26in wheel in 27.5 fork

The Duckmeister

Eats Squid
Google pointed me to several threads on this, none of which gave me the exact info I'm looking for.

Situation: Current 26in Lyrik circa 2010 is giving me grief. Tempted to upgrade to current model Pike. I'm happy with every other part of my 26in wheel'd Nomad, but will realistically upgrade it to something 27.5 (possibly another Nomad) no earlier than a year from now, no later than 3 years.

So I'm considering getting a 27.5 Pike so that I can run it in the situation where the aforementioned 27.5 next bike is a build, rather than a full bike purchase.

According to Rockshox specs, a 26in 160mm Pike has an A2C of 542mm, the 27.5in 160mm Pike A2C is 552mm*, and my current 26in Lyrik is 547mm.

So in theory, I'll only be raising the front of my bike 5mm - which is fk all really, considering the bike is designed to run up to a 180mm fork.

My only question is, is there any downside to having the smaller wheel in the slightly longer lowers? Will the fork behave ... I dunno ... badly, from a torsion or flex perspective, with this 'unused' extra bit of lowers? My gut feeling is: no difference at all. But before I go spending $700ish, I thought I'd better off ask around.

(* these figures confound me - to allow for .75in/19mm more wheel diameter, they only have to increase the lowers by 10mm ???)

EDIT - I guess the other thing to consider is: are 26in forks going to start getting cheaper as the number of bikes needing them decreases?
Short version: it'll make fuck all (but not no) difference.

Longer version: It'll have a slight effect on the steering geometry, more when you change to the future 27.5" wheel. The longer fork will have slightly more rake - forward offset of the axle relative to the steering axis, which with a smaller than designed-for wheel will make the bike a bit flighty, but with the bigger wheel to jack it up, the trail will come back to where it's designed & stabilise the handling.

As far as flex, no effect at all by running a smaller wheel. There is no "unused" bit of lower, the axle tying the fork tips together is in the same place whether you've got a 12" or 29" wheel in there (but the latter will probably drag on the bridge!). What's between the axle & lower bridge has absolutely zero effect on the fork. HOWEVER! All other factors being equal, the smaller wheel will actually make the handling stiffer, because there's less inherent lateral flex.
 

Beej1

Senior Member
Except the OP who talked about raising the front end 5mm.

To me, when referring to raising something by X amount, the reference is always vertical.
Good point. I've honestly never thought of it that way. There'd be a bit of trig involved, but yes it would definitely be less than fk all.

Short version: it'll make fuck all (but not no) difference. .....
Thanks for the long version too. I had a feeling it would make no difference in terms of torsional forces. I guess my thinking was: when they're designing and forging the lowers for a fork meant to hold a 27.5in wheel, they'd be taking into account how a 27.5in wheel behaves, and how its behaviour translates into forces on the axle and then how the lowers respond to those forces. I know pretty much zero about how all that works from a physics point of view but I figure Rockshox do, and just want to make sure that something designed specifically around a 27.5in wheel won't be terrible if using a 26in wheel.

It seems like Moorey and a few others here have been doing it for quite a while. I've honestly no idea why I didn't find that in search results sorry.

And FWIW - I'll never be running a 27.5in wheel in there on this frame. Only 26in wheels.

And yes - I looked into both the new(er) Mission Control DH damper (which eliminates the need for the second inner damper tube I currently have) and the Charger Damper. It would be tough to spend much less than $250 on the former (you need the rebound damper as well), and $400 for the latter. Combined with the fact that I recently did a terrible job of stripping and re-painting my lowers .... and the fact that I'd like to try getting a tapered steerer fork and adjustable headset to slacken my head angle a degree (which will negate the aforementioned front-raising I imagine) which my current Lyrik can't do with it's 1.5in straight steerer ... all leads to the idea of a new fork being attractive.

P.S. Since a lot of you seem to be cluey in this stuff - don't spose anyone can explain those Rockshox figures? That is, why increasing the diameter of a wheel by 1.5in, and therefore the radius by 0.75in or 19mm, only results in a 10mm increase in A2C? It's got me stumped.
 
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MARKL

Likes Bikes and Dirt
P.S. Since a lot of you seem to be cluey in this stuff - don't spose anyone can explain those Rockshox figures? That is, why increasing the diameter of a wheel by 1.5in, and therefore the radius by 0.75in or 19mm, only results in a 10mm increase in A2C? It's got me stumped.
Cause 27.5 is marketing BS and should really be 27. Real number is your ETRTO or bead seat diameter which is:

26 - 559
27.5 - 584
29 - 622

S the actual difference is only 25mm, half is 12.5. RS decided they only need 10 so they reduced clearance by a couple of mm.

wpid-Photo-201406042220242.jpg
 

MrZ32

Likes Dirt
Only thing is that there is less of an offset with the 650 fork.. to reduce the trail so that the fork is still flicky without sacrificing any grip due to the longer contact patch.. plus the taller tyre gives the effect off an offset anyway.
 

The Duckmeister

Eats Squid
Cause 27.5 is marketing BS and should really be 27.
Because 27" was already used on old roadies so they had to call it something different. It's closer to the truth than 650B though, which although used (note past tense, being obsolete) the same rim diameter, was a considerably smaller tyre profile.
Real number is your ETRTO or bead seat diameter which is:

26 - 559
27.5 - 584
29 - 622
Indeed, and I wish the indusrty at large would actually use it, because it's a much more accurate expression of tyre & wheel size!
 

moorey

Pagophagic
Only thing is that there is less of an offset with the 650 fork.. to reduce the trail so that the fork is still flicky without sacrificing any grip due to the longer contact patch.. plus the taller tyre gives the effect off an offset anyway.
More offset usually innit? 44v51 from memory (on pikes)
 

The Duckmeister

Eats Squid
More offset usually innit? 44v51 from memory (on pikes)
Yup, jacking the bike upon taller wheels means the steering axis intersection with the ground moves further forward of the tyre contact point, so more trail-> stable but sluggish handling. So with longer forks they increase the rake (aka offset) to also bring the contact point forward to keep trail somewhat consistent.
 

MrZ32

Likes Dirt
More offset usually innit? 44v51 from memory (on pikes)

Sorry my bad.. was thinking about something else.

But essentially yes, increasing the offset to decrease the increased trail (due to the taller tyres).

With that being said, if you are running a 26fork with a large offset already, you shouldn't really notice the switch across to a 650 fork.

It would be worth working out the change in trail as it could drastically change how much confidence that you have in the front tyre if you get up to solid speeds on your trails.. if you barely ever get over 25kph on your trails, i cannot imagine it being an issue.

The redeeming factor could be that if you do slacken out the front end of the bike by a degree with the bigger forks, the change in trail due to the offset may be negated somewhat.
 
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MrZ32

Likes Dirt
I can check the trail change for you if you want.. just not going ti have time until tomorrow.

What is the current HA? What is your current fork offset and your future fork offset? What overall diameter is your tyre? Or at least what size tyre do you use?
 
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SF Trailboy

Likes Bikes and Dirt
I can check the trail change for you if you want.. just not going ti have time until tomorrow.

What is the current HA? What is your current fork offset and your future fork offset? What overall diameter is your tyre? Or at least what size tyre do you use?
Could you pretty please do it for me too????

70' current os 44mm future 51mm maxis ardent race 29x2.25 Going from 100 -120mm fork
 
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