sweeks

There is a theoretical maximum size for the sprocket, but going up 2 or 3 teeth is probably fine. I put an 18-tooth sprocket on my "winter" bike replacing the original 16-tooth.
FWIW, you can't go smaller than 16 teeth (with a 1/8" chain), because of interference with parts of the hub.
EDIT: Sheldon Brown- Sprockets

ixcuix

My sprocket is 20 teeth, and I may go to 22. I have been told that this will make low gear lower. I wart to be able to climb hills more easily.
The high gears are never used by me. Seems like the only time the highest gears would be useful is on a long downhill stretch.

DiegoFrogs

I think the recommendation from Shimano is that the front chainring must have at least 2x the number of teeth as the rear sprocket to prevent over-torque of the hub.

I had a 7 speed nexus for a few years that ran fine with exactly 2:1 gearing. I never had a problem with it.

FiftySix

What's your current front sprocket?

I have the opposite issue as you as I have no hills only head winds, so I never use the lowest 4 gears in my 2013 Norco CityGlide with a Nexus 8 IGH.

The Norco came with a 42 front and 20 rear and 700x35 tires. I changed the tires to 700x38s about a year ago when I bought the bike used, and I changed the rear sprocket to a 21 about two months ago. But, I left the front sprocket as is.

The reason I went up one tooth on the rear was to have my 6th gear on the Norco match my single speed Schwinn which is set for 66.2 gear inches. Previously, 6th gear was 70.2 in the Norco and is now 66.8. Not a big change, but moving all the gears down a bit seems to make me enjoy the ride more with my troublesome left knee.

One thing to note is since I didn't change the chain with the slightly larger tires and the 1 tooth larger rear sprocket, my rear tire lost a lot* of clearance where the rear fender's mounting bolt behind the bottom bracket is. Which adds frustration to moving the tire forward enough to get the chain off the sprocket. I did have to install a bolt with a shorter head to regain some clearance.

*Didn't have much to begin with, to tell the truth.

ixcuix

This Jamis, came with the same setup. 42T front 20 R with 70x35 tires. Do you think if I go to 22 or 23 R, I will have an easier time climbing hills? I do not have that hard a time climbing hills, but I like to watch out for my knees also. I guess is what Diego is saying in regard to the torque/sprocket size is correct, I may have to change my front sprocket.
By the way do you have any idea how many links your chain has? Like i mentioned, I plan to change the F and R sprocket and the chain. Really love these 8 speed hubs.
I also went to a wider tire, and had problems. After going to the wider tire in the rear, I had to remove the fender. Will be going back to narrower tire and replacing the fender.

sweeks

I don't think you'll cause any problems with the hub by going a couple or three teeth bigger on the rear sprocket. No reason to change the front either. You might need a longer chain.
You will certainly have less trouble on hills. That's the reason I went from 16 to 18 teeth on my "winter" bike: studded tires and going through snow need lower gears (for me!).

FiftySix

Per Sheldon Browns gear calculator, if you go from a 20 to a 23 in the rear, all the gears on your bike would move down by about one gear. Meaning your current 1,2,3,4,5,6,7,8 would effectively become 0,1,2,3,4,5,6,7. Looks like 23 would be worth a try if you actually need an extra gear on the bottom, but I don't know much about the 2:1 gear ratio situation brought up by Diego.

I don't know my Norco's chain length without counting, but since your bike's frame is different than mine the odds of our chains being the same length are pretty slim I'd think. For one thing, my frame doesn't have track ends for the rear axle like your bike frame does.

I can tell you that when I replaced the chain last spring (before the rear sprocket change) I installed a nickle plated KMC 3/32"x1/2" single speed chain, which matches the sprockets on my Norco. If I remember correctly, that chain was 110 or 115 links long before I shortened it to fit.

This is the chain I installed. https://store.kmcchain.us/p/z610hx-silver

ixcuix

Quick Link looks good. What size is the chain on these Nexus 8 hub bikes. 3/32 or 1/8?
Thanks T

FiftySix

The hub comes with a 3/32" sprocket, so you'll need to check the width of your front sprocket.

My Norco with Nexus has a 3/32" front sprocket and 3/32" rear, but another non-Nexus bike I have came with a 3/32" rear with 1/8" front from the factory. I have to use a 1/8" chain for that bike.

Wilfred Laurier

I remember seeing a recommendation from Shimano to not go beyond 1.9:1 (or maybe 1.8:1). Other, non-Shimano recommendations I found just now on the internet are 2.1:1 and 1.6:1. Shimano sells 23 tooth cogs and 39 tooth chainrings in the 'Alfine' group so I would expect that ratio (approx. 1.69:1) to work as there are no notes that I can find to not use this combination.

However, I used my Alfine 8 with 32:22 (1.45:1, well on the bad side of any recommendation) with 180mm cranks and ~260 lbs rider weight and it never hiccupped for the years I used it as my primary off roader. Their product info page also says they are designed to handle the torque from an eBike.

YMMV

Wilfred Laurier

I just read some of the 'Dealer' manual and they give recommended gear combinations that top out around 2:1 for the 8 speed, and 1.8:1 for the 11 speed.

ixcuix

I am a bit confused and math challenged. I plan to put a 23 sprocket on rear and 42 Tooth on front. This is with a Nexus 8 speed. Do you think I will do any damage with this setup?
Thanks Thomas

Andrew R Stewart 

How big are you? Do you ride smoothly and spin or pedal squares? One aspect of manufactures' limitations is that they rarely have specifics and instead are often based on worst case. And Shimano is considered to be rather conservative in this Not for wrong reasons as many riders bank on their expectations and not the reality that can often be the case.. Andy

ixcuix

I am 6 2 about 200 lbs. I ride smoothly and evenly. More or less take it easy. I will try this 23/42 setup and see what happens. Should make some noises or whatever if it is straining the hub. Thanks for the feedback.
T

ixcuix

I finally replaced my 20 tooth sprocket with a 23. 42 stayed the same in front. Really like the gear ratios. No sign of any stress.
Can go up steeper hills easier for sure. The changes are actually more subtle than I expected.
T

Winfried

BTW, are there two different "shift cable fixing bolt units", a.k.a. anchor bolts, one for Alfine hubs, and the other for Nexus hubs?

When using the TL-CJ40 ruler on its "101mm" side for use with a Nexus 8 hub, the hole in the drilled screw doesn't align, so I'm having a hard time tightening the cable:



--
Edit: Yes, the angle is different → Get the right bolt for the right hub.

sweeks

The cable-fixing bolts on my Alfine 11 and Nexus 8 hubs look identical.

To be fair, those jigs are to help effect rapid assembly of the shift cables. In a "home mechanic" setting, it's not too hard to do it by trial and error; it just takes a bit more time.