What Skidmarks? Those Aren't Mine.

​Has your car ever used its anti-lock brakes? Have you ever drifted off the pavement? Have you slid your rear tires, even just a little? If you answered no to any questions, then keep reading, if not, then you still might find this interesting anyway.

Cars aren't magical, they all have their limits. Certain cars are more capable than others, and cars could always be built to turn a little better, accelerate a little faster, or brake a little shorter, right? What you do to accomplish these things is based on the physics of a car's motion. When an object is at rest, it can exert a finite amount of friction, determined by the materials in contact and the object's weight. When a car turns, accelerates, or brakes, this friction is used to alter the car's motion.

If you're turning and try to accelerate, you are using the car's friction both to turn and to accelerate, which places more load on the tires. The same goes for braking and turning also. Regardless of what you are doing, if the total force that the tires will have to exert to make the car do what you want exceeds their limit, then the tires will slip. Once the tires are fully skidding, they have less friction then they did before they slipped, which is one of the reasons anti-lock brakes exist.

So friction is finite, right? Well, I sorta lied, there are ways to manipulate this limit. Your car's suspension supports the car's weight and connects the wheels to the body. When you brake, have you noticed that the car leans forward until you come to a stop and it jerks back? That's because of the front suspension compressing as you slow down, and then once you stop the brakes aren't doing much to the car's motion (stopped), so the springs decompress and the car kicks backwards a little (or a lot). Remember when I said friction depends on the object's weight? When you brake, your car puts more of its weight on the front tires than when it is coasting, causing the front tires to have increased traction. By braking gently during turns, you can increase your front tires' grip, helping you turn (this is called trail braking). If you brake hard enough or turn tight enough, your car may slide it's rear tires because you have taken so much weight off of them and onto the front tires (this is called oversteer).

When you accelerate, the tires push or pull the car forward (FWD or RWD), which causes the car to lean backwards due to the acceleration, similar to if you've ever floored it and been pressed into your seat. The car leaning back places the weight on the rear suspension, which causes the front wheels to lose grip. If you are accelerating and turning, especially in a FWD car, your front tires are working to power the car, but they also have less weight on them because of how the car's weight shifts when you accelerate. This shifted weight reduces the friction threshold of your front tires, which means that while you might be trying to turn, your car just skids and doesn't turn as fast as you tell it to (this is called understeer).
​
Weight distribution is why one person might take an exit ramp at thirty-five and crash due to understeer, someone else might shoot the same ramp at fifty but be completely fine. By knowing these techniques, you can be a safer driver and have more fun (at legal speeds).