No apologies for the geekiness of the following question:)

Had a demo ride of a 140mm FS bike yesterday – which involved a fair bit of suspension fiddling. I evetually found the sweet spot of the suspension by trial and error, but this got me wondering if there’s a mathematical way of determining the how different amounts of sag will be reflected in bottom bracket height. Being mathematically challenged (to put it mildly!), I’ve hit a brick wall with my calcs. Can any maths experts help please?

The ‘knowns’ I have are as follows:

Rear wheel travel 140mm
Shock travel 48.25mm
Recommended sag range 10mm to 12mm
BB height at 12mm sag = 335mm

Front travel 140mm
My preferred amount of sag on the fork is 35mm

TIA

12mm sag sounds like it should be at the shock, not at the bb? i always work on 25% of travel as sag on a xc bike, 33% of travel as sag on a dh bike – but nothing is cast in stone. If its a coil shock, just wind the coil out till its lose, then 2 full turns back in. If thats too saggy, next coil up.

Yup, 12mm sag is measured on the shaft of the (air) shock. The bike is a plush feeling 4-bar ‘trail bike’, with a relatively low static BB, so this is I guess why the manufacturer has suggested 20-25% sag.
(I was getting quite a few pedal strikes at 25%)

There’s so many variables you can choose to count or ignore that I think it renders any “formula” fairly pointless. Most important would be the axle path, but also the BB path as the suspension compresses.

If you massively over simplify it and treat suspension compression as directly proportional to BB height (ie as your wheels goes up, your bb drops the same amount) then your 12mm of sag on the shaft rough equals 35mm drop in BB height:

(140/48.25) x 12 = 34.81865…..

The same formula with give you a rough estimate of additional BB drop for given extra sag at the shock, essentially 1mm at the shock giving a bit less than 3mm lower BB height.

But there’s so many things not being taken into account that that can only be a very rough guide.

Thanks Jonathan, a rough idea is better than nothing, but I can appreciate what you’re saying regarding the axle and BB paths – and I’d forgotten about these until you mentioned it. I recall a bike mag a couple of years ago that had diagrams showing the general pattern of axle paths for various rear suspension designs. It’s been quite interesting to demoing FS bikes with different suspension designs (if only I could cherry pick the suspension system of one and combine it with the geometry of another, I’d have the perfect bike for my needs – er…but then it’d probably not suit anyone else!)

FWIW, the potential for 3mm increase in BB height for 1mm decrease in sag might be in the right ball park. On the demo bike, I decreased the sag from 12mm to 10mm and I was getting noticeably less pedal strikes, but the bike still felt relatively low and very plush compared to other 140 travel bikes I’ve tried.