Fork Spring Shortening with PD Valves

[PEZBerq]

What is the proper way to shorten fork springs?  I am fitting PD valves and the fork springs need shortening by about 15mm due to the height of the PD valve and adaptor spacer.  I tried it without shortening the springs but its too stiff.  How do you get the end of the spring flat like original when you cut them? Its for the 40mm Husky forks.

[Freakshow]

Just cut em..........

[Freakshow]

Cut ...cut ...cut.... ohh and maybe grind off a nice champher too. 

but cut ........cut.......cut..

[brent j]

If you shorten the springs you will increase the spring rate. This might not be a bad thing as most bikes are undersprung anyway.

Did you have any spring preload before fitting the PD's? A higher rate is probably better than preload anyway

I do cut springs but only to try different spring rates. I'm playing with my TM125 at present and I've made some helical spring seats so I can cut the springs bit by bit. I have three sets of TS125 springs so have heaps to experiment with.

I've flattened the ends of a couple of pairs of springs but I'd rather not go into that
My methods are not "proper" but they are ok to determine what spring works best.

Brent

[PEZBerq]

The springs are 720mm long, 58 coils wound at 33 mm OD using 4.5mm dia wire and dont run any spacers with the fork caps preloading them by about 15mm.  I tested them and they are 0.32kg/mm rate so are reasonably soft by modern standards.  An increased spring rate is probably not a bad thing.  Cutting 15 mm off 720mm is not going to stiffen them up very much I wouldnt think about 1.5 coils in 58 removed.

Once you have cut your springs to get the right length/rate what do you do Brent - buy new springs to suit? Can you post a picture of your helical spring seat?

[brent j]

The pic isn't very clear as my camera is not particularly flash.



They are made from heavy wall steel pipe. I measured the spring pitch and set a lathe to cut the same. I took a very light cut to mark them then hacksawed them out. I really don't think they need to be that accurate as the springs will deform under compression. These allow me to simply cut the the springs coil by coil until I'm happy. When I find what I want I'll buy new springs.

Your spring rate calculates out to about 16.8lbs/in or 0.29kg/mm, cutting 15mm (about 1.5 coils) will increase this to around 17.3lbs/in or 0.31kg/mm.

[PEZBerq]

Quite easy as it turned out Wasp.  Cut spring down to 46 coil active from the original 56. This increased the rate to close to 0.4kg/mm which is much better than original - though even stiffer would be nice I agree. 
Still have sufficient spring length to accommodate 300mm travel with 15mm preload. This is critical when shortening your fork springs to increase spring rate - dont want the damper rods taking the load on a bottom out . 
Made some spacer tubes out of a high density engineering plastic using Brents helical cut out design (thanks Brent for the idea;D ) to preload the spring 15mm.  Road test across the gutters on my street shows promise.  Need to get track time at next club day - cant wait. 
Heaps of mucking round I admit all to fit a set of PD valves.  Hope it is all worth it