The Science of… Tires, Part II

Tires: What Goodyear Can’t Control

I’ve reproduced the figure from yesterday to reiterate that there are two factors that determine grip: the interaction between the track and the tire, and the force pushing down on the tire. In part I, we covered the interaction between the tire and the track. Now, let’s turn to the force pushing down on the tire.

The effect of the force pushing down on the tire can be divided into two parts: How much force is pushing down and the way in which that force is distributed across the tire. Both factors are affected by the way the car is set up.

Let’s discuss how much force presses on each tire first. The total force comes from a combination of the car’s weight and the aerodynamic downforce. When the car is standing still, roughly one-quarter of the car’s weight pushes down on each of the four tires. There is some weight distribution differential between the front/back and left/right, but that is usually only a few percent.

When the car turns, more force pushes on the outside wheels than on the inside wheels. The total amount of weight pushing down is the same (after all, the car’s weight hasn’t changed); however, the weight is distributed differently. This is one reason the left and right sides of the car get different tires. Goodyear tries to account for the fact that the right-side tires generally take more abuse than the left-side tires on oval tracks. If you look up the tire information on a website like the race pages on jayski.com, you’ll see two different tire codes for the left and the right tires everywhere except the road courses.

When the car accelerates, the force on the rear tires increases and the force on the front tires decreases. The opposite happens when the driver brakes. Coming off a corner means turning and accelerating, so the right rear gets a very large force and the left front a very small force. This transfer of force can be so extreme that the left front tire can actually lift off the track. Take a look at some of the cars at Martinsville during the practice sessions–you’ll probably see this happen.

Teams can affect how the force shifts by adjusting the wedge (the crossweight) and the trackbar (the difference between these is a subject for another column), as well as the springs and shocks. The amount of shift in force is strongly affected by the height of the center of gravity (CG) above the track surface. The CG of the new car is quite a bit higher than it was in the old car, which means there is more change in force on the tires during cornering, accelerating and braking.

In addition to the car’s weight, aerodynamic downforce also pushes the wheels into the track. The old car was twisted: The left front fenders were broader than the right front fenders because giving the left front as much grip as possible is the key to being able to get back on the throttle quickly coming out of turns. The new car is much more symmetric. Each car generates slightly different amounts of aerodynamic downforce, and the amount changes depending on how fast the car is going.

Something new this year is the use of bump stops, which can affect the tires the same way that coil binding did in the old car. In the old coil-binding setups, the cars were configured so that at some point coming into the corner, the coils on the spring would compress enough to touch. The spring lost all its springiness, so the driver was driving on one (or sometimes two) wheels essentially without springs. Imagine jumping on a pogo stick and having the spring suddenly disappear. The only thing bouncy is the rubber end, so that’s all you have absorbing the force of the bounce. In a coil-bound car, the tire is forced to fill the role the spring should be serving– absorbing bumps. The problem is that tires are not designed to do this.

Bump stops (hard pieces of rubber that prevent chassis travel much the same way that coil binding does) are legal in the new car and they have a similar effect as coil binding. Bump stops can force tires to play the roles of springs, which can put a lot more force on the tires than they were designed to handle.

All factors considered, tires can experience very high forces–one report said that the right rear at an intermediate track might be subject to 3500 lbs of total downforce during turning and accelerating.

How the force is distributed across the tires is also important. A tire tread is between 11” and 12” wide, and different parts of the tire can experience different amounts of force depending on the particular way the tire contacts the track. The tire temperature and the wear directly reflect the tire’s grip (remember that friction = grip and friction produces heat and wear, which means (by the transitive property) that heat and wear indicate grip). Team members measure the wear and the temperatures at different points across the tire to determine whether the load is being borne equally across the tire. Two of the more important factors determining force distribution are the tire pressure and the camber. These aren’t the only variables. A-arm lengths, springs, shocks, sway bars, and other suspension setting will affect the way in which the tires contact the track and thus the distribution of force across the tire.

Tire pressure. Underinflating tires puts excess stress on the sidewalls because instead of the car’s weight being supported by the gas in the tires, it’s being supported by the sidewalls. Check out how flat the tires look when the teams are waiting in line to go out for practice this week at Bristol and again in Martinsville. As soon as the cars get out on the track, the gas in the tires gets hot and the pressure increases. If the starting tire pressure is too low, you can still place more stress on the sidewalls than they were designed to handle. Goodyear sets a minimum pressure and a NASCAR official checks the pressure prior to the tires going on the car. Overinflating the tires decreases the area of the tire that is in contact with the track, which decreases how rapidly heat can be dissipated from the tire and leads to higher tire temperatures and more rapid wear.

Camber. If you look at a car head on, a car with no camber has the tires straight up and down. Negative camber means that the top of the tire is closer to the centerline of the car than the bottom of the tire is. If the bottom of the tire is closer to the centerline than the top, the tire has positive camber. When the car goes into a banked turn, a tire with no camber (or positive camber) struggles to assume the same angle as the banking. Camber gives the tire a head start on matching the banking angle.
Compare the two tires below. If the tire doesn’t sit flat on the track, less of the tire tread will be in contact with the track. Less tread touching the track means less grip and more heating on the part of the tire that is touching the track. One side of the tire will wear more than the other.

The setup, which plays just an important a role in tire wear and performance, is entirely in the hands of the teams, subject to NASCAR’s rules. Goodyear has no control over these variables and they aren’t necessarily going to be the same for each car.

Whew. Did I mention this tire thing is complicated? And I’ve only included the more major factors that determine tire performance. The most important thing to get out of these two posts is that there are some variables that Goodyear has control over, and there are other variables that Goodyear has absolutely no control over. Next post, I’ll examine statements made by various drivers (mostly Tony Stewart and Dale Earnhardt, Jr.), by Goodyear and by others associated with NASCAR and you can determine for yourself whether you think Tony was overreacting and whether Goodyear would deliver a better tire if they had competition.