Traction is defined by dictionaries as adhesive friction, another name for static friction (non-sliding friction). Specifically, traction refers to the maximum static friction that could be produced between two surfaces without slipping.
The term traction is mostly found in contexts where static friction is used to produce or prevent independent motion of some system against the ground, such as "these new hiking shoes give me great traction on the rocks," or "my car wheels keep slipping because I can't get any traction in this road."
In the design of cars, higher traction between wheel and ground is generally more desirable than lower traction, as it allows better acceleration, cornering and braking without wheel slippage, giving the driver more control over the vehicle. One exception in racing is in the motorsport called "Drifting", in which rear-wheel traction is purposely lost during high speed cornering. Higher traction also allows for steeper ground inclines without wheel slippage, whether the vehicle is moving or is parked.
Just to make it simple, we'll define traction as the maximum amount of torque 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 always shifts 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 to the amount of traction between the tires and the road to the weight resting on each tire. The coefficient of friction is mostly depending of the tire compounds on the car and the type of surface the vehicle is driving on. Harder tire is more durable but gives less traction, and the softer tire gives more traction but are less durable. A racing car slick tire has a very high coefficient of friction when it is driving on a dry tarmac track. That is one of the reasons why race 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 racing 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.
- Static contact - The tire and the road (or ground) are not slipping
Comparing potential tyre grip for Ultra High Performance, FIA GT and F1 cars. (courtesy of Pirelli Motorsport Services Ltd., courtesy of Willem Toet, Head of Aerodynamics, Sauber F1 Team, Sauber Motorsport AG)
Wheel slip occurs when the torque 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 (don't do this!), 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 circle 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 or all-wheel-drive systems are most useful in low-traction situations, such as in snow and on slippery hills. The benefit of four-wheel drive is easy to understand: If you are driving (applying torque to) four wheels instead of two, you've got the potential to double the amount of longitudinal force (the force that makes you go) that the tires apply to the ground.
This can help in a variety of situations. For instance:
In snow - It takes a lot of force to push a car through the snow. The amount of force available is limited by the available traction. Most two-wheel-drive cars can't move if there is more than a few inches of snow on the road, because in the snow, each tire has only a small amount of traction. A four-wheel-drive car can utilize this limited traction of all four tires.
- Off road - In off-road conditions, it is fairly common for at least one set of tires to be in a low-traction situation, such as when crossing a stream or mud puddle. With four-wheel drive, the other set of tires still has traction, so they can pull you out.
- Climbing slippery hills - This task requires a lot of traction. A four-wheel-drive car can utilize the traction of all four tires to pull the car up the hill.
There are also some situations in which four-wheel drive provides no advantage over two-wheel drive. Most notably, four-wheel-drive systems won't help you stop on slippery surfaces. It's all up to the brakes and the anti-lock braking system (ABS).
Whenever a torque is applied to wheel (braking or accelerating) the wheel slips slightly. You will find that in a road car, peak acceleration is around 17-18% slip. This mean that the wheel is turning that much faster than the road surface is passing underneath it. Even acceleration at traffic speeds exhibits some slip (~0.5%). While static friction might be 'stronger' than dynamic friction, any torque applied to the wheel means that the surfaces will be experiencing dynamic friction. There is no such thing as no slip with acceleration.
in a RWD car, if you watch a trace (graph) on the test bench of each wheel speed, under acceleration conditions (ie. a torque being applied through the wheel), the rear wheels will be ALWAYS turning faster. The faster you accelerate, the bigger this difference will be. Until you reach a difference of approximately 17-18% (on dry asphalt), and then your difference will continue increasing, but your acceleration will begin to decline.
Differentials and Traction
The open differential always applies the same amount of torque to each wheel.
There are two factors that determine how much torque can be applied to the wheels: equipment and traction. In dry conditions, when there is plenty of traction, the amount of torque applied to the wheels is limited by the engine and gearing; in a low traction situation, such as when driving on wet road or ice, the amount of torque is limited to the greatest amount that will not cause a wheel to slip under those conditions. So, even though a car may be able to produce more torque, there needs to be enough traction to transmit that torque to the ground. If you give the car more gas after the wheels start to slip, the wheels will just spin faster.
Torque, Traction and Wheel Slip
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. Even if you have a racing car engine in your car, if the tires won't stick to the ground there is simply no way to harness that power.
Check my article about Traction Circle !!