Correct spring rate for SPV shock- Mtbr.com

# Thread: Correct spring rate for SPV shock

1. ## Correct spring rate for SPV shock

Sitting here and guessing how to pick correct spring for FR (some DH). Any formulas exist?

Thanks!

2. ## I dont remember it.

Originally Posted by Lucky13
Sitting here and guessing how to pick correct spring for FR (some DH). Any formulas exist?

Thanks!
Someone will chime in with the formula. They are going to need the shock stroke, the amount of travel, and your body weight.

3. Originally Posted by damion
Someone will chime in with the formula. .........................
it's easy....
too stiff = lighter weight spring
too soft = heavier weight spring

4. Originally Posted by Lucky13
Sitting here and guessing how to pick correct spring for FR (some DH). Any formulas exist?

Thanks!
Used to have a spreadsheet set up, but here's what it comes down to, with examples.

take the stroke of the shock
2.0 "

take the stated travel of the bike
6.0"

that gives you the actuation ratio:
6/2 = 3:1

take the desired sag
30%

and multiply it by the stroke of the shock:
30% * 2 = 0.6 inches.

take you weight
150 lbs.

each pound of your weight will be multiplied by the actuation ratio. and springs are rated in pounds/inch. so a 500# spring will compress 1" for every 500 lbs.

so (150lbs * 3 ) / 0.6" = desired spring rate / 1.0"
simplifying we get
450lbs / .6 = spring weight
which yields 750# as the ideal spring rate for a 30% sag.

At least that's how I figure it.

5. Originally Posted by butocabra
Used to have a spreadsheet set up, but here's what it comes down to, with examples.

take the stroke of the shock
2.0 "

take the stated travel of the bike
6.0"

that gives you the actuation ratio:
6/2 = 3:1

take the desired sag
30%

and multiply it by the stroke of the shock:
30% * 2 = 0.6 inches.

take you weight
150 lbs.

each pound of your weight will be multiplied by the actuation ratio. and springs are rated in pounds/inch. so a 500# spring will compress 1" for every 500 lbs.

so (150lbs * 3 ) / 0.6" = desired spring rate / 1.0"
simplifying we get
450lbs / .6 = spring weight
which yields 750# as the ideal spring rate for a 30% sag.

At least that's how I figure it.
Can you do mine for me I suck at math...
230lb guy, 8.5 x 2.5, only want 25% sag.

6. ## You forgot something VERY important...

...full body weight is not 100% distributed to the rear wheel. Remember, you have 2 wheels.

Generally, I split the person's weight 1/3 to the front and 2/3 to the back.

Also, these formula's don't take into account the characteristics of the shock. They can make a difference. Or any rising/falling rate of the actual linkage.

Although I do use the formula's, I generally use them as a guideline for picking springs when I already know what spring is on the bike already, what kind of bike it is, what kind of shock they currently have, and what kind of problem the rider is having.

Picking new rear springs is part math, part trial and error, and part experience.

7. ## And believe it or not folks...

Originally Posted by .WestCoastHucker.
it's easy....
too stiff = lighter weight spring
too soft = heavier weight spring
...in many cases, this is what it really comes down to...

8. Originally Posted by LT1
...in many cases, this is what it really comes down to...
This is my ride, shock is a pro pedal.

9. ## What is the spring rate you have now ?

It will be on the side of the spring, it will say something like "550x2.25".

10. Originally Posted by LT1
It will be on the side of the spring, it will say something like "550x2.25".
550 x 2.80

11. yep...550 x 2.80

12. ## whoops

Originally Posted by LT1
...full body weight is not 100% distributed to the rear wheel. Remember, you have 2 wheels.

Generally, I split the person's weight 1/3 to the front and 2/3 to the back.

Also, these formula's don't take into account the characteristics of the shock. They can make a difference. Or any rising/falling rate of the actual linkage.

Although I do use the formula's, I generally use them as a guideline for picking springs when I already know what spring is on the bike already, what kind of bike it is, what kind of shock they currently have, and what kind of problem the rider is having.

Picking new rear springs is part math, part trial and error, and part experience.

yep, right you are. I forgot about the weight distribution part. so 2/3rds of rider weight on the back. seems right. maybe it should be riderweight * sin(seat angle) (kidding)

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