i think "Tête de marteau" is more correct. I find it hard to explain, but I'll try: one would use 'de' if no particular 'marteau' is involved. Using 'du' would imply that this is the head of some hammer in particular.

Compare to (ok, strange comparison maybe) 'tête de veau' (veal's head, the stuff the eat in France) vs. 'tête du veau' (head of the calf, as used in the sentence, "there's a fly on that calf's head").

Wat does this have to do with bikes again?