I am also fighting with this recently, and now I can get the collision reliable on block or terrain slope.

Slope test

Control: 1-5 select camera, WASD movement, cursor left/right to turn.

I use engine default setting, I don't know is it correct but base on trial and error I discover that don't set the model too high from origin, if your model is a vehicle. Lower from the origin a bit, see the ma1.mdl in the zip.

Subtract the "distance to grand" calculation a little, like -5.5 in my example, otherwise the model will sink in the ground. so the model raise a bit to the ground. Now the model "touch" the ground nicely.

Code:

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ext_force.z = max(-35*time_step, -(c_trace(my.x, temp.x, ignore_me|use_box)-5.5));

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I am a newbie I don't know if there are better solution, I'd like to know the best practice as well, so any advise are welcome.