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Chain tightens and slackens while spinning crank

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Chain tightens and slackens while spinning crank

Old 07-20-18, 10:40 PM
  #1  
drlogik 
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Chain tightens and slackens while spinning crank

The chain on my my Wabi Classic started to "tighten and slacken" while spinning the crank. No adjusting of the rear wheel or moving the chain around to a different place seems to fix it. It didn't used to do this. I only noticed it when I had to change a tire a while back. When I mean tighten and slacken I mean the chain gets really tight then loosens. The crank and chain-ring don't "appear" to be bent nor does the rear cog. The chain doesn't have any tight spots/links either. Any idea what's causing this?
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Old 07-20-18, 10:56 PM
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This is the chainring either not being round or the bolt pattern not being exactly aligned around the BB axle centerline. (Or the bolt holes becoming elongated, perhaps due to riding with loose chainring bolts.) It is also possible that the rear cog isn't exactly concentric with the hub axle, either because the cog was drilled and threaded off center or the hub threads being off center.

Sheldon Brown has a description of a technique to better center the chainring if there is any play with the bolts. I'm guessing that perhaps the bike was set up using this technique and that since then the chainring position has shifted. (Are you doing skid stops? That might be enough to shift the chainring position.)

In my experience, the best solution by far is to spend the money for a quality track crankset. A crankset with the issue you describe is completely not OK on the velodrome. (By contrast, your issue doesn't matter at all on a derailleur bike and there is little incentive for the makers of derailleur cranksets to spend the extra money on precision bolt hole drilling.)

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Old 07-20-18, 11:25 PM
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You can’t possibly have put on a Biopace chainring by any chance? ;-)
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Old 07-21-18, 12:05 AM
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Originally Posted by Aubergine View Post
You can’t possibly have put on a Biopace chainring by any chance? ;-)
In theory, a Biopace ring can be used on a fixie without a tensioner. It might be a fun experiment.

======

As far as thoughts, If you loosen the chain slightly, perhaps the issue wouldn't be as noticeable (which is probably why it showed up when changing tires).

You should be able to tell if the issue is a chainring by putting the bike on the stand and watching if the issue follows the chainring (happens at the same part of the stroke for every crank revolution) or in the rear sprocket (jumps around with each crank revolution, a few times per crank revolution).

Are any of the teeth looking bent, shark toothed, elongated channel, etc. A worn or damaged ring might cause the chain to ride high, and thus be tight.

You could rotate your ring forward one bolt hole to help even out the wear. You probably don't need the pin to protect from dropping the chain to the outside.

Double check your chain isn't getting overly stretched. Precisely Measure 12" of chain (24 half links).
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Old 07-21-18, 11:48 AM
  #5  
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Originally Posted by 79pmooney View Post
Sheldon Brown has a description of a technique to better center the chainring if there is any play with the bolts.
The OP could give this technique a try. I had no luck with it on a bike with the same problem.


Originally Posted by 79pmooney View Post
In my experience, the best solution by far is to spend the money for a quality track crankset.
This. ^^^

