MegaSack DRAW - This year's winner is user - rgwb
We will be in touch
Have got a 27.5” full suss and toying with the idea of replacing the frame with something newer as there’s a few good deals to be had at the moment.
Most of the components on my bike are fairly new so running with the concept of an Mx frame so rear wheel fits easily then going with the assumption that if my forks have 10/20mm more travel than the 29” designed for, plus a larger volume tyre than spec, then it should more or less keep the angles much the same.
I know it’s not as the manufacturer designed, understand the ride characteristics of the different front wheel sizes and know I could sell front end to part fund new … but don’t wanna.
Are there an obvious flaw in my thoughts or am I more or less on the right lines?
As long as you adjust things to keep roughly the right geometry you'll be fine. I think you can fiddle with things an awful lot and your bike will still be rideable. You can use a longer fork or a crown race spacer if your fork isn't quite long enough.
Fwiw I've mulleted a 650b bike and I've run my 29er as a mullet and with different length fork travel and they've all worked fine
I'd absolutely give it a go! 20mm or so on the fork over recommended and go ride! I've been buggering about with geo for years and yeah, sometimes you find something that really doesn't work, but one person's 'perfect' is another's sluggish boat.
Currently riding a 29er 130mm hardtaill frame with a 150mm fork, -2 angleset, and sometimes use a 27.5 rear. It's a heap of fun!
27.5 rear on a MK1 Ripmo AF with a coil and cascade link was not so successful!
Most of the components on my bike are fairly new so running with the concept of an Mx frame so rear wheel fits easily then going with the assumption that if my forks have 10/20mm more travel than the 29” designed for, plus a larger volume tyre than spec, then it should more or less keep the angles much the same.
Are there an obvious flaw in my thoughts or am I more or less on the right lines?
BIG obvious flaw I'm afraid... You need to make up for not only the wheel being smaller, but the fork being shorter too... You lose approx 20mm in axle to floor height for the wheel (for a given tyre size), but you also lose another 20mm in axle to crown height too for a given fork length... Replacing a 29er front end with a 27.5" front end, to keep the same geometry you would have to fit a fork with 40mm more travel!!! And that's just the static geometry... A 170mm fork is going to sag a lot more than a 130mm, so in reality you'd need to go 50mm longer on the fork to make up for the extra sag to keep the dynamic ride height similar... For reference, going for a 2.6" tyre say over a 2.4" is only going to give you about 3mm extra height, and you'll run it softer for the grip anyway, so probably in real terms it won't give you any extra ride height.
There's companies out there that do deep cup external headsets that might buy you 15-20mm potentially... That would be a good start combined with at least 20mm more fork travel... But I really just wouldn't bother! Sell your 27.5" fork and front wheel and buy 29er equivalents, it'll ride way better anyway.
BIG obvious flaw I'm afraid... You need to make up for not only the wheel being smaller, but the fork being shorter too...
No, it’s the other way round. I’ll be running longer 27.5” forks than the 29” that would normally be specced.
Sounds like you’re looking at running forks with more travel, but slightly shorter static a2c length, and probably shorter still ride height.
As mboy says, a 120mm 27.5 fork is very roughly the same static a2c length as a 100mm 29 fork.
A 120mm 27.5 fork and 27.5 wheel will result in much the same ground to crown race distance as an 80mm 29 fork and 29 wheel.
MRP make some tall crown races you could use to help adjust things. But a bargain 29er fork (plenty out there at the moment) would be a better buy.
I have converted all my bikes to 27.5, two 29ers and an mx. I prefer to run a 27.5 fork with a shorter offset and 20mm more travel. I have fit a extended lower headset cup to one but wouldn't bother if doing it again.