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Chordal action/purring vibration from drivetrain

Posted: Mon Sep 23, 2019 8:42 am
by MaxPower
Hey
Long time reader, first time poster... so, hello everyone.

(did a search, without any finding)

I have a purring vibration on my bike, which is noticeable by ear and feel on the road (on smooth tarmac), but becomes almost apocalyptical on the trainer (hardwood floors don´t help). Vibration happens independent of component life.
Bike is a Bianchi Oltre XR4. Gearing is Shimano R8000 all out, 34/50 front, 11-30 at the back.
Tried with shimano HG-701 chain and KMC X11EL. Lube is Squirt.
I noticed the vibration from the beginning I got the bike (self build, 6000-8000km on it now/1 year of using, independent of chain/cassette/chainring wear).
Happens in 50 front and 13t, and 12t at the back, rpm around 85-86 and power around 300-310 watts. Slight changes in force and/or cadence or gearing and it disappears.
The vibration can actually be re-created on the work stand in the 13t and 12t (50t front) if I hit the cadence and force (by hand) just right. It seems that the top of the chain starts to oscillate. Extra torque on the pulley arm does nothing.
Only thing that mitigates the vibration is to create a “dampening” (lifting it slightly with one finger while pedaling) between the top of the cogs and the chainring.
Everything else is checked and is okay (freehub, cassette wear or damage, chain wear or damage, all bearings in the wheel/freehub/crankbox).

I´m thinking chordal action is to blame.

Which brings me to my question (sorry for the long interlude): Has anyone had luck with mitigation chordal action by changing chainring size (from 50 to 52) or the makes of the chainring (different manufacturer)?

Thanks everyone

Re: Chordal action/purring vibration from drivetrain

Posted: Mon Sep 23, 2019 1:48 pm
by alcatraz
Some bearings spin ok when there isn't a load but grind when there is. A trick is to put your ear to the frame while spinning the bearings (without a chain).

I'm guessing rear wheelhub. It has 3-5 bearings.

Simplest way to exclude it is to just borrow a wheel for a few minutes.

Also check your rear wheel and crank for side play.

Check if the chain is rubbing on the next inner cog while in these noisy gears. There might be a slight indexing issue that becomes apparent under load only.

Another issue is worn cogs. It doesnt take much effort to wear small cogs. The problem then comes when you put a new chain on. I've seen such chain oscillations on worn drivetrains. You can observe some of it by holding the rear wheel with one hand and pedaling with the other. As you hold against the rotation and look at the chain engaging/disengaging you can see some odd movement, usually at the first or last engaging teeth.

Re: Chordal action/purring vibration from drivetrain

Posted: Mon Sep 23, 2019 6:48 pm
by MaxPower
Thx for the reply.

Bearings in the crankbox are smooth when rotating at speed without the chain. No side play. Correct pre-load too.
Tried neq chain+cassette - its the same (always has been like this, even through crank bearing change, chain+cassette change, wheel change, on the direct mount trainer or with a wheel)
Checked the indexing - no touching. Small adjustments don't make a difference.

The only thing i haven't tried is a new big chainring - and i don't have one at hand (henceforth my initial question).

Re: Chordal action/purring vibration from drivetrain

Posted: Tue Sep 24, 2019 4:12 am
by kman
Substitution is the easiest step to begin with. Borrow parts that are easy to swap and test like wheels, in particular.

Rear derailleur hanger alignment OK?

Re: Chordal action/purring vibration from drivetrain

Posted: Tue Sep 24, 2019 5:44 am
by MaxPower
Thanks for thw reply.

Yes, aligned with alignment tool. Actually i have "tried" another hanger too, no difference.
(wanted an extra hanger in reserve, so swapped the original with the extra one and aligned that one too)

Sadly i cant borrow any r8000 chainrings.

But i was entertaining the thought of returning to elliptical chainrings anyway - so i might just bite the bullet and do it

Re: Chordal action/purring vibration from drivetrain

Posted: Tue Sep 24, 2019 1:50 pm
by F45
Maybe swap wheels or rotate the tire 180 degrees or add small weights to the wheel. If the wheel has an imbalance it might be contributing to the resonance.

Re: Chordal action/purring vibration from drivetrain

Posted: Tue Sep 24, 2019 7:03 pm
by MaxPower
thanks for the reply. same happens on the direct mount trainer (withour wheel), same gears, same RPM/wattage

Re: Chordal action/purring vibration from drivetrain

Posted: Tue Sep 24, 2019 9:18 pm
by NiFTY
Cassette lockring tight? And i assume the chain isn't touching the outer cage if the FD in those small sprockets.

Re: Chordal action/purring vibration from drivetrain

Posted: Tue Sep 24, 2019 10:33 pm
by bm0p700f
It could be the frame resonating. Nothing you can do about that apart from adding masses to the tubes to change the resonant frequency.

Re: Chordal action/purring vibration from drivetrain

Posted: Wed Sep 25, 2019 2:08 am
by jfranci3
What kind of pedals?

Re: Chordal action/purring vibration from drivetrain

Posted: Wed Sep 25, 2019 6:12 am
by MaxPower
speedplay - vibration can be created without holding the pedals

Re: Chordal action/purring vibration from drivetrain

Posted: Wed Sep 25, 2019 6:29 am
by MaxPower
thanks for the replies.

Cassette lockring is 40N (with torque wrench, as i have damaged lockrings in the past by being to hard on them).

i will try with oval chainrings now (just ordered) and se if it makes any difference, since the chain should be in less constant motion with these, and may prevent oscilliation of the chain. I will report back

Edit: 1: yes, the frame virbates to, but it only happens when there is a vibrating source. never had that happen when free-wheeling on the road.
2: yes, FD is not rubbing the chain

Re: Chordal action/purring vibration from drivetrain

Posted: Wed Sep 25, 2019 11:47 am
by kman
Sounds like you've been pretty thorough already. Please post back and let us know whether the chainrings make a difference.

Re: Chordal action/purring vibration from drivetrain

Posted: Wed Sep 25, 2019 1:36 pm
by jfranci3
MaxPower wrote:
Wed Sep 25, 2019 6:12 am
speedplay - vibration can be created without holding the pedals
Check the pedals again. The speed plays have a roller bearing that can cause a few issues. The grease maybe too thin, the bearing maybe fouled, or the o-ring may be allowing the roller bearing to ride up the spindle

Re: Chordal action/purring vibration from drivetrain

Posted: Thu Sep 26, 2019 8:00 am
by MaxPower
But would you not expect similar behavior regardless of front chainring use? It obly happens on the 50t front ring.