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Diagnosing mystery bike noises; Chainring photos: new vs old

Tuesday, October 28, 2014 by  
Filed under Daily Blog


My replacement Ultegra crankset arrived yesterday, and I decided to go ahead and install it. I was sort of on the fence about installing the new crankset because, as it turns out, I don’t think the original crankset was quite at the end of its life. Maybe. I’ll get to that in a minute. Before I do, I’ll recap why I ordered the replacement crankset in the first place…

Last week I was chasing down a mysterious noise that started on Saturday, October 18th, which was a hilly 81 mile (130 kilometer) ride. Originally I suspected that noise was drivetrain related, as it was intermittent, and seemed to occur most frequently when I was climbing or putting power into the pedals.

I took the usual steps to discover the source of the noise: I swapped my shoes out, swapped the pedals out, pulled the cranks and checked/cleaned/re-greased the bottom bracket bearings, oiled and lubed all moving parts, inspected the frame for cracks, made sure the chainring bolts were tight, greased the QR skewers and so on. Unfortunately none of those diagnostics bore fruit, and the annoying noise persisted–in fact, it was getting worse.

I knew my chain (I use KMC X10SL) was due to be replaced, and I hoped that would resolve the issue…

It did not. So I replaced the cassette (Ultegra CS-6700 11/28T), which had a little over 6,000 miles on it. I normally would have replaced the cassette with the chain anyway (I replace the cassette every second chain), but I was trying an experiment to see if I could get three chains out of this cassette. The new cassette did not fix the problem.

So I returned to my bike shop, and put my bike in the work stand. I turned my cranks and carefully watched the chain from above. There was a definite wobble: the chain moved inboard relative to the front derailleur cage at a particular point in the rotation. It didn’t quite hit the cage, but I reasoned that under power (which is when I was hearing the noise), perhaps the frame flexing was enough that the chain was rubbing the derailleur cage, creating the noise.

My crankset/chainrings had a little over 12,200 miles (20,000 kilometers) on them, and once I saw that wobble I figured it was time to replace the crankset. So I placed the order (Shimano Ultegra FC-6750, 50/34, 175mm).

I knew the crankset would not arrive before this past Sunday’s 100 mile ride, the Horse Farm Hundred (that ride report is in yesterday’s blog), and I was a little nervous about doing such a long and fast-paced ride with an undiagnosed mechanical issue.

So this past Friday after work I was sitting in my bike shop with a dumb look on my face, staring at my bike and scratching my head. Wiping some drool from my slack chin, I glanced around the garage, when suddenly my spare wheelset caught my eye.

I removed my rear Zipp 404, popped in the spare wheel and did a couple miles around my neighborhood. Silence. Sweet, sweet silence.

I should have performed this step much earlier in the troubleshooting process. In fact, the wheels should have been one of the very first things I checked. It’s a safe bet that you’re much smarter than I am, and would not have made this error; if not, though, I hope you learn from my mistake.

Anyway, at that point I knew the problem was almost definitely the rear hub (spoiler alert: it was). That was an entirely different nightmare, which you can read all about here.

Anyway, I got the hub overhauled in time for Saturday’s club ride, and my bike ran perfectly smooth and was very quiet. My bike also performed flawlessly on Sunday’s Horse Farm Hundred ride.

Which brings us to yesterday. My shiny new crankset arrived, and I considered not installing it just yet. I kept thinking about that wobble, though, and I decided to go ahead and replace the crankset, and would simply keep the old crankset as a backup.

After replacing the crankset the wobble I noticed is now, of course, gone. I think it was probably a good idea to go ahead with the replacement.

That said, I’d love to get some learned opinions from very experienced/professional bike mechanics, so I took a few photographs of the old and the new chainrings. You’ll notice the inner ring is in nearly new condition and shows virtually no wear, as I do 95% of my riding on the big ring. The big ring definitely shows a significant amount of use–and there’s the issue of the wobble I noted earlier–but I’ve not had any issues with skipping or shifting. Thoughts?

Here are the pictures (click to enlarge, you can progress forward and backwards while enlarged with the arrows on the left and right):

Happiness is new bike gear.

Happiness is new bike gear.

I won't be needing the NDS crankarm, as I have a Stages Power Meter.

I won’t be needing the NDS crankarm, as I have a Stages Power Meter.

Old chainrings: photo 1

Old chainrings: photo 1

New chainrings: photo 1

New chainrings: photo 1

Old chainrings: photo 2

Old chainrings: photo 2

New chainrings: photo 2

New chainrings: photo 2

New crankset installed and ready to rock!

New crankset installed and ready to rock!

John Stone Fitness Comments

3 Responses to “Diagnosing mystery bike noises; Chainring photos: new vs old”
  1. A couple thoughts came to mind looking at your chainrings:

    In an earlier post I mentioned that you can periodically rotate chainrings on the “spider” to increase lifespan, since the power stroke isn’t always hitting on the same teeth. I believe your old large chainring bears this out: In the first shot showing the “Shimano” label, the teeth are very worn but uniformly. In the second shot, the teeth are very worn but show a “wave” effect, where the chain has been pressing in the inside of the wave and machining away at the tooth.

    It’s a little hard to do, but you might want to measure from peak-to-peak — or even on a group of teeth — to compare the new ring against the original to see how much wear there has been. (There are also gauges for this.) Regardless, the replacement is well timed.

    What I can’t explain is the wobble, especially if it was only in the large ring but not the small — i.e., the crank “spider” isn’t bent, which is more common with Mt. Bikes due to the crashes. You might have tweaked it a small amount in your crash and it’s been exacerbated over time. If you lay the old ring on a flat surface, can you 1) see the bend, and 2) is it oriented near the crank arms or at ninety degrees to the arm?

    What I’m thinking is that you’re a strong riders with powerful legs who is putting in a shitload of miles. Over time you may have been gradually bending the ring, contributing to the metal fatigue.

    GD Star Rating
    • The bend/wobble is almost exactly 180° out from the crank arm. I really think it happened during one of my crashes (there have been three + dropped bike on driveside :o).

      Good reminder about rotating the chainring on the spider. I’d completely forgotten about that excellent suggestion.

      Another quality post, thanks Neil!

      GD Star Rating
  2. Mate, reply to an old post i know but…
    Any thoughts on two points:
    1) using a 53/39 rather than the compact 50/34. Stalking you on strava indicsted that significant hillage is few and far between.
    2) are you thinking of a new road bike? if so what? if not when would you start thinking of a new one.

    P.S. should I have put this in a Forum post?

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