor fitted by levering them over the rims

Attachment face

Attachment

face

Inset

Outset

Fig 41.9 This wheel can be reversed left to right, in which case the attachment face

to the brake drum is that on the other side of the nave to provide a wider track Whereas in the former case the brake drum is enveloped by the wheel, in the latter it

is not and therefore is better cooled

Fig 41.10 This is a layout widely employed for axles with two wheels at each end; these two are bolted back to back

Consequently, rim bases for this type of vehicle are either flat or almost so,and one or even both of the flanges are removable so that the tyre can be slid

on to or off the base To hold the bead securely, however, some have a veryshort 5° incline in the bead seating area close to the flange

Various forms of locking device are employed to hold the rim in placewhen the tyre is inflated The most common is a split ring which is sprunginto a groove, rather like a huge circlip, although two-, three- or four-piecelocking rings, which are themselves locked in position by the rim flanges,are used on some heavy vehicles

One-piece rims are also available for commercial vehicles These have

15° bead seatings, narrow rim flanges and deep wells They are suitable fortubeless tyres but are unsuitable for very heavy vehicles Such wheels areabout 10% lighter and tend to be better balanced than two-piece wheels.Another advantage is that they provide more space for accommodation of thebrake discs or drums than do multi-piece wheels

Six types of rim are in general use in the UK These are, wide-base, flatbase semi-drop centre, drop centre, well-base, and divided rims They alsofall into different categories, according to the method of securing their flanges,

as follows First, a one-piece rim does not have any detachable parts Second,there are two-piece rims, with a detachable spring flange similar to thepreviously described spring lock rings but of rim flange, instead of elliptical,section A three-piece rim comprises a rim base with one endless flange and

a spring lock ring Four-piece rims have two detachable endless flanges, each

of which is retained by a spring lock ring finally, there are the two-piecedivided rims already mentioned comprising two main parts bolted together.Wire braced wheels, Fig 41.11, had mostly disappeared from general useshortly before World War II, although they are still fitted to some racing andsports cars They have the advantages of light weight and the relatively freepassage of air through them for ventilation of the brakes, but their assembly

is labour intensive and they are difficult to clean Originally, they resembledcycle wheels in that they had two rows of radial spokes Later, the spokeswere fitted tangentially between the central shells and the rims, to transmitdriving and braking forces Where the two rows are attached to the shells,

Rim Wire spokes

Hub seat Splines

Outset Reference

plane

Conical seat for retainer nut Shell

Fig 41.11 Two arrangements of wire spokes enabling both transverse and radial loads

to be transferred from the rim through the shell to the axle

they are widely spaced, but at the rims they are either closely spaced orarranged in a single row This triangulates the structure and thus renders itcapable of transmitting the lateral loading between the rim and the hub.

41.4 Wheel fixing

Primary radial location of wheels can be effected either by spigoting thewheel nave on to a flange on the hub or by the use of special studs and nuts,Fig 41.12 Good location is essential to the maintenance of accurate balanceabout the axis of rotation Conical or spherical seating nuts are employed tolocate the wheel assembly and to prevent the nuts from being loosened byvibration and other forces Wire wheels can be secured by nuts and studs on

a flange on the hub, Fig 41.12, left Bolt-on wire wheels were even used onsome early heavy vehicles

For sports and racing cars in particular, the alternative method of a centre lock or knock-on fixing nut has been widely employed This is simply a large

nut that is screwed on to a threaded extension of the hub and pulled upagainst the conical section on the outer end of the shell, Fig 14.12, right.With such a fixing, the wheels can be rapidly removed and replaced whentyres are changed during pit stops These fixings are of either the cap nut

type or the ring nut type, and a hub cap may be sprung or screwed over them

to retain the grease in the wheel bearings and to keep all the moving partsclean They can be located radially either by a spigot or by being tightened

on to conical rings or seatings Nuts with left-hand threads are fitted on theoff-side and right-hand threads on the near-side hubs, so that they will notwork loose They are appropriately identified by an L and R stamped orembossed on them

