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Old 11-12-2007, 11:22 PM   #5
Luke's Avatar
Joined: Aug 2004
Location: Idiotville, OR
Oddometer: 4,241
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?
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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