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Old 11-12-2007, 11:22 PM   #5
Luke
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Joined: Aug 2004
Location: Idiotville, OR
Oddometer: 4,036
Quote:
Originally Posted by BirtDike
Anyway, I rang a suspension expert here today, and asked what he'd recommend in the way of back springs. I figured he'd probably advise something like an upgrade from the 70/260 to an 80/260 - instead, after he'd rung the WP Suspension importer, he suggested that I go for a 70-90 progressive spring.

Firstly, anyone know how the 640 would go with a progressive spring? Does the linkage already make the spring rate progressive with a linear spring?

I figure that the loaded sag of the bike is going to be pretty high with a spring starting at 70 (sorry, I've got no idea what units the 70 is . 70kgs/cm???), and would probably need a fair amount of preload to set it up right - would this create spring bind like it can on a mountain bike? Any other negative effects?

Anyone with any knowlegeable advice as to what I should put on will be really appreciated. I don't want to go fast, I just don't want my shock to blow up in in Timbuktu (literally). FYI - I probably weigh close to 100kgs fully kitted for riding (220 for those who think in old school).

One more thing - is is possible to change the spring from the shock body yourself, just by backing off the preload, or do you need the special spring compressor tool?
Cheers
Thoughts in no particular order:

The WP progressive springs the tuner is probably thinking of are slightly wider than the 640 springs, if a centering spacer isn't added, the spring will rub the shock body, which is bad.

Spring rates are quoted in either kg/mm or N/mm. kg/mm is the most common, but N/mm is the correct unit from a proper physics standpoint.
1 kiloram is 10 Newtons (sitting still on earth) so an 80N/mm spring is the same as an 8.0kg/mm The 70-90 spring is in N/mm. Oddly enough, 1N/mm is about the same as 1kg/cm.

Bill and I are still hashing over the numbers, but the first stab looks like the
linkage has way more progression than the recommended spring. As in, comperable to a 50 to 100% increase in rate. By comparison, the 70-90 spring has a 28% increase.

You need a tool to change the spring.

The progressive springs are designed for shocks with progressive damping- ie PDS.

On the practical side I'd recommend an 80 or 85N/mm straight rate spring in the back, depending on how much luggage you carry and what you change the fork springs to.
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