What’s going on with Shimano’s road cranks?

Shimano snuck in a few key changes with Dura-Ace and Ultegra cranks that should improve their reliability.

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You may have caught wind of some reliability issues concerning previous-generation, high-end Shimano road cranks, specifically Dura-Ace 9000 and R9100, and Ultegra R8000 and 6800. Basically, some of them are coming apart. Shimano won’t officially comment on the issue (perhaps due to legal constraints — and believe me, we’ve asked), but some key changes with Dura-Ace R9200 and Ultegra R8100 provide some key clues as to what’s happening, and what’s been done to fix it.

Shimano have long been masters of forged aluminum construction, and it’s by no coincidence that the company has continued to resist offering a carbon fiber crankset of any sort (aside from the very short-lived Dura-Ace FC-7800-C). Instead, Shimano’s position has always been that its various advanced forging methods could produce an aluminum crank that performed at least as well as a composite one, if not better.

The two-piece construction is a big reason why Shimano’s aluminum crankarms are so competitive with carbon fiber in terms of performance.

In recent years, Shimano’s preferred strategy for crankarms was to employ a two-piece design, with two separate clamshell sections that are bonded together to form a single hollow structure. But unfortunately, if and when that bond joint failed, the whole arm basically just felt apart.

To be clear, the actual scope of the problem is anything but certain. Social media obviously makes it seem like every one of these things is blowing up, but it’s maybe worth mentioning that none of the major retailers I spoke with reported big numbers. 

“Honestly, we have not had it happen very often, maybe three times going back a decade — two times with the last-generation cranks,” one told me. “For sure, the clients were not stoked, but for their part, Shimano squared us away no problem, and quickly.”

Once the bond has started to fail, there’s not much holding the two halves of the crankarm together.

Without official numbers from Shimano, it’s impossible to say exactly how many of these cranks have failed (though it’s maybe worth mentioning that the volume and severity of the problem apparently hasn’t been sufficient to prompt a mandatory recall). That said, there’s an awful lot of these out in the field right now, and even if a tiny percentage of them are failing, it still makes for a lot of broken cranks. Various third-party analyses have suggested a common symptom for many of these failures: corrosion. Somehow, it seems that water is getting inside these things.

On the driveside, the most likely culprit seems to be the hollow steel spindle, which is pressed into the aluminum crankarm base before the cap is bonded on. There’s some epoxy sealing off the end of the spindle from the rest of the crankarm, but it’s either an imperfect seal or one that potentially degrades over time, which then allows water to get inside the chainring spider cavity. This might not be a huge issue if the inside of the arm was anodized like the outside, but because it’s left raw, it’s more prone to corrosion if some water — or worse, salt — gets inside. That corrosion can then compromise the bond integrity, which can then potentially lead to complete structural failure under load.

Non-driveside crankarm failures have also been reported. These appear to be much fewer in number, however, probably because there’s less bond surface area to go bad, and no obvious ways for water to get inside.

This Dura-Ace R9000 crankarm has obviously failed, but there’s also substantial evidence of corrosion, too.

Either way, a careful look at the latest Dura-Ace and Ultegra crankarms suggests that Shimano is not only aware of these issues, but has implemented changes to prevent them moving forward.

The most obvious change on the driveside crankarm is the addition of a physical plug inside the chromoly steel spindle to keep water out. Although the shape of the crankarm is different from the Dura-Ace R9100 and Ultegra R8000 generations, the basic design appears to be otherwise unchanged.

The non-driveside crankarm sees a much bigger revision, at least for Dura-Ace. Whereas the two previous generations used a two-piece bonded construction like on the driveside, Dura-Ace 9200 reverts to a one-piece forged design (like Ultegra). Without a bond seam, there’s now no bond to fail — problem seemingly solved.

While these changes should greatly improve the reliability of both cranksets, there’s also a downside: they’ve gotten heavier. By Shimano’s own official figures, the new Dura-Ace R9200 crankset is 71 g heavier than before at 685 g with 50/34T chainrings.

It’s a similar story with Ultegra. The new R8100 crankset is officially 700 g with 50/34T chainrings, while the R8000 model is 674 g — still heavier, though a smaller gain since the non-driveside arm already used a one-piece forged construction.

Will these updates really put the issues to rest? Let’s hope so, because while Shimano seems to have been very accommodating in terms of taking care of affected customers, it’d be better if this problem just went away entirely. Weight weenies might not be happy that the new cranks are heavier, but if this solves the issues, this will definitely be weight well spent, and hopefully a return to Shimano’s well-earned reputation for reliability.

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