Chain ring wobble
 

I've been having some issues with chain rub on the front derailleur. I have a 3x8 bike, on the largest chainring I noticed some side to side motion on the chain when trying to set it up after watching the Park Tool video. The Crankset is an unknown model ProWheel 48-38-28T on an unknown model square bottom bracket.

When setting high and low limits to try and get about 1mm of clearance between the chain and the FD cage I noticed that depending on the crank rotation this would change. There is consistent lateral movement of the chain at the same degrees of crank rotation. I can setup the FD so with my largest chainring I can use cogs 8,7,6,5 without noise but at cog 4 there's a slight ding...ding...ding...whenever the drive side pedal is upward.

I can't post images yet but I have 2 showing the lateral chain movement in the largest chainring and smallest cog. I don't know if this would be considered within reason or if something is bent. I'm a heavier at 95 kg and am relatively new to cycling and have generally been at a lower cadence and higher torque and maybe cheaper cranks can't hold up to that over time?

It's pretty common for low cost stamped and riveted chain sets to have more lateral run out then more expensive cranks that are machine turned and cut. But even the nice stuff can Suffer from poor manufacturing or incidents that bend the rings (jam shifting being one). Rings can be aligned to a degree with localized bending efforts. 6" adjustable wrenches placed on the ring outer portion (that is the offending area) and/or screwdriver placed behind the ring "bolts" and wedged against the RH BB face are two methods to bend rings/cranks straighter. Do know that with much leverage much damage can happen:) Andy

This is fairly common. Simply put the chainring is not in plane with the axis of rotation.

If it's the crank all 3 rings will wobble together. OTOH, it's likely the one ring is slightly warped, sort of like a potato chip, but not as extreme.

I don't know your skill level, nor how bad the issue is, so it's your call whether to live with it or try to straighten it.

Ifvyou opt to try to improve it, search "how to straighten a chainring" for more info. Be advised that your goal shouldn't be perfection, just good b enough to be OK.

With a square taper, you can check if the spindle is bent by removing the right crank and installing 180 degrees, (aligned with the left arm) and checking if this helps/hurts the wobble. Try to have the crank arm torqued down onto the spindle similarly.

Based on your results, you can logically guess if the spindle is bent. Rub at pedal upwards again, spindle is OK, rings may be the problem.

You can also try the other 2 alignments and see if any helps. If something works better, then move the left arm.

A mallet on the chainring bolts is also effective.

the one thing we can know with near certainty is that the spindle is not bent. That's about as near impossible as jumping over the moon.

It is possible that the crank isn't concentric on the spindle, which is why I suggested seeing if all 3 rings wobbled together as a unit.

The key for the OP is finding if all 3vrings wobble together, meaning it's a crank or spider problem, or if only one wobbles, in which case it's the ring itself.

The proper diagnosis will determine the correct cure.

The correct cure with a cheap crank is going to be bending or hitting it or replacing it.

I always worry about the poor bearings involved when I contemplate beating on some part of a bike.

If the bottom bracket is cup and cone, or cartridge bearing, there may be slack in the bearings, that manifests as chainring wobble under load. Try wiggling the crank arms laterally. With cup and cone, you can adjust this out. With cartridge, no, that's my one issue with square taper cartridges. There may also be spalling (pitting) at one point on the inner (spindle) or outer (cup) race on either of the above. Try wiggling the crank arms at every point of rotation, at every "half-hour" mark, so 24 different positions; The spalling may be in a very small zone. This is why, with square taper, I would move the crank arm position on the taper 90 degrees every 6 months, this will spread out and minimize wear on the spindle, though not the cups.

My current hollow spindle setup with adjustable bearing preload, makes taking out bearing slack easy. I'd try to get spacers to index the external bearings periodically, but they are so inexpensive, I'll just replace when needed. But so far, 2+ years and a lot of riding, still great.

Assuming no bearing slack or adjusted out or corrected, now check chainrings. Before upgrading, I had a cheap swaged steel chainring that wobbled, I was able to carefully correct with a large, deep jaw adjustable wrench. But that was a single chainring. Multiple rings, look to see if they are both out in the same direction, and if so, the crank spider may be off, and that's a lot more difficult to correct. You may be able to use very thin washers at the chainring bolts to correct, but note it would be opposite for inside and outside rings on the same flanges, assuming equal spider flange thickness.

