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| Know Your Car : 4 Wheel Drive |
We need to know a little about torque, traction and wheel slip before we can understand the different four-wheel-drive systems found on cars.
Torque
Torque is the twisting force that the engine produces. The torque from the engine is what moves your car. The various gears in the transmission and differential multiply the torque and split it up between the wheels.
More torque can be sent to the wheels in first gear than in fifth gear because first gear has a larger gear-ratio by which to multiply the torque. The interesting thing about torque is that in low-traction situations, the maximum amount of torque that can be created is determined by the amount of traction, not by the engine.
Traction
We'll define traction as the maximum amount of force the tire can apply against the ground (or that the ground can apply against the tire - they're the same thing). These are the factors that affect traction :
- The weight on the tire : The more weight on a tire, the more traction it has. Weight can shift as a car drives. For instance, when a car makes a turn, weight shifts to the outside wheels. When it accelerates, weight shifts to the rear wheels.
- The coefficient of friction : This factor relates the amount of friction force between two surfaces to the force holding the two surfaces together. In our case, it relates the amount of traction between the tires and the road to the weight resting on each tire. The coefficient of friction is mostly a function of the kind of tires on the vehicle and the type of surface the vehicle is driving on. For instance, a high-performance tire has a very high coefficient of friction when it is driving on a dry, concrete track. That is one of the reasons why such cars can corner at such high speeds. The coefficient of friction for that same tire in mud would be almost zero. By contrast, huge, knobby, off-road tires wouldn't have as high a coefficient of friction on a dry track, but in the mud, their coefficient of friction is extremely high.
- Wheel slip : There are two kinds of contact that tires can make with the road - static and dynamic.
- Static Contact : The tire and the road (or ground) are not slipping relative to each other. The coefficient of friction for static contact is higher than for dynamic contact, so static contact provides better traction.
- Dynamic Contact : The tire is slipping relative to the road. The coefficient of friction for dynamic contact is lower, so you have less traction.
Why Does Wheel Slip Happen?
Quite simply, wheel slip occurs when the force applied to a tire exceeds the traction available to that tire. Force is applied to the tire in two ways :
- Longitudinally : Longitudinal force comes from the torque applied to the tire by the engine or by the brakes. It tends to either accelerate or decelerate the car.
- Laterally : Lateral force is created when the car drives around a curve. It takes force to make a car change direction - ultimately, the tires and the ground provide lateral force.
Let's say you have a fairly powerful rear-wheel-drive car, and you are driving around a curve on a wet road. Your tires have plenty of traction to apply the lateral force needed to keep your car on the road as it goes around the curve. Let's say you floor the gas pedal in the middle of the turn - your engine sends a lot more torque to the wheels, producing a large amount of longitudinal force. If you add the longitudinal force (produced by the engine) and the lateral force created in the turn, and the sum exceeds the traction available, you just created wheel slip.
Most people don't even come close to exceeding the available traction on dry pavement, or even on flat, wet pavement. Four-wheel and all-wheel-drive systems are most useful in low-traction situations, such as in snow and on slippery hills. In the next section, we'll see how four-wheel-drive systems can help in these situations.
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