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Question by Jusas · Sep 05, 2012 at 09:56 AM · physicswheelcollider

How does the WheelCollider brakeTorque work?

I have this very weird problem with the brakeTorque:

I have a GameObject of a forklift truck. The forklift truck has four wheel colliders and a couple of box colliders and a Rigidbody with the mass of 1000. The mesh is animated, and the forklift truck's forks can therefore animate up. I created a script to apply WheelCollider.brakeTorque to all four wheels in Start(). Then I added a constant force component to the lift truck. I press Play and push the force up high enough to overcome the brakeTorque and the lift truck starts moving. All is well and works as expected, nice. I now remove the ConstantForce, looks like it works...

Now I decided to add a container on top of the forklift truck's forks, so that it sits there nicely thanks to physics. It's a simple container with a Rigidbody with the mass of 100 and a Box Collider. I press Play, and trigger the lift truck forks up animation... and the forklift truck, without any force applied to it, starts accelerating wildly into the horizon! The only physics forces applied are the standard gravity and the brakeTorque on all the four wheels. This is puzzling.

Granted, if I drop the weight of the container to 1, the problem disappears. Now, using somewhat realistic weights, like the forklift's weight (a heavy container lifter, 34000kg) and an empty container weight (2250kg) I ran a few tests. What is weird is how the physics behave with certain brakeTorque values. Here's a set of examples:

  • Forklift weight: 34100

  • Container weight: 2250

  • Brake torque: 0

  • Result: steady, the container gets lifted, no movement


  • Forklift weight: 34100

  • Container weight: 2250

  • Brake torque: 10

  • Result: as the container gets lifted, the forklift accelerates to ridiculous speed and drives into the horizon...


  • Forklift weight: 34100

  • Container weight: 2250

  • Brake torque: 500

  • Result: as the container gets lifted, the forklift accelerates. As it reaches the top, the forklift brakes to a halt.


  • Forklift weight: 34100

  • Container weight: 2250

  • Brake torque: 5000

  • Result: steady, the container gets lifted, no movement

So... how can low brakeTorque actually accelerate, while 0 brakeTorque doesn't and a high brakeTorque does what it's supposed to do? The whole issue here is the container that the forklift carries - I just can't understand the logic of these physics calculations. If I brake a little while lifting a high load in my forks, I suddenly accelerate, what what? This just doesn't make any sense.

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avatar image Fattie · Sep 05, 2012 at 12:11 PM 0
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forget the container, and try just adjusting the weight of the vehicle but that much ?

avatar image Fattie · Sep 05, 2012 at 12:14 PM 0
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are you making this container a CHILD of the other system, or is it literally just sitting on it? cheers!

avatar image Jusas · Sep 05, 2012 at 12:18 PM 0
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The container is not a child object, it's a completely separate object that just sits on the forklift's forks.

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