Fork springs and spacers

Ingah · World Chat Champion Joined: 10 Apr 2009 Karma :

I am in the fortunate situation of having two of the same make and model of bike (2 CB500's, one to ride and one as a donor bike).

I'm using the forks from the older model on my newer daily ride (unfortunately the newer forks are pitted), and when changing the fork oil (was transferring the headlights across so it made sense as a good time to check), measured the fork springs. Haynes says they are ~308mm new, 305mm is the service limit. Mine were ~298mm! So i need to replace them, clearly.

Normally i'd just nab them from the other set of forks, but the newer model's fork springs are 100mm longer as stock, so they don't just slot in. However the spacer is much shorter.

So i put them side by side (spacer and shock vs spacer and shock) to find that the total lengths are about 50-100mm different.

So, to save me forking out the £80 a new set of Hagon springs cost, is it a bad or a good idea for me to use the longer springs and get a spacer made by a local engineering firm so that it is the right length?
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-- Ingah

stinkwheel · Bovine Proctologist Joined: 12 Jul 2004 Karma :

I would be looking at how far the new spring + spacer are normally compressed when fitted in the (fully assembled) fork they came from and trying to match this distance. That'll give the spring the same amount of preload when you swap it into the other fork.

If you want to be technical, check the static sag. Fit the springs and spacers then see how much the fork compresses from fully extended when you set the bike on its wheels. You're looking for around 30mm (extreme ballpark, varies with bike, stickiness of seals and rider preference).

Blue alkathene water pipe makes excellent spring spacers. It's light, strong, cheap and can be cut to length with a junior hacksaw.

As such, trial and error is also a valid tuning tool because the spacers are effectively disposable and infinately adjustable. Start with them long and cut them back a bit at a time until it feels right.

EDIT: The easiest way I can think of to measure how much preload is applied is to undo the fork cap and measure the gap between the top of the stanchion and the underside of the fork cap that normally butts against it with the fork totally unloaded and the cap on the end of the spring. You want to compress the spring by the same amount in the other fork.

Again this is ballpark because how much preload is required will vary with the overall weight and weight balance of the two different models but it should be pretty close.
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Fizzer Thou · World Chat Champion Joined: 06 Aug 2011 Karma :

CB500.

Do you mean a 1975 model CB500/4?
https://i295.photobucket.com/albums/mm133/npscl/bikes060.jpg

Or a 1978 CB500T?
https://i178.photobucket.com/albums/w275/toecutterat22/1974-cb500t.jpg

Or the more recent CB500s twin?
https://i285.photobucket.com/albums/ll59/miguel29646/CB500.jpg
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Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

MarJay · But it's British! Joined: 15 Sep 2003 Karma :

Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

MarJay · But it's British! Joined: 15 Sep 2003 Karma :

Don't forget you need a third of the suspension travel to be used when sitting on the bike. This is called rider sag.

If the bike is topped out then it won't deal with depressions in the road and you'll end up bouncing around the place.
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Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.