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Ưu nhược điểm của các hệ dẫn động 4 bánh Với những kiểu như 4 bánh toàn thời gian hay bán thời gian, việc lựa chọn hoàn toàn phụ thuộc vào phong cách lái và điều kiện sử dụng bởi chúng có ưu, nhược điểm riêng. Động cơ sinh ra công suất và mômen xoắn. Để truyền năng lượng tới các bánh khiến chúng quay, chiếc xe của bạn cần phải có cơ cấu dẫn động. Tuy nhiên, không phải tất cả các bánh đều trực tiếp nhận công suất và mômen xoắn từ động cơ. Tùy thuộc yêu cầu kỹ thuật giữa các loại xe và tại từng thời điềm mà người ta có những phương pháp truyền động khác nhau như một cầu (cho 2 bánh) và 2 cầu (cho 4 bánh). Dưới đây là ưu nhược điểm của các loại dẫn động 4 bánh. Dẫn động 4 bánh bán thời gian (Parttime 4WD) Đây hệ dẫn động phổ biến nhất trên các mẫu xe địa hình. Nó hoạt động sau khi tài xế nhấn nút hoặc chuyển chế độ từ cabin. Lúc đó, mômen xoắn được truyền tới cả 4 bánh. Còn khi chưa được kích hoạt, năng lượng từ động cơ được truyền tới hai bánh trước hoặc sau.
Four-Wheel-Drive Why Four-Wheel-Drive? ❚ Advantages ❙ Improved traction and handling ❙ Able to transmit torque to all four wheels ❚ Disadvantages ❙ Higher initial cost and weight ❙ Requires special service and maintenance Definitions ❚ 4X4 = Four wheeled vehicle, four wheels can receive torque ❚ 4X2 = Four wheeled vehicle, two wheels can receive torque 4WD Designs ❚ RWD vehicles equipped with an added transfer case, a front drive shaft, a front differential, and front drive axles 4WD Designs ❚ FWD vehicles with an added transfer case, a rear drive shaft, and a rear axle 4WD Definitions ❚ Transfer case - An auxiliary transmission mounted to the side or rear of the main transmission ❙ No differential action is provided 4WD Components ❚ Transfer case ❙ Transmits power from the transmission to the front and rear drive shafts 4WD Components ❚ Transfer case ❙ Will typically be a gear or chain type Types of Transfer Cases ❚ Drive chain design ❙ Has less weight to improve fuel economy ❙ Is usually used with planetary gearsets ❙ Is the most common design Types of Transfer Cases ❚ Gear-type ❙ Uses only gear-sets to transfer power ❙ Stronger & more durable Transfer Case Modes of Operation ❚ Ranges available with a part-time transfer case: ❙ Four-wheel-drive—high Transfer Case Modes of Operation ❚ Ranges available with a part-time transfer case: ❙ Four-wheel-drive—low 4WD Definitions ❚ Full-time four-wheel drive (4wd) Constantly provides power to all four wheels These systems typically NOT have a provision for “low” operating range ❙ Axles and differentials are constantly locked ❙ Usually equipped with a center “differential” instead of a transfer-case ❚ All-wheel drive - see above 4WD Systems ❚ Full-time systems ❙ Cannot be selected out of four-wheel-drive ❙ May have an open or limited-slip center differential Full-time 4WD ❚ Open Center Differential - If the wheels on either final differential loose traction, maximum torque is delivered to the axle with the LEAST traction ❚ For this reason, limited-slip center differentials are far more common today Full-time 4WD ❚ Newer computer-controlled designs allow for different operational modes ❚ Ford Automatic Four-wheel-drive (A4WD) ❙ Navigator and Expedition ❙ 4WD AUTO - full power to the rear wheels The front wheels receive power as required ❙ 4WD High - Full power to both wheels, no center gearreduction ❙ 4WD Low - Low gear ratio in center differential for (2:1 ratio) for slow, high torque driving 4WD Definitions ❚ On-demand four-wheel drive Automatically controlled 4WD One axle is driven until slip is detected or anticipated ❙ Computer controlled ❙ uses wheel speed sensors and/or axle speed sensors to detect slippage ❙ When slippage is detected, torque is sent to the opposite axle The amount sent depends on the amount of slippage 4WD Definitions ❚ On-demand four-wheel drive 4WD Definitions ❚ Viscous coupling in a center differential 4WD Definitions ❚ Viscous coupling in a center differential On-demand four-wheel drive ❚ Ford’s control-trac system ❙ normal operation - 96% of torque is sent to the rear wheels ❙ Upon slippage a computer energizes an electromagnetic clutch in the transfer case Up to 96% of torque can be sent to the front wheels 4WD Systems ❚ Four-wheel drive systems without a center differential ❘ Tire sizes must all be the same (this holds true for any 4WD application) ❘ Tread wear must be similar ❘ Tire changes on an all-wheel drive must be done in sets of four ❘ Gear ratios should be the same in the front and rear differentials Front Axle and Hub Service Guidelines ❚ Most front axles are serviced like those in 2WD systems ❚ Follow the recommended procedure for servicing 4WD hubs ❚ Some hubs are not serviceable and must be replaced Effects of Vehicle Modifications ❚ Installing larger tires and lift kits ❙ Alters vehicle handling by changing unsprung weight ❙ Increases steering effort ❙ Causes a rough ride ❙ May cause vehicle to wander and vibrate ❙ Possible driveshaft working angle issues ❙ Possible Vehicle Speed sensor issues ❘ May necessitate computer programming Maintenance Tips ❚ Check the fluid level often ❚ Check for fluid contamination any time the vehicle has been driven through deep water ❚ Use only the recommended lubricant ❚ Always follow the proper towing procedures ... service and maintenance Definitions ❚ 4X4 = Four wheeled vehicle, four wheels can receive torque ❚ 4X2 = Four wheeled vehicle, two wheels can receive torque 4WD Designs ❚ RWD vehicles equipped... operated 4WD Components ❚ Drive shafts ❙ Connect to the front and rear differentials 4WD Components ❚ Universal joints or CV-joints ❙ Are used to connect front axles to the wheel hubs 4WD Definitions... Part-Time 4WD Shift Controls ❙ Manual Shift lever ❙ Some vehicles have a low-speed range that can be selected Part-Time 4WD ❚ Locking Hubs ❙ Manual or automatic operation Part-Time 4WD ❚ Locking