November 4th, 2016 by Matt Holden
Whenever we put a race car on track, be it real or virtual, we have two things that can give feedback on how the car is performing: The driver, and the tires. Drivers are usually very literal when they communicate what the car is doing. If it’s starting to spin, they’ll say it’s loose or oversteering, and if it won’t turn, tight or understeering. Going off of only what a driver says can sometimes provide quick-fixes to chassis setup, but can also make a problem worse when something else is causing what the driver reports. Tire data, such as hot pressures, temperature, and even tread wear, can be used together with driver feedback to diagnose issues more accurately and in less time. In this article, I’ll use a few examples taken directly from iRacing events. A link to download the spreadsheet I use to record tire data during the NASCAR PEAK Antifreeze Series can be found at the end of the article.
Individual Tire Loads
With load comes friction, with friction comes heat. As we run the car on the track and expose the tires to high loads in the corners, the interaction between the tire and racing surface produces heat. The heat put into the tire can be from sliding, but will also come from the work done by the tire to maintain traction with the racing surface. As we load a tire, it will become hotter, and as a tire becomes unloaded, it will cool off. In many cases, this is the only thing that will be constant in all situations, so it’s important to understand how to see which tires are loaded more heavily than others when you finish a run. To walk through a basic tire analysis, I’ll look at the first set of tires that came off of Nick Ottinger’s #05 car in the 2016 NASCAR PEAK Antifreeze Series race at Kentucky:
To analyze the load on each tire, we need to get an average temperature for each tire. This is fairly simple since we only have three temperatures to work from, and it will often give a much clearer picture of what the car is doing when compared to the 12 temperatures from the garage tire sheet or the black-box tire sheet. The tire averages were as follows:
LF: 207° RF: 262°
LR: 241° RR: 248°
Nick said during the first run that the car was loose off of turn 4, but he was managing it well. Looking at the tire averages to get an idea of the tire loads, we see that the rear tires are fairly close in temperature, meaning they’re seeing a similar load. The front tires, however, have a very large difference in loading, with the right-front tire being much hotter than the left-front, indicating that it is experiencing a very heavy load. While he is turning left only, and it’s a given that the right-front will take more of a beating than the other tires because of this, a large difference between the front tires such as this is typically not desirable.
This type of tire readout is also an indication of a suspension imbalance. In this specific case, we have the rear end too soft (in roll stiffness) when compared to the front. This results in the rear of the car rolling over and unloading the left-front tire, placing the load removed from the left-front on the right-front. Over a run, this will result in the car progressively becoming tighter, or producing more understeer, as the right-front tire wears and can no longer support heavy loads. In a race, this is basically an un-solvable issue, but at a track like Kentucky where there is very little grip and a lot of bumps, it’s often more comfortable for a driver to have this kind of car due to the extra security from the large amounts of rear grip.
When in practice or testing, it’s almost useless to tune the car off of the tire wear, especially oval cars. Since oval cars only turn in one direction, the right-front tire will almost always be the most worn tire on the car, and one of the left-side tires will be the least worn tire on the car. Not to mention that a tire will wear when it’s loaded and working hard as well as when it’s sliding across the track with no load on it. That said, wear does have its uses, especially when combined with tire temperature analysis. Here are the wear averages from the same set of tires above:
LF: 93.7% RF: 78.7%
LR: 90.3% RR: 78.3%
Looking at these tires, the one thing that’s immediately apparent is the large difference in the left-side wear. Nick was reporting a loose-off condition, but the temperatures were showing a tight condition. The wear numbers, in this case, finish the full story and give the message that he was more than likely sliding the left-rear tire more than the others which would result in a loose condition. Since his tires showed that the left-rear was very hot, the adjustment during the pit stop was to remove weight from the left-rear corner via the spring perch, reducing the preload on that spring and allowing the left-rear tire to work a bit more than before. This proved to be the right adjustment and the second run was faster than the first run.
Alignment and Temperatures
One of the major things to look for during practice and testing is any issues with camber, which will only show up on the tire data in the garage. For ovals, we typically want the left-side of the tires to be hotter and slightly more worn than the outsides, so negative camber on the right-side tires and positive camber on the left-side tires. For road courses, we’ll typically want negative camber at all four corners, but the amount of symmetry between the sides will depend on the track’s characteristics.