+1. I fixed a kid relative's bike when he was little, instead of using the kickstand, he would just drop the bike sideways, pedal onto concrete (determined after failure analysis). Fractured the stamped cup on the cheap bottom bracket on a one piece ashtabula crank. I replaced, greased, adjusted, and instructed him on why it broke and to use the kickstand.

If beating on a chainring bolts is going to cause the BB bearings to fail, riding the bike is going to kill them even faster..

It isn't a concern.

A big-assed screwdriver makes a good prybar to straighten chainrings.

Here are the OP's pictures....

https://cimg3.ibsrv.net/gimg/bikefor...b648efee7e.jpg
https://cimg6.ibsrv.net/gimg/bikefor...b9fafde077.jpg
https://cimg7.ibsrv.net/gimg/bikefor...ef16102242.jpg

https://cimg2.ibsrv.net/gimg/bikefor...f59944d1ee.jpg

I may have gotten the same pic twice. They are all here though and you can get the larger version also..... https://www.bikeforums.net/g/album/34621645

Don't forget to check for damage and position on that Front Derailleur...

OP states on large chainring of 3X. BB issues would manifest on all of them but wouldn't hurt to check BB

I checked this morning and the side to side movement is present on the middle chainring as well at the same spot. I pulled off the cranks and rotated them 180 and the same movement occurs at the same spots.

I went out for a ride on a quiet road and realized the noise was present on a cog that is nowhere near rubbing when I stopped to check. At the spot where the chainrings move inward I get a ticking noise (Almost like a ratchet wrench). I think it may be the chainring teeth rubbing on the inside edge of the chain at this spot, The teeth weren't center in the chain and leaned to the left in this section when I checked. When riding uphill I realized the noise was a lot more prevalent, I can hear it better at a slower speed with less wind and tyre noise. It's also my understanding that a chain is under more constant tension uphill?

Could I be hearing the chain and chainring rubbing together under tension where the rings bend inward? That would explain why I can't hear it off the bike and pedaling by hand. The chain seems to be a KMC and has Zs and 8.3 written on it. It probably has about 2000 KM on it and the teeth on the chainrings do look a little worn.

I think I'd rather just replace the cheap cranks rather than fighting to bend them just right. My bottom bracket is a sealed square taper. The only writing I could find on it was TB702-2N L:120 C:28. There doesn't seem to be any sort of play in the spindles.

Specifically a rubber mallet, a hide or wooden one could cause more damage than it fixed.

I'd take that as a 120mm length. Different cranksets require different BB spindle lengths. You can either find a crank suitable to your needs that uses a 120mm (or a mm +/- either way usually) or buy a new Shimano UN300 for <$15 of the length required for the crank you choose.

Wait, what?!...

Concur. Wouldn't that just change by 180 where the wobble is?

Possibly yes, if the problem is with the crankarm or chainring. If there is a change with the 180* orientation, it may point to a bent spindle.

This is just a way to diagnose and try to see what could be the issue.

And on bike you don't want to spend any $ on, one of the 4 orientations may work OK. Sometimes 2 wrongs can equal a right. :)

I'm looking at a Shimano FC-M361 cause of the removeable chainrings. It uses a 122.5mm spindle. Is there anything else to consider other than the shell width?

Chainline, which is the distance from bike centerline (seat tube centerline) to the center of the chainrings; If 3 rings, then to the middle ring. If 2 rings, to the midpoint between the 2 rings. The goal is for the chainline spec to line up with the middle of the stack of cogs on the cassette, this will give best overall shifting.

My 2X crank with 68mm BB shell has 43.5mm as chainline (has integral hollow spindle, no square tapers, so no length choice there), which works perfect with my 130mm dropouts and 7 speed cassette (road crank, road dropout spacing). I originally spaced the crank out 4mm due to an interference issue between the front derailleur lip and my very fat seat tube, and it shifted terrible, dropped the chain a lot. Remedied the derailleur issue and took out the spacers, shifted perfect, the way the bike gods intended.

If your new crank says in specs, same BB shell width, and same dropout width or says "road" or "mountain", should have the proper chainline with the BB spindle length it specifies.

Also, Q-factor is a choice, how far apart the pedals are; Usually a "road" crank is closer/lower q-factor, "mountain" crank is usually farther/higher q-factor, crank arms a bit more splayed outboard, better to clear fatter chainstay tubes.

If the wobble moves with the chainset then the chainset is bent, but if it remains with the axle then the axle is bent. Its also possible that both are bent, in which case one of the four possible mounting positions may minimise the movement.