[EricH]

Originally Posted by Chris Fultz

I don't understand what you are saying here.

When the wheel hits the bump, it will roll up and over. When the wheel is then sitting on top of the bump, the axle is then 4" above the bump. How does the axle hit the top?

On the moment of contact, the front wheels hit with the axle at the top level of the bump. They won't roll up and over without help--their force at that point will be entirely vertical once they leave the ground, but without something to push them horizontally, they'll fall back down. The back wheels are what provide the horizontal force to get the wheel up on top of the bump.