Author Topic: Rohloff hub to frame torque  (Read 3137 times)

macspud

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Rohloff hub to frame torque
« on: September 20, 2016, 05:14:56 AM »
There is an interesting article on Sheldon Brown's site about the torque that the Speedhub transfers to the bicycle frame:
http://sheldonbrown.com/twist-internal.html
It shows why the Rohloff torque arm is so long, something that I'd wondered about in the past, interesting reading.

Peter_K

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Re: Rohloff hub to frame torque
« Reply #1 on: September 20, 2016, 11:31:35 AM »
On the first picture on the link I see something that is even way more likely to break a frame: a disc brake:


Since the torque from the hub was resolved to the disk brake mount there is no way to see the difference. Braking with disc brakes on surfaced roads is very demanding for the wheel and the frame.

A second aspect is that the shown seat stay seems to be pivoting on the pad, and thus probably not contributing in resolving any torque. Nowadays full suspension frames have their pivot mainly at the bracket.

A third aspect is the frame material: when the frame is aluminium the stay is not allowed to give in at all, otherwise it will always break.

BTW: I bought steel Thorn Nomad after I my aluminium KTM frame broke.  :)

macspud

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Re: Rohloff hub to frame torque
« Reply #2 on: September 20, 2016, 12:17:21 PM »
Yes, I wouldn't expect it to be a problem with the Thorn Raven range as they are specifically designed for Rohloff use. It may be a good idea to keep the high torque in mind for those wanting to fit Rohloff to a frame not designed for one, though. I'd certainly rethink using a monkeybone rather than the torque arm if I was planning a build using a non Rohloff frame especially, as you say, an alumimium one.
« Last Edit: September 21, 2016, 10:34:10 PM by macspud »

mickeg

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Re: Rohloff hub to frame torque
« Reply #3 on: September 20, 2016, 02:58:14 PM »
In the photo posted by Peter_K, the dropout area of the frame appears to have rotated clockwise in the failure.  If the disc brake caused the failure, I would have expected the dropout area of the frame to have rotated counter clockwise as that would have been the rotation of the wheel (assuming the bike was going forward) at the time of failure. Thus, I am inclined to think that the failure was caused by Rohloff torque in a very low gear.


macspud

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Re: Rohloff hub to frame torque
« Reply #4 on: September 21, 2016, 10:36:24 PM »
Yes, that's my thinking Mickeg, and that of the auther I think.

Peter_K

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Re: Rohloff hub to frame torque
« Reply #5 on: September 22, 2016, 10:50:15 AM »
The remark of mickeg would be right when the photo would have been made immediately after the rupture took place.
The way I see it is that the pad is still attached to the rest of the frame only via a pivot: it could easily be moved in any direction.

I just wanted to bring forward that worrying about the torques of a Rohloff hub is a bit silly when mounting disc brakes: the torque caused by the disc brakes will be higher.

The combination Rohloff hub / disc brakes is naturally even worse: the frame experiences torques from opposite directions very frequently.

The torques caused by disc brakes are the main reason for the through-axles seen on many new frames and forks.

MacLeod

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Re: Rohloff hub to frame torque
« Reply #6 on: December 08, 2017, 11:51:51 PM »
Do we have the same torque with pinion gearbox on the frame ?

mickeg

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Re: Rohloff hub to frame torque
« Reply #7 on: December 09, 2017, 02:24:13 AM »
Do we have the same torque with pinion gearbox on the frame ?

Since Thorn does not make a Pinion, I think you will find nobody here that has the knowledge to answer that.

Rohloffs can be installed on just about any frame that has the right dropout spacing, even if the frame was not designed for the torque.  But a Pinion gearbox has to be installed in a frame designed for it, so that would suggest that they should be thinking about the torque when they build the frame.

ledburner

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Re: Rohloff hub to frame torque
« Reply #8 on: February 06, 2018, 02:51:55 PM »
Interesting the rotation of the frame after it cracked will be floating on the pivot, so we cannot draw any conclusions; unless we have a picture of attempting to mate the 2 parts again at the crack and see which way it is now inclined.
it getting out in the fresh air that counts.

MacLeod

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Re: Rohloff hub to frame torque
« Reply #9 on: June 07, 2019, 12:34:26 PM »
Can a steel frame be broken or the material to wear out in time from the rohloff torque or this is only on alloy frames ?

Andre Jute

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Re: Rohloff hub to frame torque
« Reply #10 on: June 08, 2019, 12:00:03 AM »
A third aspect is the frame material: when the frame is aluminium the stay is not allowed to give in at all, otherwise it will always break.

Your example is not only ali but appears to be clumsily welded, raising a stress point where, as we can see, the ali didn't flex but parted.

On the question about the Pinion gearbox, which mounts at the bottom bracket, resolving its torque would be considered in the design of the single-purpose frame, as George says, but it seems likely to be a lesser problem as long as the frame is built of steel, because the triangle in which the Pinion mounts is of larger area so that it will flex before it shears off, and the tubes are in any event of larger diameter there, plus there is another stiff triangle of the chainstay and seatstay bracing one side of it, so multiple paths to resolution.

However, it is important to understand a confounding factor that doesn't arise often in bicycles, which is that multiple force-resolving paths must not resolve unequally or the weaker path will break sooner or later. (Actually the possibility is there all the time in bicycles, but not often discussed because most bicycles are built of off-the-shelf tubes of diameters and wall thicknesses long, long since settled by trial and error in steel and just copied in ali by analogy and due regard for the different characteristics of ali; ditto in plastic bikes.)

That broken bike looks like a conversion to Rohloff and in addition one which in its original derailleur incarnation used rim brakes. I don't believe a commercially made bike would be that incompetently designed without a brace between the stays on the disc side. It may be that the incompetent welding referred to above was an aftermarket job to fit the Rohloff and disc brake, and besides adding a stress-raiser furthermore embrittled the ali by careless heating/cooling procedures. In short, a breakage waiting to happen.

On the 8th page of this PDF photo essay of one of my bikes you can see a close-up of how a disc brace can be turned into a decorative feature; it's the curve with the holes punched into it.
http://coolmainpress.com/AndreJute'sUtopiaKranich.pdf
Notice that this was done on a bike where the rear triangle was already tremendously strong in three dimensions because all those stays are differently angled. I don't run disc brakes on my everyday bike because a) I hate the on-off suddenness and dead feel of the discs on my other bikes, and b) I already have disc brakes on my fave bike, but they're a rim-size disc, Magura's hydraulic rim brakes and c) I really prefer the smooth progressiveness of the Magura hydraulic rim brakes. But, if for some reason I had to fit normal discs, I'd have no hesitation fitting them to this bike. On that broken bike in the photo up the thread -- nah, I'd take one look at it, flick my fingernail against it to be sure it sounds like what it looks like, ali, and nix it as mad, bad and dangerous to ride on.

Can a steel frame be broken or the material to wear out in time from the rohloff torque or this is only on alloy frames ?


I believe a steel bike would have stood up better to the same mistreatment, probably much better. Steel can take a lot more abuse than even properly-scaled and processed ali can. Normally this wouldn't matter with a properly designed ali bike (most of us would reject an ali bike, if we do, for other reasons, for instance its dead feel) but here we're discussing an ali bike that appears to have been recklessly messed about. The pictured bike might have survived either the Rohloff or the disc brake, but both at the same time was simply a torque load too far.
« Last Edit: June 08, 2019, 12:14:03 AM by Andre Jute »