Advanced Tyre Models
Even though Rush Rally 3 was heavily praised for it's handling model and physics, one thing that kept coming up in the reviews was that the difference between the surfaces wasn't as noticeable as it could or should be. When coding up the Rush Rally 3 model I always had gameplay and fun at the front of the specification and so I made sure that the handling would be suitable for everyone playing it, by making the surface changes a bit more forgiving. Lots of people also mentioned how great the game could be with the addition of a tyre choice for this game.
For Rush Rally 4 I really want to address this and the best way of doing this in my mind is to build a tyre model for each surface, and then couple that with how the model would behave with a given tyre.
I decided then that the first job would be to change the way the tyre model works for a small set of surfaces and get the tyre model to change whilst driving between these surfaces.
The Rush Rally 3 tyre model is still in use, and applies mainly to the tarmac base roads. The way a tyre behaves on each surface is incredibly different.
For example we all know that an asphalt road will behave differently when wet or dry, we expect a wet road to be a bit more slippy and in simple terms you could just reduce the friction for a wet asphalt surface. In reality though it is not simply just friction that makes a wet road different to drive on, you have to consider aquaplaning and a whole load of other things that impact the model, not to mention you need tyres that displace the water (try driving in the wet on slicks, it's not pleasant). If you compare this to a gravel based road, you might be surprised to know that actually in the wet or dry, the gravel behaves mostly the same, so there isn't really a wet tyre choice for gravel, but the model for gravel is vastly different from a tarmac based road (it is a lot more sliding in general and the car will likely slide sideways a lot more).
With all this in mind I cam up with a number of surfaces that all behave slightly differently to one another to give that nice transition feeling when going from one surface to another. I decided that mud and dirt can be thrown into the gravel category to keep it a bit simpler.
Each of these surfaces basically use the same mathematical model, however they all use slightly different input to adjust how the model behaves. The asphalt surface will have a quicker response indicated by a sharper gradient in the base curve. A snow tyre will be a lot more "floaty", indicated by a much shallower gradient.
These curves are not used directly, they are input into a more complex tyre model based on the Semi-Emperical Brush Model for Combined Slips model which I studied a long time ago. This basically changes the way the tyre curves work based on wheel spin, skidding etc etc and make the car feel exactly like a real car in the Rush Rally simulation.
Now we have the tyre models per surface, we needed to change the way they all worked based on what tyre you have put on the car which I will address in another post shortly.
