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24" Gear Ratio


Fishy

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I'm running 22:16 with 175mm cranks, 137.5% gearing. It works well for me! A harder gear than I was running on my old stock frame. I had to use this 24" set-up because the freewheel was 16t and firmly attached to the rear axle, wheras on my stock it was easy to swap sprockets on the custom cassette I had.

Just to experiment I will be taking the front sprocket down in size slightly to 20:16, 125% gearing which will be closer to my old stock preferred set up of 22:17, 129.4% I can easily swap back if it doesn't have enough Ummph!

Steve

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I run 20-16 with 165 cranks and it feels pretty much perfect in my opinion. I have also use 175 cranks with this ratio and it felt good too, definitely felt a little better for trialsy stuff, but to be honest you can’t feel much difference once you get used to it.

I may be going back to 175 soon so have been thinking of going to 20-15 just to give a gear with a bit more speed, but while still getting plenty of torque but I probably won’t bother.

On a side note how do you work out your gear ratio, when taking into account the wheel size and crank length?

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On a side note how do you work out your gear ratio, when taking into account the wheel size and crank length?

Here's a rough chart that I've made up. The graph was intended for a Stock set-up because of all those posts from the past where mainly stock ratios were being compared. Of course the actual ratio neglects to include the crank length or the contact radius of the tyre.

[attachmentid=6505]

The "Gear ratio" (not including the crank/tyre) is given on the y-axis. The 3 curves are for each of the front sprocket sizes with 170mm cranks, and the ends of the vertical bars are the 175mm (top) and 165mm (bottom) differences to this ratio (+- 3% compared to 170mm)

This chart is useful if you're thinking of changing one of the following aspects of your drive chain on a set bike, for example:

  • Front sprocket
  • Rear sprocket
  • Crank length
For example:

I've got a 22:17 set-up that I like on my stock with 175mm cranks. I'm going to be replacing them with another set of cranks with 170mm arms and 20t front sprocket, Q: What size rear sprocket would be the best match to my previous favourite ratio.

A: From the chart, following the dark blue 22t line up, and taking the upper point on the vertical line gives a ratio of about 1.33. Following the pink 20t line up and considering the centre point of the vertical line (170mm cranks) shows that 15t would be a very close match.

To convert a ratio between wheel sizes, simply multiply by the percentage difference. For example, taking the 26" as a starter, 24" wheels are 8% smaller, 20" wheels 23% less. so if you transplanted your stock drive train to your 24", if you understand how this effects the ratio, use this percentage to recalculate your drive train set up.

Steve

P.S. ^ The above could contain a huge flaw somewhere since I only got 3 hours sleep last night!

P.P.S. If my lunch time was longer then an extra diagram would help make it crystal clear

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