Chainring to sprocket ratio, Shimano Nexus 3 speed

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pioneer
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Chainring to sprocket ratio, Shimano Nexus 3 speed

Post by pioneer »

Hi all, anybody know please if the Shimano 3 speed ratio is the same as the Sturmey Archer? I.e. if using a 42 tooth chainring, then max possible sprocket should be 21 tooth?
Also, is the gearing roughly an increase from DD of 33% and decrease of 26%. I don’t find the Shimano docs that easy to figure out. Thanks.
Brucey
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Re: Chainring to sprocket ratio, Shimano Nexus 3 speed

Post by Brucey »

gear ratios of a Nexus 3s hub are slightly wider spaced than an SA 3s

1. 0.733
2. 1.000
3. 1.364

try using this gear calculator

http://www.gear-calculator.com/?GR=SNI3&KB=46&RZ=19&UF=2185&TF=90&SL=3.5&UN=MPH&GR2=SAAW&KB2=46&RZ2=19&UF2=2185

cheers
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pioneer
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Re: Chainring to sprocket ratio, Shimano Nexus 3 speed

Post by pioneer »

Brucey, thanks, got it. Though near enough to the SA ratios as to make little difference. As they used to say in the dockyard, close enough for government work.
But.... it’s the max sprocket size compared to the chainring that I really want to know.
I’ve got a couple of Nexus 7 hubs geared 38/23 and they are both fine. But this Nexus 3 is a new beast to me.
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mjr
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Re: Chainring to sprocket ratio, Shimano Nexus 3 speed

Post by mjr »

https://electricbike-blog.com/2015/11/2 ... h-a-bbs02/ sounds like taking it near the limits, including 32/19.
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pioneer
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Re: Chainring to sprocket ratio, Shimano Nexus 3 speed

Post by pioneer »

Thanks mjr. Interesting, if a little long winded! Think I’ ll stick with 42/21. This is on a Genesis Day One. Dropping to a 26” wheel.
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barrym
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Re: Chainring to sprocket ratio, Shimano Nexus 3 speed

Post by barrym »

pioneer wrote:......This is on a Genesis Day One. Dropping to a 26” wheel.


Sounds interesting. Any pics? Further details on what you've done and for what use would be nice to hear too.

Barry
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Brucey
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Re: Chainring to sprocket ratio, Shimano Nexus 3 speed

Post by Brucey »

I don't think you need to worry too much if your input ratio drops a little below 2:1.

I do wonder how well a day-one frameset will work with 26" wheels installed though; if the same tyre section is used with 559 rims as with 622 rims then your BB height will drop about 30mm. I'd ground the pedals very often if the BB was set that low.

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barrym
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Re: Chainring to sprocket ratio, Shimano Nexus 3 speed

Post by barrym »

Brucey wrote:.....if the same tyre section is used with 559 rims as with 622 rims then your BB height will drop about 30mm. I'd ground the pedals very often if the BB was set that low.

cheers


I was assuming wider, which is why I was asking.
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Cyril Haearn
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Re: Chainring to sprocket ratio, Shimano Nexus 3 speed

Post by Cyril Haearn »

I thought a 2:1 chainring-sprocket ratio should not be used because of chain wear
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elPedro666
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Re: Chainring to sprocket ratio, Shimano Nexus 3 speed

Post by elPedro666 »

You've got me intrigued as to why the wheel swap; looking for bigger air volume perhaps? Be interested to hear how it works out.

Apart from the BB drop, the only change to the geometry I can see would be reduction in trail. I've run 24" wheels in a 26" wheel frame in the past and it handled really sweetly with a short (65mm iirc) stem fitted. Think the forks may have been slightly longer than intended as well though, so that will've skewed things a bit!

Remember that you'll effectively reduce the gearing slightly when you reduce the wheelsize, although I've just looked at my girlfriend's Day-One and there's clearance for a pretty chunky 26" tyre in there so you might not actually reduce the overall diameter all that much...

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