Why 18"?

[quaggy]

Bit of a noob question, but why do all the mx guys run 18" rear tires? Is it only a centre of gravity thing? Surely running a 20" tire would produce more grip and be better in the really rough stuff...

[PaRedneckRiders]

with a smaller tire you will have more acceleration.

I run both 20s and 18s on my 400ex depending on what im doing, but if i run the 20s on the track it still seems a little sluggish out of the turns, with the 18s i dont feel that way.

But with the 18s the top speed will be lower but most of the time your not going to top out on a track, mayb for an instant in a straight stretch.