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Hopefully a pretty simple question someone could help me with! I'm riding a early'06 Trek Fuel EX8 with this shock on the rear. Spec's. say 100mm of travel front and back. However, when I measure the exposed portion of the rear shaft I get 41mm? I'm sure I'm missing something in the travel smarts? Also, OEM says to set this shock up for about 20% sag. Is this 20% of 100mm or 20% of my measured 41mm? That 41mm measurement is take at rest w/ no weight! Any thoughts..on setting up this guy (XC riding) would be good too...I go about 140# w/all my goods! Thanks! ~Tim
 

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Do It Yourself
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The 100mm of travel is at the rear wheel. The leverage ratio of the bike based off pivot placement, chainstay length, etc detirmines the relationship between the shock stroke and the rear wheel travel. In other words, it's perfectly normal.

And on the sag, you measure that off the shock stroke. The shock doesn't use the full shaft. I would guess your shock has a 1.5" stroke since that's a common size close to what you measured. So with a 1.5" stroke unweighted, it should sink about .3" (a hair under 5/16ths of an inch). That should be close. If you're used to a hardtail, you may want to go a bit firmer at first to ween yourself on to full suspension or you can jump right in and let the suspension work like it should.

Front shock sag setup works the same way only it's 1:1, so about 20mm sag.
 

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I live to bike
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what homebrew said. 20-25% of 41mm. Don't worry, you are getting 100mm travel of the rear wheel, even though the shock itself does not travel that much.
 
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