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.
No, it’s the other way round. I’ll be running longer 27.5” forks than the 29” that would normally be specced.
Your lack of understanding for my response, and total lack of grasp at what you're doing, tells me all I need to know...
A 27.5 fork is 20mm shorter, axle to crown length for a given travel, than a 29er fork... At least according to Fox and Rockshox's technical drawings for 36/38 and Lyrik/Zeb.
A 27.5 wheel and tyre is 19mm smaller in radius (we'll call it 20mm for ease of maths) than a 29er wheel, for a given tyre size... 622 minus 584 equals 38mm, of which half is 19mm
So you need to make up 40mm of difference somehow!!! Not 20mm...
The easiest way to do this, if you have to run a 27.5" front wheel instead of a 29er (god only knows why though!) is to continue to use a 29er fork, albeit one with 20mm more travel than you would have run with the 29er wheel fitted, and still fit the 27.5" wheel... There will be an extra 20mm gap between the tyre and the fork brace, but that's not an issue functionally, only cosmetically...
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.
All well and good, as long as you're happy with the fact you've dropped your BB by massive amount, and you've steepened your bikes angles by about a degree too...?
Your lack of understanding for my response, and total lack of grasp at what you're doing, tells me all I need to know...
Someone is clearly happy to be back at work. Happy New Year!
🙄
Thinking about it further, will you be limiting yourself to what frame you can buy if it needs to specifically fit a fork you already own i.e. if it's currently 160mm then you are looking at a 140mm 29er when you might want something longer travel
> forget it <
All well and good, as long as you're happy with the fact you've dropped your BB by massive amount, and you've steepened your bikes angles by about a degree too...?
The BB will be maybe ~10mm(ish) lower, and is 1deg steeper really the end of the universe?
Don't get me wrong I likes me a Mullet bike now and a chopped out head angle just feels 'Right' on a HT these days, but -1/2" on the BB and 1 degree steeper HA is probably well within the realms of still being perfectly rideable.
MTB Geometry is so variable that most people aren't all that attuned to such relatively minor variations.
Nothing the OP has proposed doing is particularly dangerous, it just might be a tad "sub-optimal" for handling Vs the original intent for the frame.
I don't think he deserves the hard time he's getting for this...
I've been thinking about this a bit lately. I don't think you can take preserving A2C and head as being the most important factor when you are swapping wheel sizes. I think the important measurement is the normal trail. Normal trail is measured perpendicular from the steering axis to the contact patch (just so that it's not confused with mechanical trail which is from the contact patch to the point where the steering axis intersects the ground).
I think calculating the changes to normal trail (both front and rear) will give you a better idea of how drastic the changes in wheelsize would actually be, particularly since a 27.5" fork almost certainly won't have the same offset as the equivalent 29" fork.
Just because I like this stuff I made a FreeCAD sketch to calculate and visualise all this but it's on my computer at home so hopefully I'll remember to post some screenshots to show what I mean later on 🙂
Sounds like you've been overthinking it massively.
Remember when the entire bike industry swapped to shorter offset (eg. 51mm to 44mm) and all the mtb media were doing huge articles on it, forums were fill of discussion on it and everyone who bought into it and tried it exclaimed it to be a revelation?
Well it wasn't. There are positives and negatives to each. And the human body is amazing at adapting and adjusting to slight changes ... Basically. BOTH offsets are actually absolutely fine especially if you actually ride your bike more than sitting around thinking about and discussing it. Yeah. I do get the irony there.
Rather than give the OP a kicking, maybe it's worth considering what frame would be used and what numbers are involved?
And also what fork
Take the Stanton Switchback FS - it's designed to be run as full 27.5 or mullet
Here are the comparison figures
Rather than give the OP a kicking, maybe it's worth considering
Consideration is rarely the STW way for too many keyboard warriors. Part of the reason I’m no longer a suscriber.
