The Motor Vehicle 2010 Part 14 pot

936 The Motor Vehicle road. This not only increases the speed of onset of aquaplaning, but also helps to maintain a high coefficient of friction between tyre and road. Incidentally, a useful rule of thumb for estimating aquaplaning speeds for tyres without treads is: Aquaplaning speed = 9 × √tyre pressure It is based on experimental data and the fact that recommended tyre pressures are a function of, among other things, the load on the tyre and the area of its contact patch with the road. Aquaplaning occurs when the film of water on the road is driven by the forward rolling motion of the wheels into the wedge-shaped-gap between the tyre and the leading edge of its contact patch with the road. At the critical aquaplaning speed, the pressure in this wedge of water has risen to the point at which it is high enough to support the vehicle. Therefore, the tyres then ride up on to the film of water, which, of course, has a coefficient of friction even lower than that of ice, so the car is floating and will respond to neither steering nor braking forces. An important aspect of design for active safety is the minimisation of driver stress and fatigue. Another is provision for warning the driver of danger as early as possible before the situation becomes critical. To this end, good all round visibility and efficient lighting at night are, for instance, two of the measures that can be taken. Others include the installation of devices such as electronic detection systems for warning the driver that he is becoming drowsy: some of these depend on the monitoring of eyelid movements and others of pulse and steering wheel movements. Thirdly, the design should be such that, should the car become involved in an accident, its occupants will be, so far as practicable, protected from injury due to collapse of the structure. Fig. 36.10 The HITS (Head Impact Test System) rig used by MIRA for assessing the occupant-friendliness of interior components and trim 937Vehicle safety 36.6 Structural safety and air bags Since it is neither practicable nor desirable to build vehicles as strong as tanks, their basic structures must be designed to collapse in a controlled manner in an accident. A prime consideration is to prevent the steering wheel from being thrust back and crushing or penetrating the driver’s chest or neck or, perhaps, even breaking his jaw. Among the measures originally adopted were the inclusion of telescopic or concertina type collapsible elements in t he steering column. In some early instances, the lower end of the steering column tube was coarsely perforated, so that it would collapse when subjected to heavy axial loading. Another of these measures was the incorporation of two universal joints, one at the lower end of a shortened steering column shaft and the other on the steering box, the section between them being set at an angle relative to the axis of the steering column. In the event of a front end impact, the section between the two universal joints would displace laterally instead of pushing the upper part of the column back towards the driver. Subsequently, two further changes were made. One was to increase the area of the hub of the wheel, to reduce the intensity of loading locally on the chest. The other was to reduce the stiffness of the rim of the wheel, so that, if the driver was thrown forward on to it, it yielded rather than severely damaging his rib cage. Later, gas-inflated bags were installed in the steering wheel hub, Fig. 36.11. These are supplementary safety devices, as they are effective only in conjunction with correctly adjusted seat belts. They can be inflated by air but, to obtain rapid deployment, inflation using chemicals producing nitrogen or other gases are more commonly used. Correctly tensioning the belt is important, otherwise it will fail to guide the driver in a manner such that his face comes down on to the air bag instead of slithering over it and striking hard objects beyond. In the USA, failure of drivers to fasten seat belts has been the cause of serious injuries, which has led, unjustifiably, to doubts being expressed regarding the effectiveness of air bags. For the protection of front seat passengers, air bags are installed behind a panel in the dash fascia, and side air bags may be embodied in the seat squabs. An advantage of the latter site is that it moves with the seat when its position is adjusted, so the bag can be smaller than if it were stowed, for example, in the door. Moreover, in the door, it could be more vulnerable to impact damage. Mercedes has developed what they term window bags, 2 m long, for the protection of the heads of all the passengers, which otherwise could be injured either by hitting the side window or by intrusion. These are stowed in the sides of the roof, and deploy in 25 ms. Bags suspended from the cant rail and extending the full length for protecting the passengers in both the front to the rear seats are sometimes called curtain bags. In general, because the occupants’ heads start further away from the bags than do their shoulders, side bags at or near shoulder height, for protection against side impacts, should open earlier than those for either window bags or those for frontal impacts. To meet this requirement, Toyota have developed a system in which pellets of a chemical that generates mostly argon gas are used for inflation. The sensors are mounted low in the centre pillars and the air bags are stowed in the front seat squabs. 938 The Motor Vehicle Padded lid Bag Casing Inflator Dash Steering wheel pad Enhancer Squib Bag Gas generant Screen Inflator Fig. 36.11 Two Toyota gas bag installations: left, in the dash for the front seat passenger and, right, in the steering wheel hub, for the driver. In both instances, an electrically fired squib generates the heat to fire the pellets which generate the gas. As the bags inflate, they push away the padded trim panels beneath which they are housed Since the primary impact may be over within 10 ms, all the bags have to deploy within 20–30 ms. To obtain rapid deployment, most manufacturers employ pellets of sodium azide which, when heated, produce large quantities of nitrogen to inflate the bags. Sodium azide is a salt of hydroazic acid (N 3 H 3 ). Initially, air bag deployment was mostly triggered by deceleration force acting on some very simple form of mechanism, such as a ball in a tube, mounted adjacent to, or within, the steering wheel hub. Subsequently, electrically fired gas generators have been triggered by computers in response to its receipt of appropriate deceleration signals. The deceleration sensors are usually mounted on a front transverse member of the vehicle structure. An advantage of this system is that the whole sub-assembly, including the gas generator, can be housed compactly within the steering wheel hub assembly, and the deceleration sensor can be placed in any position where it will be most effective, Figs 36.11 and 36.12. Perforations in all bags allow the gas to leak out at a rate that increases with internal pressure, thus modifying their spring rates so that the occupants’ heads do not rebound violently. This at least reduces, and hopefully even completely obviates, the possibility of spinal whiplash damage. Moreover, the deflation and collapse of the bag, within a few ms after inflation, leaves the steering wheel relatively clear of obstruction so that the driver will have a better chance of regaining control after the impact. In the event of a multiple collision, the air bags are, of course, effective in only the first impact. Steering wheel 939Vehicle safety 36.7 Passenger compartment integrity The compartment that houses the driver and passengers should remain intact after an accident. Four measures are necessary: one is to incorporate crush zones at the each end of the car; the second is to stiffen the door and its immediate surroundings so that, in the event of a side impact, it will not be penetrated or deflected violently inwards and strike the occupants; third, the door trim must be soft or side air bags must be installed so that, if the occupants are flung against it by the lateral acceleration, they will not be seriously injured; and fourth, the door frame and not only its joins but also those between the pillars and cant rail must be strong and stiff enough to react elastically to absorb the shock loading. Basically, the occupants must be housed in what amounts to a strong cage, which will protect them also if the car rolls over. This generally entails the use of substantial fillets, and perhaps the fitting of reinforcement plates, at the joints between the pillars and the cant rails and sills. With the current need to reduce overall weight, the use of thin gauge high strength ductile steel, instead of the traditional thicker gauge high ductility material for structural members and some body panels can help to improve both crushability and integrity of structures. It is important to design so that the loads due to an impact (whether front, rear or side) are, so far as practicable, spread uniformly throughout the whole structure and that the proportions of all the principal members of the cage containing the occupants are adequate to react those loads elastically. Diagonal and transverse members may have to be incorporated under the Direction of impact Spring Igniter Driver’s seat bag Nitrogen Firing pin InflatorMechanical sensor Air bag for driver Airbag sensor assembly Power Igniter Inflator Inflator N N Igniter Direction of impact Sensor at front of vehicle ECU (Sensor at base of door pillar) Weight Front passenger’s air bag Fig. 36.12 Top, mechanically actuated bag firing mechanism: bottom, electrically actuated alternative. The latter has the advantages of greater compactness of the parts that may have to be accommodated in the steering wheel hub and the sensors and electronic control unit can be sited in the most appropriate positions 940 The Motor Vehicle floor and, possibly, in the roof to transfer some of the loads from one side to the other especially, although not solely, for catering for side or offset frontal impacts. If the shock to the occupants is to be reduced significantly, a considerable proportion of the total kinetic energy of the moving vehicle must be absorbed by the crush zone as it collapses. At the front, the space between the grille and engine is inadequate for absorbing that energy, except in very minor collisions. Consequently, in the more severe accidents the engine will be pushed back, and it is important to prevent it from thrusting the dash and toe board back until they strike the occupants and possibly trap them in their seats. Consequently, the engine is generally mounted in a manner such that it will be deflected downwards and slide under the toe board. In particular, if the engine is on a sub-frame, the attachment of the longitudinal members of that frame to the toe board and front floor can be designed to shear, to enable