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Discussion Starter · #1 ·
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.
 

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But...

tubadude said:
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.
Don't things change with different bikes with different suspension rates and even other factors? Maybe even more important than the ratio of travel to stroke is the type and rate of the compression. Some designs have a falling rate, some a linear rate, and some a rising rate. I have two bikes with 6" of rear travel. The stroke on both shocks is the same to produce that travel. One is an FSR design and requires a 600lb. spring. The other is a straight single pivot with no linkage and requires a 400lb. spring. These two bikes have a very different suspension rate even though they have the same ratio of travel to stroke. Rate has a bigger effect than ratio...I think.
 

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Discussion Starter · #3 ·
TNC said:
Don't things change with different bikes with different suspension rates and even other factors? Maybe even more important than the ratio of travel to stroke is the type and rate of the compression. Some designs have a falling rate, some a linear rate, and some a rising rate. I have two bikes with 6" of rear travel. The stroke on both shocks is the same to produce that travel. One is an FSR design and requires a 600lb. spring. The other is a straight single pivot with no linkage and requires a 400lb. spring. These two bikes have a very different suspension rate even though they have the same ratio of travel to stroke. Rate has a bigger effect than ratio...I think.
Did not think about rising rate. I was thinking in linear terms. I believe my Ellsworth Moment has linear compression through the travel so I did the math based on that assumption.

My numbers worked out on my bike using calipers to double check my guesswork. I also got the same recommendations from Fox Racing Shox when I called them to find out which coil weight is recommended for a few body weights.
 
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