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Weird problem - chain tightens and loosens while spinning

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Weird problem - chain tightens and loosens while spinning

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Old 01-21-18, 11:57 AM
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drlogik 
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Weird problem - chain tightens and loosens while spinning

I don't have the deep knowledge that a lot of you have with single speed bikes even though I worked in shops for years...never worked on many. I got a weird problem that has me scratching my head.

Bike is a Wabi Classic with the rear wheel drop-out protector washers. When I tighten both axle bolts no matter what adjustments I make when fully tightened the chain will tighten (a lot) and loosen while I am spinning the crank.

I have taken the washers off and it still does it but not quite as much. I have noticed this in the past but it was much less pronounced than it is now. When I start to snug up the sprocket side of the wheel the wheel starts to pull to the drive-train side more than it used to.

I have around 2,000 or fewer miles on the drive-train so I don't think I'm facing worn parts. Starting to think that maybe the axle is bent. The wheel is in true and does not wobble at all when I crank the drive-train.

Got me stumped. Any ideas as to what may be causing this?
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Old 01-21-18, 12:57 PM
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Do the axle nuts have floating washers built into them ? If so, do they move fleely on the nut ?
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Old 01-21-18, 12:57 PM
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This is from the chainring not being exactly centered around the centerline of the bottom bracket. (Most cranksets out there have this issue to some degree. The best velodrome cranksets are very good. Cranksets intended for road use on derailleur bikes less so since with a derailleur with its spring -loaded cage, it does not matter at all.

Cranksets intended for single speed/fix gear should be pretty decent as far as getting all the machining to be accurate dimension-wise, but there still is you-get-what-you -pay-for. Accurate maching costs time and money. Now, you can improve most cranksets by going through a routine to adjust any slop in the chainring bolts and holes to improve the location of the chainrings. Go to Shelden Brown's website. He describes the procedure very well. (I don't have the link but maybe someone here can find it fast.)

The nuts and washers at the hub have little to do with what you are seeing. Best you can do is adjust the chain tension so it never goes tight at its tightest and still has little enough slack that it doesn't fall off. (I know, your bike will now flunk the "perfect, oh-so-tight chain that it seems every Portland hipster has to have. But if you to Portland's velodrome where the really fast guys race, you will not see any tight chains. Not one. Yes, on the road you want to be a touch tighter than on the track because the track does on t have bumps, but that is all the tighter you need.)

So, if you can find that magic spot where the chain stays on and never goes tight, you're there. If not, try Shelden Brown's approach. If that doesn't work, upgrade the crankset. The Sugino 75 is the gold standard but you will pay. You will be able to get any decent Sugino crankset, even quite cheap road, to work using what I shared above.

I have the 75 on my best fix gear and use quality Sugino and Campagnolo rings. Setting the chain tension is a joy. My workhorse fix gear and my retrofitted Peter Mooney use old Sugino GT and AT 110 bcd medium value road cranksets. I have to do as above but they work very well. Just 30 seconds more time to dial in the chain tension. (I've gotten pretty good at it!)

Ben
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Old 01-21-18, 12:59 PM
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Originally Posted by drlogik View Post
When I start to snug up the sprocket side of the wheel the wheel starts to pull to the drive-train side more than it used to.
"Pull" meaning the wheel alignment wants to shift? This sounds like your dropouts are not aligned.

The chain becoming tighter and looser at certain points during rotation is pretty common. Most chain rings and sprockets are not perfectly round. If both are pretty "out" and they hit their worst spots at the same time, you'll experience the changes in slack that you have.


Sheldon Brown has pages on how to address both issues.
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Old 01-22-18, 08:06 PM
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Thanks for the replies, folks. Looks like I may be upgrading my crank.
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Old 01-22-18, 08:15 PM
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Originally Posted by drlogik View Post
Bike is a Wabi Classic with the rear wheel drop-out protector washers. When I tighten both axle bolts no matter what adjustments I make when fully tightened the chain will tighten (a lot) and loosen while I am spinning the crank.

Got me stumped. Any ideas as to what may be causing this?
This is normal to a degree. All sprockets are eccentric, so the chain will tighten/slacken as the high and low zones go past. It's purely a question of degree, with the best equipment running very (but not perfectly) true, and cheaper stuff being off by a mile or so.

Sometimes there's some slop where the rings mount, so you can loosen the chainring bolts until they hold firm but not tight. Then turn the crank until the chain is tightest. That's where the high spot is toward the front of the bike, so use a mallet to give it a rap backward, and hopefully slacken the chain. Repeat the process, working toward smaller corrections until you believe you have it as good as it's going to get, then tighten the bolts, reset the rear wheel position accordingly (so there's minimal slack a the tightest point).

FWIW - the problem is often the result of an excessively tight chain, which forces the chainring out of position.

Note - if this is a recurring issue, you'll want to take everything apart, clean to dry bare metal, and repeat the centering process. When finished, apply some wicking adhesive to help lock the chainring in position.
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Old 01-22-18, 08:22 PM
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Last fall I discovered that my single-speed has a seriously "out of true" crank. I haven't had a chance to diagnose it more closely, and it doesn't look like it's been bent or the square tapered hole damaged. And the spindle runs true. So it's a mystery, but the chainring sloshes from side to side by about 1/8 inch. It's off the bike for now, as I had a spare.
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Old 01-22-18, 08:36 PM
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Originally Posted by Gresp15C View Post
Last fall I discovered that my single-speed has a seriously "out of true" crank. I haven't had a chance to diagnose it more closely, and it doesn't look like it's been bent or the square tapered hole damaged. And the spindle runs true. So it's a mystery, but the chainring sloshes from side to side by about 1/8 inch. It's off the bike for now, as I had a spare.
The problem isn't that rare, and has become more common with the expansion of BBs to all sorts of makers beyond the original few that sold cranks also.

