Gear ratios Nexus Inter 8 hub

anotherkiwi

Esteemed Pedelecer
Jan 26, 2015
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For my next build I am looking at gear options and thought I might put a Nexus Inter 8 behind the GSM kit. The bike is a 28" wheel I think I will be running 50-622 Big Bens on them. As the bike has vertical drop-outs I will be using the Alfine chain tensioner.

As I have lots of long up-hills which are often followed by long down-hills (doh!) I am considering mounting two chainwheels on a spider mounted to the motor. The most common options are 50/38 and 52/42.

Reading the Shimano dealer manual they recommend a 2.1 gear ratio. With the standard GSM 46 tooth that means mounting a 22 tooth sprocket in back. If I mount my preferred 52/42 option the 42 will work with a 20 tooth sprocket but the ratio with the 52 tooth will be 2.6.

Question: will this damage/ increase wear on the hub after taking into consideration that I will be using the big sprocket on downhill and flat roads (with no wind or tailwind) only? A lot of my riding will be done with the 42 tooth chainwheel which just happens to be in the direct ratio gear at cut off speed in cadence!

I am not allergic to derailleurs I just though I would try something different.
 

flecc

Member
Oct 25, 2006
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30,604
Reading the Shimano dealer manual they recommend a 2.1 gear ratio. With the standard GSM 46 tooth that means mounting a 22 tooth sprocket in back. If I mount my preferred 52/42 option the 42 will work with a 20 tooth sprocket but the ratio with the 52 tooth will be 2.6.
That's not the problem Shimano have, and that won't be harmful in any way.

Shimano's concern is with torque, so it's the low ratios that concern them. If the rear sprocket relative to the front sprocket is too large and the resulting gear ratio too low, that can push the torque loading beyond the hub gear's safe limit.

For example, if you had a triple on the front with the smallest chainring 24 teeth, and a 24 on the rear hub, that 1 to 1 ratio would result in huge torque on climbs, well beyond the hub gear safe limits.
.
 

Artstu

Esteemed Pedelecer
Aug 2, 2009
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Is this going in an assisted bike? you can get away with some pretty high gearing and still get up the hills in 1st with some assistance. I can pedal to 38 mph on mine.
 
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anotherkiwi

Esteemed Pedelecer
Jan 26, 2015
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OK so I could take it down to an 18 or 19, I would like that.

Yes I am still good for pedalling. For the moment :D. The day my strength goes out the window I can keep the bike and change the gear ratios! :cool:

Thanks once again!
 

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