the whole installation to slide back under the floor. Even so, the dash and toe board structure must still be stiff enough to prevent significant engine intrusion into the saloon. At the rear, there is more space for a crush zone, but the fuel tank must not be ruptured, which is the reason for the modern trend towards installing fuel tanks much further forward than hitherto. Ideally, the structure should collapse progressively at a constant rate, as if it were a sprung buffer, Fig. 36.13. One design method that has been successful is to bow the longitudinal members so that they either spread outwards or collapse progressively inwards when heavily loaded in compression. Another is to incorporate vertical swaged grooves in the side walls of straight members so that they collapse in a controlled fashion. Ideally, the swages would be distributed alternately, along each side, over the length of the longitudinal members of the frame or sub-frame. However, the zig-zag, or concertina type of collapse thus aimed at is extremely difficult to achieve in practice. Once the first kink has formed, usually at the foremost swage, the member is already bowed and therefore is more likely to continue to do so than to concertina. One manufacturer has notched the corners of the rectangular section longitudinal members to initiate progressive collapse. Each notch Swages Fig. 36.13 Diagrammatic representation of front longitudinal frame member carrying the suspension and engine. The lengths of the swages, in each set of four (in the top, bottom and two sides of the frame), become progressively smaller, from the foremost to the rearmost, so that the frame will offer progressively increasing resistance to collapse in a frontal impact. The lower diagram shows it only partially collapsed 941Vehicle safety extends from the corner only a very short distance down one face and a long distance across the other face. However, one should be wary of introducing notches in such structural members subject to fatigue loading, since cracks are liable to be generated by and spread from the stress concentrations thus induced. It is preferable to encourage simple bowing by siting all the swages along either the outer or the inner face rather than the top and bottom of each member, to cause both to bow respectively either inwards or outwards. If both bow outwards, the restriction imposed by the body panelling attached to them will help considerably in providing a progressive reaction to the crushing force, If they bow inwards, they are similarly restricted, but perhaps by the presence of the engine between them. Inward bowing, however, tends to absorb more energy per unit of length of collapse. This might or might not be what is desired, hence crash testing is essential for proving designs. An aspect that should not be overlooked is that swaging the sides of the longitudinal members will reduce their stiffness for reacting to side loads. This need not be serious if the ends of the vertical swages terminate short of the junction with the top and bottom plates, each of which will then become, in effect, a separate U-section member. The ends of the arms of each U terminate where the swages begin, Fig. 36.14. Incidentally, box section longitudinal members can be welded fabrications. Alternatively, they could be square section tubes, the swages being produced by hydroforming, using internal hydraulic pressure to expand the tube into a mould. 36.8 The problem of the small car In an impact with a large car, a small car is inevitably at a disadvantage because the inertia of the former is greater than that of the small car. Moreover, the provision of a crush zone of adequate length at both the front and back of the small vehicle is, of course, much more difficult. For this reason, the principle of designing for the engine so that, when thrust backwards, it slides B C AD EH FG 16t 16t AD EH FG BC Fig. 36.14 Sections through two box section frame side members, one tubular and the other fabricated. Although the swages in their sides weaken them so far as taking side loads is concerned, these loads can be taken mainly in the sections ABCD and EFGH. A useful rule of thumb is that a length equal to 16 multiplied by the thickness of the metal represents the maximum length that is stable on each side of each angle under compression, the measurement being taken from the inner face in each corner or, for the fabricated section, the centres of the bends 942 The Motor Vehicle down beneath the toe board and floor is the only practicable course. Furthermore, maximum use should be made of transverse members to distribute the loads appropriately between all the longitudinal members, including the body panelling, in a manner such that they are all equally stressed, as in Fig. 36.15. An interesting feature in this illustration is the pair of gusset struts, one each side, between the front transverse member and each longitudinal side member. If an impact occurs as indicated by either of the two thick arrows, the corner affected by the impact will be pushed back. The gusset strut will stabilise the front end of the side member so that, assuming it is designed to collapse concertina fashion, it will not bow. Moreover, the transverse member will tend to pivot about the opposite corner, which will be stiffened by the gusset strut. It therefore will offer more resistance to the pivoting movement, and therefore a larger share of the impact loading will be transferred to that side than if there were