How doe we set up a bike (both dirt and track) to handle as well as this kawi 250gp weapon?
Doesnt matter if its Road Race, MX, Dirt Track ar any in between, There are a few basics that have to be not just in the ball park but on the money before we can even start considering going fast.
Just like riding skills, anyone can go fast on anything, just like blodgening a corner will get you around the bend, when you have the basics of handling or the basics of riding skills down pat, you can get around corners easier with less chance of crashing and with more repetition.
First up is springs, it’s actually a tossup between springs and geometry, but my thinking is you can have all the geometry in the world but if the springs are not what you need for the job your not going to hold the geometry when you need it. As a very (very) general rule on dirt race bikes 98-102mm rear sag 30-45 on the forks, 4 strokes 110mm ish rear sag, it’s a starting point- not set in stone. On the track, well we don’t measure sag but I appreciate not everyone has the MotoSPEC program and it is expensive and tape measure isn’t so aim at 25mm sag both ends, again it’s not set in stone.
If your springs can not achieve sag numbers with less than 10 mm of preload on dirt and 16mm on tar then the rate is wrong to start with, by the way 3 mm preload is min.
From there tar guys need at least 1 rate stiffer AND 1 rate softer front and rear as well as the means to change springs quickly, it’s so as the track changes grip levels you can change springs to suit and either bring grip back or take advantage of increased grip levels on the day.
So a very close second. Geometry. We have all had that tip toe feeling, the front doesn’t feel “there” the rear spins or the bike has a mind of its own. Welcome to geometry.
Now there area some ideal numbers or window that most riders feel nicest or darn close to perfect for them. On tar that is 95mm trail 92-98 is the window, below 92 is understeer ish above 98 like a truck (exaggerating there) on dirt 115-130 trail window. Now things that work the window is, spring rate (funny enough) fork hight, swing arm length, tyre size.
Next port of call is swingarm angle, now for all dirt bikes your not going to achieve good angle, the length of travel means don’t bother, but track guys and dirt track guys swingarm angle is grip and drive, magic mike is 12.7 deg 12.3-12.9 window. Most road bikes are in the 8-9 deg area stock, so there is a bit to do in that regard. Oh and changing sprockets changes swingarm angle as well just so you know.
And I’m going to throw bolt tension in geometry as well, quickest way to slow a rider down is over tighten the lower triple clamp bolts or axle pinch bolts. Buy a toque wrench.
So these are the basics, truth is damping isn’t on the list so it’s no use pulling the screwdriver out until the basics are done.
Because damping only changes the speed it happens, not what happens to start with. - craig