I have a home built bike design and the rear shock absorber that came with the part bottoms out so I want to get one that is more appropriate. But I have no idea how to calculate which one to buy as far as the tension of the spring is concerned.

At Amazon for example there are many from 550lbs to 1500lbs shocks. I don't want an immovable spring nor do I want another weakling so how do I calculate what shock to get?

I found this calculator https://www.tftuned.com/spring-calculator
I put in 210lbs - shock stroke 1.5" - wheel travel 4.5" - 33%sag

I don't have a clue how to read these results.

  • 1
    Are you saying you made your own full suspension bike? Also, are you confusing the shock absorber (or damper) with the spring? Commented Mar 27, 2018 at 2:05
  • Have you got a rating number for your current shock? Is it adjustable at all ?
    – Criggie
    Commented Mar 27, 2018 at 2:33
  • 1
    @Criggie I never noticed before that there is a rating on my shock for 550lbs written on the spring. OK,that's a good starting point now I need to add more, but how much more? Commented Mar 27, 2018 at 3:09
  • 1
    @Argenti Apparatus I took parts from several bikes and the suspension of the rear tail is from a 20" wheel mountain bike. The spring is without a shock absorber yet is sold as shock absorber online so I use the same nomenclature. Commented Mar 27, 2018 at 3:15
  • 1
    @Criggie I already got this, I am a licensed mechanic. I just need a number for my spring tension. Commented Mar 27, 2018 at 7:08

2 Answers 2


I found a couple of websites with calculators on that may be useful for you:

https://j-techsuspension.co.uk/pages/spring-calculator http://www.bearandwife.com/bear/cycling/springcalc.html

The first unfortunately doesn't have an option for setting sag. The 2nd has more details, but spits out the same number if you set the sag to 25%

Based on these, I think we can say you will need a spring of at least 750lb/in, and no more than 1000lb/in.

Edit: These results tie up well with the information in Criggie's answer. An 850lb/in spring seems to be a safe choice.

  • The distance difference from the tail's pivot to the contact patch of the wheel and the attachment to the tail to the spring is a multiplier for the weight capacity. In my case 3 times further away means the spring needs a weight capacity of the bike and passenger X3 to bottom out the spring. And then add more to keep it from getting crushed. Commented Mar 29, 2018 at 22:21

From that spring calculator site "The spring rate is the amount of force required to compress the spring one inch, and is measured in pounds."

Your current spring is rated at 550 pounds according to the label

Optional - to test it, put the spring somewhere you can measure the compression accurately, and load it with 55 pounds of weight. It should compress 1/10 of an inch and then rebound to the original length. Likewise it should extend 1/10 of an inch if you hang 55 pounds off it.

As long as you're not exceeding its elastic limits then the spring will rebound to its empty size (so says Hooke's Law) Otherwise it will permanently deform, but that will take a lot of weight.

Based on the numbers in your question the answers from the calculator are:

Single Pivot / DW-Link

CCDB/5th(CVT)/Man(SPV)    764 lbs  
Fox/RS/Maz (shims)       859 lbs

This is a single pivot suspension: https://cdnmos-bikeradar.global.ssl.fastly.net/images/news/2010/11/16/1289905871746-xpg6sxc9nxig-630-354.jpg

4 Bar / VPP

CCDB/5th(CVT)/Man(SPV)    840 lbs
Fox/RS/Maz (shims)      935 lbs

This is a pivoiting suspension:


The other titles are brands

  • CCDB is probably Cane Creek
  • 5th CVT is probably Progressive Suspension Fifth Element
  • SPV is Manitou air shock
  • Fox is Fox Suspension
  • RS is RS Suspension
  • Maz escapes me sorry
  • Shims is one way of adjusting the spring, by adding more washer-like obstacles to compress the spring sooner.

Given your bike is slightly non-standard, at the end of the day you're going to have to take a guess. If your bike has a see-saw pivot in the suspension, go higher or if its a straight squash then go lighter spring rate.

You could also consider the normal load you exert on the saddle when it bottoms out. Then imagine half that weight, and extrapolate from there.

  • Not eligible for bounty - I started writing this before you added that. Also, this answer doesn't give any calulation, its just trying to interpret the calculator's jargon which is not really an answer, but it was too long for a comment.
    – Criggie
    Commented Mar 29, 2018 at 0:57

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