In between the bluster are messages that actually answer the questions and confirm the difference is relatively minimal to make it worthwhile trying without dropping a whole load of cash just because the manufacturers say so.
For what it’s worth, Chat GPT reckons front end will drop by 20mm for a 27.5” fork that’s +20 more travel than a 29”. Mostly recoverable with an high stack lower head race.
Even without that, 20mm difference is 1 degree on the angles and less than 10mm on the BB.
Worth a punt in my book. YMMV
All that ‘Chat GBT reckons’ is already in this thread.
The big LLM machines are tweaked to not be an arse when their help is ignored, real people tend to be more spiky!
“Take the Stanton Switchback FS - it's designed to be run as full 27.5 or mullet”
Those Stanton charts have been made whilst forgetting to change the front wheel size as well as the fork axle-crown (and the A2C data is a bit sketchy too)…
If you know what you want/like in terms of geometry then you can figure out how changing a bike from stock will work for you, especially if you reference from a bike you know well.
For instance I have a 160mm fork on my hardtail (which it is designed for) but I don’t need quite that much travel, 140mm would be fine judging by where the O-ring ends up. But I knew the 20mm longer fork would put the angles, reach, stack and BB height in a better place for me. And my old Levo was designed to be 150F/150R full 29 but I now run it 170F/157R mullet and it rides better than ever.
It’s not often (ever?) that I’ve preferred how a shorter fork / smaller front wheel behaves but maybe I would on a bike with a very slack tall front end?
Quite the opposite Chief
A shorter fork, shorter wheelbase and steeper head angle combined with smaller wheels makes for an infinitely more maneuverable and playful bike.
It's definitely a preference I have.. But it's not for everyone. And if you're used to long slack and big wheels you'd probably find it a sketchy as hell set up until you got used to it... But then it should make a whole lot more sense.
Ps. Who's being accused if giving the OP "a kicking"?
Seems a fairly civil discussion to me with plenty of good info to consider. (well apart from the odd nonsensical bits here and there)
"A shorter fork, shorter wheelbase and steeper head angle combined with smaller wheels makes for an infinitely more maneuverable and playful bike."
A fine example of how one rider's manoeuvrable and playful is another's skittish and nervous! 😉 Give me a bike that's a stable steam-roller and I will unsubtly manhandle it to make it do what I want, and have the confidence to chuck it about in a way I don't on a faster-handling bike.
But I don't think I can go longer or slacker without struggling to weight the front on flatter trails or getting around tight slalom turns at speed. That's what I'm telling myself anyway which stops me looking at new bikes!
Everything else aside - after going 29, I'd never go smaller again (on the front at least), it was a revelation for me - both mulleting an existing 27.5 bike and then buying new 29'ers
A fine example of how one rider's manoeuvrable and playful is another's skittish and nervous!
Yeah. I guess. TBF tho I'm lucky enough to have both smaller wheeled shorter bikes and modern longer full 29 And still ride both fairly equally. Do I prefer each in different situations? Yes. Of course. And this means I can still manage to be fairly subjective about it Rather than just ditching all smaller bikes because I've now found a massive increase in stability and completely forgetten how much easier a smaller bike is to pop, manual and turn.and how much fun that still is.
Rather than just ditching all smaller bikes because I've now found a massive increase in stability and completely forgetten how much easier a smaller bike is to pop, manual and turn.and how much fun that still is.
this absolutely. our local used to have way more corners and i put this largely down to bikes and bars getting longer and wider and harder to steer. and eebs because eebs are bad for everything right!
anyway i fancy a shiney new HT frame soon and probably it will have to be a Mullet frame (as a 27er doesnt exist). so i`ll put a 27 on the front at least to start. mainly as i dont have any 29er front wheels. it`ll have a slighyly lower BB but that just means it`ll go around corners faster right?? haha! BTW it'll be rigid fork as i`m a sadist - so dont need to worry about bottoming out the fork and exceedingly low BB heights. .