Motorcycle Modifications, Repairs, Restores, and Upgrades:

inferno

A primary issue with springs, front and rear, is to simply spring the ends so that when you push down on the footpegs (where most of your weight is when you aggressively corner).

Both ends of the bike NEED to go up and down exactly evenly, both in travel and rate. That's without damping!

General order:
Select a rear spring that will just bottom out over your harshest bump that you are riding on with "normal" sag. Soften it up till it just bottoms out, then tighten preload 1/4. Check the sag and see if it's in general range of 35mm to 20mm.
That's with no compression damping.

Select front fork springs that allow the front end and the rear end to travel equally and evenly when you bounce on the footpegs (helps to have someone hold the bike upright). That's after you have set the rear spring as in the above step.
That's with no compression damping.

Rear rebound: Use just barely enough to prevent wallowing in high speed sweepers.

Front rebound: Use just enough rebound to keep the front end from wallowing and drifting wide when exiting low speed corners at full throttle.

Compression damping: The goal is to use as little as possible. Use the proper, perhaps heavier than stock spring to do the major work during non-braking mode riding.

Use compression damping to trim if necessary and only if required.

Excessive dive under braking: Use increased oil level to firm the front end under hard braking. 5mm is a significant change in modern forks.

Goals are to always use the softest rear spring that works within "sag" range limit.

Use just enough rebound damping to control the springs.

Use as little compression damping as possible and only when required. Not everybody has a supply of alternate fork and shock springs, so you will probably have to improvise and use a bit of extra preload or a bit more compression damping to compensate for too soft of a spring.

All in all, there is nothing like heeling a bike over in a 120mph sweeper with a sharp edged bump in the middle of it and knowing that you can back of just a bit over the bump, get bounced in the air and calmly land heeled over, in control, straight and unperturbed and remain at highest possible speed with no fuss or muss.

That just doesn't happen if the springs aren't perfectly matched!

adrian

I started putting some posts up that describe different terminology for suspension tuning, but it will be faster if I just post a link to the page in my blog with all the information:

Understanding Motorcycle Suspension and Terminology | adrian's blog

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"I just know that someday someone is going to start a thread here"

the spindoctor

i would suggest anyone who is planning on heading to the track to get their bike professionally tuned beforehand and save the time and trouble of figuring it out yourself. you will spend MUCH more time yourself than letting a pro to get it close on the first shot. i knocked 6 full seconds off my fastest lap after letting someone bounce my bike and play around with it.

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Justin