It's not really a matter of heat that I foresee being a potentual problem with the exhaust exiting straight into the rear tire. It's exhaust soot, which by the way carbon makes for a nice dry lubricant. Also, if you were ever to encounter an engine problem were it might blow oil out, guess where it's aimed at? Other then that, I've been wanting to do this (with angled tips) before I even saw the GB thread and now this one. Great job!
~Mike......