Quote: My a6 car works OK on big shaded ground(block). When i change this block's shaded flag to FLAT , then my car will always bounce. What is the problem ?
The wheels have a tendency to bounce at polygon edges. If your non-shaded surfaces are a lot smaller than the shaded ones the car will be more bouncy. You can check the polygon size by hitting F11 twice.
If you encounter problems only with flat surfaces even though they are the same size as shaded ones then that would be a bug. I am not aware of that problem though.
@Marco - I hope you'll be able to improve the problem of wheels bouncing on polygon edges. Currently, I have to sacrifice some lighting and design to keep the vehicle from bouncing on my levels. Since lighting is done per-vertex(per-pixel is too taxing), lower poly racetracks do not look as nice with dynamic lights.