A good starting point for camber values is around 10-15° difference between the inside and outside edge temperatures with the wear numbers showing close to the same for all three zones (a few percent is okay). At larger tracks with long straights, such as Michigan, Indianapolis, or Pocono, the optimum camber can provide a larger temperature split than normal, since the tires will ride on the left-side edge for the straights. Small tracks like Bristol or South Boston may require a smaller split, since the distance between turns is short and the outside edges won’t cool off between turns. Optimum camber is simple enough to find though, and is usually the setting that provides the most grip with the lowest lap times. If you find what works best for your car, write down the temperature and wear split in the Notes section of the garage so you can fall back on it later or at a similar track.
While the minimum pressures are usually dictated by the tire manufacturer (and reflected in the iRacing sim), we still have a bit of room to work with starting pressures. In the broadest terms, we would like to have the pressures set so that the center temperature on the tire sheet is between the inner and outer temperatures. As of writing this article, the current tire model averages the center temperature and will show the center temperature higher than what would be measured in the real world (Read Mr. Kaemmer’s explanation here on the forums: http://members.iracing.com/jforum/posts/list/1808602.page). With that issue known, the best setting seems to place the center temperature around the temperature of the inside edge of the tire. With that set, further pressure adjustments can be made based on the wear numbers: If the center is wearing too much, reduce the pressure. If the center is not wearing, increase the pressure.
Starting pressures can also be changed for the situation as well. Our fastest laps will occur when the tire is at optimum working temperature and pressure, so getting those to match is key to a fast car. For short runs, especially qualifying, it’s common to see pressures raised quite considerably in an effort to get the car up to speed quicker. Longer runs will need lower pressures to allow for pressure buildup over many laps, with the aim of hitting the optimum pressure at maximum buildup.
Tire pressures can make or break a race, and among the many things we have to deal with through a race, pressure buildup is one of the most frustrating. As the tires heat up, the heat in the tire will heat the air within the tire and cause the pressure to rise. Over a full run on a set of tires in oval racing, it’s possible to see upwards of 20 psi buildup on the right-side tires, so it’s important to make sure the tires are building up to the proper pressure through a run. The garage shows us Hot Pressures next to our Cold Pressures, and the hot pressures can be just as important to diagnosing a car as the temperature numbers themselves.
The next article will take an in-depth look at tire pressures, but tire pressure changes the “spring rate” of the tire. This spring rate is very similar to the suspension spring rate, and it has a big influence on how the car drives. The biggest thing to look at is the difference between hot pressures on a given side of the car. For instance, if we set the cold pressures like this…
LF: 25psi RF: 45psi
LR: 25psi RR: 45psi
And hot pressures end up like this…
LF: 32psi RF: 65psi
LR: 34psi RR: 62psi
We have a problem! Knowing that the tires on each side of the car are the same for oval racing, equal pressures would produce equal spring rates. Thus, when we start, we have equal spring rates on the left tires and equal spring rates on the right sides. However, once we’ve run the car a while, we start getting higher spring rates on the right-front tire and left-rear tires. Plus, the left-front and right-rear tires will be a little flatter on the track than their counterparts and will produce a little more grip because of that. These two issues will undoubtedly lead to a tighter car late into a run.
For this situation, we need to adjust the cold pressures to compensate for the difference in pressure buildup. For the right-sides, we have a 20psi build on the RF and a 17psi buildup on the RR, so we can either add 3psi to the RR, take 3psi from the RF, or go halfway with each and take 1.5psi from the RF and add 1.5psi to the RR. Any of these will help the issue, and we can do the same type of change to the left-side tires. Getting the pressures to build up evenly can help address issues with the chassis, since a difference in pressures would add another set of spring rates to the car’s behavior.
The temperatures shown in the garage are the temperatures that would be taken using a pyrometer in the tire carcass. Essentially, these are the temperatures of the tire’s tread based on how much work it’s done during the run. On the other hand, telemetry exports display a more instantaneous temperature reading, and can differ from the garage temperatures quite considerably. Basically, the telemetry temperatures can be thought of as being taken from infrared sensors around the tire, and can give a good indication of how much of the tire is in contact with the pavement at a given moment. If the telemetry trace is showing a steep drop in temperature, there’s a good chance that section of the tire isn’t contacting the pavement and is doing no work at all. These temperatures, and especially the rate at which they are changing, can be extremely effective in tuning corner performance, while the garage temperatures are better for tuning long-run handling and overall balance.
While each set of data for tires has its own advantages and disadvantages, it’s important to consider all of it when setting up your car. This is all given to you with no work needed and is the only way your car can tell you accurately what’s going on, so don’t ignore it!
To get the spreadsheet I use in the NASCAR PEAK Antifreeze Series for tire data analysis, head over to the iRacing forums where you can download it yourself!
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