Axle to Crown question (caution: MTB content!)

[a4racer]

Ahoy there!

Looking at swapping my low-end Judy TK Silver from my 27.5+ bike for a Niner rigid carbon fork. Both are Boost.

The Judy claims an ATC measurement of 510mm, whereas the Niner rigid is 490mm. 20mm seems like a noticeable difference in ATC, but I'm curious--does ATC measurement on a suspension fork account for sag? I'm guessing that the sag on the Judy (130mm travel) would be around 20mm anyways, so that the ATC with sag on the Judy would be about the same as the ATC on the Niner...

What say ye--oh brilliant minds of PL?

Not using this bike / setup for anything more than riding around rail trails with my kid. So, frankly, I'm not all that worried about minor changes in handling!

Thanks all!

[Jaybee]

Sus fork quoted ATC lengths are almost always unsagged, since there's no way to precisely dial in how much sag a given user will be running. In cases where the ATC is quoted as sagged, it usually calls out the sag measurement (i.e. 507mm at 25% sag)

You are probably running more than 20mm sag (that's only 16%), unless your fork is unusually stiff, but I would expect no detrimental changes to handling with your proposed swap and uses.

[BikeNY]

As mentioned, suspension fork A-C measurements are unsagged.

Are you sure about that measurement for your fork? 510mm sounds too short for a 130mm travel fork.

But, if it's correct, sag should be set around 25% of travel, so about 32.5mm. 510mm - 32.5 = 477.5.

That's less than the rigid fork, but it will work fine.

Looking at a current SRAM chart, I see a 27.5 Judy with 120mm of travel(that's the max) with A-C of 512mm

That works out to 120 x 25% = 30, 512 - 30 = 482mm. That's even better.

[a4racer]

I think I'm going to go ahead with the Niner fork...

I'll let the massive 3.0" tires be the suspension instead of the Judy!

Thanks all!