The centre-lock type has either a serrated or a hexagon head so that thenut can be tightened with a large spanner, and the knock-on type has a

Spigot

Fig 41.12 The two diagrams on the left show respectively how single and double wheel assemblies can be located by means of a spigot machined on the attachment flange on the axle Those on the right illustrate location by conical seating nuts Double wheels can be carried cantilever fashion on large diameter studs, the gap between the inner nave and the attachment flange reduces the rate of transfer of heat from the brakes to the wheel An alternative method of location is the use of part spherical instead of the conical seatings, but without the gap between the inner nave and attachment flange These seatings may be either integral with the nut, as are the conical ones illustrated, or separate washers; the inner and outer seatings are of different thicknesses and therefore not interchangeable

diametrically opposed pair of lugs, or wings, projecting outwards from theperiphery of the nut In the early days, the last mentioned were in someinstances loosened and finally tightened by knocking the lugs with a hammerand drift However, the removal and replacement was more usually donewith a special spanner, and the final tightening done by striking with ahammer the arm of the spanner or, if a box spanner is used, its tommy bar.Occasional fractures of studs and loss of wheels on the road led to thedevelopment of the more sophisticated, manual and powered, torque spannersnow employed for virtually all wheel tightening.

Security of wheels is, of course, of paramount importance While carsgenerally have either four or five wheel nuts and studs, commercial vehiclesmay have six, eight or ten To ensure that they are tightened uniformly andwithout distortion of either the wheel or its mounting flange, diametricallyopposite nuts have to be tightened progressively in turn, first at 6 and 12o’clock and next at 3 and 9 o’clock, etc or in an order stipulated by thevehicle manufacturer Spherical seating nuts must not be tightened on toconical seats and vice versa, otherwise fracture of studs and loosening ofwheels in service may follow BS Au 50: Part 2: Section 7 gives a code ofpractice for the selection and care, including fitting, of wheels for commercialvehicles and Section 8 is for cars

41.5 Light alloy wheels

So far we have covered the design of steel wheels The same principles apply

to cast aluminium or magnesium wheels, but there are other considerations.These wheels generally have one-piece rims They may have inset, central oroutset naves, as for steel wheels, Fig 41.13 An inset nave is one in whichthe centre of the rim profile is inboard of the nave mounting face, and anoutset wheel has its rim centre outboard of that face

Definitions, based on BS Au 50: Part 2: Section 5, are as follows Between

the wheel hub and rim is the centre member, which can be of either the disc, spider or spoked type The nave is the portion of the centre member comprising the mounting face and the fixing features Materials used should preferably

comply with BS 1490 or BS 2970 However, other alloys may be usedprovided they comply with performance requirements specified in considerabledetail in BS Au 50: Part 2: Section 5

Outset Central

Inset

Fig 41.13 Left to right respectively: inset, central and outset cast alloy wheels

disturbances generated by its rolling along the road Lower frequency highamplitude deflections are absorbed by the suspension springs Becauseacceleration is proportional to the square of velocity, a 30 cm diameter steelrimmed wheel riding over a small obstacle at 4 mph may accelerate vertically

at a rate of 1g and, at 60 mph, at 200g and thus consuming considerable

energy Moreover, the wheel leaves the ground and grip on the road is reduced

to zero, with adverse effect on road holding

Tyres, however, have to perform several other functions, all of which areequally, if not more, important For example, the tyres must be capable ofsupporting both the static and dynamic vertical loads applied to the contactpatches between their peripheries and the road Only 5% of this load is taken

by the sidewalls of the tyres, the rest being taken by the air pressure acting

on the inner surface of the tyre in the contact patch Therefore, if a wheel vehicle weighs 2000 lb (56.63 kg), and the tyres are inflated to 1.72bar, the total load on the contact patch is 80 lb (36.3 kg), of which 0.04 lb(0.18 kg) is taken by the side walls

