I don't know jack shite about photography, but my friends that do it for a living say that black vehicles are the most difficult to photograph.
It's true. Black reflects EVERYTHING. You don't want reflections in the paint. You want clean highlights.
EDIT: Here's a very informative link about lighting vehicles from one of the most famous guys who used to be in the industry. I don't care for his removal of the backgrounds, but the guy knows his stuff. Helped me out a lot.
http://photography-on-the.net/forum/showthread.php?t=70290
OP: Here's a tip that I've been meaning to try. Get a couple of white bedsheets and stretch them out above the bike. Light the sheets with a few lights to create one big softbox. If you place it above the bike, then you should get some nice highlights showing off the curves. That's basically how they do it for the studio shots.
Also, are you pointing the lights at the bike? It's much better for vehicle photography to light the ground and things around it. The reflections will show up in the paint to give it the definition you're looking for.