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Changing gear ratios

Old 08-16-10, 04:55 AM
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cL0h
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Changing gear ratios

Hi all,
I've got a gearing question that I'd like to run by the folks here.
I have a Specialized Globe Comp 1G8 (2007 model) commuter bike. It has a Shimano Nexus 8 speed internal hub and a single 42 tooth chain ring.
I find the top gear is a little too low for daily use and I never get down to the bottom 2 gears either.
Am I right in thinking that if I got a new chainring with more teeth (and a new chain) it would increase the gear ratios? I'd like to move each gear up by one.

Does anyone have any idea what might be a good size chainring to get for a small step up like this?

Will this cause any problems with the internal gearing?

The current chainring is a 104mm BCD 4 bolt style with 42 teeth.

Thanks a million
C.
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Old 08-16-10, 05:08 AM
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Why not check if you can get a smaller sprocket for the rear instead? Usually cheaper than getting a new chainring. And for the number of teeth, why don't you head over to www.sheldonbrown.com and tinker around a little with his online gear calculator?
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Old 08-16-10, 07:30 AM
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Wow I didn't think of that at all. Thanks dabac. That would mean I wouldn't need to replace the chain either.
Also thanks for the great link. I found the gears calculator there at https://www.sheldonbrown.com/gears/internal.html

So that's 2 out of 3 of my questions.
Last one:
Will this cause any problems with the internal gearing? For instance my bike is not going to start missing gears or grinding or clicking if I make this change. Will it?
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Old 08-16-10, 08:53 AM
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Originally Posted by cL0h View Post


Last one:
Will this cause any problems with the internal gearing? For instance my bike is not going to start missing gears or grinding or clicking if I make this change. Will it?
No, the hub generates gears both up and down from a 1:1 ratio, You control the range by changing the external sprockets, which control the input speed. What happens inside and outside are otherwise totally independent, and you're doing the right thing by changing either the sprocket or chainring to adapt it to your own needs.

BTW- when using the gear charts, also look at the Nexus ratio chart here. To move up one value, you'd want a sprocket roughly 14% smaller, though since you don't use the 2 smaller gears might split the difference and go up 1-1/2 steps.
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Last edited by FBinNY; 08-16-10 at 08:59 AM.
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Old 08-17-10, 09:12 AM
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Originally Posted by FBinNY View Post
No, the hub generates gears both up and down from a 1:1 ratio, You control the range by changing the external sprockets, which control the input speed. What happens inside and outside are otherwise totally independent, and you're doing the right thing by changing either the sprocket or chainring to adapt it to your own needs.

BTW- when using the gear charts, also look at the Nexus ratio chart here. To move up one value, you'd want a sprocket roughly 14% smaller, though since you don't use the 2 smaller gears might split the difference and go up 1-1/2 steps.
Thanks a million for that info FBinNY. Now my head is hurting from the math.
Could you explain how you got the 14%
If there are 8 gears then there's 12.5% between each gear

Then if I have 42 teeth on the chainring and 20 teeth on the sprocket. That's a 2.1:1 ratio or a 1: .4672 ratio.

If I reduce that by 12.5% it's a 1: .416 ... so .416 * 42 = 17.5 tooth sprocket
If I reduce that by 25% it's a 1: .357 ... so .357 * 42 = 15 tooth sprocket

Am I way off the mark here?
BTW I can get only get 16, 18 and 19 tooth sprockets for this hub.
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Old 08-17-10, 09:20 AM
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you can test the gear range change here: https://sheldonbrown.com/gears/internal.html .

if you do consider changing the chain-ring, I like the Surly Stainless steel [QBP] replacement chain-ring I got.
long wearing, in a 3/32 width format..
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Old 08-17-10, 09:28 AM
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Originally Posted by cL0h View Post
If there are 8 gears then there's 12.5% between each gear .
The Shimano 8 speed hubs are not a math exercise. They are real mechanical devices, with real gears that have to have an integer number of teeth that have to mesh with other gears that have to fit inside the hub's form factor.

First gear inside the hub ratios the chainwheel/cog by 0.527. Second gear increases this by 1.223. Third gear increases that by 1.16, and fourth gear increases third by 1.139. Fifth gear is 1.175 of fourth gear, and one-to-one inside the hub. Sixth gear engages the same planetary as second gear, so it is 1.223 times fifth gear. You can guess then that seventh gear will be 1.16 of sixth, and eighth gear will be 1.139 of seventh, 1.615 of one-to-one and 3.066 times first gear.

tcs

PS - If you're really not using the bottom two gears, put on a 16T cog (the smallest that will fit on the Shimano 8 IGHs) and shorten the chain. If that still doesn't get the gear range high enough (although I think it will), you'll have to spring for a larger chain wheel.

Last edited by tcs; 08-17-10 at 09:42 AM.
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Old 08-17-10, 09:34 AM
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The difference between my 14% and your 12.5% is probably a matter of which way we're counting. I took the chart ratios of 1.615:1 and 1.419:1 and divided to find that the higher one is 14% higher than the lower one. If you work the other way, the lower one is only 12.5% smaller than the higher one. It's a matter of which is the basis of comparison.

Consider one and two, 1 is 50% smaller than two, but 2 is 100% larger than one. BTW- once you understand this tiny mathematical paradox, you'll never trust government or advertising statistics again.

It doesn't matter which way you measure the steps, as long as you're consistent in which way you measure.
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Old 08-17-10, 09:50 AM
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Originally Posted by tcs View Post
The Shimano 8 speed hubs are not a math exercise. They are real mechanical devices, with real gears that have to have an integer number of teeth that have to mesh with other gears that have to fit inside the hub's form factor.
You're absolutely right that it isn't a linear progression, but one of the problems in discussing gear percentage steps is whether one figures the changes counting upward or downward.

Counting up for the 8s Nexus, the increase averages roughly 17% per step, but counting down from the top the drop averages 14% per step. So, in a sense it is a math exercise after all.
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Old 08-17-10, 09:58 AM
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Originally Posted by FBinNY View Post
BTW- once you understand this tiny mathematical paradox, you'll never trust government or advertising statistics again.

It doesn't matter which way you measure the steps, as long as you're consistent in which way you measure.
Fantastic . OK I think I got it and thanks to the calculator I can see I should choose the 18 teeth as I use the current 3rd gear all the time on take off at junctions and the 16 tooth wouldn't give a close equivalent but the 18 would.
Thanks for your help!

@tcs
I get it now. The gears aren't evenly spaced but mechanical devices do operate on mathemathical principles. That's why I was attempting to do the math. Happily there is a calculator on the Sheldon Brown website as kindly pointed out by fietshub. Otherwise a lot of people would be scratching their heads trying to figure this out.
Nevertheless it's true that nothing beats experience and I thank you for your input.

C.
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Old 08-17-10, 10:08 AM
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My Gain Ratios compared!

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Old 08-17-10, 10:16 AM
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Fwiw, the sturmy archer aw3 hub has one set of planetary gears , always meshed.

High is 4/3. 1.33, Low is 3/4, .75 , and inbetween in 1:1.

a driver and driven swap ..
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