Tires get more conversation than probably any other item on a racecar, yet they are largely misunderstood. Everyone has theories about how to use tires to go faster, but for any given set of circumstances, a set of tires can only make so much traction. The goal of the astute racer should be to not screw up the total available traction, but the sad reality is that most racers do not maximize their own tire’s maximum traction potential.
The reality of life on the racetrack, assuming you want to win races, or at least improve your lap times and finishing position, is that everything you do to the chassis and aero package, if you have one, is to increase overall traction of the four tire contact patches. That includes what the driver does with controls while on the track. No driver can increase traction of a given tire. A skilled driver can manipulate the controls to keep traction as close to maximum as possible, but a less-skilled driver can reduce traction.
The laws of physics are not open to interpretation. They must be followed if success on the track is your goal. What you read here is based on developing an understanding of physics through education, reading and experience on the track and in the shop. It takes hundreds of hours and thousands of miles on the track testing and racing to really understand and apply this stuff. This story is intended to help you gain a greater understanding of tire traction and how it works.
Most racers understand that tires make traction through friction between the rubber molecules at the tire contact patch and the racing surface. And most of us understand that traction increases as vertical load on the tire increases, which is why aerodynamic downforce works so well. In addition, most racers understand that the tire will make more traction if the entire contact patch is equally loaded, which is why monitoring tire temperatures is useful. Within this basic knowledge, misconceptions and misinformation can add confusion to an already difficult topic. Let’s try to shed some light on it.
In this article, we will not address tire design or construction, since no one I know who races can change those parameters anyway. The factors we have control over include: tire pressure, tire camber, tire toe (in or out) and camber change.
Each of these items has an optimum setting that allows the tire to create maximum traction for a given set of circumstances. Then there is the vertical load on the tire, which is crucial to understand, but also the most misunderstood element of tire traction. Traction increases as the vertical load on the tire increases, but it is important to understand that the relationship between vertical load and traction is not linear. Being nonlinear means that if the load on the tire is increased, the traction also increases, but it does not increase as much as the load. For example, if the vertical load is doubled, then the traction increase is somewhat less than doubled. If the vertical load increased, say 200 pounds, the traction increase may only be about 175 pounds. Now is a good time to explain traction in terms of pounds of force and vertical load on a tire.
One way to look at traction is in pounds of force. The most convenient way to do this is to look at the car as a whole, and measure the force that the tires create. Most racers have heard the term g force. If a car accelerates at 1.0 g, and the car weighs 3,000 pounds, then the tires are producing 3,000 pounds of traction force. This applies to acceleration forward, braking (negative acceleration) and cornering (lateral acceleration). A late model stock car can produce a cornering force of about 1.4 g in a flat corner, about 1.25 g under braking and somewhere around .50 g under acceleration on a short track with a very low final drive ratio. For a 3,000 pound car cornering at 1.4 g, the traction in pounds is 4,200 (3,000 x 1.4 = 4200). That is a lot of force from those four tire contact patches.
Vertical load is the load actually seen at the tire contact patch. This includes the weight resting on the tire contact patch plus any aerodynamic downforce. If the car creates any aerodynamic lift, then the vertical load on the tire will be less than the weight on the tire, since the car is lifting instead of being pushed down. Aerodynamic downforce is good because it increases traction without increasing the weight of the car. Let’s look more closely at this, since it is another area of some confusion.
Downforce is pretty much a traction freebie. It costs a little in acceleration at high speeds and reduces top speed somewhat, but it adds no weight to the car. Adding weight to the car actually reduces the relative amount of traction compared with the total weight of the vehicle. In the previous example, the 3,000 pound car made 4,200 pounds of cornering force at the limit. Let’s say we add 500 pounds to the car with nothing else changed, including the weight distribution.
It’s easy to understand the car will not accelerate as quickly because it weighs more and the engine is making the same horsepower. It is less obvious that cornering speed will reduce. Here’s why. The 500 pounds of weight adds 500 pounds of vertical load to the tires, but because the relationship between the vertical load increase and traction increase is nonlinear, the amount of traction increase will only be about 400 pounds. That means the tires now make an additional 560 pounds of traction (400 x 1.4) for a total of 4,760 pounds (4,200 + 560) of traction. This works out to a cornering force of only 1.36 g. This equates to a loss in cornering speed due only to the effect on the tires, not on the dynamics of the suspension. This is due entirely to the characteristic of tires where traction does not increase as fast as load. This nonlinear relationship also becomes more significant as the design load of the tire is approached.