no gusset member there. At the rear, the design is such that the spare wheel will help to take some of the loading from a rear end impact and transfer it to the main structure. At the rear, the main requirement again is to utilise transverse members to the best advantage. Also important is a robust C-pillar and a good supporting structure for the rear axle. Double skinning the rear quarter panels can enormously strengthen that part of the structure, although this does raise problems as regards repair to minor damage. In general, the overall strength and integrity of the occupant cage may need to be higher than that of a car with long crush zones front and rear. Front B A Fig. 36.15 Below: plan view of a Toyota frame designed to spread the loads imposed by front and rear end impacts uniformly throughout the structure. The combination of the front transverse member and the diagonal members, A and B, one on each side, triangulate the front end of the frame to constrain it to collapse concertina fashion, as shown in Fig. 36.13. Scrap view above: elevation of a different frame, showing how the loads are distributed as viewed in a vertical plane. The triangulation struts shown in this example are fitted in the door frames 943Vehicle safety 36.9 Side impacts As regards side impacts, there is not enough space within doors to serve as a crush zone, so the emphasis is on the use of transverse members between the sills and cant rails to share the loading between the structural elements on both sides of the vehicle. Within the doors themselves, horizontal beams the ends of which are securely fixed to the front and rear frame members of each door are widely used. However, it is difficult to make them stiff enough to help much unless the frame and especially its waist and bottom rails are very stiff, so that vertical or diagonal beams can be fixed to them to support the centre of the horizontal ones. The longer the door, the more intractable is the problem. Of particular importance is that the B-pillar be strong enough to prevent it and, with it, both doors from being pushed inwards in a side impact situation. In general, if the central portion of the outer panel of the door is thrust inwards, it will tend to pull not only its front and rear edges, but also the waist and bottom rails towards each other. Consequently, all these members must be adequately stiff. Another measure that has been adopted, for example by Volvo, is to fill the space between the outer and inner panels of the door with a plastics honeycomb. If the hexagonal elements of the honeycomb are fairly thick, the filling as a whole will offer significant resistance to penetration. Moreover, it also transfers some of the loading radially outwards to the door frame members and thus further reduces the tendency towards penetration of the door. It would appear, however, that structural stiffening alone will not be sufficient to satisfy future legislation, so the installation of side air bags to supplement the door stiffening measures will probably be inescapable. Arm rests which could be forced against the vulnerable areas of the lower ribs of the occupants, should not be installed. 36.10 Smart air bags Some early work with air bags revealed shortcomings, but these have been overcome. First, the occupants of the car must be accommodated in fully supportive seats, with their seat belts fastened. Second, the bags in front of the driver and passengers must not deploy in any situation other than a serious frontal impact. Third, because the impact in a crash is usually over in about a tenth of a second, the deployment of the air bags must be accurately timed. Deployed too soon, they might strike the occupants’ faces and cause the driver to lose control earlier than he might otherwise have done and, if too late, they may be ineffective. Research to overcome these problems has demonstrated that first the precise shape of the impact acceleration pulse must be determined. This is a function of the crush characteristics of the front end of the car. Then the characteristic of the performance characteristics of the bags is ascertained, so that the deployment and collapse can be synchronised with that of the pulse. Gas- inflated bags deploy in about 20–30 ms, but they have perforations in them so that they subsequently deflate to enable the driver to maintain control after the impact. In any case, if they did not deflate, the heads of the occupants might bounce back from them, possibly causing neck injury. An outcome of this research is the development of computerised controls for regulating not only the deployment, but also the tensioning of the seat 944 The Motor Vehicle belts. These are the smart air bags referred to in Table 36.1. Signals transmitted to the computer include seat belt tension, rapidity of brake application, and the deceleration detected by a sensor mounted on a front transverse member of the structure of the vehicle. If the belts are too loose, the occupants are accelerated forwards before being suddenly restrained by them, which can cause injury. Incidentally, the acceleration sensor for side air bag control is generally mounted at the base of the door pillar. Testing is now carried out initially using computer programs, which are followed by full-scale crash tests both to prove the validity of the computer modelling and to enable any fine tuning necessary to be done. Smart air bags are still under development, so further sophistication can be expected. A recent advance