Kontact 

Clearly, no thanks for any insight.

I've been a shop mechanic for a very long time - I'm not guessing. And I watched the installation video before commenting. Why bother asking for help if you are smarter than everyone?

FBinNY 

No problem, I miss mid-thread posts all thd time. Doubly so when they feel to long to read.

Duragrouch

Possible insight, not sure. I don't know if this is a cone-and-cup bearings, or cartridge, I'm gonna assume the latter, but use the same terms for its internal components.

You apply max force when the crank arms are horizontal, forwardmost. Let's freeze that position with the right crank arm forward. The BB axle (inner race) will feel force on its bottom, and so will the bearing cup (outer race). Let's assume over time, both points get spalled (pitting on bearing race due to repeated load), so there is a gap at the bearings when the crank is at that point, but the gap is at the *bottom*; You wiggle the crank when the arms are horizontal, and this would detect a bearing gap at the *front or rear* of the bearings, so *no slack, even though damage*. NOW, you rotate the crank forward to driveside arm down, the axle damage point is now pointed *back*, but the cup damage point is *still down*; You wiggle the crank arms, feel looseness. So my guess is some wear has occured at the bottom bracket, this is normal, and normal to happen at particular points in the power stroke. You can reduce this by rotating the BB so that the damage points are not both facing down during the power stroke; I don't know your crank interface with the BB, but if square taper BB, pull off both crank arms and put them back on 180 degrees different, this will rotate the BB spindle. If you can somehow rotate the cartridge, such as installing a spacer that is 1/2 or even 1/4 thickness of the thread pitch (one full thread), that will rotate the outer bearing races. One you have any looseness in a BB bearing, you lose the preload which distributes the force more evenly on the bearings, and wear will accelerate, but at least you shouldn't *feel* the wear as much during your power strokes.

If you put the crank arms horizontal, driveside forward, but grasp the chainring top and bottom and wiggle that, you will probably detect looseness.

Qualifier: Due to chain tension, during the power stroke, it will pull the crank driveside aft, so bearing damage may not be right at the bottom. But rotating the BB axle and cups with respect to the power stroke, and that will be taken into account.

FBinNY, your thoughts on the above? I've come to respect your experience on stuff.

choddo

Diagnosis of the cause seems plausible but I assume that kind of mitigation isn’t possible as it’s asymmetric? A year seems a short time to get through a BB bearing though this suggests there’s no sleeve between the cups to protect the bearings from any abrasive contaminants that get inside the shell and as it’s a GRX crank, maybe this bike is used offroad?

Duragrouch

A year is not at all a long time for a BB cartridge with frequent long rides and climbs. The problem is, you cannot adjust the internals to take up bearing slack. If you could, the cartridge would last 5-10 years. This is one of the advantages of a hollowtech II system, when it starts to loosen up, you undo the left arm clamp bolts, (clean between arms and bearings on both sides), retorque the end cap, tighten the left arm, and no more slack. That preload is *critical* for bearing life; No preload and you are loading at most 2-3 bearing balls per side at any given time; WITH preload, you are loading almost 180 degrees of the bearing balls per side.

Asymmetry, I can't comment on as I don't really know about the design. I don't even know the style, whether internal cartridge, cup and cone, or external bearings.

EDIT: If the OP's BB is cup and cone, old style, very possible to wear quick, but also can adjust out slack. First, pull off crank and spin BB axle in fingers to see if smooth, and adjust that way, or rebuild. AND, bearing life is greatly improved by tossing out the caged bearings and installing all loose balls, many more balls greatly improves durability. Must be all new balls, do not reuse any old one, must be same within 0.0001". With plenty of grease in cups, loose balls stay in place until installation.

choddo

It’s not a cartridge or cup&cone bearing, it’s like hollowtech II with sealed bearings set in cups as per the pdf I linked.

Duragrouch

Oh. OK. I try to avoid downloads for security reasons. In that case, the inner race on the external bearings, that is a close slip-fit with the crank axle, so will vary in rotational position with respect to the axle. The outer race stays put, so my guess is more damage likely to be there, and as reasoned earlier, and the bottom of the race. So could try to rotate that with a spacer that is 1/4, 1/2, or 3/4 thickness of the thread pitch. BUT...

If it's a hollowtech II style, the first thing that should be done is try to take out the slack by readjusting the preload. I'd even go so far as to adjust the preload with the crank in whatever position has the most slack, take that out, I think the larger preload in the less slack positions will not be a problem.

choddo

But because it’s eccentric, it can’t be rotated.

Duragrouch

Ah, thanks for that info. Well, at least, if it's a hollowtech II style, they should be able to adjust out that bearing slack. If too damaged already, then replaced. But in either case, staying on top of bearing preload is key to long bearing life; Check tightness every 2-3 weeks, it takes all of 10 seconds to do, just try to wiggle the crank arms laterally at different arm positions, and if any slack, adjust it out right away, and that takes about 2 minutes, including cleaning between the arms and the bearings. I'm about two years on mine on my daily townie, still smooth, I think I had to adjust preload once so far since new.

choddo

Have to agree that’s good discipline after my NDS crank arm came off mid-ride.

Duragrouch

Only time mine came loose was when I had the left arm 4mm off spindle due to same spacers on right side, to accommodate front derailleur. I then ground off top lip on same to allow removing the spacers. This also corrected the chainline.

See my post #24 on this thread, marking the bolts, to help prevent that:

https://www.bikeforums.net/touring/1...hile-tour.html

Kontact 

Aside from wear caused by the eccentrics not being lined up, this problem could be from one of the eccentrics coming loose - either because they weren't tightened into the frame enough or because the grub screws got loose.

Unlike regular BB threads, this eccentric is subject to pedal forces working to loosen the way the cups are threaded into the frame. The off center pedaling forces up and down on the spindle work like a pedal in the cups.