repairing rusty fork tubes

[ajensen]

mr72 has some interesting insights into fork spring length and spacers.

[RichDesmond]

...If your forks are an early 1989-1991 then I recommend installing the longer springs from 2000 and cut down the spacers accordingly.

Just out of curiousity, why?

[mr72]

mr72 has some interesting insights into fork spring length and spacers.

Do I?

I cut the springs in my fork to stiffen them by about 25-30%. This required cutting about 1/4 of the length of the spring and then replacing the dead space with additional spacer made from PVC tubing plus a stainless 1" fender washer between the new spacer and old.

You have to recalculate the fork oil quantity if you do this because the PVC pipe takes a lot more volume than the spring coils you cut off would have.

You need a dremel to cut the coil and a bench grinder to flatten the cut end.

In my case I needed to change the fork oil anyway so I was doing most of the work to get it apart. It was trivial to get the springs out, cut them, and put them back with the spacers added. Before I did this the fork would bottom frequently over speed bumps and of course at the bottom of my steep driveway every time I rode down it.

BTW I replaced the rear shock with a Katana 600 shock which has a higher spring rate, then did the math to stiffen the fork by the same ratio as the rear spring change.

In reality this may get the stock springs into the 0.72-0.75 kg-mm range, and any more cutting (for stiffer springs) would likely risk both breaking the coil and also reduce suspension travel because the spacer required would be too long. Ideally I'd prefer springs in the 0.85-0.90 kg-mm range that are full length but I didn't really find any options out there that were not prohibitively expensive. Every time I consider spending $100 on this bike for a non-essential "upgrade" I have to seriously consider whether that money would be better spent investing in a replacement motorcycle that didn't require the upgrade.

Anyway if you are doing the katana 600 shock swap then I think cutting the springs or replacing with 0.75 kg-mm fork springs is recommended to keep the suspension balanced front to rear.

One thing to note, the "pre-load" in rear and length of spacer in front don't change the spring rate or stiffness, they merely change ride height, unless you put so much pre-load that the suspension is topped out with rider weight. That will make the ride more harsh but still doesn't not change stiffness or spring rate. And ride height is rather critical for shock performance. You want some "sag". The "preload" adjuster and adding spacers in the fork is all about getting sag correct. As long as there is some sag, preload == ride height. If you want to change spring rate, bottoming resistance, etc. you have to change (or cut) the actual spring.