Without grip — aka ‘friction’ — there is no racing. But how do you know how much grip a racetrack has?
The primary tool for tuning racing grip is the tire. Goodyear produces different tire compounds for different tracks, as well as different left- and right-side tires. Teams use tire pressure and small differences in circumference to further fine-tune grip.
But what if that’s still not enough?
NASCAR turned to PJ1 Trackbite in 2017 to selectively increase grip at tracks with one-groove racing. PJ1 Trackbite is a sticky, water-resistant substance that increases grip. The success of early experiments made PJ1 Trackbite one more tool in NASCAR’s toolbox for making racing better.
But how do they know when they need it? Where to apply it? How much of it to apply?
The Physics of Grip
Grip depends on two things:
- The force pushing the tires into the track
- How the tires interact with the track.
PRO NOTE: This linear relationship doesn’t hold for large downward forces. You reach a point where you don’t get as much frictional force for increasing downforce.
How hard the tires are pressed down into the track is determined by the combination of aerodynamic downforce and mechanical downforce (i.e. the weight of the car). Optimizing that is up to the teams.
The Coefficient of Friction
The other factor — how the tires interact with the track — depends on the specific of the tires and the track.
We characterize this interaction by the coefficient of friction, represented by the Greek letter mu (𝜇), which is pronounced ‘mew’, like a kitten sound. (When making spherical-cow jokes, and only when making spherical-cow jokes, you may read it as ‘moo’.)
𝜇 tells you how easy or hard it is to slide one material on another.
Let’s say you have a 100-lb metal file cabinet you need to move from a wood floor to carpet. Let’s say it takes you 30 lbs of force to push the file cabinet on the wood floor. That means the coefficient of friction between your metal cabinet and the wood floor is 30 lbs/100 lbs = 0.3.
Now let’s say you need to move your cabinet on a carpet: a deep shag, 70’s-era carpet. It’s much harder to slide, so it takes you 75 lbs of force to move it. The coefficient of friction between the metal cabinet and the shag rug is 75/100 = 0.75.
The higher the coefficient of friction, the harder it is for the two materials to slide against each other.
- If you’re moving file cabinets, you want a low coefficient of friction.
- If you’re racing, you want a high coefficient of friction.
Here’s a few examples from real life.
Coefficient of Friction & Racing
Let’s examine the coefficients of friction, 𝜇, for tire rubber on various surfaces.
These are all for normal tires, like the ones on your passenger car. The wide slicks used for racing are designed to maximize the coefficient of friction between the tire and the racetrack.
Did you notice that all the real-world numbers above are less than one?
I didn’t know you could have coefficients of friction greater than one before I got into NASCAR. Think about it like this: a coefficient of friction greater than one means it’s easier to pick up the object than to slide it.
A coefficient of friction greater than one means it’s easier to pick up the object than to slide it.
A coefficient of friction above one would be like trying to push your file cabinet along a really hot asphalt road if the bottom of the file cabinet is made of chewing gum.
PRO NOTE: There are three components to the coefficient of friction when dealing with soft, stretchy materials like rubber: adhesion ( molecular interaction between tire and road); hysteresis (due to deformation of the tires) and cohesion (due to wear and tear).
In addition to the usual friction, hot race tires molecules interact with rubber on the track. This is why:
- A green track has less grip
- A cold track has less grip
- Cold tires have less grip
The actual values of coefficients of friction for race tires are trade secrets and closely guarded. You hear number for stock-car tire coefficients of friction around 1.0 to 1.5.
If the coefficient of friction of our file cabinet on some surface is 1.5, it means you have to pull the 100-lb cabinet with 150-lbs of force to slide it.
Goodyear tailors the frictional properties of their tires to each track. But, of course, Goodyear engineers can’t just make the stickiest tire possible, because soft tires wear out faster
- Right-side tires must deal with more force, so they are harder and don’t wear as quickly.
- Left-side tires are softer, but wear faster.
When Tires Aren’t Enough
Since Goodyear builds each race tire by hand, the tire structure and compound have to be determined well in advance of each race.
And, since treating tires chemically or physically (siping) are prohibited, the only other place NASCAR can tune the coefficient of friction is at the track.
Track surfaces change with temperature, humidity and the aging of the race surface. So it’s not like you can say that Bristol always needs x amount more friction in the upper lane. If it’s hotter, it may need less. Unseasonably cold weather may require more friction. What worked in spring won’t necessarily work in fall.
So how does NASCAR test for coefficient of friction?
Remember the example of measuring how much force it takes you to slide a file cabinet? That’s pretty much what they do: they drag an airplane tire around the track and measure how much force it takes.
Dividing the weight of the tire assembly by the force needed to pull it gives them the coefficient of friction.
NASCAR has to do this in a slightly more advanced way: They’re interested in the grip all the way around the track, so they map the coefficient of friction values and collect them on a computer with a GPS index. They’re probably also reading the temperature of the track at each spot.
This Seems Too Simple
Could NASCAR use more complex techniques to make this measurement?
Sure. But the advantage of the way they’re doing it is that they can make measurements quickly enough to determine whether they need to apply more PJ1 Trackbite before a race.
When we’re back to normal and they’re trying to get qualifying and practice for multiple series before a race, there isn’t time to shut down the track for a four-hour measurement.
Why Not Use the Race Tire?
First of all, which one? The lefts and rights have different coefficients of friction.
Secondly, there has to be some weight on the tire besides the weight of the tire itself. I’m guessing an airplane tire satisfies the bill without a lot of extra fiddling about.
Finally, Goodyear constantly improves their tires. If NASCAR makes measurements one year at Las Vegas with tire D1234 and the next year Goodyear bring tire D1235, how do you compare the two measurements? Minimizing variables is always a good thing.
An Experiment You Can Try at Home
You can do the same coefficient of friction measurement if you have a spring scale. Since most coefficients of friction are less than 1 (even tires wouldn’t be more than 1.5), you need a scale that goes to 1.5 times the weight of the object.
Using the spring scale, drag the object along the surface you’re testing. When you’re pulling at constant speed, note how much the scale reads. Divide the weight of your object by the force you’re pulling with and… voila! You’ve measured the coefficient of friction.
This video, from Harvey Mudd College, shows the experiment pulling a block, but c’mon… a tire would be much more interesting, wouldn’t it?
Caution: Physics Humor
An experimental physicist working for NASCAT (The National Association for Scientific CATs) places two identical kittens on an inclined tin roof. Which one falls off first?
For More On This Topic (Friction, not Bad Physics Jokes):
- I highly recommend Paul Haney’s book The Racing & High-Performance Tire: Using the Tires to Tune for Grip and Balance.
- This open-access journal article talks about the different types of friction in tires. It also shows you how coefficient of friction measurements are done in a laboratory setting.