I am building a cart that needs to vary it's length to accommodate rough terrain. How can I keep the cart's front face parallel to its back face? At first this question seems like a simple parallelogram. However in order for a parallelogram to keep all edges parallel the edges must all be of fixed length. In my case (looking at the cart from the side) only the front and back edge lengths are fixed. The top and bottom edges can vary in length, although they will always be the same length relative to each other.

At this point I am stumped. Can someone suggest a mechanism that can keep the front and back edges parallel?

Any help would be much appreciated!

Steve