Chain drop during Sprint

[leej88]

I took a tumble last Saturday as my chain dropped (to crank side) during a sprint.
Was pedalling down a slight incline on 52-11 and launched the sprint out of saddle. I jumped hard starting with my right leg and heard a loud snap followed by an immediate loss in resistance in the pedal which sent me crashing to the ground.

Inspection of the bike after the crash made it more perplexing. Asides from the scuffs on my bartape and broken saddle rail, the bike was in good shape. Chain was intact.

Here’s some contextual information that might be of help as I am unable to pinpoint the root cause:

*SETUP*
Frame: Time Scylon Medium (Never crashed, well taken care of and no cracks AFAIK)
Wheels: ENVE 4.5 SES (True with no stress/cracks in rims)
Hubs: Chris King R45 (Bearing preload setup properly with no free play)
Groupset: Campagnolo Super Record EPS V3 (tuned perfectly by Campy Pro Shop)
Cranks: THM Clavicula SE (Proper preload with Loctite & C.R. bolts tightened to spec)
Chainring: Praxis Buzz 52/36 (New Warranty Replacement with less than 800km)
BB: THM BB386 EVO (newly replaced BB after Ceramicspeed one I was using died)
Casette: Campagnolo Record 11-27 (Mileage: 3000km)
Chain: Campagnolo Record (Rock N Roll Gold Lube Mileage: 2500km)
Total Bike mileage: 15,000km with regular servicing

*WORK DONE/CHANGES TO BIKE*
I replaced my Ceramicspeed BB with THM ones about 2 weeks ago as the former BB had pitting and rust in the NDS bearings.

During installation, I noticed that the THM BB CUPS weren’t made to the same tolerances as Ceramicspeed’s as the drive side cup could be pressed in easily with one hand without the need for a press.

Note: Chainline on new THM BB cups was changed very slightly (Entire Crankset is now outboard by (0.5~1.0mm).

*CHECKS PERFORMED after BB change*
1. BB preload/adjustment-No free play detected
2. All Chainring bolts checked and tightened in Star Pattern incrementally to 7Nm
3. FD adjustment is spot on. As entire chainline is moved out slightly to the right, the FD outer cage sits even close to the chain (protects against chaindrops to the outside)
4. Rear shifting and indexing spot on

*CONDITIONS RIGHT BEFORE CRASH*
1. Gearing is in 52-11
2. Cadence (was doing about 90ish RPM)
3. Hand position on Drops
4. Rolling down a slight decline (-1 to -2%) @ ~55kph


I did a comprehensive and thorough check on the bike following the incident and brought it down to the local Campy Pro shop and they too were not able to figure out the issue. I've done a couple hundred of sprints on this setup and it's the first time this has happened to me.

Being a larger rider at 87.5kg, I peak at around 1720w with a 5sec p-max of 1500w so things do flex around a little sometimes but I believe bikes these days should be more than enough to handle these loads.

Have not checked for:
1. Chainstretch (campagnolo chains use a different method to check for chainwear <132.6mm across 6 roller pins?)
2. Bent chainring tooth (Had a quick glance and it seems that only a few teeth had gouge marks on the outside facing side)

[TobinHatesYou]

The chain probably temporarily jumped onto the small ring, but since the FD cage was in the outboard position, it jumped right back. It likely has to do with the very close high-limit you have set.

[Calnago]

Might be one of those mysteries you never really figure out and have to chalk it up to just a freak mishap.
However, there’s a few things that “might” have contributed to your mishap:
1) Campy chain on non Campy rings;
2) Chainline not to Campy spec; related to this is also the fact that Time Scylon chainstays are one of the shortest in the industry... setting up a Campy EPS V3 took a few fine tunings to dial before I could get clean shifting from the small ring to big ring while in the big rear cog. I know that’s a shift you’d rarely if ever perform but it should still be capable of completing the shift quickly while riding aggressively.
3) Worn chain (2500km, depending on your weight, power, and conditions it’s ridden in could very well mean time for replacement.
4) when you say the cups can be inserted by hand, do you mean the cups can be inserted into the bare BB shell of the frame without a press? That’s not good. Either BB shell is out of spec or worn, or the BB is. You should not be able to insert the BB cups by hand. It’s a pressfit.
5) worn/damaged teeth on the rings.
6) just bad luck. Stuff happens.

Edit: Reread your initial post. You say front derailleur adjustment is “spot on”. I doubt it. You’ve already said the chainline is further outboard than where it should be, so that the front derailleur cage is very close to the outer side of the chain. I suspect the shift I described in point (2) above is not easily achieved. There’s some overrun in the shift. I think it’s likely settling a little too close to the chain (outside) once the shift is completed. Your weight and power is putting a lot of stress on the system and it may have skipped a couple teeth. Check the chain stretch. I like to stick pretty close to the 132.60 measurement for determining replacement, especially for a bigger sprinter with some good power. And if there’s any flex in the whole thing as a result of a loose BB, be it the frame shell or the BB itself, that could be just enough to cause something too. Basically, impossible to say for sure, but that should give you a few things to think about.

[leej88]

1. Chain fell outboard (not inwards onto small chainring).
2. Yes, the 404mm chainstay length does not help but we've managed to get it dialled perfectly so it's shifting great.
3. I'll definitely change the chain as a precaution
4. Yes, the cups can be inserted without a press. That's for the THM BB. Ceramicspeed ones i took out where both a b*tch to remove as well as reinstall.
5. Can a chainring wear out that fast?

