Here are some shots of the progress of the Wheely King towards a decent-strength bumper:

This 2008 version in metal didn't last a day (or more precisely the epoxy resin didn't hold it together long).

Shapelock is a pretty cool stuff: malleable from 60°C upwards and pretty strong when cooled down. This metal-plus-plastic bumper survived a good two years and quite a few rammings caused by a certain Kid Who Loves Full Throttle ;-) - but eventually the metal supports succumbed to said kid (the shapelock parts were still fine).

And this is the latest version: a bumper make from shapelock only, no more batteries high up but shortened packs low and on the side of the frame, a fully locked rear diff, a grease-and-blutack-LSD in front, and decent tires on beadlocks. Looks dud but it works pretty well now :-)