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What is the maximum input torque that can be sustained by sturmey archers internally geared hubs. The website is surprisingly short on info about this.

I'm specifically interested in the hybrid gearing hubs that allow fitting a 9 speed cassette onto a 3 speed IGH. But I expect the internals of all the 3 speed hubs are the same. http://www.sturmey-archer.com/en/products/detail/cs-rf3-silver

The website mentions a maximum sprocket size of 34 teeth, but without the minimum chainring size that's rather meaningless.

Thanks

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A pro cyclist putting out 1.4 kWatt at 60rpm at the starting sprint has a torque of almost 2000 lb-in or 222 N-m.

A recreational bicyclist may be normally only be capable of just 1/10th that but even an out of shape Clydesdale (heavy but strong rider) just getting off to a start on a hill may momentarily hit that level. If they weighed 200kg but jumped on the pedal and momentarily put 800 newtons of force on a 170mm crank, that’s 136 n-m, or still a lot of torque.

If I was an engineer, I’d want to make sure that my components could handle that level peak on an occasional basis. If you’re really worried, get a Rohloff as they’re really well designed.

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  • I know the gearing limit on the rohloff is designed for the peak torque output of a pro tandem team. I am not that. However the hub of interest is less than 1/10 the price. It's reasonable to assume the build quality is not quite the same. Then again, the lack of a torque spec implies it's not critical. – maxf130 Feb 20 '19 at 20:14
  • The chain might be the first to break! – Carel Feb 21 '19 at 9:11
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    The 200 kg person puts 1962 newtons of force by just standing on the pedal without doing any work. With bicycles where minimum RPM is zero you really need to take into account that power and torque are two separate things. – ojs Feb 21 '19 at 9:16
  • The instantaneous torque will be even higher as power readings are usually an average over one or multiple rotations. Given the none symmetrical distribution of torque over a pedal stroke I wonder how much higher the peak would be. – Rider_X Feb 21 '19 at 19:12
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    @Carel No, chains won't break from transmitted force: Typical bike chains won't break before you put some 8kN on them. I.e. you could hang an 800kg car on it (yes, small cars in the 1980s were that light). To get this kind of force on a bike chain, you would need a 200kg person standing on a pedal with a 20 tooth chainring attached... – cmaster - reinstate monica Jan 25 at 9:57
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I suggest you study what St. Sheldon had to say about Sturmey AW 3 speeds failing in top gear under heavy load.

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  • Welcome to Bicycles SE. We're looking for answers with more detail. Please consider editing, your answer to summarize what "St. Sheldon" had to say. Additionally, some users may not know who that is. A short, one-line answer like this is likely to get downvoted, flagged for moderator intervention, and possibly deleted. – jimchristie Jan 25 at 14:27
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I recall the old Sturmey company used to give a maximum rear cog / minimum front ring ratio limit, possibly a 24T? rear while assuming a 40T front as was common on roadsters in England. I don’t think they disclosed the assumed rider weight or crank length (IIRC a 6-1/2” or 165 mm crank was typical on roadsters). You’ll have to dig deep into Sturmey or Raleigh history to find this.

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