i wanna upgraded my bike to commuter bike, by change my gear to nexus 3s, it's 18T.. so i just confuse which crank i should choose? and how much tooth..

How about a Shimano Alfine to go with the 3 speed in the rear. The number of teeth go from 39-45. If you are strong, get the 45.

It depends on what you want. Some people like to set up 3 speeds to use the middle gear on flat terrain, the low gear for climbing and the high gear for downhills or strong tailwinds.

Other people find 3 speeds more useful with the top gear used for level ground leaving the lower gears for climbing and accelerating.

There isn't any particular reason you need to change the crankset on your bike. You can either leave the extra ring(s) on or get chainring bolts designed for using a single ring on a double. If you have a triple, just remove the smallest ring.

You may wish to use a different size chainring but how many teeth depends on your terrain and whether you want your most used gear to be 2nd or 3rd. I would check Sheldon Brown's gear inch calculator for some help deciding.

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thanks for info guys..
i have ride my nexus inter-3 for a couple of months.. but i missed calculation.. i use old sugino 48T crank..
and yes it's freakin heavy but it make me more strong for pedalling tho', but anyway i rarely to shift to 3rd gear, even i shift to 2nd just for race with someone..
do u think i have to change ma crank ? cuz it's bad for accelerating and the hub is pretty heavy too, plus my bike is old all steel mtb..
what size of crank i should change??

It's not the size of the crank, it is the number of teeth on the chain ring that is going to help you with this problem. You could change the crank to something lighter but Sugino make good cranks. You need to find out the BCD of the ring, which is the bolt circle diameter diameter. Google it and there are ways to measure it or take it to a bike shop and ask. Then find the same size BCD ring with the number of teeth that you think might work. Without making this overly complicated by using a gear chart, my guess is a chain ring between the sizes of 40 and 45 would give you the range you need. I would say closer to 40 than 45. There might be issues of fit but I will let others chime in about that.

It's almost certainly cheaper to put smaller gears on the hub than changing your chainrings or crankset.

Do you have a means of chain tension for 3spd?
My Alfine bike came with truvativ isoflow in square taper. It is perfectly sound and does the job.
Square taper is still the most durable bottom bracket system.
Avoid Power Spline and Octalink.

Actually putting larger cogs on the hub lowers the gear, smaller one raises it .. 3 spline cogs are made from 13-22t.

a 110 BCD crank would start at 34t and go upward, a 130CD road style crank its 38t, 39 if its a Campag 135 bcd style/type .



how much tooth = Math ... did you learn about the ratios calculation math? No? there is apps for that written by people who do.

yeah the problem is my sugino crank ddnt have a bolt like nowadays crank.. it's old skool sugino no.3 crank.. it's look good tho' it look like gundam, but its 48T too heavy for my Internal gear..
if i change the cog of nexus, i cant find it anywhere in my country