[uphiller]

I have a dreambike: a 29er with 16" (or even shorter) chainstays.
I am not an expert, but think it is possible.
The (factory-produced) Nimble 29er has 16.5" chainstays.
Chris Igleheart said he could get the stays down to 16.2".
I have seen fat-tire 26" trials bikes (= 27" real rear wheel diameter) with 15" stays and shorter.
What, really is the absolute minimum on a 29er if we pull out all the stops:
-83mm or wider bb shell to push the cranks away from the frame; corresponding rear hub
-ditch the front derailleur if necessary
-chainstay yoke
-seat tube shifted forward
How low can we go?

 

[Blaster1200]

I'll say you're looking at 15-1/4" minimum with about a 2.0" tire.

Wheel radius of about 14-1/2" plus the BB radius of 3/4". Of course, that will net you with about 0 tire clearance. You could have more clearance or a shorter chainstay with a smaller tire, such as either a 44mm or a 1.8"ish tire. Clearance to the side of the tires will also be very tight, unless you don't mind fairly flexy chainstays, but chainstay clearance or stiffness wasn't a criteria.

And if you do go this route, make sure to tell the builder you want to be able to fully insert a 410 mm seatpost into the seat tube, if that's the case.

 

[uphiller]

I always suspected that framebuilders who say 16.5" is a limit are BSing me.
I think it's a question of economics: you can build a lot of standard frames in a certain time frame. But if somebody demands something special, you need to re-align the jig, buy different tubes, make drawings, think... None of that is pleasant.
15.5" would be absolutely amazing. I would miss having two front chainrings, though. I also wonder about the seat tube- the farther forward you shift it to clear the rear tire, the closer to horizontal it gets. This means you need a fatter seat tube, which risks impinging on the tire again.
Because the seat tube would have to be shifted forward, each inch you move the seatpost up or down would mean a bigger horizontal distance. That means you really need to build custom- you have to have all the lengths right for the rider who is going to use the bike.

 

[Blaster1200]

Adjusting the jig is no big deal. It's easier than lowering your seatpost.

Seat tube aside, there are challenges with chainstays if you want to add the criteria of "relatively stiff." Again, not impossible.

All in all, I think most builders wouldn't want to go much shorter with their CS because there isn't much demand.

 

[dbohemian]

15.5" would be absolutely amazing. I would miss having two front chainrings, though.

Can't really do it. You could barely do that on a road bike and it wouldn't work most of the time there. (interference issues)

I think what most builders are telling you is that the sacrifices you have to make (all of which have been mentioned) outweigh just getting a short stay. You may be able to use a 26'' tire and a 700 front to do it.

I believe that chainstay length should be varied based on the size, height and style of the rider so ultra short is almost never really the preferred length.

 

[uphiller]

What interference issues do you mean? I can think of the following:
1) Front tire bumps the chainstays- use a machine yoke; use a wider bb shell and matching rear hub
2) Front tire bumps the seat tube- use a forward-shifted seat tube.
3) Front tire bumps the front derailleur- ditch the front derailleur; use a wider bb shell and matching rear hub
I don't know, quantitatively, how much those problems can all be solved, but I do know that there are trials bikes with giant rear tires and 15" or less stays. For that reason, I think 16" is possible, especially with an internally geared rear hub.
The reason such short stays are desirable is that they make the front end easy to lift. The same can be accomplished with other means- a slack head angle by default puts the rider's weight behind the front axle, making the front end easy to lift.
Just think of it- the jumpability of a 26" wheeled bike, plus the rollability of a 29" wheeled one. It's something worth a pound or more of extra frame weight, in my view.

 

[Blaster1200]

For that reason, I think 16" is possible, especially with an internally geared rear hub.

You may have just added a big problem to the mix. I don't know of any IGH that is wider than 135 mm. Also, I'm pretty sure the Alfine uses a very narrow chainline, which means your front chainring is very close to the tire even with a 73 mm shell, leaving very little room for chainstays - especially short ones. And since you keep mentioning trials riding, have you tried an IGH for trials?

The reason such short stays are desirable is that they make the front end easy to lift. The same can be accomplished with other means- a slack head angle by default puts the rider's weight behind the front axle, making the front end easy to lift.
Just think of it- the jumpability of a 26" wheeled bike, plus the rollability of a 29" wheeled one. It's something worth a pound or more of extra frame weight, in my view.

I'm not really sure what your objective is by going really short. Lofting the front? Raising the bottom bracket will do that as well...with other consequences.

I'll tell ya what...For the longest time, I hated 29ers. All the normal production bikes were extremely boring to ride. They felt dead. Couldn't manual, couldn't bunnyhop, and couldn't whip in the air. Just some of my background, I race DH, 4X, and BMX. I ride XC like I'm riding any one of those bikes. I built my 29er SS with 16-5/8" stays, and I've been surprisingly quite happy with it. My opinions of 29ers changed. I go back and forth between this bike my Black Market Mob (with the wheel slammed forward) and my 20" and 24" BMX race bikes (the dropouts on the Intense Podium 24 are filed to allow the wheel to be even further slammed forward). As odd as it sounds, I can get the front end of that 29er up easier than the other bikes. Most of this may also be due to the taller front end. I'm just saying this as you may be chasing something that you may not need or want. Try a 29er with around 16.5" CS that is set up for you and you may be surprised.

Although I haven't taken my 29er to the BMX track, I do take my Mob there often. And when manualing and tucking over larger whoops and tables, my butt often hits the back wheel. I'm sure it would be way worse with a 29er! So bad that I think that when my butt hits, it would buck me forward. Some day, though, I will regear it and try it.

