in a nutshell: because i've posted on this subject half dozen times...
the 21''/17 raises front wheel circa 3/4-1'' depending on the tire = slight increase in trail= slightly slower response increased straight-line stability most folks enjoy specially off-road
21/18 raises both ends up about the same amount ,,thus leveling the bike to the original configuration...that's the simple version of it
the biggest impression you will have is that 'WHOA!!!1' this really is easier to ride off-road and come to think of it ,,,it ain't bad on the highway either''.....i have dozens of testimonials /ride reports attesting to the transformation of the bikes handling ,,wait til Jimmy Lewis does a report in Dirt Bike magazine here in the near future,, putting it mildly he was pretty impressed..
ok gotta go..any more specific questions ya canpost em here:
in vendor 'ask woody about your wheels''