Racing, whether it happens in the real or the virtual world, is all about tires. That’s why Project CARS is powered by the revolutionary Seta Tire Model (STM) that plays a key role in making the WMD-powered title the most authentic virtual racing experience.
But what exactly does the Seta Tire Model do? Slightly Mad Studios’ physics expert Andrew Weber takes us for a look deep under the hood of the tire technology that drives Project CARS!
Seta Tire Model Described
Seta Tire Model (STM) is a full dynamic tire simulation. Actually, it is three coupled simulations, one for the tire carcass, one for the tire tread and contact patch, and one for heat transfer simulation. It is also modular, where different carcass and tread simulation techniques can be used interchangeably. For example, off road racing may use a different tread simulation.
Carcass Simulation
The carcass simulation used in Project Cars is a finite element simulation with specific computational optimizations specific to real time tire simulation. The carcass is discretized into small connected “elements”, each one flexing and deforming due to forces.
Features:
- Elastic behavior changes with speed, temperature, and pressure
- Rolling resistance changes with speed, temperature and pressure
- Sidewall buckling at low pressure
- Bias Ply, Radial, or Hybrid construction
- Gyroscopic Effects
- Dynamic response such as vibration, telescoping, and twisting
Tread Simulation
The tread simulation used in Project Cars is a finite difference simulation of the contact patch, with the tire tread “flowing” through the contact patch. The whole tread itself is discretized into elements much like the carcass, but the contact patch itself is a finite difference grid.
Features:
- Flash Heating, which is the change of temperature in the outermost rubber layer through the contact patch.
- Componentized grip model. Each component is affected differently by road surface conditions, wetness, and temperature.
- Deformation – the rubber deforming in and around asperities, resisting sliding motion.
- Adhesion – the rubber bonding to surface rubber and material.
- Tack – the sticky tacky grip you can feel on your shoes when walking a rubbered in track, related to adhesion.
- Tearing – the ripping of rubber from the tire
- Cut – grip from the geometry, edges, grooves, and siping of the tread, with particular effect in dirt and gravel
- Tread channel depth and water handling.
- Discretized and temperature sensitive wear
- Curing
- Temperature sensitive elastic properties
The carcass and tread simulations are coupled such that there is no roughness or “stepping”, while still preserving the detail of both simulations. The contact patch size, shape, and pressure distribution is determined by the carcass simulation and is used by the tread simulation. The forces on the tire from the road surface are simulated in the tread simulation and transferred as external forces to the carcass simulation.
Heat Transfer Simulation
The heat transfer simulation handles heat flow between brakes, wheel well, rim, carcass, and tread layers. The heat transfer amongst tread elements, from tread elements to the road surface, and from the tread elements to the air are handled directly by the tread simulation (including advection and evaporation). The pressure of the tire is maintained by the carcass simulation via the ideal gas law.
Emergent Effects
Most effects just “fall out” of STM without explicit coding for effect:
- Fy, Fx, and Mz vs slip angle curves, complete with realistic nuances, such as Mz inversion
- Inclination effects such as camber thrust
- Complex and sometimes subtle changes in behavior due to load, heat, pressure, and speed.
- Proper behavior at a standstill and very slow speeds, although due to limitations of consumer force feedback devices, oscillations may still occur.
- Many tire models break down at a standstill.
- Flatspots
- Hydroplaning
- Changes in behavior due to surface differences, such as surface roughness, track rubbering in, wetness, and dirt.
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