Hello

I've posted about this issue on another forum and I didn't seem to get anywhere with a solution.

I get an intermittent but lengthy vibration felt through the pedals from what seems to be where the chain is meshing with the chainring. This is felt mainly the drive side but also the NDS too and varies in intensity around the crank cycle. This can get bad enough to prevent you from 'spinning' properly. I also get an accompanying chain grindy noise coming from the same place which changes pitch in time with the vibration intensity.

The FD is clearing fine and the virbration occurs across the entire rear sprocket (10 speed) but only on the middle 39T ring up front. It does not occur when standing up afaik, does not occur when freewheeling and I also cannot replicate it on the stand.

This is a new bike <2 months old with about 400 miles on it. It did it pretty much from new and has been back to the LBS 4 times with no solution. It is a Shimano Ultegra 6703 Triple.

Replaced so far:

Pedals (twice)
Cleats (twice)
BB
Chain (symetrical)

Swapped out:
Rear wheel + casette.

I have taken 4 links out of the chain and that has helped enormously but the issue still persists.

Not rubbing on FD.
RD appears aligned, not bent.
Shifts cleanly without problem.
The casette locker has been checked and was tightened, made a slight difference.
The chainring bolts are tight.
Frame inspected, no cracks.
No stiff links in chain.
Visually, I can't see anything wrong it it when on the stand, all seems well, its not catching or rubbing anywhere and sounds all normal. Always kept clean and oiled too.


I've noticed there is some casette wobble when freewheeling and that it moves a little when pedaling but there is very little to no play in it and no movement in the sprockets.

Any ideas ?

Cheers
Peg

Logic is king when diagnosing a problem, and attempted solutions do not seem to honor that rule. The middle chainring is the only one on which the problem occurs, yet you have not indicated at all that it has been inspected for signs of damage or replaced. It makes no sense to me to proceed to replacing other items until the primary variable has been replaced. The key is that the problem occurs only on the one chainring, so it makes the most sense to change the chainring just to see what happens.

Chain length should have very little effect, as the length of chain under tension remains the same, and the freewheel wobble you mention is also not a problem. There is something called "chordal action" whose symptoms are very similar to what you describe, but typically it occurs only on small cogs in the rear.

+1

I'm with CNY-Bikeman in saying that there is a total failure to use logic in what's been done so far. The common denominator in the issue is the middle ring which is the only place the problem manifests consistently, and where it manifests regardless of all other factors.

Logic would dictate that the first thing replaced would be the ring itself.

But be forewarned that that may not resolve the problem entirely. As mentioned there is chordal action which you may feel because of the way you use the ring (higher pedal load compared to when on the outer ring).

It may also be a question of the ring's surface and tooth cut which will resolve with wear and your choice or lack of chain lube which makes it more sensitive to less than perfectly smooth engagement. I'd wait and give it time to resolve through wear, and/or (consider the source here) try a heavier, more lubricious chain oil.

I agree with FB's suggestion. It was a bit early in the a.m. for me to think of the idea of letting it wear in with a better chain oil. He was too principled to mention that his Chain-L (link in his signature) is an excellent option.

The sound came later, originally it was solely an intermittent vibration through the pedals.

I've inspected all rings and the cranks for cracks, gouges and defects etc. There is nothing to see, everything meshes runs and sounds perfect when turning the cranks by hand. It's only detectable when on the bike and i'm sure you know how difficult it is to determine the location of a problem unless you've experienced it before.

I'll go and order a new middle ring and fit it as soon as possible. I don't really know what i'm doing with crank spiders etc but i'm sure i'll figure it out.

Thanks

Stop throwing money at a problem until you're sure of the causes and remedies. Based on your report, logic points to the middle ring rather than other things, but that doesn't ensure that replacing it will solve anything. Give it a chance to break in, and give yourself time to eliminate any other factors. Chain vibration in and of itself isn't an issue, so you may be pursuing something that doesn't matter, and nay not be 100% solvable.

Of course, it's your bike and your money, but I'd hate for you to waste it based on my blind analysis of the issue.

and how long is break in? 1000 miles? I've looked at and swapped out so much there is little left to do.

Unfortunately, this isn't some buzz or hum that is slightly annoying. Before I removed the links from the chain it was pretty much verging on unridable. It's far too bad to ride another 100 miles on, let alone 650. I've spent ages with it on the stand, there is nothing to see or replicate and i've changed or swapped out everything except the RD, FD and chainrings.

Not a very nice introduction to Ultegra or modern cycling :/

Thanks for you help again.

I can't imagine a problem so severe as to cause enough drag to interfere with pedaling cadence. But I'm working totally blind so can't have a sense of what's happening. Given that the dealer can't solve it either, I see two broad possibilities.

Since it can only be felt on the road, have a knowledgeable person ride the bike until it happens, then work from there. Or, it may be minor, and not be an issue except in your mind (the opposite of the placebo effect), and if you learn to ignore it, or muffle it with a different chain lube, it will go away.

Of course it may be an actual chainring issue where replacement is the only solution, but this is very rare with new high quality rings.

So, you can try your luck with a new ring, but I'd let someone else take a spin and see if they can find the same problem.

Update, for those interested:

Switched everything out with a mechanic, spent a couple of hours on it. Eliminated all components one by one, including the wheel untill the whole drivetrain was changed apart from the BB, which has already been replaced and it still does it. He got it to partly present on the stand in the big ring, said it was bad and seemed to be coming from the rear.

Eliminated shoes and cleats by transfering to old bike and rode that, which was somewhat of a relief tbh.

The only assumption left is that it's some kind of frame defect/misalignment. Being returned to manufacturer.