Vibration when pedalling in high gear

[spymaster]

Hi all,
I've been noticing on my old 10-speed that whenever I'm in the highest gear (biggest front, smallest rear) or the next one down and accelerating, there's a vibration feeling from the crank (or maybe the back wheel?). This goes away when I reach constant speed. I also don't tend to notice it when I'm in lower gears.
I don't sense any play in the BB. I've also tried disengaging the chain and just listen to the BB turning the crank. The turning felt smooth and easy. The bearing doesn't produce any grinding noise.
My chainrings aren't perfectly true. They wobble 3mm or so... Nothing that would cause accidental shifting and I'm not getting chain rub on either side of the front derailer.
Has anyone experienced this vibration before? What can be the likely cause? Should I try to get my BB serviced? Or maybe the rear hub? Thanks in advice for your reply

[DaveM]

Hard to diagnose, but here are a few congectures:
1. You're chain is worn so the links don't line up with the spacing in the teeth of your chainrings anymore. Under load on the large chainring you're feeling the chain shift as it settles into the teeth unevenly. Look to see if you chain lines up with the teeth evenly all the way around your large chainring or seems to "ride up" the side of some teeth.
2. The spokes in your back wheel have too low tension. Under high loads and high speed, a sheel with loose spokes will subtly change shape and may be vibrating a little. Alternately, the rear wheel could be a bit off balance and is vibrating at higher speeds, but that probably wouldn't be affected by accelarating or coasting.
3. Pedal bearings
Stuff like this is very hard to diagnose since it only occurs under very specific conditions that you cannot achieve on a stand. It doesn't sound like something that will cause an immediate problem. I would start going through the bike doing general maintanance and see if anything changes. Also, try to really pay attention to the conditions that cause the vibration - is it really only happening in those gears, or do you just push harder/longer in those gears than when you're in a low gear. Is it really related to speed or just to pedalling pressure? The answers might rule out some parts of the bike.
Check the chain though, that's my best guess.

[spymaster]

Thanks Dave. My feeling is it's not gear specific, but as you said, pedalling pressure related. I tend to push harder/longer in those high gears. Another piece of evidence is that I can only feel the vibration when I'm pressing down on the pedals during a pedal cycle. I don't feel the vibration when the cranks are on the top/bottom.
I've just checked the chain. No stretch observed. Don't have a chain measuring tool but it's exactly 12 inches from the center of a link to the center of another one. The chainring and freewheel didn't show any wear either (no sharkfins).
I'll be overhauling the rear hub, LBS or DIY one way or the other. Let see if that improves things.

[mcullum]

I'm seeing these symptoms on my bike too. My gears have no visible wear and the behaviour has not changed with a new chain. My spokes don't seem loose either.
Did you find a solution with further investigation/overhaul?

[spymaster]

Sorry mcullum, I took my bike apart last year for a complete overhaul and still haven't put it back together (happens when you have another bike in good service). So I don't know if the BB/rear hub overhaul will do the trick.
One additional thing I'd also try (if I ever get to it) is to try out all 4 positions on the square taper BB spindle so I get the least amount of chainring wobble. I hope a straight(er) chainring might help the situation too.

[cyclerUK]

I wouldn't expect trying the four positions to make any difference to chainring wobble. The wobble will come from a warn BB, loose chainset or just flexing in the chainset itself. That's assuming the chainset was manufactured true in the first place?

[spymaster]

Hi cyclerUK, I actually read this from Barnett's manual and have tried the method, with success, on another bike of mine to minimize the chainring wobble. A true chainring that wobbles is a result of imperfections on the square taper surface of the spindle and/or the square hole of the crank, so that when the crank slides up the taper, it doesn't go in perfectly perpendicular. Out of the four positions on the square taper when you install the drive side crank, there would be one that gives minimal, at best no, wobble.
This might not apply to high end components however. I was dealing with VP spindle and Sugino cranks.