4WD Passenger Cars

4WD Passenger Cars

While most 4WD trucks and utility vehicles are design variations of most basic rear-wheel-drive vehicles, most passenger cars featuring 4WD were developed from a front-wheel-drive base model.

This feature a transaxle and differential that drive the front wheels, plus some type of mechanism for connecting the transaxle to a rear driveline. In many cases this is a simple clutch type engagement.

4WD passenger cars normally differ from heavier-duty 4WD trucks and vehicles in several other ways. First, there is no separate transfer case; any gearing needed to transfer power to the rear driveline is usually contained in the transaxle housing or small bolt on extension housing. Four-wheel-drive passenger cars do not have lockout hubs, traction is not quite as good in the worst conditions and tire wear not so severe should the driver forget to take the car out of four-wheel drive. Interaxle differentials are also very uncommon.

Most passenger cars  with 4WD are not designed for the rigors of off-road driving, in which clearance over rocks and debris is needed. For example road driving there is no clearance or durability problems. As with 4WD trucks, the passenger car 4WD mode should be used on dry pavement due to increased tire and drivetrain wear.

Limited Slip and Open Differentials

To help compensate for the lack of locking hubs many 4WD passenger cars use limited slip or open differential designs for the front and rear differentials. These systems allow enough differential slippage to substantially reduce wheel scrubbing and driveline windup.

A limited slip differential (LSD) uses a friction clutch to join the right and left rear axle shafts to the differential case. This system ensures that most power is supplied where the traction is greatest. It permits the inner and outer wheels to rotate at different speeds when turns are made, just as in general differentials.

In addition, the LSD provides these features. When one front and one rear wheel, which are diagonal to each other, slip on snow-covered roads and driving force cannot be delivered, or when one rear wheel is caught in a ditch and runs idle, the LSD delivers strong torque to the other wheel so the vehicle can run normally. Even if the vehicle bumps with one rear wheel off the road, such as when driving on rough terrain, gravel, or snow-covered roads, the LSD ensures easy straightforward driveability by its limited differential function.

Other systems use an open differential. Here the greatest power is supplied where there is the least resistance. This is an excellent system for fuel economy and gear wear in full-time, four-wheel drive, but it does not provide the true traction advantages of four-wheel drive. For example, the greatest power would be supplied to the slipping wheels (least resistance) on icy road conditions when it is actually needed at the wheels facing the most resistance (for example, where the greatest traction is).

Systems using an open differential have a drive mode on the selector, which permits the differential to be locked up in such conditions. This causes the drivetrain to function the same as a part-time four-wheel-drive system. The power is distributed equally to all the wheels. Vehicles should not be driven in this mode, except under poor road conditions, for the same reasons that part-time systems should be used only when needed.

More Four And All Wheel Drive

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