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  1. #1
    mtbr member
    Reputation: tubadude's Avatar
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    Formula to roughly estimate DHX spring weight needed.

    Playing around with my DHX 5.0 and think I may have a general formula to determine recommended coil spring weight.

    Rider = riding weight with camelback + everything else
    Distribution = rider weight distribution ratio on bike (I go with 2/3's)
    Saddle = estimated weight on seat
    Across = riding weight across lever
    Shock = shock travel
    Bike = bike travel
    Leverage = leverage ratio for rear travel
    NoPreTrav = no preload travel
    Coil = coil spring weight
    Sag = shock travel sag %

    Leverage = Bike/Shock
    Leverage = 6"/2.25"
    Leverage = 2.6666666666

    Rider = 208lbs + 16lbs Camelback
    Rider = 224lbs

    Saddle = Rider * Distribution
    Saddle = 224lbs*.6666666666666666
    Saddle = 149.333333333333333lbs

    Across = Saddle * Leverage
    Across = 149.3333333333333lbs * 2.66666666666666666
    Across = 398.22222222222222222lbs

    NoPreTrav = Across/Coil
    NoPreTrav = 398.222222222222222lbs / 650lbs/inch
    NoPreTrav = 0.612649"

    Sag = NoPrevTrav / Shock
    Sag = .612649" / 2.25"
    Sag = 0.2722884 = 27.22884%

    The Preload dial on the top of the DHX 5.0 coil spring after being turned 2 full rotations, which is the maximum recommended by Fox to prevent coil bind, compresses the spring about .08". Take .08 / 2.25 and you end up with .0355555555555 which = 3.5%. So the preload adjuster is good for removing up to 3.5% of the sag safely.

    So for the example above I weigh 208lbs and wear a 16lbs Camelback. I estimated that 2/3's of my weight rests on the saddle. I also used a 2.25" travel shock on a 6" travel bike. This when factored in with the rest of the formula means that a 650lbs coil would give me 27% sag with zero preload. With full preload I go down to about 24% sag. A 600lbs spring gives me 29.5% sag without preload. I can get this one down to 26% sag with full preload.

    So you get the picture. This scheme should work with just about any bike as long as you know your riding weight, weight distribution on the bike, shock travel, suspension travel, and spring weight.
    Last edited by tubadude; 04-19-2005 at 03:51 PM.

  2. #2
    Brass Nipples!
    Reputation: Bob the Wheelbuilder's Avatar
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    Quote Originally Posted by tubadude
    This scheme should work with just about any bike as long as you know your riding weight, weight distribution on the bike, shock travel, suspension travel, and spring weight.
    Looks good to me. It also appears if you couldn't get the right sag with a given spring, you could work backwards to find out your actual weight distribution and then calculate the correct spring you needed with that value.
    {Principal Skinner} Hmm. Whoever did this is in very deep trouble.
    {Martin} And a sloppy speller too. The preferred spelling of 'wiener' is w - i - e - n - e - r, although 'e - i' is an acceptable ethnic variant.

  3. #3
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    I ahva a program that takes all of your inputs into account and gives you spring weights for different sags....Send me a pm if you want it.

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