thanks for the response guys - so many ways!
i haven't had a chance to try anything yet, but i like the idea of just laying down the tread pattern on a surface and just revealing it using the transparency node - if i can figure that one out
the tire uses a bump map for the tread, so getting it to leave an indentation would just leave a flat groove (i think)
at the moment, i have rendered out the tread trail as a separate file with alpha, and am now rendering the main animation without the track - then i'll comp them together in after effects, using a mask to hide the bit in front of the tire
this seems to be the easiest way for me at the moment, as i still get a bit confused by the hundreds of ooptions in maya
by the way does anyone have any input on the custom brush thing? - that would seem like a cool way to do it if the tread pattern could be made in to a maya paint brush, then it would be quite easy to animate the brush on a path
will post result of comp when done