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Vehicle Dynamics

TIRES

Everything you do with the controls of your car — the steering wheel, gas and brake pedals, transmission — affects the amount of traction your tires have.
It is important to note that the only thing connecting you and your car to the road/track are the four tires — and specifically the “Contact Patch” of each tire, which is the small area of the tire that is actually in contact with the road/track surface.

Weight Transfer & Traction

Assuming a car has equal weight distribution — front to rear — when it’s moving at a constant speed, each tire has about the same amount of traction. The car is balanced.When a tire has more load or weight placed on it, the Contact Patch expands slightly, increasing the traction on that tire.

When you brake, weight transfers forward, putting more load on the front tires, and giving them more traction than the rears (which have reduced traction because they’ve had load taken off them).

When you accelerate, weight transfers to the rear tires, and they now have more traction than the front tires.

THINK ABOUT IT

PUSH DOWN ON A RUBBER ERASER AND PUSH IT ALONG A PIECE OF PAPER. THIS IS HOW TRACTION WORKS.

Weight transfer also happens when going around corners.

Weight transfers to the tires on the outside of the turn, causing them to have more traction than the inside tires.

As the outside tires have more load put on them, they gain traction; as the inside tires have load taken off them, they lose traction.

But here’s the big thing: As this graph shows, the more load on a tire does not result in a corresponding increase in traction. 

It’s not a linear relationship — they gain grip, but not at the same rate as the load increases.

When looked at from the perspective of all four tires on your car, the tires that gain more grip from the extra load don’t gain as much as the tires that are un-weighted lose. So, just when you need the traction the most — when going around a corner, for example — you actually have less traction. This has to do with the physics of how rubber tires interact and grip the track surface (which is a step beyond the scope of this guide).

Traction Unit Number

To help you better understand how traction and load work, let’s use a “model” or analogy, and call it the Traction Unit Number concept (as shown in the illustration to the left). 
If you could measure the amount of traction each tire has when the car is balanced and traveling at constant speed down a straightaway, you would see that each tire has 10 “units of traction” (again, this is a model, a concept, and not something that you can actually measure like this). 
But as you turn through a right-hand corner, weight transfers to the outside, or left-side tires. They gain grip, resulting in 15 units of traction. But the inside tires have lost traction, resulting in only 3 units of traction each. Overall, when the car is cornering, it has 36 units of traction (3+3+15+15=36), instead of the original 40.
Every time you brake, turn the steering wheel, or accelerate, you cause some amount of weight transfer, resulting in less traction than when the car was steady and perfectly balanced.

The smoother you brake, turn, and accelerate, the more precisely you move the weight to the different wheels, meaning you have more traction to work with. That is why smooth is fast!

EVERY MOVE THE CAR MAKES RESULTS IN LESS TRACTION THAN WHEN IT’S BALANCED AND STEADY.

Engine Location

Where the engine is located in your car will have some impact on the dynamic weight transfer when driving.

Think of holding a dumbbell with 10 pounds on each end. You’re holding it above your head with one hand. You rotate it, twisting your arm, stop it, and rotate it back in the other direction. As you can imagine, with that much weight out at the very ends of the dumbbell, stopping the twist and changing direction takes a lot of effort.

Now imagine sliding those 10-pound weights in towards the center of the dumbbell bar, until they’re nearly touching your hand on either side. Again, twist your arm and rotate the bar in one direction, stop it, rotate it back in the other direction, then back the other.

With the weights located closer to the center, it’s easier to change direction of the twisting, right?

The same concept applies to cars. If the weight of your car is located at the far ends, it’s harder to get it to change direction, such as from a straight line into a corner, or from a corner in one direction to one in the opposite direction. But if the car’s weight is concentrated more in the center of the car, it’s easier to change direction. That is why race cars are built with as much of their weight concentrated in the center of the car.

UNDERSTANDING VEHICLE DYNAMICS WILL HELP YOU UNDERSTAND THE LIMIT.

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