Analyzing Tire Models in Sim Racing

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It can be debated that tires are one of the most important parts of a race car, with them providing the connection between the road and the car. Likewise, a racing simulation’s tire model is arguably the most important part of a sim, and one of the trickiest to make. I decided to look into some of the tire model technical aspects, to see what makes it so darn important.

A Tire Model Needs to Be Universal

If you think about the physics that apply to tires, you can come to this conclusion: they are consistent. Gravity will (usually) stay at 9.8 m/s² while stationary. The laws of physics will not change, so a tire model should not change. Regardless of tire type, it still abides by the laws of physics, so you need to analyze the physics and forces that are enacted on a tire.

Magic_Formula_Curve

That is exactly what Hans B. Pacejka, Professor emeritus at Delft University of Technology, has done over the past twenty years or so. Pacejka created what is known as the “magic formula”, which is used in many racing simulations, including rFactor, netKar Pro, and NASCAR Racing 2003 Season. While the Pacejka Tire Model can be a solid method for replicating tires at race speed, the formula struggles replicating low-speed tire behavior. Because of that, racing sims tend to combine PAcejka’s model with other formulas. One notable one is produced by Milliken Research Association. Most notably, NASCAR Racing 2003 Season and Grand Prix Legends used a “Milliken & Pacejka ‘Similarity’ Tire Model”.

Shifting to Proprietary Tire Models

With the latest generation in racing sims, we’re seeing a departure from Pacejka’s Tire Model, and a transition into more proprietary tire replications.

For Project CARS, they’ve been relatively open with showcasing their Seta Tire Model, giving us a written feature on on how the virtual rubber is simulated in their title. However, they don’t go too deep into what they use.

Sims like iRacing, rFactor 2, and Assetto Corsa have made significant departures from their Pacejka roots, and have worked on developing innovative new formulas. Dave Kaemmer has been hard at work bringing his “New Tire Model” to iRacing. rFactor 2 has its own tire model, which dynamically simulates flat-spotting, tire flex, and dirt pickup.

Which Tire Model is Best?

Then on to the big question: Which tire model works the best? The simple answer is that each tire model has its strengths and weaknesses when it comes to replicating real life. Its commonly mentioned in forums that iRacing can struggle with low-speed cornering and lack of flat-spotting. Some people feel that Assetto Corsa’s tire model doesn’t have enough flex, while rFactor 2 may have too much. Whether these claims are true remains to be seen, but bottom line is, drive what you like.

My Thoughts/Analysis

However, to give a more “tangible” answer, I’ll share my opinions on the matter, and analyzing:

Tires Need to Flex

One important element of tires is that they flex, and the area of contact with the road (the “contact patch”) shifts around. You need a proper simulation of that to get the most realistic slip angles and realistic deformation of tires. rFactor 2 does a great job simulating the flex, but the contact patch simulation is lacking (but will be implemented in an upcoming patch).

Tires Need to Wear Out

Tire wear is a crucial element in racing, that can be a determining factor in many events (see the latest F1 race at Sepang for a good example). Tires sliding and scraping on the pavement causes friction, and friction causes wear. Most racing sims can simulate that effectively, but again, I’d say rFactor 2 does the best job.

Tires Need to Feel Like Tires

You may be thinking this is a “no DUH!” statement, but really think about it. In some sims, driving a car doesn’t feel like a car. While some of that can be attributed to Force Feedback, some of it can be traced to tire model issues. In my opinion, iRacing can be guilty of that, but they are improving. Assetto Corsa can feel like you’re on “bouncy balls” at times, but they have a feeling of driving on tires.

Conclusion

In conclusion, I would say that tire models have greatly advanced in recent years, to really innovate. A solid tire model combined with solid Force Feedback can make for a very authentic driving experience, where it feels quite close to driving a real car (at least the feel through the steering wheel, that is).

What are your thoughts on the different tire models in racing sims? Which do you feel is best? Let us know in the comments!