Thx Eric.
I built a TT02D as a rally car for bashing. I got it as parts from ewippler who I think buys kits for their bodies and then sells all the chassis parts nice and cheap.
It has a Trackstar 21.5 in it. Qatmix (thercracer.com) built his with a 17.5 and it was fine. So I think 13.5 would be OK. 10.5 might be too far.
The cool thing about the TT02 is that you can build it with different options for wheelbase, track, and ride height. I built mine with a standard TC wheelbase, 185 mm track, and high ride height.
thercracer.com shows what parts to use to allow standard 64p or 48p spurs and get a nice low FDR for lower revving motors like 21.5. You might have to modify the gear cover a bit, but you can get down to 3.0 FDR if you need it.
I used the Tamiya spur gear adapter and metal suspension balls, but everything else I got was Yeah Racing blue alloy stuff... drive shaft, steering linkage, knuckles, rear hub carriers (2 deg toe in).
The single biggest thing that helped the car was the front diff oil. I think I actually have 300k in mine. The diffs are not designed for oil, but 300k is so thick that it won't leak. The car wanted to spin out constantly on-throttle until I added the oil.
It's still not anywhere near as precise as a race TC chassis, but it goes OK. Mostly it's very durable and has a sealed gearbox which makes it great for blasting through dirt, leaves, jumping curbs, etc.