Uneven chain tension: binding!

Nosemaj

Suffering from uneven chain tension. If I go for about 1-2cm of play as is normally advised then the chain tends to bind a bit at the tightest point. Sheldon says this is quite common: chain wheels are not always exactly round. His solution is to loosen the chain-set bolts and to work through a process of tapping the chain and re-tightening the bolts. Have tried this to no avail.

I wondered if there are other potential causes. Bent or incorrectly fitted bottom bracket (unlikely)? Something weird with the rear hub or cog? Frame is custom-made fixie from about six years back so should be true.

Chain ring is good quality TA one so should be exactly concentric right? Or perhaps the fault lies there and the solution is to replace it. (Is it possible to measure how concentric a chain ring is ... are we talking millimetres or fractions of millimetres?)

Anyone had the same problem and found a solution?

Thanks.

nicklouse

bent crank axle?

and you are talking about a Single speed set up?

Nosemaj

Fixed gear set up.

I have bashed the pedal on the curb at speed a few times ... but I was thinking that the crank axle is a fairly tough bit of kit ...

nicklouse

take chain off and spin the cranks and see how the chainwheel moves.

if it seems OK then look at the cog mounting on the rear hub.

strap a zip tie to a chain stay to use as a marker for the chain wheel. Could also work with the drive cog.

meagain

"Chain ring is good quality TA one so should be exactly concentric right?"

Wouldn't make any such assumption! And quite apart from "round", is it "flat"?

And is the cog as good a quality - they ain't always "round".

Nosemaj

Just had a thought. The moron (at a well-known shop on West Hampstead high street) who screwed in the bottom bracket claimed he had a hell of a job. Perhaps he put it in on a cross thread (I remember thinking it looked a little off). This would set it up the cranks at a slight angle right? Would cause binding? Any thoughts? (apart from: I now have the option of facing expense of returning bike to frame builders to re-thread ... or just putting up with the uneven chain tension).

Monty Dog

Cross-threaded BB would certainly cause you a problem - remove chain and spin cranks slowly - and radial movement of the centre bolt shows it's 'off'. Depending on the frame material you may be able to recut or helicoil the BB shell - a decent shop or framebuilder should advise and your LBS should pay!

KeithGlos

Monty Dog wrote:

Cross-threaded BB would certainly cause you a problem - remove chain and spin cranks slowly - and radial movement of the centre bolt shows it's 'off'. Depending on the frame material you may be able to recut or helicoil the BB shell - a decent shop or framebuilder should advise and your LBS should pay!

Unles you are lacking ball bearings the bottom bracket should turn true. Is the chain worn unevenly? you might tell by laying the chain round the chainring and see if you can lift the chain up a bit, this could check a worn section. The crank arms are a likely cause.
Its possible the bracket axle is badly worn one side.

stickman

I was reading about this today, chainrings purposely being made slightly oval to cancel out the two dead spots of the cranks' rotation.

stickman

Have a look at Bradley Wiggins' extremely oval chainring in the current cycling weekly, pages 54-55

!

toshmund

stickman wrote:

I was reading about this today, chainrings purposely being made slightly oval to cancel out the two dead spots of the cranks' rotation.

I still have an old Dawes with the oval chainring, so you applied more power in the downward motion...something along that line. That is Ok with a derailleur, no good for Fixed though. Apologies, if I am stating the obvious to you there.

stickman

It's ok, i'm just discussing stuff that exists.

bigflangesmallsprocket

Despite modern technology, many chainsets are still lacking in perfect roundness, so only a very slight off-centre ring can pull a fair amount of chain in one direction. Also, cheap sprockets, especially the old Cyclo brand, are notorious for not being very round.

I have a Stronglight track chainset and a reasonable quality sprocket, but still get tight/loose spots. I've seen threads about this in the past and am surprised it hasn't been suggested yet.
As long as it's not too loose and you have a reasonable chainline, you shouldn't get any problems. If it's really bad you could swap out your chainring and see if that helps, or it may be worth your while simply forking out for a better quality crankset. You WILL need to check your chain regularly though, as a new chain will bed in and stretch quite quickly initially; and even with well tightened nuts, your wheel can still go off-centre over time, being pulled to one side, and adding more slack.

OOPS, just re-read your post and you reckoned the ring is a good-un, so it could be the crank itself with the boltholes not aligned perfectly. One bit of advice I've seen elsewhere is to only partially tighten the chainring, and then turn the wheel. when you get these binding spots, try tapping the ring inwards, as some boltholes are a tad loosefitting and you may be able to adjust tension this way to suit, and then fully tighten when/if this works.

Tom753

Sheldon says this is quite common: chain wheels are not always
exactly round. His solution is to loosen the chain-set bolts and to work
through a process of tapping the chain and re-tightening the bolts.
Have tried this to no avail.

One bit of advice I've seen elsewhere is to only partially tighten the
chainring, and then turn the wheel. when you get these binding spots, try
tapping the ring inwards, as some boltholes are a tad loosefitting and you
may be able to adjust tension this way to suit, and then fully tighten
when/if this works.

It's about time this advice is laid to rest; it does not work!

At least not with any halfway decent quality chainset, good ones do not have
any slop, the ring fit on the spider exactly. The only way it'll work is to make
the crank or ring holes bigger and file where the crank meets the ring. Even
then it might still not work if the ring is not round.

bigflangesmallsprocket

Well, I've never used that technique myself, but posted it as I thought it might help others, just giving more options.

meagain

"I still have an old Dawes with the oval chainring, so you applied more power in the downward motion...something along that line. That is Ok with a derailleur, no good for Fixed though. "

Are you sure? THINK I've seen fixeds running with Biopace.

Altho' completely equal tension is unusual - whatever the quality of the kit - not often sufficiently unequal to cause a dropped chain. In fact, only time I've ever done so was down to a warped ring, rather than a non-concentric one.

toshmund

meagain wrote:

"I still have an old Dawes with the oval chainring, so you applied more power in the downward motion...something along that line. That is Ok with a derailleur, no good for Fixed though. "

Are you sure? THINK I've seen fixeds running with Biopace.

Altho' completely equal tension is unusual - whatever the quality of the kit - not often sufficiently unequal to cause a dropped chain. In fact, only time I've ever done so was down to a warped ring, rather than a non-concentric one.

Cannot say I am a true expert, but it seems to go against the principles. Biospace - yes, that is the one. It is not noticeable when pedalling. I have had the bike since 1990 and still a cracking bike.

bigflangesmallsprocket

When I was younger I used to drop my chain regularly, and soon learned to skid-stop whilst whilst simultaneously loosening my toeclips, I had to! Mind you, back then I'd never head of the concept of a correct chainline...the wonders of the internet ey.