What I often see is a spindle shaving one side of the square tape as the crank is pressed tight. It takes very little to drive the system off center to a degree that's very obvious at the chainring. If you go back and look at the spindles produced by the likes of Shimano and Campagnolo, you'll see a machined lead taper to ensure that there's no sharp edges. However many other brands skip this (IMO) critical step and produce spindles where the taper runs to the end, leaving a sharp corner.

Skis have the tips curved up so they float on the snow. Can you imagine skiing if the flat boittom went all the way to the front.
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Old 01-22-18, 09:47 PM
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Originally Posted by a wise man View Post
This is from the chainring not being exactly centered around the centerline of the bottom bracket. (Most cranksets out there have this issue to some degree. The best velodrome cranksets are very good. Cranksets intended for road use on derailleur bikes less so since with a derailleur with its spring -loaded cage, it does not matter at all.
This is not uncommon at all with lower end chainsets and conversions. In most* cases you will not die.




*all
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Old 01-22-18, 10:46 PM
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Originally Posted by FBinNY View Post
The problem isn't that rare, and has become more common with the expansion of BBs to all sorts of makers beyond the original few that sold cranks also.

What I often see is a spindle shaving one side of the square tape as the crank is pressed tight. It takes very little to drive the system off center to a degree that's very obvious at the chainring. If you go back and look at the spindles produced by the likes of Shimano and Campagnolo, you'll see a machined lead taper to ensure that there's no sharp edges. However many other brands skip this (IMO) critical step and produce spindles where the taper runs to the end, leaving a sharp corner.

Skis have the tips curved up so they float on the snow. Can you imagine skiing if the flat boittom went all the way to the front.
That makes perfect sense. Sadly, the crank is second-hand SunTour, so I can't look at the original spindle.
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Old 01-23-18, 02:32 AM
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Originally Posted by Gresp15C View Post
That makes perfect sense. Sadly, the crank is second-hand SunTour, so I can't look at the original spindle.
Next time the crank is off, take a close look inside the hub, and check for any evidence of shaving. Itll be most evident where the spindle ends, and you'll often see the burr resulting from metal pushed forawrd.
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Old 01-23-18, 02:22 PM
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There are several variables that can cause the tight/loose chain effect. Using better quality parts helps but it's not a guarantee it won't still be there to some degree. Try adjusting the chain tension while it's at the tightest spot and if it runs smooth and chain isn't falling off I wouldn't worry about it too much.
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Old 01-23-18, 03:48 PM
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I have this issue as well, and it's just because of what has been said above- the chainring just isn't perfectly round. It's an Origin 8 crankset and while not dirt cheap, it wasn't high end by any stretch of the measure.
My tightest area of the rotation is when the crank arm is passing 1-2 o'clock. When I am adjusting my chain, I make it so it's just barely not too tight it will bind at that position, then the rest of the rotation the chain has acceptable slack (but not so much it could just bounce off). Has worked great since I built the bike 4 years ago.
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Old 01-23-18, 04:15 PM
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Originally Posted by the sci guy View Post
I have this issue as well, and it's just because of what has been said above- the chainring just isn't perfectly round.....
This isn't intended as a correction, but a clarification to avoid misunderstandings and possible arguments.

While chainrings aren't perfectly round, they're pretty damm good, even cheap ones.

The problem isn't related to any out or goundness, but to how they mount on the crank, and in turn how the crank mounts on the spindle. this is where then error originates, so you end up with a round ring turning off center. Imagine drilling a new hole in one of your records (remember those) and you'll see the point.

Sometimes that eccentricity can be corrected by progressively working it to center. Of course this calls for some slop in the mounting, but the right side is that it's that slop that likely is causing the problem in the first place.
-------------------
I detailed how to do this earlier, but to save hunting for it, here's how it's done. Adjust the chain slack so it's still slightly slack at the tightest point. Loosen the chainring bolts, then retighten them so the ring stays put, but still allows you to move it. Turn the crank to find the tightest position, and hold it there.

When the chain is tightest, the high zone is to the front of the bike, so use a rubber or wooden mallet and drive it back. Repeat this trial and error finding the high spot, and driving it back. Unless you're totally hamfisted, the corrections will get progressively smaller, and you stop when you stop seeing improvement. Now tighten the bolts as tight as possible without breaking them, and reposition the rear wheel to reduce slack if needed.

------------------

For future reference, when buying cranksets, look for those where the ring is machined and supported on shoulders machined into the spider. Those that depend entirely on the bolts for positioning are less likely to stay concentric, and not suited for fixed wheel use. (actually, IMO they're not suited for anything, but I'm cranky about this stuff)
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Old 01-23-18, 08:18 PM
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Originally Posted by FBinNY View Post
For future reference, when buying cranksets, look for those where the ring is machined and supported on shoulders machined into the spider. Those that depend entirely on the bolts for positioning are less likely to stay concentric, and not suited for fixed wheel use. (actually, IMO they're not suited for anything, but I'm cranky about this stuff)
I don't deal with very much new equipment so I'm not familiar with how contemporary high end mates together, but it gives a lot of satisfaction when I work with old high end rings that snap into place on crank spiders and stay in place without bolts. Feels good, man
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