bobotech

I'm doing a budget build for my son. I have a large Hardrock frame with rear disc tabs and I want to build it up as a disc brake bike. The only holdup is the front fork. My son is big like me and I don't want to use the cheap Suntour fork. I do have a very nice air-charged front fork that would work great except that its a 29er fork. The overall difference is about 2 inches longer.

How badly would that affect the usability of the bike? Would it be okay or will the bike be pretty much unusable at that point?

dabac

It used to be that people were very concerned about the increased load on the joint between down tube and head tube when someone suggested to fit a longer fork to a frame not explicitly intended for such use. But I haven't heard that objection in quite awhile.
Maybe the absence of gory accidents reliably attributed to longer forks has pushed this scenario into the "true, but not important"-category.

I haven't tried a 29er fork on a 26" myself, but I have run sus forks on frames designed solely as rigids before, with 63, 80 and 100 mm travel. The result - IMO - is a bike with sluggish steering in comparison, with comparison being the operative word here.

If my previous ride was a bike with "normal" geometry, I'd spend maybe 200 metres being conscious of the slower handling, and then the brain's auto-adjust would have kicked in and I'd do the rest of the ride in blissful ignorance of geometry details. Any limitations in performance would be far more reliably blamed on me rather than the bike.

Going back to a bike with more upright fork, it's the same but in reverse. A moment of "wov, that's snappy", and then it's forgotten.

If I really concentrate, I can find some differences in ride characteristics. A slacker fork angle makes for more relaxed steering on bumpy descents. But in terms of actual performance, it's me, not the bike that sets the limits whether it's a slack or a steep fork angle.

Unless the intended use is bike polo or competitive riding, I'd say you're good to go.

techsensei

Installing a long fork is going to make the head tube angle slacker, and also probably make the amount of trail shorter. The results being a bit twitchier steering. I don't think it would be unusable; people ride on bikes with bent frames and/or forks all the time and never realize anything has changed.

cobba

https://www.cotic.co.uk/geek/

Slide the slider under the frame from 100mm to 150mm and you'll get an idea on how 2" / 50mm changes the geometry of a frame.

a77impala

I put a 29er fork on my Trek 930 replacing a suspension fork, works great with a 2 by 29 tire. I put the same fork on another 930 that had rigid fork and it was very hard to balance.
The 29er fork was basically the same length as suspension fork so no change in geometry.

bobotech

Well I guess I could try it and see how it responds. If it is awful, we will just put something else back in its place of the 29er fork but since the 29er fork is free to me, might as well try it.