four-If we ignore the effect of the side walls, the total area of each of the fourcontact patches would be 20 in2 (129 cm2) These figures are of majorsignificance in relation to wear of the road Whereas a man would impose,through the soles of his shoes, approximately the same pressure, a solidwheel with a steel rim would impose between 10 and 100 times as much.The contact patch also transmits all the other loads, such as steering,braking, acceleration It therefore influences to a major degree qualities such

as stability and handling, as well as ride and road holding

Driving and braking torque are transmitted through the contact area betweenthe bead and the wheel rim, to the sidewalls, and thence to the tread andcontact patch of the tyre to the road The forces and stresses, especially onthe sidewalls, can be very large, as also can be those involved in steering andcornering During cornering, a lateral force must be applied through the rear

as well as the front tyres, otherwise the front wheels would go round thecorner and the rear pair would carry straight on The situation is aggravated

by the fact that the lateral forces on the laterally advancing leading side walls

of each wheel are higher than those on the trailing ones

Lateral forces are generated by the elastic deflection of the tread, thepattern of which influences the steering characteristics of the tyre, its rate ofwear and its effectiveness in squeezing water from between the contact patchand the nature of the road surface The overall effect of this elastic deformation

is what is termed the slip angle Without slip angle there can be no steering

effect What happens is that the tyre, following Newton’s first law, tends toroll forward in a straight line but the deformation of the rubber in the contact,due to the angle at which the wheel is deflected by the steering gear,progressively moves it laterally Thus, by providing the external force necessary,according to Newton, to change the direction of motion and, incidentally,both generating heat and causing wear

Slip angle is defined as that between the direction along which the tyre isaligned and that along which the vehicle is travelling Provided the wheelsare accurately aligned relative to each other on the vehicle, the slip angle will

be zero when it is travelling in a straight line The lateral force created at the

slip angle is termed the cornering force It increases with slip angle until the

tyre can no longer generate extra cornering force, at which point it begins to

slide laterally Cornering force is a function of both slip angle and inflationpressure Consequently, decreasing the inflation pressure on the front tyres,for example, will increase understeer This is because, to produce the samecornering force, the slip angle will have to be larger In other words, thesteering wheel will have to be rotated further.

Tyres wear more rapidly in hot than in cold weather This is because of thecombination of high ambient, and therefore static, temperature with heatgenerated by the effects of flexure of the rubber and the rapid changes inlocal pressure under dynamic conditions, and the abrasion between the tyreand road, causes general softening and, in the extreme, even local melting ofthe rubber in the contact patch In cold weather, and especially when wet,rates of wear are much lower

41.7 Tyre construction

Details of the construction of a Dunlop crossply tyre are shown in Fig 41.14and radial ply tyres in Fig 41.15 In each type, the beads comprise parallelhoops of steel wire bound with fabric to form a tightly wrapped bundle in theform of a ring The ends of the plies are wrapped around them In crossplytyres, there are generally several plies, or layers, of fabric reinforcement inthe form of cords of rayon, nylon or polyester The cords are rubberised andset at an angle of approximately 45°, each alternate layer being orientated in

Fig 41.14 Relative flexure of the plies laid alternately at 45 ° to each other in crossply tyres tends to generate more heat than do those in the radial ply type

Tread

Casing plies

Inner liner

Filler Casing

plies

Bead wrap Bead wires

Casing plies Chafer

Wall rubber

the opposite sense to those above and below it Flexure of this layer generatesheat, which is why radial ply tyres, which have fewer layers and in which thecords are parallel, tend to last longer A breakthrough in tyre design camewhen, in the 1930s, Michelin found that they could bond rubber to steel wire

by first coating them with brass This was the breakthrough that enabledthem to produce plies of rubberised steel wire

Rayon is the material most widely used for radial plies for cars and steelfor heavy vehicles Other materials include nylon, polyester, glass fibre andaramid fibre Polyester is stronger than Rayon, but is difficult to bond torubber and is less stable dimensionally when hot Nylon has similar problemsand has tended to fall out of favour since the introduction of radial ply tyres.Glass fibre is stronger but has poor fatigue resistance Aramid fibres such asKevlar are very strong in relation to their weight but are costly

