Technical Q&A with Lennard Zinn: Why did Schleck’s chain come off?

Dear Lennard,

Why did Andy Schleck’s chain come off today? It looked like he was just pedaling and not shifting.

Jed

Dear Jed,

I’ll venture my theory as to why it came off, but that’s all it is: a theory. But, I think I can definitively explain what’s unique about his derailleur that caused his rear wheel to jump up and made it impossible for him to pedal the chain back on and forced him to stop.

First, it may seem amazing that his mechanics had installed an inner stop, which, if adjusted properly, one would have thought would have prevented the chain from dropping in the first place. But I think the chain came off from the bottom and not the top, and the inner stop could not prevent that.

As you can see on the video of Schleck’s mechanic working on his bike after the stage on VeloCenter, the bike had an inner stop, and it looks like a K-Edge Chain Catcher. Assuming the Chain Catcher was adjusted properly so that it just barely did not touch the chain when in the small-chainring/large-cog combination, the only way the chain could have come off over the top of the chainring is if something flexed, like from a crack somewhere, perhaps in the frame or a crank spider, significant enough that the top of the ring could flex outward far enough to allow the chain to drop between it and the inner stop. Otherwise, the Chain Catcher would have pushed the chain back on. However, I don’t think that happened, because once a chain gets past one of those inner stops, it is very hard to get the chain past it and back on the chainring. It would have taken a heck of a lot more struggling from the Mavic mechanic to get it back on than you saw on TV; in fact, he probably would have had to loosen the front derailleur mounting bolt to get the chain past the arm of the Chain Catcher.

While it may have looked like Schleck was not shifting at the time on TV, the video appears to show him pushing his shift paddle inward with his right forefinger. And a chain can be dropped due to a changing chainline as well as from a sudden reduction in chain tension on the bottom at the same time the right leg comes over the top and flexes the frame, crank and chainrings.

Here is what I think is the most likely scenario: the chain came off the bottom of the inner chainring due to hard pedaling combined with shifting the rear derailleur, presumably to a higher gear (with SRAM, the same shift paddle does the shifts in both directions, so there’s no way to say that he was downshifting or upshifting from the photo of his finger) and not easing off on the pedals to accomplish the shift, and perhaps even a small bump in the road to add a little bounce to the chain helping it to come off the bottom of the chainring. Since Schleck’s speed was increasing, the shift most likely would have been to a smaller cog, which would have dropped the chain tension and forced the rear derailleur to pull up some slack at the same time he was throwing his weight into his attack.

The video also seems to show that the rear derailleur twisted way around (clockwise), and then his rear wheel jumped up. I think these are cause and effect. I think that his chain came off of the bottom and not the top, meaning that he still would have had a taut chain over the top. However, I think that the rear derailleur locked up his rear cogset and prevented it from turning forward, thus causing the rear wheel to jump up when the chain tension pulled the cog forward and up.

I propose that the reason this happened is unique to SRAM rear derailleurs. When you drop a chain with any rear derailleur, the derailleur, and particularly the lower jockey wheel, pulls back. This is due to the sudden release of the chain tension that would have otherwise been pulling the lower jockey wheel forward. When the chain drops in front with a SRAM derailleur in the rear, however, something unique happens. Note the long loop extending backward off of the cage around the lower jockey wheel. When the derailleur’s lower jockey wheel swings clockwise around to pull up the chain slack, that loop catches on the next largest cog (unless the chain is on the largest cog, which Schleck’s was not). The cog then pulls the derailleur around in a clockwise direction so it comes way up high in back. Once the limit of rotation of the rear derailleur is reached, the loop cannot rotate any further with the cog, and it locks up the cogset and prevents it from turning any further forward. This is what I think Schleck’s did.

By comparison, on a Dura-Ace rear derailleur, there is no loop, simply parallel cage plates alongside the lower jockey wheel. Notice that on a Campy rear derailleur there is also no such loop, only a small tab, and note how it only extends off of one cage plate and not very far from it.

(However, if Schleck was using this upgraded version, that loop might not have been there, and my theory may have a huge hole in it. But the video is inconclusive whether he has this cage [I actually think the beginning of the video with the mechanics shows the loop], and you’ll find plenty of photos online from the Tour showing unadulterated Red rear derailleurs on various of Cancellara’s and Schleck’s bikes, save for special painting – yellow for the yellow jersey, or gold for Cancellara’s world and Olympic medals.)

I’m also pretty sure that the chain dropped completely off to the inside, and it did NOT fall between the chainrings as some have theorized, since the rear derailleur sprang back so far, indicating there was lots of chain slack to take up.

Other possible (unlikely) causes:

Chains can drop off of the chainring due to a change in chainline due to frame flex. In other words, the cogset moves inward or outward relative to the plane of the frame and chainrings as the chainstays flex with pedaling load. Unless Schleck’s frame was cracked, this seems unlikely, as one of the features of the Specialized Tarmac SL3 is a one-piece monocoque bottom bracket/chainstay assembly intended to grant it high rear-end stiffness. According to Specialized, this design, an upgrade from the SL2, was praised by Tom Boonen as giving him all the stiffness he was seeking.

With Schleck’s lean body and the UCI weight limit making it unlikely that Specialized would have made him an extra-light version of the already-featherweight SL3 (around 900 grams), it seems unlikely that flex caused it unless the frame had been damaged. Remember Lance Armstrong’s second bobble with Iban Mayo on stage 15 to Luz Ardiden in 2003 after he and Mayo went down when a fan’s musette bag strap hooked the yellow jersey’s handlebar? Mayo’s wheel running into Armstrong’s right chainstay had cracked it, and when the Texan stood up on his pedals to chase back to the leaders, the broken chainstay caused the rear end to move laterally so much that his chain skipped and he dropped down onto his top tube and clipped out, causing Mayo to nearly run into him a second time.

A misadjusted front derailleur could cause the chain to drop, especially when paired with frame and/or crank flex. If the front derailleur had been too far inboard and touched the chain, it could have pushed it off to the inside. It seems unlikely that it could have been adjusted this way, because, it has to be way off to get it to happen (think of how many times you’ve had a rubbing front derailleur that did not derail the chain).

But, if there had been enough flex in the combination of frame, chainring, and crankset that, when stomping down with the right foot, the chain moved over far enough to the right that it touched hard against the inside of the right cage plate of the front derailleur, that could have pushed it off to the inside.

The chainring moving laterally can also cause chain drop, but that also seems unlikely. Schleck’s BB30 cranks cannot move laterally as its shims are installed properly, which it’s hard to imagine that they weren’t.

A loose chainring could move back and forth enough for the chain to drop. Also unlikely, given the constant attention to details like that by pro mechanics.

Flexing chainring spider arms could cause the chain to drop, but those are pretty stiff cranks, and riders do what Schleck did all of the time on them without the chain coming off.

A bent tooth on a chainring can push the chain off. Also unlikely, given the attention his bike gets.

And all of these scenarios, except perhaps the bent chainring tooth, would be knocking the chain off of the top of the chainring, not the bottom. I’m ruling that out because of the unlikelihood of getting the chain off past the Chain Catcher, and, more importantly, because of the extreme difficulty of getting it back out from behind the Chain Catcher and onto the chainring again.

So, we’re left with my original theory. LZ’s Schleck chain-drop theory in a nutshell: ‘twas a perfect storm of upshifting under load with a derailleur that has a big loop on it to snag the cogset when the chain drops off the bottom to the inside of the small chainring.

Lennard

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Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.