by Jim Kerr

The snow is melting quickly, but not quickly enough for drivers on my block. Deep snow, rain and running water have combined to make streets almost impassable. Even some 4×4 trucks are getting stuck if they drive off the beaten path. One wheel drops into a hole and the other wheels spin on the ice. The engine’s torque is being put to the ground, but unfortunately, it is all going to the spinning wheels.

Torque, the twisting force that moves our vehicles, is divided between a vehicle’s drive wheels, depending on the vehicle. When an engine puts out torque to a two-wheel-drive vehicle, it’s divided, with half going out to each drive wheel. Four-wheel-drive models typically have the torque split evenly in the transfer case and then again at each drive axle, so each tire receives one-quarter of the total engine torque. All-wheel-drive and some 4×4 vehicles use transfer cases that can provide an uneven torque split; more on that later.

Basically, vehicle drive axles and transfer cases can be separated into two groups: “open” axles that allow one wheel to turn faster than the other, and “limited slip” or “locker” axles that are designed to limit or control speed differences between the wheels on one axle. For vehicles with “open” differentials (drive axles), equal traction at each tire will allow the vehicle to accelerate quickly. However, there is seldom equal traction for each drive wheel. The wheel with the most traction slows down and the torque is transferred to the other wheel, which spins on any loose surface. If the road surface is very slippery, such as the ice on our street, one wheel spins easily and no torque is delivered to the wheel with traction – in other words, you are stuck. The same problem can occur in loose sand or mud.

To get the vehicle moving again, you need to either provide traction for the spinning wheel, or slow that wheel so torque will be transferred to the other side. An old trick used by experienced drivers is to apply the brakes slightly when one wheel is spinning. This slows the spinning wheel so the other wheel can drive. On rear wheel drive vehicles, use the parking brake, instead of the regular brake. Do not apply the brakes too hard or the vehicle will simply stop. Care must be taken not to overheat the brakes, so use this technique only for a few seconds at a time. Traction control systems use the same concept as applying the brakes, except the system can apply the brake only on the spinning wheel, so the other driving wheel gets all the torque.

Four-wheel-drive vehicles can also get stuck easily. If one front wheel and one rear wheel are on slippery surfaces, no torque is delivered to the other two wheels. The vehicle doesn’t move. Limited slip or locking axles will help keep that vehicle moving. When one wheel starts to spin, the limited slip axle will slow that wheel and the torque transfers to the other wheel. It works just like traction control does, but this time it is all done mechanically.

Unlike traction control, limited slip and locking axles don’t help if both wheels on one axle are on slippery surfaces and both wheels spin. It is up to the driver to limit the torque and wheel spin with the gas pedal. Traction control limits the torque electronically and automatically, so if you drive in conditions where wheel spin would help keep the vehicle moving, it’s better to switch the traction control off.

Limited slip and locking axles are not usually used on front-drive axles. With both front tires turning the same speed, it becomes very difficult to steer the vehicle around corners. Some vehicles, such as the Mercedes-Benz G500, have driver-controlled locking front and rear axles for ultimate traction. Torsen-design differentials are sometimes used in high-performance front-wheel-drive axles, because this design allows the wheels to turn at different speeds for easy steering but works like a limited slip axle as soon as a wheel starts to spin.

Finally, many all-wheel-drive vehicles use a transfer case design containing a planetary gear set to split the torque between the front and rear axles. The design allows uneven torque splits such 60/40 or 70/30. By using various gear ratios, the desired torque split is achieved. Today, many of these systems also use computer-controlled clutches in the transfer case to hold part of the planetary gear set and vary the torque split automatically. Combined with limited slip axles and good tires, you have the best traction. That’s what I will need for the next week or two!

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