slack - tight

[rootes]

had the flip the wheel round on my lankster (big bike!) on the free wheel as could not get the chain tension right on the fixed..


at certain points in sprocket / chain combo it goes slack...

almost like the sprocket / chainwheel is out of round...

check they are not so perhaps weird wear in the chain / sprocket?

 

[rustychisel]

chainring not sitting flush and true on the crank spider. It's not uncommon, and sometimes the result of poor manufacturing.

Read sheldonbrown.com on fixies for how to minimise the issue.

 

[MrGrumpy]

just another thing ive only just discovered on langster is the chainline !! 42mm at the back as it should be but 45mm at front ???

 

[Mr Phoebus]

I'll always now pay close attention to centralising the ring on the spider before doing up all the bolts tight.
Critical on a fixed set-up. The first time I wasn't so thorough and the difference
in chain tension per revolution was very surprising.

 

[rootes]

I shall check the run out on chainwheel and sprocket..

 

[GrahamG]

I had better luck once I changed the cheapy chainring I was using - now have a lovely (and bloody expensive!) TA track chainring and it has made a noticeable difference.

 

[samid]

had the flip the wheel round on my lankster (big bike!) on the free wheel as could not get the chain tension right on the fixed..

at certain points in sprocket / chain combo it goes slack...

almost like the sprocket / chainwheel is out of round...

check they are not so perhaps weird wear in the chain / sprocket?

http://www.sheldonbrown.com/fixed.html

Scroll down to the "Chain Tension" part (it's about 1/3rd down the page), and follow Sheldon's advice.

 

[davidad]

It can also help to have the bottom bracket shell refaced and the threads chased, particularly if you have an external bearing type bottom bracket.

 

[ASC1951]

It can also help to have the bottom bracket shell refaced and the threads chased,

I don't see how that can help. Surely the BB alignment doesn't affect chain tension, because it will be constant even if squint? To get a slack/tight chain it has to be a non-round sprocket (common, but usually too little to matter), a non-round chainring (much more common), a non-centred spider or chainring bolts.

I have exactly the same problem on my fixie but haven't found the cause yet. A Miche chainset shouldn't be off-round but I think mine is. Loosening and tightening the stack bolts, as Sheldon suggests, doesn't work but I haven't yet tried moving the chainring onto different holes. The trouble with going for a new chainring or chainset is that you can't really tell if it is properly round and centred until you get it on the bike.

 

[peekay76]

If the chain tension is OK on the freewheel but out on the fixed I would suggest the problem is with the hub or sprocket. I don't see how the chainring could be causing the problem on one and not the other.

 

[davidad]

I don't see how that can help. Surely the BB alignment doesn't affect chain tension, because it will be constant even if squint?

It sorted mine out after hours of faffing about loosening bolts and hitting the chain with an assortment of blunt instruments.

 

[ASC1951]

Well I'll give it a go. It'll be cheaper than a new chainset.

 

[davidad]

Well I'll give it a go. It'll be cheaper than a new chainset.

My LBS charged me £7.50.

 

[rustychisel]

It sorted mine out after hours of faffing about loosening bolts and hitting the chain with an assortment of blunt instruments.

I doubt it. If the BB shell required facing to sort out an error of that magnitude it suggests things were out of whack by several degrees, at least!

Much more likely is that the crank & spider were installed 'skew-whiff' in the first place, and when removed then replaced they went on 'square', and just as likely, on a different facet/position of the BB axle. Sometimes curiously, the respective mating surfaces do matter.

There is another possibility, however remote... the rear wheel axle is bent. Commonly used to happen with screw on cluster hubs, on the drive side. This causes a subtle oscillation around the bent axle and binding of the bearings at one point. The giveaway is knackered bearings and cones, oh and a bent axle when you eventually remove it for servicing.

Is it bent? Roll it on a pane of glass or mirror.!!!

 

[ASC1951]

I'm with you, rustychisel. A bad BB shell facing simply cannot affect chain tension unless it leaves the BB loose, which you would notice straight away. Varying chain tension can only be because the distance between the BB and axle centres varies, or between the chainring and sprocket circumferences. There can be lots of explanations for that, but simple geometry tells me that a squint and rigid BB cannot be one of them.

I hadn't thought of a bent axle. I don't think it's the case with mine, because the wheel runs steady at the rim, but I'll have a look.