has been the provision of sensors and a control system that will inhibit deployment of bags in front of empty seats. This will reduce costs for the owner, since only those for the occupied seats will need to be reinstated. A further refinement that has been proposed is automatic assessment of the size and weight of each occupant and his or her belt restraint status, and setting the deployment characteristics accordingly. Yet another factor that can be brought into the equation is the direction and severity of the crash. Compartmented air bags have been produced that could be selectively inflated, according to the severity and direction of the impact, and perhaps the weight of the occupant of the seat, or whether a child seat has been fitted. Following instantaneous assessment of the weight of the occupant relative to vehicle speed or the severity of the impact and seat belt status, such a system might be able to inhibit deployment if it is unnecessary. TRW Automotive has developed what they call a heated gas inflator (HGI), in the form of a vessel containing a weak mixture of hydrogen and air at a pressure of 175 to 310 bar, as a substitute for explosive pellet type inflators. This device would be difficult to accommodate in a steering wheel hub, but it would be suitable for passenger and side air bags. Two or more such devices might be used for multiple rates of deployment or for compartmented air bags. Further in the future, we might see radar-based systems for gauging the closing speed of the car with the vehicle ahead, or any other object with which the car might be approaching, and setting the air bag control system appropriately. This could entail also the fitting of an acceleration sensor in the crush zone. Current provisions for adjustment of the driver’s seat and steering wheel could, if he had short legs and a long body, place him too close to the steering wheel for safety in the event of air bag deployment. It therefore could become desirable to mount the pedals on a base plate that could be moved horizontally to adjust its position relative to the seat. This, together with the usual seat adjustment facility, would give the driver the means of positioning himself optimally in the horizontal sense relative to his steering wheel and other controls. Vertical adjustment of either the steering wheel or seat might also be desirable, however. 36.11 Seat belts Ideally everyone would have a safety harness of the two shoulder strap (four point) type, worn by aircraft pilots and rally drivers. However, this is commonly regarded as too restrictive to be acceptable by the motoring public. It is also 945Vehicle safety more costly than the adjustable combined lap and single shoulder strap (three point) type harness. The latter type is satisfactory if the lap belt fits snugly round the pelvis, the upper anchorage for the shoulder strap is low enough to prevent strangulation or damage to the neck of the person it is supposed to protect, the whole harness is a reasonably close fit around the body, and the occupant can instantly release himself from it in the event of, for example, a fire hazard. To obtain a snug fit without discomfort to the wearer, and so that the strap automatically winds back on to its reel when not in use, the harness reel is, of course, spring loaded. A further refinement is an acceleration sensor, either mechanical or electronic which, in the event of an accident, triggers a ratchet to lock the belt and thus hold the occupant firmly and snugly in his seat. If this trigger mechanism is too sensitive, the belt may lock unnecessarily and cause discomfort to the occupant: on the other hand, if not sensitive enough, it may fail the occupant in a real emergency. The shape of the seat bucket must be such as to prevent him from sliding down through the lap belt, sometimes termed submarining, and the combined effects of both the lap and shoulder straps should restrain him from being shot either forwards or upwards out of the seat. Another requirement is that when the belt is retracted, the buckles must be in a position such that, when the occupant is seated, he can easily reach them. The upper buckle is usually mounted on the B-pillar, and has to be pulled down and snapped into the socket beside the seat pan Attachment to the B-pillar may present a problem, especially to cater for the seat’s being slid forward for the benefit of driver with short legs. To overcome this problem, the belt is in some instances carried on the otherwise free end of a short arm pivoted to the B-pillar. Alternatively, either a manually or electrically actuated adjustment device may be installed. As previously indicated, if belts are not fitted snugly, violent acceleration forces can propel the occupant forward at high speed until the slack in the belt is suddenly taken up and he strikes it with such force as to injure him. To avoid this situation without having to rely on the occupant’s making the appropriate adjustment, Toyota offer a system incorporating an automatic device for increasing the pre-tension in an emergency, Fig. 36.16. When an electronic sensor detects an acceleration rapid enough to throw the occupants violently forward out of their seats, the electronic control causes gas at high pressure to be released into a cylinder which is part of the seat belt tensioning mechanism. A piston in this cylinder pulls a cable wound round the belt pulley, which it rotates to pull the harness tight. Subsequently, the gas escapes through