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Criado em Sexta, 26 Agosto 2011 23:01

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Hoje, Dave Kaemmer, publicou um longo texto sobre a nova física de pneus (NTM) do iRacing. O texto está em inglês mas relata datalhadamente a nova física lançada neste mês de agosto e que passará por diversas atualizações. Confira.

It appears there is some unhappiness (but also some happiness) with the changes to the new tire model (NTM) that were introduced at the beginning of the current season. I’m going to try to explain where we are with the NTM and address (at least some of) the current concerns. This will be a long post, but bear with me, if you are interested.

It’s important to keep in mind what the new tire model is, and what it isn’t. It is a theoretical model that makes very good tire performance predictions from a few easily measurable quantities. It is not a perfect reproduction of the tires supplied last weekend in the real world for any particular series. While the data (lap times, cornering speeds) that we see on television are certainly helpful in figuring out whether we have the theoretical model predicting stuff properly, it is most important to me that the same model works as we move from a NASCAR tire to a Formula Mazda tire, and that we only have to change the numbers that really do change in the real world, such as the curvature of the tread and sidewalls, the rim and tire radii, the amount of carbon black, silica filler and/or extender oils that might be in the tread compound, the glass transition temperature of the tread rubber, etc. The most important thing is that they behave like tires that might be supplied on a given weekend for a given series under as many conditions as possible. Actually, the most important thing is that they behave like tires under as many conditions as possible. The NTM is already a very good model in that regard. It is not perfect, nor is it completely finished. I’d be the first to tell you that there are areas where it still doesn’t predict as well as I would like, but I’m working hard on those areas, and I’m pretty happy with where it is at the moment.

I’ll move on to more details, with some explanation of the tire temperature readings, wear, and a number of other issues that are being debated in this forum.

Tire temperature readings
These are a source of quite a bit of confusion. In real life, temperatures are taken as soon as possible after a car has rolled to a stop in the pits (sometimes after removal of the tire), by pushing a probe into the tread rubber at three points across the tire, typically at 25% (left), 50% (middle), and 75% (right) of the way across the tread. Sometimes a team will take temperatures at the shoulders (edge of tread) to see if they are in danger of blistering the shoulder from running too much camber. These temperatures are taken at single points laterally on the tread. I refer to these temperature readings as “carcass temperatures”, because they represent the temperature a few millimeters into the tread rubber. In telemetry output, the readings are taken by infrared sensors and so represent the “surface temperatures” of the tread, which can be quite a bit cooler (or hotter) than the carcass temps.

In the iRacing NTM, the carcass temps (which are what you see in the garage or in the tire pop-up info, as opposed to in the telemetry output) actually represent an average temperature over one third of the tire surface, instead of the temperature at a single point. This is something I’ll need to fix someday, because it makes the temps look “wrong”, but the carcass temps are useful as is, because they include the shoulder temps, in a way. What’s wrong is that this almost always results in a higher center temp than edge temp. For now, don’t worry about this. It doesn’t necessarily mean your pressure is too high. In the sim, if you have an edge temp that is higher than the center temp, you are probably running too much or too little camber, or too little pressure.

Depending on the tire pressure and the weight on the tire, there is a patch of tread (the contact patch) that is flattened against the ground. What the carcass temperatures tell you is roughly where your tread contact patch is, and how hard you are working that contact patch. With higher pressure or lower weight, the contact patch gets smaller. With more weight or lower pressure, the contact patch gets bigger. If you lean the tire to the left, the contact patch moves to the left. If you lean the tire to the right, the contact patch moves to the right. To some degree, you can tell roughly how big and where the contact patch is by looking at the temps. The center temp is usually highest because often the contact patch covers a good bit of the center third of the surface, even if it has moved left or right, so all of the center is doing work, all the time. Comparing center temps around the car is a good way to see if the car is balanced, or if one or more tires are doing too much work. The left and right third of the tread are not usually both completely in contact with the road. If they were, then you would see nearly equal carcass temps (actually they could be a little higher at the edges, since the tread is often a little thicker at the shoulders). You will usually get more grip, if this is the case, by increasing the pressure. That makes the contact patch smaller, and gets the shoulders a little off the ground, which makes the contact more efficient (less edge loading).

