Been there, done that. It's VERY hard to get a mag wheel to start burning. The alloy used is ~95% magnesium so it will burn if you can ignite it. The problem was getting it hot enough. Magnesium is a good conductor of heat so the heat from any "point source" such as a propane torch and road flare was dissapated throughout the rest of the rim and so it never reached ignition temperature. Wood and rubber don't burn hot enough unless you have a BIG fire with a good wind blowing on it. A large amount of charcoal might do it, again if you have wind blowing on it (think a forge). Basically, anything most people carry in their car won't do the job.

A pound of thermite works though.
<img src="/images/graemlins/grin.gif" alt="" /> <img src="/images/graemlins/grin.gif" alt="" /> <img src="/images/graemlins/grin.gif" alt="" />

-Blast