# WheelCollider friction explored

Discussion in 'Physics' started by Ghidera, Aug 9, 2015.

1. ### Ghidera

Joined:
Mar 6, 2013
Posts:
53
I've been experimenting with wheelcolliders and I'm having a bit of a hard time understanding the settings for friction.

I assume that Extremum is synonymous with "Static" and "Asymptote" with Dynamic.

So the Extremum slip point <Static Friction> is where an object will start to slip or "Break Free". Like pushing a desk, you have to push hard to get it started.

The Asymptote slip value <Dynamic Friction> is the resistance to movement once an object has started moving. The desk, once moving, takes less effort to keep moving.

Next is the "Slip" and "Value" items. I've got a suspicion what they are which I'll explain and I'm hoping someone can confirm or correct me.

"Slip" appears to be the amount of slipping that can occur before reaching the break point where dynamic friction takes over. So even though the object hasn't "broken free" it still slips.

"Value" appears to be the amount of force required to "break free" for Static Friction and the amount required to "level out", or stop dropping, for dynamic friction.

So an approximate slip amount before breaking free would be something like

Force / ExtremumValue * ExtremumSlip. In reality the slip amount is on a curve and this formula is plain proportional.

At this point I haven't a hunch what Stiffness means. I took physics a few times and learned about coefficients of friction but no "stiffness" was ever involved.

So am I right, wrong, close, waaay cold, or what?

Thanks!

Edit: I just had a thought. Is stiffness the "curviness" of the static curve? The higher the stiffness the sharper the curve is around the slip/value points?

Last edited: Aug 9, 2015
2. ### Ghidera

Joined:
Mar 6, 2013
Posts:
53
Well I did a lot of experimentation today and the above doesn't seem to be the case. I cant make any sense of the settings at all. If I lower the Extremum slip it slips more, if I increase it the slip also increases.

Unity really needs to explain what the settings actually do.

Joined:
Jun 2, 2015
Posts:
290
4. ### Edy

Joined:
Jun 3, 2010
Posts:
1,580
I've also been unable to extract any logic out of the WheelCollider friction. While the docs explain how should it behave "ideally", the experiments show other results. Moreover, it looks like forward slip and sideways slip as reported by GetGroundHit are using different units among them.

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5. ### Ikaro88

Joined:
Jun 6, 2016
Posts:
69
i have the same prole, every one can help me to add friction?
in the guide NOTHING is clear how to increase, decrease friction

Last edited: Jan 5, 2017
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6. ### Marcos-Schultz

Joined:
Feb 24, 2014
Posts:
297
I realized exactly what you said. The lateral slide and slide forward does not seem to relate to each other.

Another thing I also noticed is that information is missing from the Unity documentation. Let me give you an example: You can use the following values for forward friction:

1
2
1
2
1

or

10000
20000
10000
20000
1

For a curve, this should yield equal results, but that is not the case. I am very curious to know how the engine works internally with these values.

7. ### WickedCube

Joined:
Jun 4, 2016
Posts:
2
For anyone looking to simulate wheel collider friction
In versions over 5.4, wheel collider comes with "Wheel damping" set it to desired value.
Ex: For mass of 75 wheel damping of 200 looks neat.

8. ### JeyP4

Joined:
Mar 6, 2018
Posts:
16
I think below cases are identical:
1
2
1
2
10000

or

10000
20000
10000
20000
1

9. ### Marcos-Schultz

Joined:
Feb 24, 2014
Posts:
297
Yes. I did some tests with exactly the same vehicles, changing only the wheel colliders.

Apparently they behave equally.

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