As much as I love the idea of capturing the sound of the bikes in a garage or staging area it's not strictly documentary. I would do it, however; you cannot ever have enough audio fodder. The problem is that it's going to be difficult to match to picture, unless you create a complete cue list and record them after the race.
I wouldn't want to depend upon "long distance" wireless systems; the odds on having drop-outs is just too high. You could use a boundary mic and wire it directly to a digital audio recorder. You would have to experiment with wind protection and to see if a dynamic or cardioid is the better choice; my bet would be on the dynamics - they can handle very high SPL levels, and are usually more robustly built. The biggest "problem" would be to find a place to put the mic (probably on the back fender) and the recorder (beats the hell out of me!) and securing them firmly without interfering with the rider or the operation of the bike. You absolutely should tap into the comms as suggested; hearing the rider and his team communicate would definitely bring the audience into the action at a much more personal level. I would also have mics and recorders at various positions around the course if you have the budget.
When working on "Johnny Montana" I spent a day doing pass-bys, starts, stops, etc. and two more days riding double on a Harley with various mic combinations. Rather exciting at 100 mph.....
