Shimano nexus 8 sprocket size

[Anchorman]

Hello everyone
I am a new member and have had an ebike for 4 years. When I bought my Kalkhoff Tasman Impulse 8 with Nexus 8 speed hub in 2016 it had a 38T chainring and 21T rear sprocket. I have been reading that Shimano recommend a 2:1 ratio to reduce wear on the hub internals so should I now fit a 19T rear sprocket as the 38T chainring is the only size available. I have done 13,000 miles and have replaced the drive train 3 times and serviced the hub every 2,000 miles so my question is fit 19T or stay with the 21T
Thanks in advance
Dave

 

[Gringo]

Hi and welcome
So you have done 13 thousand miles without a problem on your present ratio and your now worried about hub longevity.
I’d say it’s proven itself to be reliable already so why change.

 

[Anchorman]

Thanks for the reply Gringo, it is only after reading the article in the CTC mag on low gearing this month that I found out about the 2.1 : 1 ratio. It did not say whether the strain on the hub internals would be just on low gear climbs on ebikes or standard bikes

 

[Nosweat]

Might be missing something here but thought Woosh and others warn against going too large on the crank ie 42 is better than 44. By the same argument 21 on the rear sprocket would surely be kinder to your Nexus than 19, though you wouldn't be able to go so fast in top gear.

 

[flecc]

Might be missing something here but thought Woosh and others warn against going too large on the crank ie 42 is better than 44. By the same argument 21 on the rear sprocket would surely be kinder to your Nexus than 19, though you wouldn't be able to go so fast in top gear.

That actually isn't Shimano's worry, it's about how much torque their hub gear can stand. The lower the gearing, the higher the torque, hence the recommendation that the rear sprocket should be no larger than half the front one.

In Dave's case though, the excess isn't large enough to be concerned about, I'd consider 38 - 21 to be ok.
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