Beneath the tread of a radial ply tyre there are normally two or threecrossply layers of steel, or four or six of textile such as Rayon These are

termed bracing plies, and the cords are set at angles much lower than those

of the plies of crossply tyres In some instances the steel wires have beensupplemented by nylon or polyester supporting threads The function ofbracing plies in general is to prevent the tyre from being distorted by centrifugalforce at high speed, to stabilise the contact patch and to increase the resistance

Bead toe Bead core

Centring, or fitting line

Fig 41.15 Beneath the treads of radial ply tyres are usually two or three steel or four

or six textile cross braced bracing plies These support the tread against centrifugal force, stabilise the contact patch and increase resistance to punctures

crossply tyres termed bias belted tyres was introduced, mainly in the USA,

to combine the best performance features of the radial and crossply types.Their tread regions were braced by additional crossply layers at a cord angleslightly lower relative to the circumferential plane of the tyre than that in thetread bracing layers of the radial ply type

Radial ply tyres tend to be stiffer in resisting lateral loading than crossplytyres This reduces under- and oversteer It is illegal to fit radial ply tyres tothe front if there are crossply tyres on the rear wheels, because this combinationwould be liable to accentuate to a dangerous degree any tendency to oversteer.Because the stiffness of radial ply tyres is also greater in the fore and aftdirections they tended, when first introduced, to give a harsher ride Thistendency was subsequently overcome by the fitting of suspension busheswith increased flexibility in those directions

41.8 Tread design

The tread itself is of a special rubber mix formulated to provide good wearresistance, firm grip on the road surface and high strength to cope with thevarious loads applied to it in the contact patch area Indeed, different types

of rubber may be employed for the treads, side walls, beads and fillers Treadmaterials and patterns are determined by the type of duty required Racing

cars, for example, have what are termed dry and wet alternatives which may

be interchanged during a single race in response to weather changes.The earliest tyres had plain treads In the 1920s, however, simple blockpatterns, in many instances with a single peripheral rib for longitudinal gripand stability, were widely used A problem was irregular wear of the blockpattern and taper facing of the individual blocks The latter results in theirbehaving like Michel bearings on wet roads, with a consequential loss ofgrip By 1930, the emphasis become increasingly on peripheral ribs, withblock patterns on the shoulders where the loading was not so severe exceptduring cornering Up to the 1940s maximum speeds of the small family cartended to be of the order of 60 to 65 mph By 1950, these had increased to

70 to 85 mph, so the edges of the peripheral ribs were in most instancesserrated to increase the grip

In the 1950s, aquaplaning began to be recognised as a serious problem Ithad previously been found to affect the landing run necessary for aircraft onrunways Consequently, some research into the phenomenon had alreadybeen done, and a useful rule of thumb for estimating aquaplaning speeds foraircraft tyres without treads produced It is:

Aquaplaning speed = 9 × √tyre pressure

This result is based on experimental data and the fact that recommended tyrepressures are a function of, among other things, the load on the tyre and thearea of its contact patch with the road

Following a great deal of research with road vehicles, the need becameapparent for the spaces between the tread blocks and ribs to be increasedand, in some instances, the blocks and spaces to be tapered from their roots

to the periphery of the tread, to provide passages of progressively increasingwidth through which the similarly progressively increasing volumes of watercould be squeezed out from the contact patch To the same end, V-sectionslots were formed extending inwards from the outer edges of the ribs Lateralbars in the tread pattern improved the grip for acceleration and braking

Incidentally, it is a legal requirement that tyres must not be used if theyhave less than 1.6 mm depth of tread Among the other requirements is thatthere be no damage to the plies in the side walls, nor bulges on them Thelatter indicate that the plies have separated from the rubber of the carcass.Exposed plies too are illegal regardless of their position on the tyre.

