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Bent chainring

Old 02-16-21, 07:27 AM
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Spellscape
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Bent chainring

Hi allLarge chainring on this is slightly bent on it of frame in one quarter of rotation.
should I try to straighten it out search for new one.
Not of great importance but this is french threaded.
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Old 02-16-21, 07:42 AM
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Bicycle Research makes a special tool for a bent chainring but I have used an adjustable wrench or a large screwdriver to pry it over straight.
Good luck fining a new one if needed.
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Old 02-16-21, 07:43 AM
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Do you have a solid bench vise? A straightedge? A large adjustable wrench? A keen eye?

Work slowly and measure often. It won't take much to straighten this. You should be able to complete the job in 10 minutes.
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Old 02-16-21, 08:12 AM
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Before you attempt the repair, reinstall the crank. Trying to true a chainring off the bike is as difficult as truing a wheel without a bike or a truing stand.
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Old 02-16-21, 09:37 AM
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Originally Posted by Trakhak View Post
Before you attempt the repair, reinstall the crank. Trying to true a chainring off the bike is as difficult as truing a wheel without a bike or a truing stand.
First off, yes, that's steel, bend it back to straight, no trouble. Even if you have never done it, if you work slowly you quickly will feel how the steel bends and if you over bend well a little you just bend it back (more slowly this time). Sight along the arms, often you can see where the bend is, likely where the arms are the thinnest or the farthest from something else like the arm or a bolt/rivet to the other chainring. If you are sighting along the plane of the teeth check you are not fooled by the teeth themselves being bent to one side or another for part of the ring (fix them first).

As well, yes mount the crank (is the BB adjusted?) - but without the rings, and like truing a wheel put something on the chainstay - a bit of stiff card clamped will do - and check the arms are true; then mount the rings, if possible singly, and do them.
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Old 02-16-21, 10:31 AM
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Errrh ....just a thought. From the foto it looks like the crank is kaput. The hole at the base of the crank where the spindle rests appears to be badly eccentric. It is supposed to be perfectly round and a tight fit on the spindle prior to being secured by a cotter pin. Hopefully it is a optical illusion but it appears to be badly worn and no amount of chain-ring straightening will fix that.
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Old 02-16-21, 02:21 PM
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Originally Posted by Johno59 View Post
Errrh ....just a thought. From the foto it looks like the crank is kaput. The hole at the base of the crank where the spindle rests appears to be badly eccentric. It is supposed to be perfectly round and a tight fit on the spindle prior to being secured by a cotter pin. Hopefully it is a optical illusion but it appears to be badly worn and no amount of chain-ring straightening will fix that.
It doesn't look oval to me, the perspective of the inner wall as the only thick section could be illusory. Futher, there is precious little mechanism for that hole to become oval [1], and even if it were as long as the axis of the ovoid section lined up with that of the axle all it would do is change the crankarm length slightly.

[1] absent violent impact, of which there is no sign.
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Old 02-16-21, 02:35 PM
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It's not oval for sure. It was tight mounted on BB. I noticed that chainring is bent only in upside down position after the wheels removal. In general I've ridden it few minutes and didn't notice anything bad with shifts on front.
BTW it's not rust but some very old grease. I think previous owners used grease as chain lube.
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Old 02-16-21, 07:36 PM
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Stupid question, but being these are basically like what Nervar was putting out forever, have you tried rotating the suspect ring to the next mount? I've found several times where someone took it apart at one point and bent one of the arms fore/back, or someone used the proper tool to adjust the ring, so it was no longer straight unless mounted in the correct orientation. It's a tedious 15 minutes loosening, shifting the rings, tightening, mounting, rotating, but ... meh.

Also, depending on how well-seated and fit the cotter is, the crank can have a pitch making things wobble off-center.

