Second idea is to attach a crawler on to the front tire. Attaching a second small wheel in front of the front wheel, and connecting them by a crawler will make it able to lift the front of the chair on to obstacles.
pro's:
Steady structure. After attachment it will be able to function without complex movement.
Can use power from wheelchair itself.
Rugged look and feel.
cons:
Less shock absorbing then an air tire.
Sticks out in front of the chair
the back wheels still some way to get over the obstacles as well