41.9 Off-road vehicle tyres

Off-road tyres, such as those fitted to many 4-wheel drive vehicles, generallyhave deeply grooved block patterns with perhaps one central peripheral rib

to give respectively good grip and directional stability in soft ground Forsnow tyres, the emphasis is more on peripheral ribs to improve directionalcontrol and stability and with lateral bars to improve grip Where mud orsnow is liable to become jammed in the tread, the sides of the blocks and ribsare appropriately tapered so that it will readily be flung clear by centrifugalforce and the relative movement between the inividual blocks and otherelements Such tyres generate a great deal of noise on well-surfaced roads.Heavy tyres with chunky treads generally have lower speed ratings, give

a less comfortable ride and are noisier than the specialised tyres designed for4-wheel drive cars To avoid wind-up of the transmission, all four wheels ofsuch vehicles should have identical tyres This is advisable even if there is acentre differential gear, to avoid its being loaded unnecessarily Virtually allspecial off-road tyres, and especially those designed to maintain traction andflotation on soft sand, have lower speed ratings on metalled roads than thoseoffered for use mainly on the road with only occasional forays off it

41.10 Noise

Noise is generated by the impacts of the individual elements of the tread onthe road surface, and it can be reduced in several ways First, the treadelements are pitched at irregular intervals, to avoid resonance of components

in the vehicle structure at the frequencies of their contacts with the roadsurface Second, tread materials of high hysteresis are employed, to provideinternal damping This, of course, has to be a compromise, since high hysteresis

is associated with the generation of heat Third, patterns, shapes and sizes ofthe tread elements are selected that do not generate noise For example,lozenge-shape tread blocks may be employed, so that the whole leading edge

of each block, in turn, rolls down on to the road surface progressively,instead of simultaneously

Block size is important too: large blocks hit the road harder than smallones On the other hand small blocks imply high crown stresses and thereforepoor wearing qualities so, again, a compromise is necessary Peripheral ribs,however, are relatively silent and give good lateral grip for steering and theavoidance of skidding, although the clearance of water from between themmay be difficult Indeed, the design of both tyres and their treads involvesmany compromises between noise, grip, water ejection, coolness of operationand rate of wear

41.11 Aspect ratio and tyre markings

The aspect ratio is the inverse of the ratio of the width to the height of thecrown of the tyre above the edge of the bead that seats on the rim Modern

tyres are all wider than they are high Before World War II, Avon, for example,produced their Super Safety range, which were Super Balloon type tyreswith an aspect ratio of 95% By the time that radial tyres were introduced, inthe late 1940s, most of the crossply tyres had aspect ratios of 88% (LowSection) or 82% (Super Low Section) These reductions in aspect ratio werethe result of increasing maximum speeds and weights of cars in general, and

a consequent demand for better stability and control plus greater wear resistance.Other incidental advantages of low aspect ratio are that, for a given load anddiameter, the lower the aspect ratio the wider must be the section, so thelarger is the contact patch and the more aesthetically pleasing is the appearance

of the tyre Trends in body design have played a part too: for example, lowbonnet profiles have demanded low aspect ratios, and low platform commercialvehicles call for low aspect ratio tyres

The first radial ply tyres were of the Super Balloon type, distinguishedfrom the crossply types by specifying metric section dimensions with inchrim widths and diameters In the 1960s came the Ultra Low Section, at 77%aspect ratio, followed progressively by the 70 Series with 70%, 65% aspectratio and so on at 5% intervals right down to 35%

Car tyres have been marketed in 4, 6 and 8 ply ratings These ratingsoriginally represented the actual number of plies, but no longer do so Theynow signify the load carrying category of the tyre They are therefore anindication of the strength and inflation pressure of tyres Indeed, a 4 ply ratedtyre may have 2 or 4 plies and a 6 ply rated tyre 2, 4 or 6 plies The 6 and 8ply rated tyres are known as reinforced radials, and are, of course, capable ofcarrying heavier loads than the 4 ply rated equivalents

The data marked on the tyre wall, including those for tyre size and aspectratio, are illustrated in Fig 41.16 Tyre size and aspect ratio markings followeither of two standard patterns The first which is obsolescent, was based onthe then common radial tyres with an aspect ratio of 80% It took the form:

165 SR 13where:

165 was the nominal width, in mm, of the section

S indicated a speed rating of 113 mph, or 180 km/h

R indicated radial ply and 13 was the rim diameter in inches

The later system of marking is of the form:

165/80 R 13 82T MXT80where the first four symbols give the same information as before, and:

80 is the aspect ratio

82 is the load index, indicating that the tyre can carry a load of 475 kg at

a speed of 119 mph, or 190 km/h

T is the speed symbol

MXT80 is a Michelin tread pattern

Both load and speed symbols are listed in the British Standard There are

118 load symbols, ranging from 45 to 1360 kg: too many to be listed here.Speed markings used to be S for normal, H for high and VH for very high.Now, however, markings according to BS Au 50: Part 1 are used These startwith F for 80 km/h and L for 120 km/h and then, through the alphabet, to M,

l

Treadwea

r 160tra ction

Tem peratu

yo

Pli es

ad 2

S Re in r

ce

d

Rad ia

SR 12

11 5

D

F 67

reat ritain

W

TW I

N, etc right up to U for 200 km/h, followed by H for 210 km/h and V for 240km/h, 150 mph

For crossply tyres, the R was, of course, omitted from the side wallmarkings The tyre sizes were expressed in inches: for example, 5.6–13,where the 5.6 was the nominal section width and the 13 the rim diameter.Some crossply tyres with an aspect ratio of 77% were produced in the 1970s,and these had the markings 6.2–13

41.12 Tyre design considerations

The design requirements of tyres are as follows:

1 Adequate capacity to support both the static and dynamic loads

2 Ability to withstand centrifugal loading

3 Structural stability, to resist degeneration

4 Reasonable protection against abuse such as striking kerbs This impliesside walls of adequate strength Some tyres have a rib around each sidewall to take some of the wear and shock loading involved

5 Cooling, to avoid overheating This implies good thermal conductivity,low hysteresis and absence of chafing between plies

6 Good grip on the road, to withstand acceleration braking and cornering

in both wet and dry conditions Tread pattern, cross-sectional proportionsand shape, and rubber mix are the major factors

7 Good ride comfort and silent running These are dependent largely ontread pattern, rubber mix and carcass design

8 Light weight, to help provide good ride comfort, ease of control and

Speed symbol Load index

Uniform tyre quality markings (a US requirement)

Tread wear indicators, not

on all tyres

Old stye speed marking, including speed symbol S

ruction EEC type approval

Const-Manufacturer Not needed

in UK

Commercial name

Only if tubeless Country of manufacture

N American tyre ID No.

N American compliance

Fig 41.16 Diagram showing the markings to be found on the sidewalls of tyres

fuel economy Heavier tyres absorb more energy as they roll and bounce

The primary design parameters are load and speed rating All the otherslisted above have to be a compromise chosen according to the needs of theapplications and operating conditions

Vehicle manufacturers specify the physical dimensions, including aspectratio, to suit the dimensions of their brakes and wheel arches They also havethe choice of casing, tread construction and tread pattern and even rubbercompounds Again a compromise is necessary to match all these requirementsagainst cost

For satisfying legal regulations, BS Au 50: Part 1: recommends the following

formula for determining rolling circumference CR for all car tyres except

winter tyres (M + S):

CR = F · d

where:

d is the overall diameter of the tyre when new

F is 3.5 for radial tyres and 2.99 for crossply tyres.

This formula is based on a speed of 60 mph (96.6 km/h), and tyre loadsand pressures according to tables given in the British Standard Tolerancesquoted are 3% for crossply and 2.5% for radial ply tyres

This Standard also gives guidance on allowances necessary to counter theeffects of camber angle on tyre performance For camber angles between 0°and 2°, the tyre load capacities given in tables in the Standard are appropriate.From 2° to 3°, the tyre load capacity should be 95% of that recommendedand from 3° to 4° it should be reduced by a further 5%

41.13 Run-flat tyres

If a tyre is punctured or bursts, and therefore its pressure drops, any sideforce applied to it will cause one or both of the beads to drop into the well.Consequently, it is easily ripped off the wheel Even if it remains on thewheel, it is unstable and the driver cannot exercise steering control with anydegree of accuracy The outcome, especially at high speeds, is frequently aserious accident

Many manufacturers have produced tyres that can be run and steeredreasonably accurately if the inflation pressure falls The aim is at preventingthe tyre from leaving the wheel Devices ranging from a simple band tightenedround the rim to cover the well in the rim after the tyre has been fitted butbefore it is inflated, to others such as spacers to be expanded between thetwo beads to lock them in their seatings

The simple band device is among the lightest, but it does little to promoteaccuracy of steering control A variant of the bead locking device is one thatembodies what in effect is a solid rubber tyre the diameter of which is

considerably smaller than that of the inner face of the tread band, Fig 41.17.

If the tyre runs flat, the wheel runs on the inner rubber component All thesedevices, of course, increase the weight of the tyre and wheel assembly to agreater or lesser degree If the beads are locked in their seatings, at least areasonable degree of steering control can be exercised

Fig 41.17 In common with most of the run-flat systems, the TSS design is aimed at keeping the tyre beads on their seats if the tyre deflates It is suitable for 2-piece wheels and 3-piece rims The bead spacer seals against the tyre beads thus, in effect converting a tubed into a tubeless tyre Among its considerable advantages for military applications is the ability to run on the bead spacer even if the tyre walls are totally destroyed

41.15 Manufacture

A Michelin manufacturing process is illustrated diagrammatically in Fig.41.18 First, the raw rubbers, both natural and synthetic, are removed fromtheir bales and fed individually, according to type, into a premastication

machine from which they are delivered into what is termed a batch box Here

the various compounding materials are added The materials in the batch boxare then fed into a Banbury mixing machine, whence the rubber mix isdelivered to a mill for forming into sheet material Approximately 8% powderedchamicals, 23% carbon black, 3% texile, 18% steel cord and 48% rubbers gointo a modern tyre

The output from the mixing machine is cut into rectangular pieces of sizeseasily handled, to form the rubber stock This stock is reheated, and thensome of it extruded to form strips of rubber for use as tread, walls and fillers.The remainder of the stock is also extruded and the strip thus producedcalendered and laminated with the rubberised cords, or rubber coated steelwires, to form the plies Incidentally, the steel cords are wound from extremelyfine wire, to obtain adequate toughness coupled with flexibility

Building up the tyre begins with the bead wires, around which the casingplies are wound All the components of the tyre are then assembled, in turn,

on to a rotary jig Then the rubber components of the side walls and, last ofall, the tread are added Finally, the complete assembly is placed in the

Fig 41.18 One of the Michelin tyre manufacturing sequences

Natural Rubbers

Bale

cutting

Premastication Weighing Batch box

Steel rayon or cotton

Compounding ingredients Synthetic

Finished tyre

Banbury

mixing

Reheating

Fig 41.19 Goodyear remoulds a substantial number of first life casings at its Dudley plant, under contract for bus and coach operators

vulcanising mould, where it is heated while pressure is applied, both radiallyand axially, to mould it into its finished shape

41.16 Retreading worn tyres

Retreading car tyres is practicable but, because of the high speeds at whichthey can be operated, not recommended, and is illegal in some countries.They are in fact speed limited, but it is difficult to guarantee that these limits

will not be exceeded On the other hand, commercial vehicle tyres are verycostly and, because they are of heavy section, some are suitable for beingroutinely retreaded for certain applications Again they are speed limitedand, of course, must not be overloaded Excessive flexure and heat is liable

to cause deterioration of the bond between the tyre and carcass, with theresult that the retread will break up and be flung off by centrifugal force.Carcasses for retreading must be clean and in sound condition, whichmeans free from both mechanical damage and degradation of the rubber Thetask is generally undertaken by companies specialising in this type ofproduction First, the clean tyre is placed on a rotary jig, Fig 41.19, and theresidual tread machined off Next, the surface to which the new tread strip is

to be bonded is roughed and finally checked to ensure that it is truly circularand concentric with the mandrill on which the jig is mounted After the newtread strip has been wrapped around the carcass, in a manner similar to thatfor applying it to a new tyre, the complete assembly is placed in a mould inwhich the replacement tread is vulcanised and the tread pattern formed

