MTB fork change, from suspension to rigid

pep.fermi

I want to change the fork of my old 26" MTB hardtail to rigid.
I understand with everything engineery there are many standard regarding tube size shape, taper, not tapered, bla bla, different standard generally not compatible with each other.
How do I know what standard I have? And which is compatible?
Thanks,
Alberto

pep.fermi

for the record: disc brake, carbon, 10yr old

andrew_s-2

Drop the fork out and measure the steerer diameter both at the top where the upper headset bearing goes, and at the bottom by the lower headset bearing.
Standard on an old bike would be 1 1/8" (28.6 mm) top and bottom, but very old bikes may have 1", and newer bikes may have 1 1/4".
Ideally any replacement fork should match, but spacers to pad out a slimmer fork for larger bearings do exist.

You also want a fork that's "suspension corrected". These have axle to crown similar to that of a suspension fork, allowing for sag with the rider on board, so that the bike handling doesn't change too much.
Example: https://www.sjscycles.co.uk/forks/26-inch-steel-mtb-vbrake-disc-fork-1-18-inch-280mm-steerer-black-seconds/

pep.fermi

andrew_s-2 said:

You also want a fork that's "suspension corrected". These have axle to crown similar to that of a suspension fork, allowing for sag with the rider on board, so that the bike handling doesn't change too much.
Example: https://www.sjscycles.co.uk/forks/26-inch-steel-mtb-vbrake-disc-fork-1-18-inch-280mm-steerer-black-seconds/

Thanks a lot.
In reality "suspension corrected" is just wording, in actual fact as far as the axle-to-crown distance is the same (or very similar), and of course top and bottom diameters of the tube match, any fork should do, right....? Or wrong...?

pep.fermi

OK I'm doing some fork studying right now...
Suspension forks have obviously longer axle-to-crown distance than rigid forks, for obvious reasons, typical 465-490mm. Whereas 26" rigid forks are typical 410mm. But rigid forks are like 425mm (26"), 445mm (27.5"), or 475mm (29").

So replacing a 26" suspension fork, with a 29" rigid fork, the axle-to-crown distance should be ok.
Or am I wrong?

Thanks, Alberto

whyamihere

You need to take sag into account to get the same handling. Say you have a suspension fork with 100mm travel and 470mm axle to crown. Typically sag is set to 20%, so 20mm in this case, so a rigid fork with axle to crown of 450mm would be about right.

pep.fermi

OK, I just ordered a new fork, 29" rigid, 28.6mm straight upper tube, 9x100mm.
This to replace my 26" suspension. Both forks for disc brakes.
I have no chance to measure the old fork right now (bike is at Mum's place where I'll visiting next week).
It could well be the new fork does not fit, in case bad luck I'll have to return it, worth a try....