Jerking Under Load
 

Currently running 105-5800 on a BMC SLR02 about 2 years old. No issues with bike so far. I am currently doing a lot of hill repeats on some pretty steep stuff here. The other day, halfway up a fairly steep section and while pushing pretty hard, I experienced a jerking sensation in the general area of the right crank--or so it seemed. There was a loud, metallic pinging sound, a deeper pitch, not high-pitched, but still, metallic sounding. Then, the drivetrain continued functioning just fine. Since then, it has happened a couple of more times, always when under quite a bit of load.

I had the chain length checked a few hundred miles ago and it was stillf fine. The cassette looks okay still. But I do ride about maybe 200 miles a week right now, so that could have changed.

Any thoughts for me?

Frame flex? (I'm not a mechanic)

Replace the chain.

If the problem persists, replace the cassette.

Also, if the front chainring(s) are worn, this could be an issue. When you replace the chain, if it gets worse, it is most likely the cassette and/or the front chainring(s).

I have heard of that before, but I really don't know what it is.

Well, good question. On the hill repeats I know the answer, because I am always in the little up front and teh 4th easiest in back. It has happened once or twice while on more flat terrain and I have no idea what I was in, other than that it was little in front.

That is what I thought you were going to say. Will do that. And I do know what new chain skip is, I think, so will watch for that. The cassette still looks pretty good, but it does have a lot of miles on it. Thanks a lot for the feedback.

Thanks!!

Sounds a lot more like the drive dogs in the freewheel not fully engaging the drive pawls. Or a loose bottom bracket/crank arm.

what type is your bottom bracket: square taper or HollowTech? I’ll bet on loose BB/crank, particularly if square taper.

Is that you, Ms Cleo?

I'm absolutely incapable of detecting wear visually.

I also should have said that it is unlikely both front chainrings and all of the rear cogs would be worn equally, so if you can get the problem to go away by shifting, that is usually a good indication.

While it sounds like this isn't your issue, here is a description of frame flex.

Frame Flex | Kirk Frameworks

Bottom Bracket/Drivetrain
I believe that this is the most talked about and least understood of the three flexes. You often hear people say that the stiffer the better and that any bottom bracket flex results in a loss of energy. I firmly believe that to be untrue. Let’s look at what causes bottom bracket flex and what it leads to.

Crank starting to fail?

I had it happen to me on a local climb. I thought it was the BB as it clicked intermittently but always while my right foot was at 11 o'clock. Really strange because I was expecting crank problems. I removed and re lubed everything. Finally I removed my rear wheel and checked the cassette.

I had repaired a flat 2 weeks earlier then the noise started that weekend on the climbs.

Turns out that my lock ring had somehow come loose but not loose enough to feel by hand. Used the proper tools to tighten and sure enough, a little loose.

Put the wheel back on, noise problem solved. That one really stumped me so whenever I remove the rear wheel, I always double check the lock ring.

If it always happens under hard load, it's related to an interaction between frame flex and drivetrain almost for sure.

Maybe when pushing hard your chain rubs on the FD?

Check this:

Set your bike up against something and turn the crank backwards while watching where the chain exits from the derailleur arm. If you see the arm move a little bit, you have a tight chain link. if that's the case, find and loosen up the tight link and you'll be good to go.

Check your frame for cracks too.

Cheers

I believe it is a BB86 pressfit. How do I tell if it is square taper?

Pop the dust caps off, remove the bolts. Put a touch of anti-seize on the cleaned threads, reassemble and tighten to 30-35 lb-ft *IF* it is a square tapered bb.

Sounds like the small chainring is worn. I had to replace one on my gravel bike after I replaced the chain. It worked fine until it was under load from climbing. Under load, it had a bad vibration/grinding noise from the worn teeth hitting the roller instead of the space between the rollers on the chain.

I was thinking along the same lines as kimmo. Could the crank be failing in some way? Don't know if spline failure (its a 105 setup if I read the OP correctly) or perhaps the crankarm tightening bolts weren't tightened properly (too loose or too tight so that one broke)?

Well, little update on this. I replaced the cassette and chain with brand-new ones. Just went with an Ultegra chain. It's fine. Everything was fine for a couple of weeks, but now, it is occurring again. Difference today is, it is not just when under a fair bit of load. Even when just applying fairly modest power, it just slipped, I guess, is a way of putting it, earlier this morning. Then, it returned immediately to normal.

I will go through every post one by one and do each intervention, one at a time. Then, if it's still there, I'll be back. Actually, I'll be back either way, just to report on what seems to have worked (and not worked).

Thanks to all for your help. I really appeciate it!!!!

As a couple of others have said. Have you checked that the crank arms are tight?

There can be a tiny movement you cant see, but it will make a noise.

It's actually SLIPPING not just making a noise? Check the chairing teeth for excessive wear. Also check the freehub.

Cheers

Years ago I bought a used bike and took it for a short ride before completely checking everything over. Going up a moderate incline on a long hill I got this nasty crunching sound and sometimes some slippage. When i got home I opened up the bottom bracket which was a square-taper cup-and-cone unit with caged bearings. the bearing cage on the none drive-side was in pieces. I hope it's not your bottom bracket bearing s going.

Cheers

Sorry--yes, absolutely. Everything is snugged up/greased, etc. Thanks!

Yeah, I have not been able to set my eyes on the crank when it is actually occurring, because it is so sporadic. But the feeling is that there is a sudden loss of ability to transfer power from me to the crank/BB/whatever it actually is. Usually with the left one at the 9 o'clock position. It feels like there is maybe, oh, probably only a centimeter, maybe two (?), where the crank just moves forward freely, zero power, then, there is a metallic ping, it reengages, and I ride off on my way. Weird.

I have had a good look at the chainrings and I think they are still just fine. But will keep it in mind. Have not looked at the freehub. Will have to read a bit, as I am no hub expert.

Thanks very much for the guidance--I really appreciate it!