chain not sitting properly on chainring (brompton)

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chain not sitting properly on chainring (brompton)

Postby tinribs » 6 Apr 2019, 11:53am

Hi there

My Brompton hasn't been riding very smoothly lately, when peddling sounds and feels clunky. I had a new bottom bracket fitted only a year ago, but after looking at the bike this morning I'm wondering if it might be the chain and chainring (see pic)? I had it on the bike stand and when turning the pedals looks like the chain isn't sitting properly on the chainring, leaving a gap.

Any advice? Would this be the likely cause of the rough sound and feel when peddling?

Any help much appreciated
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Cyril Haearn
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Re: chain not sitting properly on chainring (brompton)

Postby Cyril Haearn » 6 Apr 2019, 11:56am

Looks like the chain is worn, 'stretched'
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Re: chain not sitting properly on chainring (brompton)

Postby Gattonero » 6 Apr 2019, 12:09pm

That is called:

You will need new sprocket(s) too, but they're cheap for a Brommie.

Chances are you've worn out the chainring, too, its teeth look a bit too "pointy". Now, that is one-piece with the chainset, and is a discontinued model but may be able to find one somewhere.
It takes a while to wear out that chainring, though! So try a new chain on that old chainring (clean it first!) and make sure the new chain doesn't have much play on it; on average, you should be able to pull one link of the new chain off the old chainring by no more than 2-3mm.
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Re: chain not sitting properly on chainring (brompton)

Postby Brucey » 6 Apr 2019, 1:36pm

yup, you need a new chain, new sprockets, and quite possibly a new chainring too. I'd try the extant chainring with a new chainring and rear sprockets first, it might be OK.

I'd also comment that enough miles to do that is also enough miles to significantly wear

- tyres
- rims
- swingarm bushings

on a brompton so when you deal with worn parts of one kind you should be aware that other parts may need to be changed too.


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Re: chain not sitting properly on chainring (brompton)

Postby vsmith1 » 6 Apr 2019, 8:26pm

I'd add that it might be worthwhile checking the bottom bracket is OK.

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Re: chain not sitting properly on chainring (brompton)

Postby alexnharvey » 7 Apr 2019, 11:23am

You can isolate and test BB roughness from other drive train problems by derailing the chain at the front and spinning the cranks.

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Re: chain not sitting properly on chainring (brompton)

Postby backnotes » 7 Apr 2019, 1:19pm

Has the chain ever been replaced, and have you just noticed this (so it may always have been like that) or did it gradually get worse over time? There were two different versions of the Brompton one-piece chainset. One of these needed a wider chain than the other: 1/8" vs. 3/32". I would expect that using a 3/32" chain on a 1/8" chainset could cause seating problems a bit like the picture. I will go and find out - I have one of each type, nearly new (so unworn) in the garage!

Here we go. The two chainsets look the same to the casual observer. On one, a 3/32" chain sits properly. but on the other, it doesn't bed properly and looks a bit like your picture. I wonder if you are perhaps using a "narrow" chain on a "wide" chainset?


The wide / narrow chainsets look very similar


A narrow chain doesn't seat properly on the wide chainset


A narrow chain seats properly on the narrow chainset

So in addition to the above suggestions, it might be worth trying a 1/8" single speed / BMX chain against your chainset before replacing everything. This may not be the cause, so the sensible advice from others above may be more relevant. If the chain is the wrong width and you have done a lot of miles on it, there may also be some collateral damage to the chainset. I think the wider chainset was the older one, and may pre-date the advent of the 6 speed Bromptons.

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Re: chain not sitting properly on chainring (brompton)

Postby Brucey » 7 Apr 2019, 1:30pm

a 6s Brompton would never have been fitted with a chainset for 1/8" chain and indeed nor would it have worked with such a chainset fitted; 3/32" chain wouldn't run on that chainring and 1/8" chain would be wrong at the back.

Anyway an unworn 3/32" chain cannot 'sit on the tops of the teeth' all the way round the chainring; it would be the wrong length for that. The OP's photo pretty clearly shows that the chain is meshing with the teeth in a vaguely similar way all the way round the chainring, but it is sat up on the teeth by about 5mm or so. The only way it can do this is if it is ~1.5% too long, i.e. it is badly worn.