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Same Gear Inches, Different Wheel Sizes. One Feels Spinnier.

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Same Gear Inches, Different Wheel Sizes. One Feels Spinnier.

Old 09-17-21, 04:17 PM
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Rolla
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Same Gear Inches, Different Wheel Sizes. One Feels Spinnier.

I have a 700c singlespeed with 62.5 gear inches.
I also have a 650b singlespeed with 62.3 gear inches.
In Sheldon Brown's Gain Ratios, they are 4.53 and 4.52, respectively.
In meters development, they're 4.66 and 4.36, respectively.

All this tells me that they're so close that they should feel virtually the same, but the 650b bike seems a lot easier to spin up and out.
The only thing I can deduce is that the smaller wheels are lighter, so they accelerate more efficiently, and then I'm just going faster and thus spinning faster.

I'll embark on some trial-and-error, but was wondering if anyone here has a theory. Or maybe it's all in my head?

Last edited by Rolla; 09-17-21 at 04:32 PM.
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Old 09-17-21, 04:22 PM
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Crank length, position of where you sit relative to the BB.

Did you change the wheel size when you did the gear ratio and other calcs?

Weight will go a long way, but total weight will seem more the thing than just slightly lighter wheels. Otherwise we're going to get bogged down in the rotational mass stuff that we love to argue here. <grin>
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Old 09-17-21, 04:30 PM
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Originally Posted by Iride01
Crank length, position of where you sit relative to the BB.

Did you change the wheel size when you did the gear ratio and other calcs?

Weight will go a long way, but total weight will seem more the thing than just slightly lighter wheels. Otherwise we're going to get bogged down in the rotational mass stuff that we love to argue here. <grin>

Thanks for the reply. My wheel size numbers are accurate for all the calculations. Crank arm length only figures into the gain ratio numbers, not the others.

Rotational mass may have something/everything to do with it, but I don't want to go there!
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Old 09-17-21, 04:38 PM
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Originally Posted by Rolla
Thanks for the reply. My wheel size numbers are accurate for all the calculations. Crank arm length only figures into the gain ratio numbers, not the others.

Rotational mass may have something/everything to do with it, but I don't want to go there!
Just because they aren't taken into account in those calculations doesn't mean that crank length won't make it feel different.
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Old 09-17-21, 07:16 PM
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Originally Posted by Iride01
Just because they aren't taken into account in those calculations doesn't mean that crank length won't make it feel different.
Agreed. But 170 vs 172.5 seems pretty minuscule.
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Old 09-18-21, 01:14 AM
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Tire quality? Worn out chain on one? Etc.
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Old 09-18-21, 01:31 AM
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Originally Posted by Rolla
I have a 700c singlespeed with 62.5 gear inches.
I also have a 650b singlespeed with 62.3 gear inches.
In meters development, they're 4.66 and 4.36, respectively.
Well, something is definitely amiss. You're claiming that the first bike has around a third of a percent greater gear inches than the second bike, yet somehow simultaneously has nearly seven percent greater meters of development. This is impossible, because gear inches and meters of development are directly proportional (by a factor of .0254*Pi, or about .0798).

62.5 gear inches is ~4.99 meters of development. 62.3 gear inches is ~4.97 meters of development.

What are the actual gear ratios, and the measured inflated tire diameters?
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Old 09-18-21, 01:46 AM
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Tire pressure ?
tread pattern ?
bike fit ?
frame geometry?
bearings (pedals, bottom bracket wheels) of similar friction ?
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Old 09-18-21, 05:26 AM
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I think the 650 bike feeling spinnier is in line with other people's results. Lots of theories about why that might be
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Old 09-18-21, 09:34 AM
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Originally Posted by HTupolev
62.5 gear inches is ~4.99 meters of development. 62.3 gear inches is ~4.97 meters of development. What are the actual gear ratios, and the measured inflated tire diameters?

Yes, I stand corrected on the meters of development calculation.

Bike 1: 700 X 35. 39 X 17. Diameter: 27.2". 170mm crankarms.
Bike 2: 650 X 47. 40 X 18 . Diameter: 26.7". 172.5mm crankarms.
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Old 09-19-21, 11:09 AM
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If everything else is equal .... Never mind.
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Old 09-19-21, 04:14 PM
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If the smaller wheel is lighter, it will feel "spinnier".
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Old 09-20-21, 11:54 AM
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one factor will be the rotational inertia difference in the wheels
Wheels w larger rotational inertia will take more energy to spin up. Rotational inertia is related to the effective rotating mass and the square of the effective radius.
https://en.wikipedia.org/wiki/Moment...tia#Definition
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Old 09-20-21, 04:27 PM
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Originally Posted by tyrion
If the smaller wheel is lighter, it will feel "spinnier".
Originally Posted by stuff shredman
one factor will be the rotational inertia difference in the wheels Wheels w larger rotational inertia will take more energy to spin up.
Yep, that's what I guessed:
Originally Posted by Rolla
The only thing I can deduce is that the smaller wheels are lighter, so they accelerate more efficiently.
Thanks!
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Old 09-22-21, 12:44 PM
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Different riding positions, wheel base, rotational weights, crank arm lengths are a couple ideas....
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