I have an XR250R and Im considering dropping the 21" front rim to a 17" the rear rim is OE 17" my question is: if I drop the front from 21" to 17" even though my rear rim is 17" would I need to lower the rear to equal out the changes? seems to me just dropping the front would make the rear light by shifting most of the weight to the front. mabey thats how they are sliding arround so much with slicks! In other words can I upset the front t/rear balance by lowering one end and not the other? if the front goes down the frame will be tilted down, I know thats gotta adversly effect the handling but not how, can someone please explain to me how? not technically but from seat time experiences on the edge I would think that if I dropped rim size to a 17" from 21" w/out any other changes I would then need to raise the front some to get the frame back level but the forks are not long enough. since this is the case now I want to lower the rear to equal things out. is there a good book that explains this stuff (and chassis design) even better one written by a rider/racer, those engineers try to loose you.. makes them feel smarter I guess.. HAhaha. thanks ///// Oh yeah, Ive allready searched for hours on the net and cant find the answer. HELP SUPERMOTO!
I'm no expert on this stuff, but FWIW: It seems like most dirt bikes are way high in the front compared to "normal" roadrace geometry. Lowering the front is definitely the direction you want to move in. It's possible that the ~2" lowering the 17 will give you is too much, but I doubt it. On my DR-Z125 motard I've got the rear up about 1.25" and the front down abour 1" via spring and fork adjustments, and it's still too high in the front.
Racer-X had an article on McGrath's CRF450. He had the front suspension shortened (they didn't say how).
Everything I've been reading suggests lowering the rear using a linkage such as Kouba Links and running the forks up through the triple clamps. The setup I've read on the DRZ400 is the Kouba Link #3 (lowers the rear 1.75") and moving the forks up 1.5" above the top triple. I'm warry of anyone that claims to know setup and uses inches as a measurement but I'm willing to give it a shot. Check out ThumperTalk.com for the Supermoto board. Those guys have alot of good technical info on suspension and economical improvements.
