Join Date: May 2004
Location: Toronto, Canada
I did a lower/stretch on my gen 1 to make it a better drag bike, but I still wanted to keep it streetable with acceptable steering and suspension characteristics. No nitrous, and no plans to.
Started with some math: wanted to be near the top of 5th gear at the end of a quarter mile, and with some estimates of what the trap speed ought to be, that led to the gearing selection. I prefer to do all the ratio changing with the rear sprocket. I ended up at 17/45 (stock is 17/39) and "actual field experience" has been that this seems about right. Keep in mind that not only is this no nitrous, but also no power adders at all. If you have major power adders then use a different estimate of trap speed, and you can always go through at the top of 6th rather than 5th, crunch the numbers.
Now some estimating: 17/45 ought to accelerate about 15% harder than 17/39 (45 divided by 39).
To compensate, we need to make the bike 15% less "wheelie-prone" and a bit more for good measure. If the center of gravity height stays the same height then the distance (in the fore/aft direction) between the center of gravity and the tire contact patch needs to be 15% more (plus a bit more, just to make sure). The C of G of the combined rider and bike is a little rearward of halfway between the front and rear wheels (i.e. about 26ish inches ahead of the stock rear wheel contact patch). Add 15% to get it to where we want it to be and it sounds like a 4 inch stretch. My target was 5 1/2 inches and this is without any lowering.
If you have major power adders that result in the engine making more torque than stock then it needs to be factored in just like the above.
If you want decent steering characteristics for normal road riding then lowering needs to be minimized and you need to keep suspension travel (including the sag measurement) in a normal range. In particular, with the front suspension at full compression, the front fender must not collide with the upper fairing or radiator. If you want to maintain full suspension travel then this sets a boundary on the amount you can lower the bike, and it isn't very much! 10 millimeters or thereabouts. That's what I did.
Now it gets tricky; you need to set nominal rear ride height such that it keeps the rake and trail in the same range as stock. The down-angle of the swingarm will raise the rear if you just extend the swingarm, so you need lowering links - in combination with ride-height shims.
I used swingarm extensions, fixed-length lowering links (I seem to recall that they give about 2" lowering if installed without doing anything else), 190/55 rear tire, and some washers under the upper shock mount to fine tune the rear ride height. Initially I used three 1/8" thick washers but I'm now at two 1/8" and one 1/16" washers - I fine tuned this based on how the steering felt.
It took some fiddling with ride-height washers and spring preload to get it worked out but I'm happy with the end result. Steering feel during normal riding is almost normal; it's a wee bit slower/heavier than standard but it's not a bother. Cornering clearance is still good (because it's only lowered by about 10mm ...), suspension compliance seems same as stock (because full compression and rebound travel is available), it doesn't bottom on bumps any more so than it did when stock, and there is still at least *some* swingarm down-angle to give at least *some* anti-squat geometry. And the front fender won't collide with anything, and the suspension will hit mechanical limit before the oil drain plug smacks the ground ... it probably has enough ground clearance to drive off a normal kerb, although I'm not about to try it (again, it's only lowered 10 mm).
It is never going to have as much grip exiting corners as a stock-wheelbase bike. But for a street ridden bike ... it's fine and is actually a silly fun street bike, because the shorter gearing lets the engine get into the power band at sane road speeds ... and because it doesn't steer like a school bus.
A bike with more extreme power adders will need more stretch and/or more lowering ... and more rear tire grip to avoid spinning the tire on takeoff.
Helibars, MRA screen, Ohlins damper, reversed shift pattern, sorted suspension, braided lines, Michelin Pilot Power, all else stock 'coz it's fast enough!
Last edited by GoFaster; 06-28-2011 at 05:18 PM.