# Thread: Bullet car help

1. ## Bullet car help

Hello guys
I'm experimenting with Bullet to rig a car with shockers and steering control. I've looked all the examples I could find, some videos, including a scene posted here by one of our users. I try to find the simplest way possible and what I came with so far is to connect a wheel linker (rigid body cube) to the main body with a 6DOF constraint, which enable me to glue the linker to the body and add a spring effect at the same time. Then I connect the wheel linker to the wheel using a Hinge with motor active, to make the wheel spin.

Now. while It works, the wheels tend to be unstable , like "elastic" and don't keep a strict axis. (look at the video)

2. I don't screw around with dynamics much, so I can't help, but it looks pretty convincing to me.

3. Yes, looking good
I don't have much experience with this kind of rig myself, but just wanted to point out Ryan Roye training (Bullet dynamics for LW) in case you missed it. Making a Vehicle is included.

4. Wheel slippage primarily occurs if your simulation's FPS is below ~250, or the weight distribution is highly disproportionate to what the vehicle can carry, i'm willing to bet those bricks are way too heavy as far as the sim is concerned. It helps significantly if you have hinges holding each wheel structure together to eliminate wobbling.

When dealing with vehicles I recommend using a weight distribution of Given Mass instead of density, given mass means it will not take the volume (in terms of size) of the object into consideration when calculating its weight. Depending on the size of your vehicle, those wheels may be too small for an optimal result as well.

Something else that's important to understand is that Bullet has no concept of a 100% un-movable rigid body. Just because you lock down rotations does not mean the object won't rotate, it means it will resist rotations and rubber-band back into place when not subjected to forces that are greater than the mass that influences it. So, if things are shifting out of place, you generally want to check a few things:

- Is your dynamics framerate high enough?
- Are the forces influenced by mass and gravity too great for the object to withstand?
- Are you within the scale threshold of functional bullet simulations?

5. Yeap, I'm aware of the mass vs density issue. The cubes are 5kg , the car "body" 20 kg, the wheel "linker" is 35 and the wheel Itself 20kg, all given mass. I found out that putting the wheel linker at 35, even if It's not very realistic, stabilize the wheels (It was worst before). I think that everything is scaled properly (car body is around 1m50 long)
Ok so, I understand there is no magic here, just tweaking the numbers until It looks ok because the constraints are not sticking things to each other in a definite way. I thought my setup was bad. I just changed the gravity to 7 and the dynamic framerate to 360 and the wheels don't mess around anymore. The car behave like a crazy horse (spring acting up) but It shows what you say, too much weight on the rig.
I bought most if not all Ryan's video, best lw tutorials around with the Rebelhills series. Clear, fast to the point, advanced.

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