Internal gear size

Internal gear size

I am considering buying a Genesis Fortitude Adventure. It has 29 inch wheels, a 32T chainset and 18T sprocket on a Shimano Alfine 11-speed hub gear. What is the lowest gear on this model? About 16 inches?

Sheila Tuckwood

With internal (hub) gears, first you calculate the ‘direct-drive’ gear size in the usual way: Wheel (29) x Chainwheel (32) ÷ Sprocket (18) = 51.6 inches. Then, to calculate the size of any of the gears provided by a gearbox, for example in the hub, you multiply the size of direct-drive by the ratio of whichever gear you’re interested in.

The table here has data for every hub-gear you’re likely to come across. Here you’ll see the bottom gear ratio of a Shimano 11-speed hub is 0.53 x 51.6 = 27 inches. (There’s no point in decimal fractions unless you’ve measured the tyres, which on a ‘29er’ could be anything from 27½ to 31in!) A similar sum with a ratio of 2.15 for top gear gives 111in. So that’s the range: much higher than you thought and rather too high, I think, for an off-road adventure bike!

Assuming you’re not too strong and heavy you’ll probably get away with lowering those gears (without breaking the hub internals) by fitting a larger sprocket. Shimano offer a 20, but that’s just tinkering at the edges, or there’s a 23 if you also change the chain to 1/8in. This hub appears to be the standard fitting, so Sram’s biggest 24 tooth 3/32 sprocket should also fit, perhaps with some different washers to get the chainline correct. That would give a more ‘adventurous’ 20 to 83in range.

Chris Juden​

This was first published in the February / March 2013 edition of Cycle magazine.

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