A quick one as I’ve heard contradictory opinions, so seeking answers from the STW Hive Mind…

Does adding volume reducers (in the +ve chamber) affect the psi needed to reach a certain sag %age. I.e smaller token, more psi needed, larger token, less needed – or vice versa, or even has no relationship…

Tried to engage my physics brain, but think it’s atrophied away…

Cheers,

Rich

If you run a lot of sag and add a lot of spacers then yes. Less sag and only a small change in spacers and there will still be a slight decrease in sag for a given psi but it may be too small to measure.

Consider the datum for the shock to be the equalisation point. Then you have a single pressure in the model of the system. Makes the physics much easier. With tokens you are altering the compression ratio of the positive chamber only.

Not going to spoon feed. What do you think happens?

Ta – much appreciated both of you.

Smaller aircan volume (with a bigger space) means higher compression ratio, which with the same pressure would lead to less sag… from what I can work out thinking about it like that.

So flipping that, bigger volume spacer in theory would mean less pressure for the same %age sag than you’d need with a smaller spacer. Sounds about right?

Cheers,

Rich

Equal sag when swapping around spacers is a bit of an arbitrary constraint. You’ll probably find there are better compromises than just equalising sag. Exactly what to do depends on where you feel you’re hitting the limits on your current setup.

I developed a physics model: https://www.desmos.com/calculator/5ovufiqale

It isn’t necessarily specific to your scenario (and to some extent uses arbitrary units) but in a longer travel application the model suggests a ~1.5% pressure drop per token will equalise sag but exploring up to 4-5% of pressure drop may provide a good compromise of characteristics. 8% pressure drop is indicated as probably introducing negative characteristics throughout the stroke.

Take all such numbers with a generous pinch of salt… and as stated before, your best action depends very much on the limitations you’re facing with your current setup.

same psi for me. air is very progresssive in compression so a wee air chamber alteration makes very little difference to the initial compression. 3 tokens here. I`m on a Focus which is regressive initially then ramps after – your frame kinematics may be affected differently.

i wanted to stiffen it up through the travel.

Cheers – thanks all for the input. Best case seems to suck it and see. I’ve got (it’s a Fx DPS) smaller, middle (supplied) and larger volume reducer, so will have a play and see what happens/how it feels.

Peaslaker – that’s a really interesting model – a bit (read: way) above my head, but I’ll have a bit more of a look/play and see if I can wrap my brain around it.

All the best,

Rich