If you find no matter which position the rings are mounted, it still isn't straight, then get it to the best possible orientation to straight, and if you don't have the correct VAR or Park or other ring adjustment tool, use a big adjustable wrench like this except you'll want to slide the wrench until the teeth contact the back of the wrench's throat. If you have not one large enough, But you have a circular saw, you can cut a 1" slit into a 2x4 and use it in the same manner as this tool.

RJ shows the proper tool being used here:

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Old 02-17-21, 08:47 AM
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PS Spellscape : Another trick I've learned over the years. If you want to make sure the right part is being corrected instead of bending wantonly, it's easy to determine whether it's the chainring that's bent, or one of the three crank arms. Like I mention, it's really easy for those three short crank arms to bend, believe it or not. If bent, they'll throw your rings out through 1/3 of your crank rotation which isn't far off from the 1/4 rotation you claim. This trick will also help you isolate where you need to make your correction if it's the rings, you can isolate the high spot, mark it, re-mount the rings, then use a proper tool for the job..

Remove the rings. Take them to a known-flat surface with good lighting. Lay each ring down. Push down on the ring at the 3, 6, 9 and 12 o'clock positions and see if the opposite side lifts up at all. If not, flip the ring over and repeat. If you push down on any given spot and the other side of the ring lifts off the flat surface, you know it's the ring that's bent. If you prove the rings aren't bent, you know it's an issue of the crank arm itself, either its orientation to the spindle, or one of the three arms (it's usually the lone arm pointing up) that's bent. This is so utterly common with this style of crank.

To show an example of checking the rings, here's an example from some minty T.A. 3-arm rings I've got listed in the sales forum. Someone corrected these, probably incorrectly, when a crank arm was probably the culprit. The arrow is where the correction was done. Keep your eyes peeled on the right-hand side of the ring, how much if lifts off my butcher block counter when I push down left of the arrow. Notice before pushing, there was less than 1mm of shadow under the ring to the left of the arrow. Notice how much lift the right-hand side gets out of that <1mm correction.



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Old 02-17-21, 12:17 PM
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This^^^^^, you need to determine what is bent before effecting any corrections.

Especially if the small ring is still running true, then the big ring itself is almost certainly the culprit.

Bending metal is a practiced art, one has to have a feel for how and where the metal tends to flex and at what point it bends/yields in a way that doesn't just spring back.
It gets trickier yet when bending frame tubing, which has a buckling mode looming wherever/whenever the tubing is near the yield point.
So it's always good to approach the force needed for bending very gradually, perhaps ten or more times so as not to over-correct. Minor corrections of excess corrective bending are not problematical, and actually serve to stress-relieve the metal into more-stable dimensions, but gross over-corrections can result in unusable parts or can damage adjacent parts of the structure/assembly.
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Old 02-17-21, 12:28 PM
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Note that the OP's crank is tricky in regard to having the small ring attach to the large ring, instead of to the spider. This complicates assuming that the spider is straight just because the small ring may be running straight.

Note also that a longer wrench, clamped onto the outer hoop/ring of the big ring, will effect more of a pure twisting force, which would better localize the corrective bending to the outer hoop versus the spider. This is what I would do here if the small ring and spider appear to be running true.
This is the opposite of what I usually do while truing chainring runout on cotterless alloy cranks, where I typically use a 3# hammer and wooden dowel drift to impact directly on the chainring bolts or nuts. Alloy chainrings tend to be both strong and springy, and bending them with a wrench can snap the tabs off of a cast crank's spider.
But there are always a lot of different possibilities why a chainring doesn't run true, hence my deferral to francophile's methods above.

Last edited by dddd; 02-17-21 at 12:38 PM.
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Old 02-17-21, 01:22 PM
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I recently went through a similar exercise.....long story short my issue was not the rings themselves (though I took them off and did as above to check them for flatness), and not the spiders, but the crank arm itself had been slightly bent inwards and the pedal end twisted enough that I could feel it when riding. It took a lot of patience to set up a way of hold the centre part of the spider steady while exerting both a bending and twisting force on the other end of the arm to get it realigned. as dddd noted you have to be aware of what the metal wants to do (that said I am not "experienced" like some on this forum).

