Having Bumpers mounted at different Heights?

[C.Lesco]

So I read the rule book and I have an idea to help cross the stone wall and moat by having the first thing for the robot to hit when reversing being treads or wheels to help climb over, like a rock climbing car To do this however Bumpers would get in the way. Could I lift the bumpers higher in the rear than in the front? Say life 5" from the ground in the rear and 3 inches in the front?) Is this against the rules? I've read and I didn't see anything specifically but what do you guys think?