There are TWO indications to a bumpy surface.
The first, and obvious one, is that the plank will flash yellow on the car
in your***pit showing that you are bottoming out. The more flashes, the
more you are bottoming. if it stays a constant yellow, then you are so
low that you are driving with the car touching the track. The plank will
soon turn red from the friction and the car will slow down.
The second, and not real obvious indicator, is that the car will 'bounce'.
This can be SEEN because the view will slightly shake. It's not like in
Duke3D when an earthquake goes off, but you should notice a shake to the
view as you are driving. This is seen while in the***pit and viewing
the normal driving view.