It only takes a tiny bit of eccentricity in the crank to make a big difference in chain tension as the drivetrain rotates. If you have a cheapo crank and chainring, it can be very frustrating, and sometimes the only solution is to spend the cash for good stuff. The cranks that work really nice seem to start at around the $150 price point, give or take. At any lower than that, you might get a good one, but it's kind of a crap shoot.
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Old 07-21-18, 12:39 PM
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Every time I put on a new chain and rotate the crank the chain tension is perfectly equal throughout the entire rotation. After the first ride there are loose and looser spots. Very noticeable with my finger, but completely invisible to my legs. I don't care or worry about it. I adjust the slack to what I want at the tightest spot and forget about it. I've experienced the exact same thing on every motorcycle chain I've ever installed. I used the highest quality sprockets and chains that money could buy on my motorcycles and it was always the same thing. First time you gassed it hard the chain was looser in some spots. Personally, I believe the chains are the culprits, not the sprockets. 90% of the single speed bikes I see have the chains adjusted WAY too tight.
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Old 07-21-18, 12:41 PM
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It probably always did it you just notice it now. It's fairly common, better quality components will help but no guarantee that it will ever be perfect. The method I use to fix the issue is adjust chain tension at the tight spot so it doesn't bind and then move on and forget about it. Long as it's not so bad the chain is falling off at the loose spot it won't affect anything,
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Old 07-21-18, 01:14 PM
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this is pretty normal. you might just want to try using a different/better chain on your crank.
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Old 07-21-18, 03:40 PM
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I'm running a KMC710SL chain on a Wabi Classic. It has maybe 500 miles on it and clean with no apparent tight links. Like I said it had a bit of a tight spot when new but now it more pronounced. When I spin the crank without the chain I don't see any wobbling at all. Same for the rear sprocket. I'll try the original chain to see if it does it also. Thanks for the help. Like one post said, I'll adjust to the tight spot and keep on truckkin'.
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Old 07-21-18, 05:35 PM
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Yes, it's quite typical, and as other have noted, due to the axis of the ring not being perfectly coaxial with the axis of the bottom bracket. Also as noted above, high quality track cranks (e.g. Campagnolo or NJS-certified) will minimize the issue, but unless you're riding on a track it's not necessarily a deal breaker. The important caveat is that when you set your chain tension, be sure to check throughout the entire rotation of the crank, so the tightest point in the rotation isn't so tight that it damages your crank or hub bearings.
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Old 07-22-18, 06:10 AM
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If your bike is set up as a singlespeed with a freewheel, then there is no reason to have a tight chain. My SS bikes are adjusted such that the chain has at least 1/4” slack at its tightest point, which means as much as 1” slack at the loosest point, and I’ve never had a chain fall off even on really rough roads. The only time chain slack really matters is on a fixed gear setup, and even then there should always be some noticeable slack in the chain.
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Old 07-22-18, 08:47 AM
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Originally Posted by SquidPuppet View Post
90% of the single speed bikes I see have the chains adjusted WAY too tight.
Originally Posted by TejanoTrackie View Post
The only time chain slack really matters is on a fixed gear setup, and even then there should always be some noticeable slack in the chain.
I was guilty of a too tight chain until hearing this from both of you.

A different sized sprocket went on my bike this morning and I thought of this thread. It took a few minutes to get the chain right and even Dura Ace cranks with KMC S710 chain has noticeable tight and loose spots. I adjusted it with a small amount of slack and won't give it a second thought.


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Last edited by TimothyH; 07-22-18 at 01:41 PM.
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Old 07-22-18, 12:14 PM
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I re-adjusted the chain this morning and will run it. This is driving me nuts though. I gotta find out why it's doing this. Crank seems ok, sprocket ok, chainring ok, chain ok. The only component I haven't check is the crank spindle. If it was bent though, I would think that the crank itself would wobble. I'm also going to string-check my frame to see if it's out of alignment. I'm going to keep digging. Thank you for all your help though!
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Old 07-22-18, 01:40 PM
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You could try centering the chainring.

Centering the Chainring - Sheldon Brown


It didn't do anything for my Dura Ace cranks/chainring but some say that it worked for them.
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Old 07-22-18, 09:07 PM
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I have a crank that's out of true. It's a mystery how it happened, but it was second-hand. To figure it out, I took a caliper and measured from each arm to the seat tube.
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Old 08-01-18, 05:46 PM
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Originally Posted by 79pmooney View Post

In my experience, the best solution by far is to spend the money for a quality track crankset. A crankset with the issue you describe is completely not OK on the velodrome.
This is not my experience. I have a bike with a campy record crank, phil wood bottom bracket, phil wood hub and it is the worst I have in terms of inconsistent chain tension.

My bikes that have a bolt on cog allow for centering of the cog and these bikes have the most consistent chain tension I have ever seen.

My point? You need to be able to center the chainring and cog to get chain tension to be consistent in my experience.
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Old 08-01-18, 06:06 PM
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Originally Posted by mrmb View Post
This is not my experience. I have a bike with a campy record crank, phil wood bottom bracket, phil wood hub and it is the worst I have in terms of inconsistent chain tension.
So you are aware, Campy track chainrings are notorious for going out of round very easily. You're also not saying what you're using for a cog.

Additionally, your hub and bottom bracket have absolutely nothing to do with chain tension.
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Old 08-01-18, 06:10 PM
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Originally Posted by Scrodzilla View Post
So you are aware, Campy track chainrings are notorious for going out of round very easily. You're also not saying what you're using for a cog.

Additionally, your hub and bottom bracket have absolutely nothing to do with chain tension.
Cog = Phil Wood

The campy chainring set was like this from day one and is virtually "uncenterable". Very strong and very nice fit and finish however.
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