 

[uphiller]

I wasn't thinking of trials riding, although I did do it in the past- had to stop because of back issues and a lack of time. It is a discipline that really requires a ton of time to maintain one's skill level. I don't want to do trials on this bike, I just want to have that front-end lofting ability which is so useful when riding steep stuff or even doing minor drops.
There are other ways of making the front end easier to lift- slacker head angle (already talked about it), higher bb (less stable, higher center of gravity on a bike that already has big wheels), and making the rear wheel smaller (which is what I don't want to do- I specifically want a 29er).
I didn't know about the Alfine's narrow chainline... I heard there is a 145mm threaded rear axle for the Rohloff, although I don't know what kind of chainline it has.
An 83mm bb shell might solve the problem.
As a non-framebuilder, all of this is pretty theoretical- someone needs to make drawings.

 

[dbohemian]

As a non-framebuilder, all of this is pretty theoretical- someone needs to make drawings.

YOU need to make drawings. that will help to cement it. I already know it won't work without going through contortions and frustrations. The good news is that this is a framebuilders forum, meaning that you should try and build it. You have already realized no pro is going to touch this one meaning if you think it can be done than the only one to do it is you. Grab a torch, the rest of your savings and prove us wrong as you manual all over the mountain. :cornut:

 

[D.F.L.]

With a BB shell cut-away, you should be able to get below 15.4, easy.

 

[Blaster1200]

YOU need to make drawings. that will help to cement it. I already know it won't work without going through contortions and frustrations. The good news is that this is a framebuilders forum, meaning that you should try and build it. You have already realized no pro is going to touch this one meaning if you think it can be done than the only one to do it is you. Grab a torch, the rest of your savings and prove us wrong as you manual all over the mountain. :cornut:

That is such a good suggestion. :thumbsup:

At the time I wanted a short chainstay 29er, there were no production ones available, and at the time I didn't know any custom builders who were doing them, so I picked up a torch and started. It's just a bicycle; it's not brain surgery.

 

[sean salach]

If you really want to get crazy, throw a 100mm shell or elevated chain stays into the mix to increase your options for silly short.

If you want flickable and stable, copy the geometry of your favorite 26" bike relative to the axles. Then draw in a 1.5" lower top tube(for standover), and whatever manipulations are necessary for the larger wheel/tire radius. For a bike that get's ridden in a somewhat xc like manner(AM, Trail, Trials/Trails), I don't think you would need to go any lower than 16.25" or so, which D.F.L and I think others(?) have done on a custom basis in the past.

 

[D.F.L.]

If you're of stature that requires sub-16.25-16.5, then go to 650b on the back. That'll bring it way down.

 

[todwil]

Can't really do it. You could barely do that on a road bike and it wouldn't work most of the time there. (interference issues)

I think what most builders are telling you is that the sacrifices you have to make (all of which have been mentioned) outweigh just getting a short stay. You may be able to use a 26'' tire and a 700 front to do it.

I believe that chainstay length should be varied based on the size, height and style of the rider so ultra short is almost never really the preferred length.

I'm with Bohm if it's doable build it and bring it here and post it up with pics....lots of pics!!

 

[unterhausen]

Not sure why we take posts like the OP seriously, seems like a weakness. You could at least make a sketch before insisting something can be done that experienced builders say can't be done. Of course, one meaning for "can't be done," is "you can't build it and I'm not going to."

I'm not an expert on mtb geometry by any means, but it's a real chore to get the cranks, chainstays and tires to line up in such a way that they aren't intersecting each other. And that's just on paper. Actually fabricating something is another chore.

there used to be a real fetish for short chainstays on road bikes, but that's pretty much gone away other than people making one-off bikes for show. Turned out that it wasn't that great of an idea.

Builders have many good reasons for not building something like this. It's a tough business and nobody in it is raking in the bucks. So turning down the opportunity for wasting time on something they wouldn't want to build twice is a no-brainer.

 

[uphiller]

I didn't say it was possible. I asked.
I pushed and insisted, because everybody knows that's the way to see if things are possible. Having pushed and insisted, I see that what I am up against really is impossible- although one builder specifically said that 16.2" is doable, without mentioning anything like wide bb shells or bent seat tubes.
You guys are framebuilders- I am not. I came here for answers. I got them- what I want seems to be impossible.
This forum is a place to share information among people in the know- but I also thought I would take advantage of its secondary service, to give information. Sorry for ruffling any feathers.

 

[DWF]

I have a 29er with 16" stays. It's not a big deal as long as you don't want to run both multi-chainrings or big tires with conventional BB widths. You can do one or the other, but not both with the current state of affairs with front derailleurs. You could always run a 83mm BB & 150 rear spacing or a Hammerschmidt or an internal rear hub. Effective chainstay length shorter than 16 on a gearie don't really have much going for them unless you're vertically challenged + the chainline will suck and shifting gets ghosty too. If you're average size, stays longer than 17" effective pretty much suck balls on trail bikes too. Note that I say "effective chainstay length." Stays will always measure longer (by industry definition of the measurement) on a 29er than they will on a 26" even though the rear axle is the same horizontal distance away from the BB center.

I guess what I'm saying is that getting hung up on arbitrary numbers is a pretty silly way to design or build a bike frame.

 

[uphiller]

DWF- Thanks for the info. Who built this frame for you? Can you post pics?

 

[Clockwork Bikes]

This is what 15" looks like with a 26" wheel and 2.3" tire.

 

[Hand/of/Midas]

This is what 15" looks like with a 26" wheel and 2.3" tire.

View attachment 609712

MEH....That's nothing.

Candida. Tire can touch the BB is wanted, run it 1mm from the BB, whatever.
This is how to make the CS length as short as the tire will permit, just do it with a 9er.
Sometimes having a computer with 1,000+ random bike pics is useful(if i can find them).

Also, note that a Euro BB gives the shortest possible length, since the distance to the center of BB can be closer.