Cause and diagnosis of wobbling crankset/chainring

Question

I was replacing the chain recently.

When spinning the crank I noticed that the chainrings are slightly wobbling along the axis. I was able to see the wobbling with my eyes.

First check

I kind of verified the wobbling by holding a screwdriver next to the largest chainring's teeth. When spinning the crank it was touching the teeth when the right hand side crank was the bottom.

The wobbling was in the range of 1mm - 3mm.

I removed the crank from the bottom bracket. Then I pulled and spun the bottom bracket axle to verify it's not loose or grinding or wobbling itself. It was OK, no slackness, fluently spinning, no noise.

Note that, though it looked fine, back then I had problems fitting the new bottom bracket due to a damaged bottom bracket thread.

Fortunately I had a spare crank set at hand. I attached the left hand side crank at the right hand side. Now I had two cranks attached, none with chainrings. The idea was to match the cranks for their masses and eliminating any imbalance of the chainring crank compared to the left crank.

Then I gave the crank a hefty kick assuming that, if the bottom bracket were off-axis, the cranks must wobble, which I could feed and maybe see.

Holding the bike firmly perpendicular I felt no major wobbling of the bike. So I assumed the the bottom bracket is on-axis.

Not knowing how to further diagnose the problem I reassembled everything and finished the chain replacement.

Second check

After a couple of kilometers cycling I checked for the wobbling again using the "screwdriver method" (this time with the chain in place) and the wobbling appears to be gone.

Additionally I did another check with another method:

Pointing a laser beam perpendicular onto the chain's teeth "bottom land" orthogonal to the axis. Then I spun the crank. The idea was, that if the chainring was wobbling, the laser beam must "touch" the chainring plane.

This time I was unable to detect any major wobbling.

Other issues

Before dis- and reassembling the crankset I had a clicking sound for months when pedaling.

It only happened when putting force on the right hand side crank and only along the way from the crank arm was parallel to the ground to when the pedal was at the very bottom. (Inspired by one intermediate answer: it might have occurred when the force reached its minimum.) I narrowed it down by putting force on the pedals separately and at different positions (angles).

The clicking sound is gone now. Re-seating the cranks was on my to-do list for isolating the clicking sound anyway.

I also had the problem of a slipping chain covered in another question. Though it might was caused by another issue (mentioned in the other answers), that issue might be related to this one too.

Further: I now realized that a periodic scratching sound of the chain on the front derailleur arm may have had its cause in the wobbling.

What was wrong?

Could I have attached the crankset badly during the previous assembly? Is it likely that a octalink crank may misfit the axle (mounted at a non-zero degree angle for example)?

Is it unlikely that there was a wobbling in the first place that could have been cured by re-assembling? I mean, if there was a wobbling and a clicking that are now both gone, I find it very likely that there was definitively a fault.

I want to understand possible faults and causes I should look out and avoid in the future.

May the misalignment, if there was any, have permanently damaged the crank or the bottom bracket?

Since I had trouble with the bottom bracket thread, might the bottom bracket be misaligned by some millidegrees?

Setup

• Crankset FC-T521 (octalink)
• Bottom bracket BB-ES51 (octalink)

_{Edit: added a little more explanations and clarifications to observations and questions}

Answer 1

From your description something was wrong with setting crankset onto the bottom bracket axle. It could be not tightened enough, there could be some dirt between octalinks, that affected the whole set and caused it not to be laterally true.

If the bottom bracked is tightened properly and you, despite thread issue, are able to tighten crankset, you don't have to worry about.

Answer 2

For me, that clicking sound nails the diagnosis - the only thing I have ever seen that causes that is an issue with your bottom bearing - either it isn't tightened up enough, or the bearings are worn and can slip. As you put maximum torque on a pedal, a loose fit on your bearing allows the crank to force movement in the bearings outside the bearing race. They can then click back in as pressure is released.

This matches up with having a "wobble" in your chain ring.

Tightening things back up can very easily have solved your issue - although be aware of any future problems, as the time you spent with the clicking noise would have induced more wear. I had this problem many years ago, and ignored it (I know...) and by the time I got round to looking at it I had to replace a number of components that were damaged...

Answer 3

Octalink cranks are very easy to mis-align the crank arm on the spindle. By taking it off and reinstalling it, you solved your "wobble" issue. Just take care when doing any more maintenance on your crank set.

Answer 4

I just read your story. The octa link system has a flow and crankset never sits perfectly on bottom bracket. Shimano has realized this and the are phasing octalink out and replace it by one piece crankset. Jus stay away from octalink system as it doesn't matter how many times you try and fix, eventually it would go wrong.