So if im 160 geared up what spring rate should I go with the calculator on Racetechs site says to stay with the stock spring but I dont think that is for drag racing. Id like to go with a solid strut but then I cant ride it on the street. To much work to swap. And also how do you get a wheelbase measurement on a bike? Then I could have better data to work with.
Wheelbase is measured from the center of the front axle to the center of the rear. Stock is about 88.5
A solid strut is going to cause way more problems than it will solve.
The problem you run into when you stretch the bike is we usually lengthen the rear swingarm. With a stock mono-shock system …look at the pivot points where it hooks up. Now add 6 inches to the swingarm. We didn’t move the shock. The leverage the rear tire has on the bike under acceleration now changes.
Think of a See-Saw when you were a kid… what happens when you move the pivot point away from your side. It gets easier for you to control. You can even support a heavier kid with your lighter weight. You effectively soften the spring rate and reduce the ability of the shock to control the movement.
Also when you lengthen the bike look where the rider weight center is, you are moving the weight forward on the bike. If you scale it you can see how many pounds of bike and rider moved to the front of the bike from the rear. We usually lengthen the bike to prevent wheelies, at the same time it also affects weight transfer to the rear we were gaining as the front lifts.
The data you are getting from race techs site won’t help you much because you are no longer working with a stock swing arm.
Further if you added longer links to the rear to lower it, you have changed the leverage ratio of the whole system. Then if you look a a swingarm on a stock bike at a stock height and compare it to a longer arm on a lowered bike. Look at the angle of the swing arm. When these angles change it can impact what happens when accelerating. The chain is pulling the top of the sprocket… what kind of effect does this have? The angle of the rear swingarm will have some influence on this.
The internal construction of the stock shock severely limits what can be done for drag racing. But… buying a Penske or Ohlin’s is out of the budget from the most of us. Even if you buy one you need to make sure you get some good support with it so that it works with your application. I’ve got 3-4 inch shorter bike, 30 more hp, and 70 more lbs. What works for me is not going to work for you. Getting someone to fix your stock shock could help. Then again… a guy needs to determine if he needs that much stretch.
