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So I'm looking for a bike that has the following: Belt Drive, 11 speed Internal hub, Mudguards and can fit a rack at the back.
The Contenders
Canyon Commuter 8
BMC Alpenchallenge AC01
Ghost Bikes SQUARE URBAN X7.8 AL

Canyon and BMC have a 50T Crank in the front and 24T sprocket at the back. Ghost has a 46T in the front and a 24T at the back. All 3 have Shimano Alfine 11 speed.

On my road bike I normally cycle 28-30 km/hr and my cadence is normally between 90-100 (maybe on windy days 110).

I'm just wondering 1. is there a system to measure gear ratio / cadence for belt drive bikes?

Not sure if this is relevant but here's someone asking about changing the front sprocket on a belt drive bike: Gates belt drive: installing larger front sprocket

Thank you in advance for your thoughts.

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    Cadence will be dependent on your speed and the gear you have your bike in, it has nothing to do with the belt drive compared to a chain. The important thing here is to combine the gear ratio computed for the front and rear gear with the gear ratio in the internal hub (of which I have no experience). To figure out your cadence, the best way is just to get an actual cadence sensor. – Jonathon Apr 21 '18 at 6:33
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    Just to clarify - this isn't a shopping question between three bikes, its more general question about drive belt bikes and ratios? There's a good question here but it smells like product recommendation which is Off Topic. – Criggie♦ Apr 21 '18 at 6:35
  • Cadence is odd - I used to think I did about 60 RPM, but on getting a proper sensor found I was doing 80. If you don't have a sensor, count your pedal strokes in a minute using a stopwatch or clock. – Criggie♦ Apr 21 '18 at 7:52
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    Gear ratio for belt drives is just the ratio of teeth on the belt sprockets, same as it is for chains. – Argenti Apparatus Apr 21 '18 at 17:57
  • @Criggie No it's definitely not a shopping question – Alton Apr 25 '18 at 4:12
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A Gates drive belt uses an 11mm pitch between teeth. A conventional bike chain is 1/2 inch or about 12.7mm.

The upshot is that for the same tooth count, a drive belt component will be slightly smaller than the comparable roller-chain unit with the same tooth count.

Good news - since the front chainring and rear cog use the same size, then the ratio is all that matters, and you can use all the normal formulae to figure out cadences.

The Shimano Alfine 11 has these ratios:

Ratio   0.527   0.681   0.770   0.878   0.995   1.134   1.292   1.462   1.667   1.888   2.153
Step        29.2%   13.1%   14.0%   13.3%   14.0%   13.9%   13.2%   14.0%   13.3%   14.0%

So assuming your sprocket is 24 tooth that equates to an 11 speed cassette with these tooth counts:

12.6, 16.3, 18.5, 21.1, 23.9, 27.2, 31.0, 35.0, 40.0, 45.3, 51.7

I couldn't find a nice calculator that deals with non-whole teeth, so I've rounded to these numbers instead. Loss of precision detected.

13, 16, 19, 21, 24, 27, 31, 35, 40, 45, 52

I've also assumed a 28mm tyre with a 700c wheelsize.

Poke these numbers into http://www.bikecalc.com/speed_at_cadence and you'll get this table:

Calculated at http://www.bikecalc.com/speed_at_cadence and copyright tag left in place

If you put the same info into http://www.bikecalc.com/cadence_at_speed then you get this table:

Calculated at http://www.bikecalc.com/cadence_at_speed and copyright info in place

ANSWER: on a 50:24 gear combination, with an 11 speed Shimano Alfine hub and 28-622 tyres on a 700c roadwheel, you will be doing about:

  • 30 km/h in gear 9 with a cadence of 89
  • 30 km/h in gear 8 with a cadence of 99
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  • You can calculate the 46:24 tables for yourself. – Criggie♦ Apr 21 '18 at 7:47
  • Note: I have emailed Sandi from Potato Canyon Software attempting to gain approval for use of these two screenshots. – Criggie♦ Apr 21 '18 at 7:50
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    that's fantastic! Thank you so much for the explanation. Last week i test rode the BMC with my cadence sensor on it. Gear 8 and 9 on the Alfine 11 at cadence 100 and 90 respectively got me to around 30km/h. (as the table points out). – Alton Apr 25 '18 at 4:20
  • I've just received permission to use the above screenshots, from the software authors. – Criggie♦ Jun 28 '18 at 20:03
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A gear ratio is a gear ratio. It doesn’t matter if the gears are connected by a chain, a belt, meshing with one another or anything else. All that matters is the ratio of the number of teeth.

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