Friday, April 22, 2016

Gear hubs and high torque

So gear hubs are a bit limited compared to derailleur gears when it comes to input torque.  I've been playing with gear hubs and high torque for some years, and two things come to mind:

  • torque can plain break the hub (done that)
  • torque can loosen the axle nuts (Nihola has front-angled dropouts, so the axle can move if its loose)

I've only broken one hub with torque, a SRAM P5 with 38f and 24r sprockets, where I was standing on the pedals while trying to rock a Nihola over something in the forest.  I've used a similar input ratio on two examples each of Shimano Nexus 7 & 8, one SRAM P5 I didn't break, and now one SRAM S7.  Having been using a ratio like that on various hubs in the hills here, I'm starting to feel like I have information to share.  So lets do that.  On the two Nexus 7's, even with the 38x24 input ratio I need to work fairly hard to climb the steepest hill of my commute, and the gears that give 1st 'gear' and contribute to 3rd 'gear' do get a bit nasty after a while, rough and unpleasant, but still functional. (As always, fresh grease seems to help.)  The SRAM S7 hasn't been through long enough abuse yet (and no winter use), but 1st and 2nd do feel a bit rough when pushed.  Its nice that '1st gear' isn't being reused as part of '3rd gear' on S7, like it is on Nexus 7.  S7 by comparison has a lovely smooth 3rd, not to mention 4th and all the high ratios.  So I can say Nexus 7 is fairly tough, but also it gets nasty, the P5 & S7 siblings are lovely but probably not great with torque, and also out of production.

Nexus 8 seems uneffected by a few seasons of hard climbs, though of course the 3rd & 4th 'gears' are sufficiently unpleasant right from day 0, and the 4th-5th shift is always a bit iffy.  The 1st gear on Nexus 8 is really a great gear for high torque, isn't bothered at all, doesn't get unpleasant when worked.  Nexus/Alfine 8 are often used with crank-mounted electric motors, which is a big vote of confidence.  Still I don't like these hubs on the hills because everything sitting between 1st and 5th seems inefficient, and I need to spend a lot of time there on my commute.

My one line verdict: Rohloff is flawless, or an Alfine if you don't need perfection.

I have a theory about gear hubs and torque.  Of course the input torque goes up as you increase the ratio of the rear sprocket to the front sprocket, and torque breaks hubs at some point.  (Would break anything, at some point.)  But the work being performed is actually determined by the radius of the tire & wheel, and the load which is bring pushed up the hill.  So the problem isn't the input ratio, the problem is the tire, the load, and the hill, and the input ratio simply makes it easier for the rider to apply the needed torque to climb the hill, given the tire, and the load.  So if you're already going to climb hill X (maybe you live on top of it) with a 20-tooth sprocket and thats a lot of work, its not more abusive to your bike to make your life less painful by jumping to 22 or 24 teeth.  (Having said that, I do try to be more careful on hubs with high input ratios.)

Loose axle nuts are the real day-to-day irritation of high input torque on a common gear hub.  In particular, it effects bikes that do the standard Danish thing: using front-facing dropouts and no means to hold the axle in place except the nuts.  This can manifest in various ways.  For example, this spring I have switched to using a freshly-greased S7 to commute the 21kms between home and work.  I have some experience about how to install these things, so I cleaned the axle threads, applied some grease, and torqued the nuts good.  After some time I had noticed that the it seemed an internal component of the hub would rotate some distance when I shifted into 5th & 6th, and again when I shifted into 2nd & 3rd.  It didn't manifest in 4th as far as I could tell.  I could get 'the rotation' in 3rd gear, shift right to 5th, and feel it again.  I had initially checked the axle nuts when this started happening, and found them tight, so I supposed the well-used hub was showing its age.  After some time I started getting a noise while climbing in the low gears.  I stopped many times to look for its cause.  I noticed the kickstand passed pretty close to the spokes, but didn't touch when I tested, and the noise only happened in the low gears.  At the same time, I'd been having some trouble with the chain hopping off the sprockets.  Eventually I took 15 minutes and dug into it, and the nuts were loose, but it took some effort to actually move the wheel by hand.  I needed to use a flat screwdriver to pop the anti-rotation washers free of the frame, needed to loosen the brake torque arm, then worked a bit on getting the wheel pulled back tight and torqued good.  That solved the rotation, the noise, and loosing the chain.  This was an especially tricky case of loose nuts, usually I can see the wheel has gone a bit crooked in the frame, but not on this bike.  This bike apparently allowed/forced the wheel to re-center itself every time I got off to look.

Many times I've dealt with this on Niholas as well.  About the only solution at hand is making sure the axle nuts are applied tight, which presupposes that the threads are clean and lightly greased, so that the nuts don't get bound up on the threads themselves.  The grease of course should not get all over friction surfaces between the anti-rotation washers and the frame.  Been a while since I've had to mess with this on a Nihola actually, but its worth paying a bit of attention each time the rear wheel is installed, so its done right, and the axle stays where you intended.  Also I think its worth carrying a 15mm wrench.


  1. Just stumbled upon your blog and this post. Made me think of something else I stumbled upon last week. Don't know if you've already read it or if it helps in your need for knowledge. Anyway:

    1. Wow, I had missed that link (have read some of Sheldon Brown's stuff though). That is a really excellent writeup. I'm a bit tempted to save a copy in case the site goes offline some day.