Model breaks at high velocities

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Hello Guys,

i am rebuilding the azcar car model from here: http://cps-vo.org/node/25319/browser

But for some reason with my own controllers and added steering joints for the rear. My model breaks at high speeds! - Meaning that the rear steering joints waver around the z axis, but are not fixed on it anymore.

I have implemented the "rear axle motor" as a velocitycontroller. Has sb. experienced sth. similar. I am currently thinking, that my mass distribution is wrong. But i couldnt find anything regarding that topic.

Thanks for your help