the clearances around the piston and cable rapidly enough to release the tension so that the occupants can, for example in the event of a fire, i mmediately unlatch their harnesses and escape. Without seat belts, or even if they are inadequately tensioned, the occupants can be thrown violently in virtually any direction. Small children are best belted into rearward-facing safety seats of appropriate sizes. Such seats can be anchored either to the adult seats or to the dash or backs of the front seats. This implies ensuring, at the design stage, that suitable anchorages are provided and that seat backs are strong enough to take the weights of both a front seat occupant and a child plus its safety seat. Where air bags are installed, care must be taken to ensure that they will not strike the children if they deploy. [...]... into the resilient trim on the squab and then rebound forward against the seat belt In the meantime, his head is supported by the head rest With Volvo’s new system, the forces between the body of the occupant and the seat squab activate the WHIPS system The new recliner allows the squab to move backwards, and therefore reduces the forces between it and the body without increasing the distance between the. .. while the force W2 between the back wheels and the ground decreases by an equal amount Q The forces +Q and –Q constitute a couple which balances the overturning couple FF1 The magnitude of the latter is F × OG, so that other things being equal the smaller the height OG the less the overturning couple The magnitude of the righting couple QQ is Q × SS, so that the greater the wheelbase SS the less the. .. pad is fixed directly, the other pad is carried on the cylindrical outer member assembly E, which slides in a bore in the calliper It is moved by the piston A acting through the connecting piece D, the lever B and the inner part of the assembly E which is in two parts screwed together By holding one part and turning the other the effective length of the unit is increased and the wear is taken up This... the less the alteration in the perpendicular forces between the wheels and the ground When going down a hill the conditions are changed From Fig 37.2 it will be seen that the vertical force W, the weight of the car, can be resolved into two components H1 and K The component K is the only part of the weight of the car that produces any perpendicular force between the wheels and the ground, and is, therefore,... out in accelerating In the latter the heat energy of the fuel is converted into the kinetic energy of the car, whereas in the former the kinetic energy of the car is converted into heat Again, just as when driving the car the torque of the engine produces a tractive effort at the peripheries of the driving wheels, so, when the brakes are applied the braking torque introduced at the brake drums produces... shoes apart The shoes are thus pressed against the inside of the brake drum, and frictional forces act between them, tending to prevent any relative motion This frictional force thus tends to slow down the drum, but it also tends to make the shoes revolve with the drum The latter action is prevented by the pin B and the cam M The pin B is therefore called the anchorage pin The magnitude of the frictional... which the cam surfaces bear In the second example, Fig 37.6, a wedge T is used and is pulled inwards towards the centre of the vehicle by the rod R in order to apply the brake The wedge operates through rollers that reduce friction and forces the plungers or tappets U and V apart The body W that houses the tappets may be fixed to the backplate of the brake assembly in which case the forces applied by the. .. The shoes bear against the flat faces of an abutment carried by the backplate at the bottom and against the ends of the plungers of the hydraulic actuation cylinder at the top They are held in place by two springs, only the bottom one being shown Flat strip springs S at the middle of the shoes bear against the webs and hold the shoes against projections (two for each shoe) formed on the backplate The. .. than the force P applied by the piston to the upper end of the servo shoe For backwards motion the shoes will change their functions and the shoe B will become the servo shoe The slight rotation that it then gets will cause the lever D to rotate about its pivot E on the shoe because the upper end of the lever is constrained by the wire D which is anchored at its top end If undue wear has occurred the. .. differentials (LSD) in 950 The Motor Vehicle Chapters 31 and 39 If the wheels lock, the coefficient of friction between the tyres and the road becomes lower than when they are rolling, and the vehicle is liable to become unstable and skid To prevent the wheels from locking when the brakes are applied, the sensor in a simple ABS system signals to a computer the speed of rotation of the wheels In the more primitive . to increase the area of the hub of the wheel, to reduce the intensity of loading locally on the chest. The other was to reduce the stiffness of the rim of the wheel, so that, if the driver was. (LSD) in 950 The Motor Vehicle Chapters 31 and 39. If the wheels lock, the coefficient of friction between the tyres and the road becomes lower than when they are rolling, and the vehicle is liable. this research is the development of computerised controls for regulating not only the deployment, but also the tensioning of the seat 944 The Motor Vehicle belts. These are the smart air bags