Thinking of putting a pair of uber cheap s/h SID 29ers on my Genesis fortitude which is designed for rigid forks. Can I space down rock shox dual air forks to any travel using the all travel spacers ?

I’ve done in the past a pair of revelations from 150 to 120mm.
Can I do a pair of 100mm SIDs down to 60 or 50mm? Just to offer a token bit of travel, with big tyres without upsetting the geometry???

Anybody help with this?

Loco in the house?

should be fine, but geo might be a bit steep as forks will be pretty short. not sure you’ll fit huge tyres in SIDs.

Geometry would be slack rather than steep, it’s 68 degrees with a 445mm rigid forks. Hence me thinking of running the SID with like 50mm of travel to shorten the axle to crown.

So it still has a little bit of travel, not much heavier as the rigid forks are 1000g already.

Don’t want to raise the bb height to much by the forks hence running so little travel, but it should be enough to take the edge off and run 2.4 tyres.

You can always shorten air forks – you just lengthen the top out bumper to prevent them extending fully. As you say, if they already have a system for adjusting travel (eg all travel spacers) then it’s usually fairly simple as you can just use more spacer to reduce the travel further.

FWIW with dual air you could get it to sit pretty far into it’s travel without any messing around with spacers just by increasing the pressure in the -ve chamber

445mm axle to crown measurement is the same as a 100mm fork with 20% sag.

So a 100mm SID would be perfect. If you don’t want the fork bouncing up and down so much, get SID with a RLT damper and keep it on lockout. The “T” bit means threshold, which means the fork will stay locked out and only move and absorb large bumps.

Every 20mm on the axle to crown is roughly 1 degree on the head angle. So reducing the the travel down to 60mm would result in the head angle becoming 2 degrees steeper so roughly around ~ 70degrees that’s quite a change and bike may not handle very well at all.

increasing the pressure in the -ve chamber

I’m not sure that’s the best solution. The fork would also become overly sensitive and end up blowing through is travel a lot.