Difference bwtween 511 and 502?

[Ray Kindstrom]

Whats the difference between the 511 and the 502? (please dont state the obvious...belt and shaft) I read that Tamiya didnt replace the 511 with the 502, they just added the 502. Is one meant for tight tracks or loose tracks. I mainly run on med. to high bite indoor clay track. Prob wont see much outdoor use.

Thanks in advance for the insight lads.