Suspension principles

Obviously, if the loads applied to the rolling wheels of a vehicle were transmitteddirectly to the chassis, not only would its occupants suffer severely but alsoits structure would be subjected to an excessive degree of fatigue loading.The primary function of the suspension system, therefore, is to isolate thestructure, so far as is practicable, from shock loading and vibration due toirregularities of the road surface Secondly, it must do this without impairingthe stability, steering or general handling qualities of the vehicle The primaryrequirement is met by the use of flexible elements and dampers, while thesecond is achieved by controlling, by the use of mechanical linkages, therelative motions between the unsprung masses – wheel-and-axle assemblies– and the sprung mass These linkages may be either as simple as a semi-elliptic spring and shackle or as complex as a double transverse link andanti-roll bar or some other such combination of mechanisms

42.1 Road irregularities and human susceptibility

Some indication of the magnitudes of the disturbances caused by road

irregularities can be gained from Surface Irregularity of Roads, DSIR Road

Research Board Report, 1936–7 From this report it appears that surfaceundulations on medium-quality roads have amplitudes in general of 0.013 m

or less, while amplitudes of 0.005 m are characteristic of very good roads.The average pitch of these undulations is under 4 m while most road vehiclewheels roll forwards at about 2 m/rev In addition to the conventional tarmacroads, there are pavé and washboard surfaces, the latter occurring largely onunsurfced roads and tracks Representative replicas of these two types of

surface are described in The MIRA Proving Ground, by A Fogg, Proc A.D Inst Mech Engrs 1955–65.

Obviously the diameter of the tyre, size of contact patch between tyre androad, the rate of the tyre acting as a spring, and weight of wheel and axleassembly affect the magnitude of the shock transmitted to the axle, while theamplitude of wheel motion is influenced by all these factors plus the rate ofthe suspension springs, damping effect of the shock absorbers, and the weights

of the unsprung and sprung masses The unsprung mass can be looselydefined as that between the road and the main suspension springs, while thesprung mass is that supported on these suspension springs, though both mayalso include the weights of parts of the springs and linkages

Two entirely different types of shock are applied to the wheel: that due tothe wheel’s striking a bump, and that caused by the wheel’s falling into a pot-hole The former will be influenced to a major extent by the geometry of thebump and the speed of the vehicle, while the major influence on the latter,apart from the geometry of the hole, is the unsprung masses and spring rates,speed being an incidental influencing factor.

Human sensitivity to these disturbances is very complex, and a more

detailed discussion can be found in Car Suspension and Handling by Donald

Bastow, Pentech Press, London, 1980 It is widely held that vertical frequenciesassociated with walking speeds between 2.5 and 4 mph – that is, 1.5 to 2.3

Hz – are comfortable, and that fore-and-aft or lateral frequencies of the headshould be less than 1.5 Hz Dizziness and sickness are liable to be experienced

if the inner ear is subjected to frequencies between 0.5 and about 0.75 Hz.Serious discomfort may be felt in other important organs at frequenciesbetween 5 and 7 Hz

42.2 Suspension system

A suspension system can be represented, in simplified form, as illustrated inFig 42.1 The natural frequency of the sprung mass – that at which it wouldbounce up and down if momentarily disturbed and left to bounce freely onits springs – is determined by the combined rate of the tyres and the suspensionsprings in series, which is—

1

1 + 1

where R is the overall suspension rate

Rs is the suspension spring rate

Rt is the tyre rate

In Fig 42.1, the shock absorber is the hydraulic damper at D Any friction

in the suspension system will be additional to the hydraulic damping However,whereas the hydraulic damping force of the shock absorber can be taken asproportional to the square of the vertical velocity of the sprung mass relative

to that of the unsprung mass, the dynamic friction damping force is, in effect,constant regardless of velocity It follows that while small amplitude, small