In other words, if a tire has a maximum load capacity of 2,000 pounds, but normally carries only 750 pounds, doubling the load to 1,500 pounds is approaching the design limit. Here the traction may only increase by about half the extra load. If the design load is exceeded, the situation gets worse. While there is nothing you can actually do to a tire or suspension to change this nonlinear relationship, there are plenty of factors you need to understand to minimize its effect and to allow your race car to create the maximum possible amount of traction.
These factors are crucial to maximize traction for each individual tire:
- Camber angle at the front and rear (depending on rear suspension type)
- Tire pressure
- Toe settings front and rear (depending on rear suspension type)
- Roll steer and axle squareness at the rear
- Bump steer at the front
The uniform goal in every case is to have the entire tire contact patch equally loaded across its surface. If the entire contact patch is not equally loaded, you are not getting all the traction possible from that tire. If you look at the tire contact patch as a series of one-inch squares, one square compared with another acts just like one tire compared with another tire. Reducing the load on one square increases the load on another square. The square losing load loses traction more quickly than the other square gains traction from the increased load. In other words, the tire contact patch as a whole is making less traction than it could if the contact patch were equally loaded over its entire area. This is hard to achieve, but the team doing the best job has its tires working most effectively.
Once you have the entire tire contact patch at each corner working to its maximum traction potential, then the goal is to get all four tires creating the maximum amount of traction possible for the whole vehicle. To accomplish this requires an understanding of weight transfer.
Weight transfer occurs anytime the tires create a force. Acceleration, braking and cornering are the three axes on which weight transfer occurs. Only four factors affect the amount of weight transfer. Using cornering as an example, those factors are:
- Center of gravity; height above ground
- Vehicle track width
- Cornering speed vs. turn radius
- Vehicle weight
A higher center of gravity means more weight transfer as do a narrower track width, higher cornering speeds and greater vehicle weight. Why is weight transfer a problem? More weight transfer means a greater load change on the tire contact patches. Some tires lose load, some gain load, and just like we have seen previously, that nonlinear relationship means that maximum potential traction occurs only when all four tires are equally loaded. A tire losing weight will lose traction more quickly than the tire gaining weight will gain traction. That equals a net reduction from the maximum possible traction. So we want to minimize weight transfer as much as possible, for starters.
To reduce weight transfer means keeping the center of gravity as low as possible. We also want to keep the track width as wide as is permitted, although other factors must be considered, like handling balance, aerodynamics and racing surface conditions. We want to run at minimum weight or as light as possible if there is no minimum. Going through turns slower than possible is contrary to performance, so eliminate that as a way to reduce weight transfer.
In addition to reducing the amount of weight transfer, we also must look at static weight distribution and how that affects weight transfer. For road racing, a 50/50 left-right weight distribution is desirable. Why? Let’s say you run 55 percent left weight on a road course. The car will be faster in left turns than in the equivalent right turns. Taking into account weight transfer in the turns, in a left turn, the inside will lose weight and the outside (right) gains the weight so that in the turn the dynamic weight distribution is about 50/50. That is near the ideal situation. But in a right turn, the dynamic weight distribution becomes 60 percent left, 40 percent right. This means the right side tires are not working very hard and the cornering force is much lower.
So what is ideal? How do you determine tire loads? Tire temperatures. First they should be within 10 degrees Fahrenheit across the tread surface so that each tire is optimized. Then the average temperature of each tire should be within 5 degrees of the other three tires. This is difficult to achieve, but that is the goal.
Weight transfer is much less of a problem fore and aft during braking and acceleration because the wheelbase is longer than the track width and resists weight transfer better. A disparity of up to 5 percent in front-to-rear static weight distribution is not much of a concern. If front or rear static weight exceeds 55 percent, then the tire size (actually the area of the contact patch) at the heavy end should increase to compensate for the higher vertical load.
Many factors contribute to traction, like construction characteristics, design, peak slip angles and track conditions. The things the driver controls to some degree, such as weight distribution and chassis setup make all of the difference on the racetrack. The team making best use of the potential traction at all four tires is the team with the best chance to win.
In this infrared imaging footage from Stack Motorsport, you can see how quickly temperatures rise and fall across the tire’s surface.
Video courtesy of Stack LTD, U.K.
Great write up, thanks.