I'm 13 stone.
I have a 450 pound spring.
Leverage ratio (if it makes any difference, prolly not) is 2:1 on a 4 inch travel Kona.
Sag is only 11mm.

The sag isn't really enough on my 50mm stroke shock and I want to get to around 30%, so 14mm or so. How can I work out if a 400 or even 350 pound spring will get me there or overshoot too far? I'd rather be firm than too saggy so don't want to swap the spring out if lighter springs will be too soft.

Forum maths boffins…begin!

You want a spring that is 27.5% softer (11mm x 127.5% = 14mm) so the 350lb one meets that criterion. (350 x 127.5% = 446)

Assuming linear rates etc.

the term 'spod' is offensive to us, we prefer 'geek'.

your 450lb/inch spring yields 11mm.

so thats about 195lbs on the spring => (11/25.4)*450lb

trying a 350lb spring…

195/350 = 0.56

0.56*25.4 = 14mm.

I can confirm the above spods to be correct-uh-mund-oh.

http://www.darvill.clara.net/enforcemot/springs.htm

Thank-you geeks! I bow to your superior craniums.

ahwiles – what's the 25.4 figure in your workings?

Al – should have just asked you in the first place!

25.4 is inches to mm

Both the mojo and tf tuned websites have spring calculators for just this job.

Ah right, cos the 450 pound bit is per inch of squish.

Err, no, the mojo and TF sites don't quite do what I've asked. In fact Mojo comes up with 450 and TF 400, but real worldy figures are seemingly showing 350.