Gearing ratios

General cycling advice ( NOT technical ! )
tyreon
Posts: 936
Joined: 4 Oct 2012, 4:39pm

Gearing ratios

Postby tyreon » 27 Jul 2013, 5:05pm

Can work out my own gearing ratios using cycling chainwheel/sprocket numbers. But confused about how to arrive at hub ratios/gearing when wanting a number in inches.

Most recent CTC mag details Isla Rowntree's steed. Has 29" tyres,32 chainring with 32 sprocket and Shimano 11 speed Alfine gearing. What's the lowest gear here?...in inches. And what's the rest of the gearing system(in inches)given this set up. How's the computation done?

Brucey
Posts: 35951
Joined: 4 Jan 2012, 6:25pm

Re: Gearing ratios

Postby Brucey » 27 Jul 2013, 5:31pm

chainring/sprocket x wheel diameter = direct drive gear.

the ' 29" ' wheel probably isn't really 29" but if it is then the direct drive gear is 29".

The alfine ratios are;


Gear 1 0.527
Gear 2 0.681
Gear 3 0.770
Gear 4 0.878
Gear 5 0.995
Gear 6 1.134
Gear 7 1.292
Gear 8 1.462
Gear 9 1.667
Gear 10 1.888
Gear 11 2.153

multiply the 'direct drive gear' by each ratio and you have your gears in inches. So bottom gear is 29" x 0.527 which is about 15", and top gear is about 69"

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

tyreon
Posts: 936
Joined: 4 Oct 2012, 4:39pm

Re: Gearing ratios

Postby tyreon » 27 Jul 2013, 5:45pm

Ah! Brucey! I know you from your feeds!! Respect!!!! Thanks very much.

15"! That's one hell of one low gear!

loafer
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Re: Gearing ratios

Postby loafer » 27 Jul 2013, 8:25pm


jackt
Posts: 38
Joined: 30 Jul 2008, 11:13pm

Re: Gearing ratios

Postby jackt » 13 Aug 2013, 1:27pm

According to Shimano, the recommended ratio of chainring to sprocket size for the Alfine 11 is 1.9:1. This has something to do with the torque that the hub can cope with.

It seems that Isla Rowntree's set up with 32T/23 does violate this 'recommendation', at a ratio of 1.39. Following Shimano's recommendation she'd run a 32T/17. I've seen Alfine 11 equipped bikes specced with 32T/18, which is also a greater ratio than the Shimano recommendation (e.g. http://www.genesisbikes.co.uk/bikes/mou ... -adventure ).

For the uses that Isla's bike is designed for, it would need the low gears of 32T/23, particularly with 29er wheels and fat tyres.

Anyone have any experience with running an Alfine 11 with 32T/23 in this way? Or knowledge of torque and hub gears (and Shimano recommendations) that they might be able to share?

NOTE TO MODERATORS: THIS THREAD MIGHT BE BETTER SUITED TO THE TECHNICAL FORUM
Author of Lost Lanes, Lost Lanes Wales and Lost Lanes West. Presents The Bike Show podcast.

Brucey
Posts: 35951
Joined: 4 Jan 2012, 6:25pm

Re: Gearing ratios

Postby Brucey » 13 Aug 2013, 1:42pm

jackt wrote:According to Shimano, the recommended ratio of chainring to sprocket size for the Alfine 11 is 1.9:1. This has something to do with the torque that the hub can cope with.....

....Anyone have any experience with running an Alfine 11 with 32T/23 in this way? Or knowledge of torque and hub gears (and Shimano recommendations) that they might be able to share?....


I did some digging a while back and found several lively debates on this topic on t'interweb. One correspondent shared in full the content of an e-mail from shimano USA in which he was unequivocally informed that there was no hard limit to the primary gearing on an alfine hub, and that 'violating' the recommendations in the Alfine SI techdoc would not affect the warranty. If madison here in the UK have a different view, I've not seen this formally confirmed.

BTW IMHO 99% of IGH troubles come from cable/cassette problems and poor shifting technique. On both Nexus and Alfine hubs it is pretty much guaranteed that the cable will go out of adjustment after a period of use (no matter how carefully you set it up...) and then the gear will slip but only under load (so it will seem fine on the workstand...). Slippage under load will quickly damage the gear.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

jackt
Posts: 38
Joined: 30 Jul 2008, 11:13pm

Re: Gearing ratios

Postby jackt » 13 Aug 2013, 1:56pm

thanks - very helpful
Author of Lost Lanes, Lost Lanes Wales and Lost Lanes West. Presents The Bike Show podcast.

Malaconotus
Posts: 1842
Joined: 30 Jul 2010, 11:31pm
Location: Chapel Allerton, Leeds

Re: Gearing ratios

Postby Malaconotus » 13 Aug 2013, 4:53pm

32:23 29er Alfine 11 illustrated by the best gear calculator out there... http://www.gear-calculator.com/#KB=32&R ... 295&SL=2.5