In the end I was successful but not without a bunch of cursing (mostly because the bolts holding the rings on were not coming off easily and I needed the rings out of the way to effect the repair on the arm).

Also to dddd 's point - I think I'd remove the small ring from the big one before trying anything and check them both for flatness.
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Old 03-08-21, 05:24 AM
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Chainrings was very slightly bent - maybe because of crank
Still not good
Maybe better to search for BB spindle for square and get new cranks?
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Old 03-08-21, 01:14 PM
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Did you check your 3 spider arms are perpendicular to the BB axle line? Assuming (a big assumption mind you) that the hole for the spindle is drilled properly, you could try a carpenter's square to see if the line of the BB axle is 90degrees to the spiders when you lay them flat on a surface. Its a quick check but perhaps worth trying as that "wow" is too much in my mind.
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Old 03-08-21, 03:33 PM
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Spider arms are bent. I think all of them in some way.
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Old 03-08-21, 04:11 PM
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Spellscape Spider arms are the easiest to fix! I mean if you want to go cotterless by all means but you should be able to get your existing crank working well.
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Old 03-08-21, 04:17 PM
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Never done this before. Any tips on what to use as center line for adjusting. It seems that all arms are bent in some way
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Old 03-08-21, 04:39 PM
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It's possible, not likely that the drive side crank bore is not correct. I see the axle spinning correctly but the whole thing seems out of kilter. imo
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Old 03-08-21, 05:34 PM
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Originally Posted by Spellscape View Post
Never done this before. Any tips on what to use as center line for adjusting. It seems that all arms are bent in some way
You are over-thinking this (and perhaps so are some others).

Take the crank off and make sure the spindle is correctly adjusted.

Mount the crank without any rings. Turn it once, measuring how far each arm is from the chainstay. If they are all the same it then mount each ring alone, and measure them the same way.

If arms are not the same bend them until they are - bend them on the bike, you can check your work each time right away.

If the rimgs are not true they are best bent unmounted, with a flat shiny surface to check against - a mirror or a bit of granite countertop will be flat enough, though if you have a surface plate by all means use it.
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Old 03-08-21, 06:14 PM
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Originally Posted by oneclick View Post

Mount the crank without any rings.
Looks like the big ring is swaged to the ds arm?
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Old 03-08-21, 06:52 PM
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Originally Posted by clubman View Post
Looks like the big ring is swaged to the ds arm?
Well, then he's stuck with it; check the arm and ring as one.
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Old 03-08-21, 07:31 PM
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Yeah, I'd take the small ring off, whack the large ring with mallet and hardwood blocks. It it straightens up, put the small ring back on. Otherwise, new crank.
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Old 03-09-21, 08:18 AM
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Originally Posted by oneclick View Post
You are over-thinking this (and perhaps so are some others).

Take the crank off and make sure the spindle is correctly adjusted.

Mount the crank without any rings. Turn it once, measuring how far each arm is from the chainstay. If they are all the same it then mount each ring alone, and measure them the same way.

If arms are not the same bend them until they are - bend them on the bike, you can check your work each time right away.

If the rimgs are not true they are best bent unmounted, with a flat shiny surface to check against - a mirror or a bit of granite countertop will be flat enough, though if you have a surface plate by all means use it.
Assuming the cotter is properly pressed and isn't throwing the arm slightly off-center, this is absolutely the best approach - same general path to checking dropoout hanger alignment. Remove rings, mount and properly press crank cotter in. Measure each of the three spider arms for distance off the seat tube at peak turn. Large adjustible wrench to get them all in sync with each other. Re-mount rings.

Big ring usually isn't swaged on these three-arm cottered cranks from Solida and the Nervarr cranks too.. The three-arm spider typically is swaged, large ring held on by three bolts, small ring held to large ring by three bolts.
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Old 03-09-21, 12:12 PM
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Should I call it done?
It's less than 1mm sometimes on some teeth out of line
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