Different tread designs of...
Different tread designs of these dirt tires indicate different manufacturers. In front is the uncut block-style tread in a Goodyear Eagle. The heavily cut block-style Hoosier is stacked behind that. On the top right rear, there is a chain pattern tread. The choices are sometimes confusing for the average racer. The real goal is to get the most tire footprint on the racing surface.
Correctly analyzing your tires can lead to maximum performance. Many factors can and will influence how well the tires work. The proper way to evaluate the tire temperatures and wear, primary indicators of how the tires are working, is what we will cover here.
The first indication of tire performance comes from taking tire temperatures while later indications involve tire wear. Dirt racers will tell you that taking temperatures is not practical. They may be right in most cases. Dirt teams usually need to wait longer, after the tires have worn considerably, to know how everything is working, but they will still have some indication.
It is important to know that the relationship between high and low temperatures is the same as for high and low wear. A high-temperature area of a tire will also be a high-wear area later on. Asphalt racers can utilize both tire wear and temperatures. For dirt racers, the wear information is probably most useful.
The tread designs produced...
The tread designs produced by the Hoosier Racing Tire Company allow the racer plenty of opportunity to measure the tread depth to determine tire wear. We can get indications of proper tire pressures as well as cambers by measuring tread depth at points across the face of the tire.
The reaction to tire temperatures must be immediate if those temperatures indicate that changes are needed. I have seen teams ignore obvious camber or pressure problems and change basic setup parameters to help the car handle better. Evaluating the setup in the car cannot begin until the tires are happy. Only then should we attempt to evaluate how the car is reacting in the turns.
We usually know the approximate starting pressures and cambers for our cars. As the car is run, these are the smallest changes that will be made. The most important reading of the tires will eventually involve tuning the setup. Let's start with the pressures.
Reading Tire Pressures
The optimum tire pressures are dictated by the magnitude of the load on the tire and the forces trying to roll the tire over as the car goes through the turns. We need to start on cold tires with a number that represents the optimum hot tire pressure minus the amount the tire pressure will grow due to the temperature increase from a cold tire (one that has yet to be run at speed) to a hot tire (one that is fresh off the racetrack after a hard run).
Using a high-quality memory-type...
Using a high-quality memory-type tire temperature gauge pays off in the long run. Tire data is the most important indicator of your car's performance. Keeping accurate records helps the crew chief make critical decisions about chassis setup changes.
There is a certain amount of backtracking involved in the process of determining proper tire pressures. For example, suppose there is an 8-pound gain in the right-front (RF) tire pressure after a run, and then a setup change is made to take some of the load off the RF tire, putting it onto the left-front (LF) tire. With that change plus the associated reduction in the amount of work the RF will be doing, the next run may see only a 6-pound gain in pressure. The starting pressure will need to be increased by 2 pounds so that our optimum pressure will be reached.
Wear and Temperature
Tire temperatures will tell how the load on the tire is distributed across the whole contact patch. The greater the pressure, the more work that portion of the tire does, and therefore the greater the temperature. From a quick glance, tire pressures and temperatures, together, can indicate how the tires are working. Immediate changes can be made based on this information.
Tire wear is more subtle. More laps must be run for indications of how the tires are working. Although this is a very good indicator, it does nothing to speed the process of chassis tuning. It can help us to understand if the changes already made are indeed correct.
Let's go through a typical test or practice session and analyze the data and the changes to be made. First run the car for two sessions to make sure the tires are clean, the engine systems are working, and that the track is sufficiently clear of any dirt or debris.
While the crew went to work...
While the crew went to work refueling during a break in a long-distance race, this Modified driver went directly to the tires. Tire wear plays an important role in a tire's performance.
Start with the RF tire. The middle of the tire surface is cooler than the outside edges. This means more pressure needs to be added. The LF middle temperature is higher than the outsides, so pressure must be reduced. The left-rear (LR) tire shows the middle again higher than the outsides, another case for pressure reduction. The right-rear (RR) tire temperatures rise as we go from the outside to the inside. The middle should be 213 degrees if the pressure is correct. This tire bears watching, but needs no pressure changes for now.
With those changes, the car is sent out for the next session. This is where the results of the change become known. The RF tire, although not even in temperature across the face, has an even difference from the outside to the inside. The LF tire is also fairly even in difference across the tire. The LR tire shows fairly even temperatures with the inside (meaning the edge toward the inside of the racetrack), showing a slightly higher temperature than the outside. The RR tire shows the same tendency as the LR tire, with slightly higher inside temperatures.
The LF tire is hotter on the outside edge (away from the inside of the track), so there is a need to increase the positive camber. Again, about a 11/416-inch movement of the upper control arm shaft to push the top of the tire away from the centerline is a good adjustment.
Those changes are made and the car is sent out for another run. Chart 3 shows the results of session 5. The camber changes have produced nice tire temperatures at the front of the car. The RF tire is nearly even across the face, and the LF tire is also very good. With those changes, the setup can be analyzed by looking at the average temperatures of the four tires.
If you can, buy a large dial...
If you can, buy a large dial tire pressure gauge so you can read whole pounds and increments. Some circles of racing have refined their setups so that half-pound changes in tire pressures will affect the setup. That's splittin' hairs.
The temperatures from session 5 would normally indicate a loose car, but knowing that the LF tire is not working hard enough, we might guess that the car is tight-loose. This is a condition in which the car is really tight, but not skating the front tires, and the driver compensates by steering harder in the middle. The added steering input causes the front of the car to quickly gain traction, and as it begins to overcome the rear traction level, the driver gets back into the throttle and the rear tires break loose.
The tire temperatures might lead us to believe that the car is loose, and the driver will confirm that because all he can feel is the car going loose on exit. The real indicator is in the degree of steering input. The crew and the driver can see that the input to the steering wheel is more than should be needed to execute the turn. A setup change is needed to reduce or eliminate the tight condition.
The setup changes that might be made include raising the Panhard bar, increasing the RR spring rate, reducing the LR spring rate, softening the RF spring rate, and stiffening the LF spring rate. All of these changes will help the chassis to be more balanced and make the LF tire work harder.
Another quick look at the tire temperatures shows that all four tires are a bit hotter on the inside (again, related to the track, not the individual tire) edges. By reducing pressure in all of the tires by 1 pound, the temperatures may even out further. We will assume that we made the correct changes to the setup and tire pressures as we look at session 6.
Used properly, a simple tread...
Used properly, a simple tread depth gauge, such as this Longacre unit, can tell exactly how each part of the tire is working. Many teams cut wear grooves, in addition to those that might already exist, to determine if tire pressures and cambers are correct.
If setup changes were tried before tire pressure and camber problems were fixed, the whole process might not have worked as well. We might still be chasing the handling balance or throwing crutches at the car to make it neutral in handling, but not balanced.
These numbers can be achieved in the real racing world. Teams have gone through this process and achieved the same results. Some racers might say they have never been able to get the left-side tires equal in temperatures, but then again, many teams have never had a truly balanced setup.
Now that we know the method to find that sought after balance, we can begin to make adjustments to make our cars better.
In chart 1, we see the results of the third practice session. The team has recorded the hot tire temperatures, the cold pressures, and the hot pressures. We see the breakdown in average temperatures and pressure gain for each tire. We want to concentrate on the tire pressures first.
With the tire pressures under control, it's time to move to the camber adjustment for the front tires. The RF has a higher temperature on the inside of the tire and therefore needs to have some of the negative camber taken out. Adjusting the upper control arm so that the top of the tire moves out from the centerline of the car accomplishes this. A 11/44 degree of camber will probably even out the temperatures. For a 12-inch-tall spindle (measured from between the centers of the ball joints), the upper control arm shaft would be moved out away from the centerline by about 11/416 inch.
The LF tire has shown a lower average than the other tires. We need to compare it to the LR tire's average temperatures and work to make the left-side tires even in temperatures. After session 5, the LF tire was 9 degrees cooler than the LR tire. The average of the front tire temperatures was over 6 degrees cooler than the average rear temperatures.
We have achieved a near perfect set of tire temperatures and tire pressures. The setup changes have resulted in the left-side tires being nearly even in average temperatures, and the RR tire has cooled down some as the tight-loose condition was eliminated. Tire pressure gains on each side of the car are equal with 4 pounds in each left-side tire and 7 pounds in each right-side tire. The front-to-rear averages are very close, and we have most likely created a setup package that will be fast and consistent as well as easy on the tires.
Many believe treaded tires run only on dirt, but some classes on paved surfaces use grooved tires. These tires have been implemented in some Modified and Truck racing classes, but altering the tread pattern is usually prohibited.