If the left temp is higher than the right temp, then the contact patch is on average to the left of center on the tire surface, and likewise for a higher right temp. If the split between left and right is higher, then either the contact patch has moved more, or it might be smaller. You will generally get better grip with the contact patch moved left of center for a left turn, and right of center for a right turn. How much is a matter for testing, since it can depend on pressure as well. Read on.

A lot of the carcass temperature is due to rolling drag, which is higher with a larger contact patch, and lower with a smaller contact patch. On a NASCAR oval, the contact patch gets a lot bigger in the corners than it is on the straights, because of the very high loads due to the banking. This increases rolling drag in the corners, and so the carcass will heat up more there. But in the corners, there is also a lot of heat generated at the surface of the tire from sliding. Even when you don’t think you’re sliding, part of the contact patch is, and that generates a lot of heat. On the straights, there’s very little heat generated from sliding, so the surface cools off quickly. This all gets more complicated when you consider that the tire tread band can move back and forth relative to the wheel rim. As cornering forces are generated, the tire tread band is deflected sideways and leans over relative to the rim, which changes the tread’s camber angle, which moves the contact patch sideways. This means the contact patch not only is a different size in the corners, but it moves to the right in a left hand turn, and to the left in a right hand turn. With lower pressures, the sidewalls aren’t as stiff, and the tread band will lean over more. So if you lower pressures a lot, you’ll need to adjust the camber to get the contact patch back where it works best. If you raise the pressure a lot, adjust the camber. Lower pressure generally means more static wheel camber is required, higher pressure means less static wheel camber is required.

You might think I’m just going down a rat-hole, and getting off topic. This is all related to what your tires temps mean, though, so bear with me. The carcass temps are influenced by rolling drag and the sliding work at the tire surface. The contact patch isn’t the same size when cornering as on the straight, though, and it’s not in the same place either. So on the straights, you might get temperature buildup on the left edges (NASCAR example), since the wheels are cambered for the corners, and are leaning over too much on the straights. But in the corners, the contact patch should move to the right and get bigger, which means more rolling drag. Plus now some of the contact patch is sliding, which heats everything up even more. You have to try to read the tire temps in the context of an entire race lap. For example, the left front (sticking to the NASCAR example) is usually set at a ridiculous positive camber, because in the corner, that gives the best contact patch location, and therefore the best grip. However, you will see the outside edge running pretty hot, because not only does that left front have to travel down the straights at its ridiculous positive camber, but it’s doing so at a low pressure, which gives a large contact patch on the straights, and lots of rolling drag. In the corners, low pressure works on the left side because a lot of load transfers over to the right side tires, which makes the left side contact patches smaller (and better for grip). What you see in the carcass temps is the temperature changes over entire laps, averaged over straights and corners. So it’s not easy to make sense of it. Looking at the surface temps in the telemetry output can help identify what’s going on in the corners, since the heat at the surface is closely related to sliding, which mostly happens in the corners.

A lot of energy is flowing around during this process. Heat is generated from rolling and sliding. Some of that heat is absorbed by the ground as it comes in contact with the tire surface. Some heat flows between the surface and the carcass. Some heat convects away into the air, some of it radiates away (but not much). Heat convects into the air inside the tire, flows through the sidewalls and rim (as well as the inner liner, and the air inside the inner liner, if there is one). Of all the numbers involved in this process, the convection coefficients and the thermal properties of the road surface have the largest margin of error at the moment, although I have very decent and defensible quantities for both. It is amazing how very small changes to these numbers can create a lot of controversy. That is just like real life, I guess. The line between “it’s junk”, and “it’s perfect” can be razor thin. In all seriousness, we’ve got things pretty close when the cries of “it’s junk” and “it’s perfect” are equally loud. I do agree that the NASCAR tires don’t seem to be building enough heat currently, although in the beta they were building a bit too much. Just be assured that these temperatures are amazingly close to correct, given all the complexity that goes into them.