dumb gearing question



rparedes

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Jul 21, 2007
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Assuming same gearing, same wheel circumference on two bikes.... say 50/12 on each, but one has a shorter chain-stay (410mm vs 420mm), the shorter chain-stay bike has a slightly higher gear ratio; correct?

Thanks!
 

alfeng

Well-Known Member
Jul 23, 2005
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No!

The chainstay length only affects the length of the chain + wheelbase's length -- the latter will have some effect on the "comfort" you may-or-may-not experience while riding.
 

alienator

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Jun 10, 2004
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rparedes said:
Assuming same gearing, same wheel circumference on two bikes.... say 50/12 on each, but one has a shorter chain-stay (410mm vs 420mm), the shorter chain-stay bike has a slightly higher gear ratio; correct?
Nope. Gear ratio is just the number of teeth on the chain ring divided by the number of teeth on the cassette cog.
 

daveryanwyoming

Well-Known Member
Oct 3, 2006
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+1, chainstay length has no impact on gear ratios.

Taking it from the pedal to the rear wheel contact patch on the ground the things that impact gearing, force, torque, and tangential velocities at either end of the system are:

- Crank arm length
- Chainring radius (for which number of teeth is a valid proxy given a constant chain pitch)
- Cog radius (again cog teeth is a valid proxy)
- Rear wheel radius including effects of loading the inflated tire with the rider aboard the bike

Those are the effective lever arms in play and from a gearing and leverage standpoint it doesn't matter if those lever arms are connected by one meter of chain or ten. From a drivetrain efficiency and loss in the drivetrain standpoint you'd likely start getting higher frictional losses with extremely long chains, especially once you added roller tensioners to keep them from sagging but it'd be negligible if even measurable for the kind of differences you'd see between a short and long chainstay bike.

-Dave
 

alienator

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Jun 10, 2004
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To add to what Dave said, a bicycle drivetrain in nominal condition (wear, lubrication, setup) has a mechanical efficiency in the high 90's......like on the order of 98%, plus or minus a bit.