tatfiend 

Chucky;

You got it. I personally would lightly file or stone the IGH cog till the chain fit rather than modifying a standard derailleur cassette cog as the IGH cog is not designed to let the chain come off of the sprocket easily while a cassette cog is.

Also the cassette sprocket is thin enough at the splined area that some sort of spacer will be required I believe to have the cog retaining snap ring on the Shimano hub keep the sprocket from wobbling a bit. I personally would try one of the Surly SS conversion cogs shown at the below link as the cog to modify if you do not do so to the original Shimano cog. It is thicker in the splined area than a normal cassette cog so may not require a spacer.

https://sheldonbrown.com/harris/singl....html#surlycog

dperreno

OK, I must be missing something here. Why in the world would you want to go and file/modify the cog, when you are clearly using the WRONG CHAIN??!! Get the right size chain! Either 1/8" or 3/32" (6-7-8 spd)

chucky

I actually just picked up the surly cog fom the LBS (notice I gave the width a few posts back). However, the thing that concerns me is the splined area is a lot thicker than the stock nexus cogs (~4.5mm vs ~2.5mm) which I don't think will leave room for the snap ring. It's also expensive, so I'd be pissing away a lot of money if I screw it up or if I decide I want/need a different tooth count. So I think I'm going to take it back.

As far as modifying the stock nexus cog there are two issues:
1. It's much easier to file off some tabs than to evenly file a flat surface.
2. I need a new cog anyway because both the 19T and 21T stock nexus cogs result in chain/frame interference.

Since cassette cogs are cheaper than any other choice, I figure they'll be good to experiment with; For example to see if I can squeeze a 14T or even 13T cog onto the Nexus 8 hub or just to find out if the more ubiquitous casette cogs make a good substitute for the 3 spline variety. Besides I found two recommendations for the spline modification on the web...one from a bike manufacturer, so how bad could it be? If I have chain derailment problems then at least I know what's causing it and how to fix it, unlike the sprocket thickness and chainline issues which I'm still unsure of.

Ideally I guess the 13T Bromption derailleur cog would be best, but I'm not sure if the hub can accommodate such a small cog nor am I sure it's narrow enough for the chain (Sheldon brown has a table of sprocket widths and they're almost all 2.0mm or less, even for 6 speed cassettes). Most importantly it costs 3x as much as the cassette cogs and I can't get it locally.

Decisions decisions...

tatfiend 

Chucky;

Sorry I did not notice the Surly cog thickness in the splined area.

I keep forgetting that not everyone has access to a lathe like I do. I would mount the Shimano IGH cog in the chuck and with the cog spinning lay a file or stone on the sides of the teeth area on both sides to narrow the cog in the required area. It would remove the chrome but chain oil should keep it from rusting.

With access to machine tools I have made a number of bike accessories and modified parts on occasion.

Even working the cog against sandpaper on a hard flat surface, with the cupped side down, could narrow it enough to work with the chain. It might take some time though.

With the typical long chain line of a recumbent chain line is less critical for an IGH. Rohloff lists +- 5mm as the accuracy needed for a modern derailleur chain.

thadcombs

I'm with dperreno. Stop wasting time and just go buy the right chain! ha ha

chucky

After some further measurement I'm almost certain the Brompton 13T derailleur sprocket will work, but I think the master links of the chain might rub the dust cap (and they don't make the sprocket in 14T). My chain is also about 3 links short so I'm thinking maybe I should forget about filing and just buy the Brompton sprocket along with an even narrower chain (newer 10 speed chains are 5.9mm whereas the pre2006 campy 10 speed chain I'm using is 6.1mm) in order to guarantee clearance. I hate buying more expensive parts, but I figure it's beter to have some room for error then to be pulling my hair out over microscopic tolerances.

If I use a thicker chain then I have to use a larger tooth count sprocket or else the chain will interfere with the dust caps and shift mechanism of the hub and if I use a larger tooth count sprocket then the chain interferes with the frame. So the only way to avoid chain interference is to use a low tooth count sprocket and a narrow chain.

2fast4u2ketchup

Great post Ultraman6970, way to keep up with the conversation. It's only like the 50th fricken time in this thread the OP has confirmed it's not a fricken 1/8" cog.

dperreno

You're right, sorta. It's a 3/32" cog but he's trying to use an 11/128" chain. Same problem. Chain is too narrow for the cog. Get the right size chain. End of problem. Sheesh!

chucky

Ok so I got the 2.2mm wide brompton sprocket, but it's still not narrow enough for the chain to fit comfortably (11/128" = 2.18mm) and I don't want to file it because that would ruin the case hardening.

Also, those of you who don't understand why I can't use a wider chain need to take a look at this picture:
13T-Alfine.jpg