Clutch not locked but not slipping??

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I am not sure I can follow your modifications.

The implementation is a simple iterative solver. You can try to increase the number of iterations, but it doesn't work well with large inertia ratios. The accumulatedImpulse is the total angular impulse applied by the solver. It is clamped to the constraint impulse limit. Usually one will have a lower and upper limit. I haven't noticed any issues with the current version though. Lambda is the correction impulse applied during a solver step.

If you want to use realistic values you have to increase the simulation rate and maybe use a different solver. I am impressed that it runs at 50Hz. I think 120Hz are actually too low.

What tire model are you using? I assume you are feeding realistic torque values? What do you mean by clutch torque lag?