Tire Pressures - The most important factor
Tire pressures are arguably the easiest thing to change on your car that can improve your performance. All you need is an accurate tire pressure gauge. They also give you insight into how your car is performing and how you’re driving. How does that work? What can you do? Read on!
Why do tires matter?
The tires on your car are the sole contact with the road. The portion of the tire that is contacting the road at any instant is called the “contact patch”. The contact patch of a tire is about 1 square foot (or smaller!), and that patch keeps you from flying off into the weeds as you corner your 3,500 lb car at 85 mph. That’s pretty wild! By maximizing how that contact patch works, you can have a significant impact on your car's handling.
How do tires work?
This has both a simple answer and a VERY complex answer*. We’ll stick to the simple here, as that will get you almost all of the improvement. Tires are a big balloon (often referred to as a pneumatic device so we sound smarter) that uses the internal air pressure to affect its behavior. The reason the air pressure affects the tire behavior is because it makes the tire overall more or less compliant to the road.
Imagine I have a balloon and put a little bit of air into it. If I press it on the corner of my kitchen counter, it’s going to easily change shape (deform) and fit to the edge of the counter. If I put a lot more air into the balloon, and put the same amount of force into pushing it into the edge of the counter, it will deform less. Tires work the same way - a tire with less air will deform more and be more compliant. A tire with a TON of air will be like the tires on Fred Flintstone’s car and be rock hard and not deform as much or be as compliant to the road.
One more thing on compliance - higher air pressure actually reduces the size of your contact patch. Think again about the balloon with just a little air in it vs. one with a lot - the part of the balloon that touches your kitchen counter will be larger in the one with less air than with a lot.
What happens to the tire when you’re driving hard on it?
When you’re driving hard on a tire, you’re putting energy into the tire and it's deforming all around as you make the tire move. That generates heat…which in turn raises the pressure Remember high school physics? If I increase the temperature of a fixed volume, the pressure of the gas inside goes up (The actual equation is PV=nRT, gold star if you remembered that from high school). What you want on the track is the ideal pressure for your car and tires once the tire has heated up.
Depending on a lot of factors, such as the track, the outside temperature, how you drive, how the weight is distributed in your car, etc. your tire pressures increase in different amounts over the course of a driving session as your tires heat up. Your goal is to get them to grow to the optimal pressure and stop. But here’s the tricky part - you’re not going to put the same energy into each tire over the course of a lap so the temps are going to grow at different rates. The track isn’t symmetrical, the track temperature changes, your tires wear, and due to other factors, you’re also going to see different rates of increase. So you’re likely always chasing the perfect pressure for each tire.
How can you tune the car with tire pressure?
You can improve the handling of your car by equalizing the hot pressures. There are two ways to do this - you can select the correct cold pressures that causes the hot temps to be equal (hard, but not impossible to do) or you can go out on track, heat the tires up, then bleed them all to the correct temperature. You can do this at the end of the session, or in the middle if you pit and have a friend (or you) bleed them down.
What hot pressure setting you use for your tire is dependent on the tire and your car. When in doubt, start with the pressures recommended on the driver's door sill of your car as your hot pressures. That’s a good starting point and you can tune from there safely.
What happens when it’s too low?
Overly low tire pressure means that the car will feel less crisp (especially in the front when you turn the wheel) - the balloon is too soft and compliant to the point of being “mushy” and unresponsive. At extremes, low pressure can lead to tires coming off the rim or damaging the tires. Too high is OK, too low can be dangerous.
You’ll get to experience slightly low tire pressures when they are cold, if you’ve got them tuned to be optimized for when they are hot. You’ll feel the car's handling improve as they warm up.
What happens when it’s too high?
When the pressure is too high, the tire has too little compliance to manage the small bumps and the contact patch is reduced - this leads to lower traction on that tire. The car will feel like it’s skating across the top of the pavement, and when you steer, often it feels like the front tires immediately start to slide…think “Fred Flintstone tires”.
How does it feel when it’s JUST right, Goldilocks?
Glorious! When the tire temps and pressure is just right, the car will be responsive, balanced, and corner well. Drivers often will say the car “takes a set in the corner” and by that they mean it’s responsive when they turn the wheel and then settles into the corner and carves through it with minimal other driving inputs. Again…this feels glorious, you’ll know it when you feel it.
So, in summary - once you’ve warmed them up, balancing your tire pressures will help your car handle both better and more consistently. From there….
How do you tune your car with tire pressures?
Let’s say your car understeers (the front has less traction than the rear), you can increase front grip in proportion to the back by lowering the front hot pressures. How much? There’s no magic answer, but start with 2 pounds and adjust in 1 pound increments from there. You can also increase rear hot pressure, reducing traction on the rear. That’s less optimal, but may be necessary in some cases.
If the car oversteers (the front has MORE traction than the rear…or it tries to “come around on you”), then you’d take the opposite approach. Reduce the hot pressure in the rear to gain grip there, or increase the front hot pressure to reduce grip there.
*OK so you want to hear part of the “more complex”? You’re the kind of person who read the whole thing, so here’s a little bit of the complexity - you can really think of tires as a spring that you put energy into. More air raises the spring rate, less air lowers it. So one way to think about it is as just another set of springs on the car…just like the ones that you have that are metal coils in your suspension. You put energy into the spring and have a pretty well defined behavior because we understand how springs work. Stiffer front springs in relation to the back…generally less front grip in relation to the back and vice versa.
Now, want to hear the crazy part? There are dynamics in the tire that the best and smartest physicists, motorsport engineers, and other really smart people STILL don’t understand (that’s why I said “pretty well defined behavior”). Developing tire models for simulation is still a VERY inexact science, because we actually don’t fully understand the physics of the tire adhesion to the road OR the forces going on inside a tire. It’s the biggest challenge that even the best Formula 1 simulations and the biggest car and tire companies can’t solve….yet. So don’t worry too much if tire behavior confuses you at some point in your motorsports career…it happens to the best of us.
