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# Vehicle Physics

Hey all,

Questions highlighted below, mixed with backstory.

Finally playing with some of the more fun stuff. One thing I’ve been trying to do is get a decent physics / arcade vehicle based on the Advanced Vehicle example.

I started my gaming career building driving models - formula 1 simulations and arcade, destruction derby and others. Armed with a physics degree and research from the racing industry early driving models were based on the bicycle model (we’re back on the Ps1 here) and later models with more horsepower incorporated better suspension vs friction influences but in the end would incorporate features of a bicycle model simulation. In total I spent five years with this being my primary focus.

One critical tweak was the tyre friction coefficient curve. Is there such a thing in Unreal? A tyre does not have a simple static / dynamic friction coefficient. The rubber on a tyre walks first, then vibrates when sliding. Initial friction response is pretty linear to force applied but then it starts dropping off. There is likely a drop in resultant force as the tyre moves from walking to slipping. See this thread for some diagrams.

The shape of the curve defines entry into sliding, and how aggressive the breakaway is.

Surface materials would affect this curve - changing the gradient, drop off period, final friction scale.

Q: Does Unreal have tyre friction curves? I haven’t found them.

A second element to feel was the relationship between longitudinal forces and lateral force curve. A handbrake drops the tyre right into its fully sliding state, but applying power might just push the lateral friction curve towards the more aggressive breakaway profile (once a tyre is broken away maintaining the power input would keep the wheels spinning a lot longer than it seems to in my experiments with unreal).

Q: Can I get the longitudinal forces to interact with the lateral force profile of the tyres?

If I could tweak the wheel friction at runtime it might be possible to layer this into the unreal vehicle sim. This was how the my later driving models operated, relying on the basic tyre frictions but overlaying a layer of tweaks to the tyre behaviors in order to be able to define a drifting experience vs a sticky experience, or more critically to be able to define a driving experience that has decent stick but isn’t out of control once drifting.

**Q: **Can I tweak tyre friction dynamically? This would require per tyre instance input.

Hi,

Did you ever find the friction coefficient of the tires in Unreal?