[Calnago]

I don’t know about chainring wear... I wouldn’t think it would be worn at only 800km. No way.
But I would take a measurement of the chain just so you know where it was at. But good idea to replace it.
It is odd that it dropped on the outside and then it had righted itself somehow. Don’t know what to say about that. Stuff happens.

[TobinHatesYou]

How smooth is the backpedaling in 52x11 and do you think you may have backpedaled slightly while coasting (to line up your cranks) before launching your sprint?

[Calnago]

Lol. Yeah... that’s what Cavendish always used to do before launching his sprint... coast and backpedal.

[leej88]

I was pedaling at around 90+ rpm cadence without any back pedaling.

Haven't tried back pedalling in 52/11 since i rarely use that gear anyways.

I do know however that at 52/27 (big big cross chain), back pedalling sometimes throws off the chain from the big ring.

This crash however was quite unexplainable in the sense that the chain dropped without shifting gears.

Without ever being able to pinpoint the root cause of this, i doubt I'll be sprinting for a while.

[TurboKoo]

I suspect it’s combination of flex in different places. You can check easily if it’s frame and chain ring flex.
Just find small incline, use tallest gear and do a standing start. You can actually see the flex and since you have very slow speed it will happen slowly and you won’t crash. In this test I used to be able to drop chain in 8/10 but nowadays BB and chain ring stiffness have gone up so it’s not so many bikes anymore.

[Calnago]

Really though, check the chain wear. If it’s worn it could have easily skipped over some teeth before catching again.
Backpedaling while in the big/big cross is never going to be good but that has nothing to do with what happened while sprinting. But the outboard chainline would certainly explain the chain not catching onto the big ring while back pedaling in the big/big cross. There is never a reason for backpedaling, ever, unless you’re trying to fix a jamup somehow, but the problem has already happened at that point. If I were you I’d save up and switch to a Campy crank, Campy cups. I’d hate wondering if what happened might ever happen again. You might not be so lucky next time.

[leej88]

Ok, i just ran through the sequence of events and remembered a very important detail which I've missed out.

After getting home, i noticed that the cassette had play in relation to the hub shell.

Grabbing the cassette with one hand and the rim with another, i could wobble it a little.

Tightened the preload and that play was gone.

Might it be possible that the loud Snap and sudden loss of engagement might be due to a preload that somehow came loose and under high torque caused it to slip?

Only way to find out is to strip the hubs apart when i get back to look for signs of wear in the ring drive i guess.

[Calnago]

Good find. Possible I suppose. Would explain a lot. Chris King hub right? Check it out. Still think it must have been very loose for that to have happened given how Chris King mechs work, but it’s worth checking out. But that explanation is consistent with what you described happened, except for the chain getting thrown to the outside. Not sure how that could have happened. But yeah, at this point you want to check everything. The hub engagement, the chain wear, and all the teeth on the big ring. Look for a twisted, bent plate edge somewhere on the chain too.

[TobinHatesYou]

Lol. Yeah... that’s what Cavendish always used to do before launching his sprint... coast and backpedal.

I was asking him to backpedal to check if there was binding in his freehub. If there is, then that slackens the top of the chain path under coasting... Coasting isn't uncommon before a pack sprint, especially if corners are involved.

And of course it at least resulted in this discovery:

I do know however that at 52/27 (big big cross chain), back pedalling sometimes throws off the chain from the big ring.

While this is a more severe chainline than 52x11, 52x11 is far from straight. Also the small circumference of the cog actually makes backpedaling tougher. So if there is any binding in the drivetrain that would result in the chain advancing over top while you are coasting, that could cause a lot of slack at an inopportune moment, like when you suddenly apply a serious amount of torque. It would also manipulate the derailleur cage as the effective chain length on the bottom is reduced.

[TobinHatesYou]

Ok, i just ran through the sequence of events and remembered a very important detail which I've missed out.

After getting home, i noticed that the cassette had play in relation to the hub shell.

Grabbing the cassette with one hand and the rim with another, i could wobble it a little.

Tightened the preload and that play was gone.

Might it be possible that the loud Snap and sudden loss of engagement might be due to a preload that somehow came loose and under high torque caused it to slip?

Only way to find out is to strip the hubs apart when i get back to look for signs of wear in the ring drive i guess.

Improper preload in a CK hub should not result in slippage / engagement issues. The coil spring is far too strong for that to happen. Just look at this thing, it's overkill. It could possibly be debris...the CK freehub design is...problematic if a single piece of debris prevents proper meshing of the "ring drive."

[leej88]

@Tobin & @Calnago

Back pedaling is smooth and not binding.

I guess I'll have to check through:

1. Chainring teeth
2. Recheck chainring bolt torque values
3. Chain plates
4. Strip apart Hub to check for signs of wear

The crash happened after hearing a loud SNAP (like the sound of something giving way under high load)

2 Possibilities

1. Chain Fell off outside of chainring (Most likely reason to least likely reason)
- Bent Chainring Tooth?
- Chain wear
- BB (THM one seems to be a little out of spec since I can install Drive Side cup with my hands without needing a press)
- Loose pre-load in rear hub causes cassette to wobble thus throwing off chain?

2. Rear Hub Engagement slip causing sudden loss of traction, bike crashed and chain dropped off to the outside as I landed derailleur side down
- Check for worn engagement rings/metal shards to confirm the possibility of a slippage in engagement