Gear ratios and crank length

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Gear ratios and crank length

Postby mitchy_ » Fri Jan 17, 2014 7:03 pm

i've been messing around with gear ratio's for the past couple of days. recently i've run 44x18 but felt i could increase the gear inches.
i tried 48x18 today, which has helped reduce my cadence (i find i start bobbing on the saddle at 120+) but obviously hurt my climbing. my top speed was higher, but my average speed was slightly lower.

my question is, the 44T ring was on a 172.5mm crank while the 48T was on a 165mm crank (different bcd cranks and rings lying around...) will going for a 48T ring on the 172.5mm crank make much of a difference, or should i go mid way for a 46T ring...?

44x18
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48x18
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by BNA » Fri Jan 17, 2014 7:09 pm

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Re: Gear ratios and crank length

Postby HLC » Fri Jan 17, 2014 7:09 pm

Crank length is a matter of bike fit.

48/18 is the same regardless of crank length. (or whatever other ratio you choose)
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Re: Gear ratios and crank length

Postby mitchy_ » Fri Jan 17, 2014 7:20 pm

yes, but you can apply more leverage/torque to a longer crank.
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Re: Gear ratios and crank length

Postby toolonglegs » Fri Jan 17, 2014 7:24 pm

HLC wrote:Crank length is a matter of bike fit.

48/18 is the same regardless of crank length. (or whatever other ratio you choose)


48/18 is 48/18 no matter what the crank length is... but to maintain the same cadence your legs will have to travel further ( make bigger circles ) with the longer crank length.
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Re: Gear ratios and crank length

Postby Nobody » Fri Jan 17, 2014 9:50 pm

I don't know if this is what you're looking for, but longer cranks reduce the effective gearing while shorter cranks increase effective gearing. So a 165mm cranks has an effective increase in gearing over 172.5mm cranks of 172.5/165 = 1.045 or 4.5%.
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Re: Gear ratios and crank length

Postby rustychisel » Wed Jan 22, 2014 9:29 am

Nobody wrote:I don't know if this is what you're looking for, but longer cranks reduce the effective gearing while shorter cranks increase effective gearing. So a 165mm cranks has an effective increase in gearing over 172.5mm cranks of 172.5/165 = 1.045 or 4.5%.


Yes, theere's a force multiplier, but you're overthinking it given the variable of leg speed (cadence).
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Re: Gear ratios and crank length

Postby Nobody » Wed Jan 22, 2014 9:44 am

rustychisel wrote:Yes, there's a force multiplier, but you're overthinking it given the variable of leg speed (cadence).
Depends if you have a variable leg speed preference. Most people don't appear to. So if a preferred leg speed is fixed, then cadence increases as an inverse of crank length. So for the same bike speed, the average person's preference will be to change down a gear to keep their preferred leg speed or (now higher) cadence with shorter cranks. If that is the case then the shorter cranks have effectively increased the gearing. I have found this in my case.
Last edited by Nobody on Wed Jan 22, 2014 9:46 am, edited 1 time in total.
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Re: Gear ratios and crank length

Postby rustychisel » Wed Jan 22, 2014 9:46 am

Sure, but if you can't maintain and adjust cadence then you can't ride fixed gear. Eh? :wink:
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Re: Gear ratios and crank length

Postby Nobody » Wed Jan 22, 2014 9:49 am

But why would I want to ride fixed? :wink:
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Re: Gear ratios and crank length

Postby HappyHumber » Wed Jan 22, 2014 11:28 am

Keep a 17t on the back for max skid joy. :P
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Re: Gear ratios and crank length

Postby singlespeedscott » Thu Feb 20, 2014 12:00 am

I have found over the years that my gearing has slowly reduced, yet my average speeds have remained the same. Currently my prefered ratio is 44x17.

As for crank length. I prefer 165's as they are easier to spin at high RPM's and they allow you to go deeper into the corner with less worry about pedal strike. They feel strange initially after using the 175's on my geared bikes but I get use to it within a few km's.
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