A customer dropped off a bike saying "the rear shifting is acting weird,"
but the derailleur itself had no problems.
The specific symptom was a clicking noise—"jak-ko"—that would happen sometimes when shifting into the lowest gear...
The bike has an FSA Gossamer BB30 crankset with a Shimano 11-speed cassette.
When you spin this crank, the chainring appears to wobble like a non-circular oval ring,
moving in and out of true with every rotation.
Shimano used to make "BioPace" non-circular rings back in the day,
but of course this chainring is perfectly round.
↑Like this
Depending on the crank arm phase, the height of the chainring teeth varies
(this same phenomenon occurs with non-circular rings too),
causing the rear derailleur's pulley cage to rock back and forth.
When the chain is on the lowest gear,
the gap between the upper pulley of the derailleur and the sprocket narrows,
but due to this runout there are phases where that gap gets even tighter,
and at those moments the sprocket and pulley teeth mesh together, causing the "jak-ko" noise
that the customer was calling "weird shifting."
Of course, it's not the chainring that's running out.
When I grabbed both crank arms and shook them, they rattled back and forth relative to the frame.
↑Like this
At this point I thought the cartridge bearing's inner race had broken
and the inner race was moving relative to the outer race,
but the left bearing's rotation was a bit gritty but otherwise fine,
and the right bearing spins oddly stiff
with no movement of the inner race relative to the outer.
So what's causing the play? Well...
The crankshaft itself was worn away.
The bearing's inner diameter wasn't enlarged or anything like that.
Just like hub spalling, it started with a small catch or snag
that gradually grew worse over time.
Inside the frame there was a claylike paste made of shredded aluminum and grease.
I removed just the right bearing.
The reason its rotation is so oddly stiff is:
that claylike debris had worked its way in through the seal's gap.
In the photo above, the seal is shown from the inside-facing surface.
I cleaned and re-greased it for continued use.
The crank definitely needs to be replaced soon,
but since the customer has a new frame on order arriving in about six months,
the bearing just needs to last until then.
The left bearing was
dirty on the outside
but that was all.
The customer had been thinking lately, "my knees have been feeling a bit wobbly,"
but it was the crankset that was wobbly all along.
but the derailleur itself had no problems.
The specific symptom was a clicking noise—"jak-ko"—that would happen sometimes when shifting into the lowest gear...
The bike has an FSA Gossamer BB30 crankset with a Shimano 11-speed cassette.
When you spin this crank, the chainring appears to wobble like a non-circular oval ring,
moving in and out of true with every rotation.
Shimano used to make "BioPace" non-circular rings back in the day,
but of course this chainring is perfectly round.
↑Like this
Depending on the crank arm phase, the height of the chainring teeth varies
(this same phenomenon occurs with non-circular rings too),
causing the rear derailleur's pulley cage to rock back and forth.
When the chain is on the lowest gear,
the gap between the upper pulley of the derailleur and the sprocket narrows,
but due to this runout there are phases where that gap gets even tighter,
and at those moments the sprocket and pulley teeth mesh together, causing the "jak-ko" noise
that the customer was calling "weird shifting."
Of course, it's not the chainring that's running out.
When I grabbed both crank arms and shook them, they rattled back and forth relative to the frame.
↑Like this
At this point I thought the cartridge bearing's inner race had broken
and the inner race was moving relative to the outer race,
but the left bearing's rotation was a bit gritty but otherwise fine,
and the right bearing spins oddly stiff
with no movement of the inner race relative to the outer.
So what's causing the play? Well...
The crankshaft itself was worn away.
The bearing's inner diameter wasn't enlarged or anything like that.
Just like hub spalling, it started with a small catch or snag
that gradually grew worse over time.
Inside the frame there was a claylike paste made of shredded aluminum and grease.
I removed just the right bearing.
The reason its rotation is so oddly stiff is:
that claylike debris had worked its way in through the seal's gap.
In the photo above, the seal is shown from the inside-facing surface.
I cleaned and re-greased it for continued use.
The crank definitely needs to be replaced soon,
but since the customer has a new frame on order arriving in about six months,
the bearing just needs to last until then.
The left bearing was
dirty on the outside
but that was all.
The customer had been thinking lately, "my knees have been feeling a bit wobbly,"
but it was the crankset that was wobbly all along.