How important is chainline?

[speedsixdave]

Hi all,

Further to this post about the bottom bracket of my new old Jack Bell frame, I've been doing some fettling and trying some bits out. The short precis is that this unusual frame has a bottom bracket that I do not really want to change unless I have to. I have dug out a suitable TA crank that seems to fit ok. I'm hoping to use a Sachs Torpedo 3-speed with coaster brake at the back, for reasons too boring to go into here.

My issue is this: even with a dished sprocket and a spacer behind it, the rear sprocket sits about 5mm inboard of the chainring. Is 5mm worth worrying about on a single-cog system? The current sprocket is 1/8th but that could change. Obviously a 3/32 chain would be more flexible and I don't doubt either would run, but should I? I know some people set great store by chainline.

A couple of snaps below:

[Brucey]

not a very reliable method of measuring chainline, that, not on a frame with adjustable length chainstays.

I would lay the straight edge against the chainring or (better) measure the frame for track and then measure centreline offsets for both sprocket and chainring.

BTW yes, 5mm is a country mile.

cheers

[NATURAL ANKLING]

Hi,
But if you cant get the chain line closer, 5mm would be ok with a six speed old fashioned chain like a sedis 3/32" I would of thought, not perfect but perfectly useable?

[alexnharvey]

Can you fit the chain ring on the other side of the little spider?

[robc02]

Hi,
But if you cant get the chain line closer, 5mm would be ok with a six speed old fashioned chain like a sedis 3/32" I would of thought, not perfect but perfectly useable?

+1. I've certainly run 'em like that.... and for long periods! Not ideal but you can get away with it.

[Brucey]

yebbut it is an IGH, not a derailleur. The sprocket won't be designed to accept a chain running into it at an angle, so it will run noisily/badly until the moment when it tries to derail, and then it will try and break stuff. With VDOs the wheel can't pull over in the dropouts, so there is no safety valve, and something (probably the axle or the chain) will just break (*).

Looking at the photos, I'd say it is quite likely the whole back end of the frame is pulled to the right and/or the RH chainstay is set shorter than the left. The sprocket looks nowhere near parallel to the chainring. Hence my suggestion that the frame is checked for track first, eg using a piece of string.

When I was a spotty yoof, I had a basin full of this stuff, finding out the hard way that running fixed gear and/or IGH on 3/32" chain (eg sedisport) was a daft idea, even if the chainline was OK. These days spotty yoofs are still finding out the hard way that most random combinations of chain, sprocket and chainline do not make for reliable singlespeed/IGH setups; the LBS scrap bin is usually full of various 'failed experiments'.

If you are going to run with a poor chainline (angled and/or offset) and/or run with some derailleur parts, I suggest that you use a setup with a spring-loaded tensioner. This way the chain will be fed onto the sprocket in a consistent fashion, and if the thing does try to unship, it won't break anything as the chain comes off. This is basically how shimano alfine hubs are meant to be run if you use more than one chainring to extend the range of the system, and also how Brompton 6s systems are run. Both have offsets of more than 5mm.

cheers

[reohn2]

Can you fit the chain ring on the other side of the little spider?

Going off the photos there isn't room with that chainring,there maybe with a smaller chainring and a smaller sprocket to compensate.

[NATURAL ANKLING]

Hi,
OK well put Brucey, I had overlooked that this set up if less than perfect is with a chain that flops around with not tension NDS of chain!
So a derailleur rear sprocket with a chain tensioner
After checking for bent stuff and chainline.

Forgetting my greenlanning days where an perfectly inline chain no matter how slack and worn stays on all day but an slightly out of line one see wear on side of sprockets and unships often.

[robc02]

Blimey - you should have seen some of the setups I used to routinely use! All parts were carefully selected on the "now what's in this box of tat" principle. Typically it would consist of a 120mm OLN frame (later 126) and a 5 (later 6) speed derailleur hub with its freewheel removed and a fixed sprocket screwed on with whatever spacers could be found (no lockring of course). Usually this meant the edge of the cog came to the end of the thread on the hub, so roughly where second gear on the freewheel had been. The chainset would normally be on a double axle, though this might vary over the years.

I would doubt if I ever ran within 5mm of a perfect chainline, but unfortunately I no longer have any of the bikes to measure. I was aware that chainline was an issue and that mine were visibly misaligned but needs must. I think the only derailment I had was the result of revving hard, downhill, with a slack chain and, I later discovered, a chainring worn to the point that only vestigial teeth remained for much of its circumference!

I would add that am much more particular these days.

[Cyril Haearn]

Like to know more about the strange frame, what purpose does the adjustment serve?

[speedsixdave]

Thanks very much chaps, some serious food for thought there. I will try the chainring on the other side of the spider although it will offend my aesthetic sensibilities!

Brucey - will also check for trueness and track when I get the chance.

I shall report back when further progress is made. However this is a gradual winter project so it might be a while before I get time to revisit this one.

Thanks again for your ever helpful input.

[vandjq]

My Nexus 8 IGH chainline is slightly off even after flipping the rear sprocket, i.e. the front is further out than the rear. As per Brucey's advice I'm going to install a 1/8" chain and Sturmey Archer dished rear sprocket. Will I still need a chain tensioner?

[Brucey]

I would hold fire on that; the most heavily dished SA sprocket


deep dish sprocket HSL827

may well not fit to a Nexus 8 hub, because it is so dished (~4.5mm) that it may well interfere with the cassette joint. I have not tried it but I remember looking at it and thinking that there was hardly any clearance with a standard dished sprocket (about 3mm dish). Both shimano and standard SA sprockets are dished by a similar amount.

One of the other methods of adjusting chainline would be a better idea.

cheers

[Gattonero]

Hi all,

Further to this post about the bottom bracket of my new old Jack Bell frame, I've been doing some fettling and trying some bits out. The short precis is that this unusual frame has a bottom bracket that I do not really want to change unless I have to. I have dug out a suitable TA crank that seems to fit ok. I'm hoping to use a Sachs Torpedo 3-speed with coaster brake at the back, for reasons too boring to go into here.

My issue is this: even with a dished sprocket and a spacer behind it, the rear sprocket sits about 5mm inboard of the chainring. Is 5mm worth worrying about on a single-cog system? The current sprocket is 1/8th but that could change. Obviously a 3/32 chain would be more flexible and I don't doubt either would run, but should I? I know some people set great store by chainline.

A couple of snaps below:

First thing: that looks a road frame to me, so may well be made with light-ish tubing, then do not clamp it in the middle of the top-tube!

Second thing, that chainset is probably going to limit you in the choice of rings and BB's. You can sell it (vintage parts make good money on Ebay from far-east buyers!) and buy a budget single-ring chainset that will also allow you to find a more suitable chainring (they often come with smaller rings like 46 or 48t) and choice of those long-lasting Shimano UN55 bb's.

On a hubgear I won't fuss the chainline to 1mm like you do on a fixed sprocket for a racing/training track bike, but 2 or 3mm are already a bad thing even on a "leisure" bike as you'll get noisy and rough feeling from the chain, let alone increased wear.

[vandjq]

One of the other methods of adjusting chainline would be a better idea.

Brucey - which one would you suggest?