We use box tubing with our own linear bearings. Each stage slides along the side of the previous stage. Every instance where two faces of metal rub against each other there is Teflon tape (not the stuff used in pneumatics), this is what allows the elevator to slide very smoothly. The elevator is also powered in both directions, so that we don't have to wait for gravity to pull it down. So far we have not tipped while our elevator has been extended, but it is always a concern for an arm design. We try to concentrate our weight in the back and drive carefully when the elevator is extended. The elevators best features are its speed (less than 3 seconds to fully extend), and the fact that it is self leveling as it extends, to prevent binding. Here are a few more pictures: