Shimano Crankset recall: bonded Hollowtech ones
- Thread starter Ajax Bay
- Start date
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That's the point. Hambini says galvanic corrosion damages the box section surfaces causing the debonding. If there was galvanic corrosion the damage would be at the steel/aluminium interface.
The difference between ultegra and the rest is that ultegra arms are a box section formed from two bonded aluminium u sections. The rest use a solid aluminium arm. The welding/bonding I was referring to is the method of joining the steel spindle to the arm, but we aren't seen any failures due to separation from the spindle, as I understand it the failures are due to the two u sections separating.
OP
OP
Ajax Bay
Guru
- Location
- East Devon
Think his preferred hypothesis is that it is galvanic corrosion is adversely affecting the glue (whereas the welding is unaffected, even is as your wife notes, there maybe more corrosion in a welding join).
Fact is the 105 cranks are not failing to any SM extent, and these glued ones have been, for over a decade.
So there has to be a primary failure mechanism. If not galvanic corrosion then what? Link please
https://www.bikeradar.com/features/shimano-crank-failure/
road.cc"an essential preliminary stage in the failure of the cranksets is the partial debonding of the inner and outer U-shaped channels that together make up the rectangular tube section of the crank arm."
Peak torque (2020):
View: https://www.youtube.com/watch?v=Rj__lexd_BI
Candidates:
Moisture ingress
Trapped moisture (during manufacture) and thermal cycling (cool indoors/garage condensation, hot outdoors)
Meniscus effect at the joins (spider arms)
Galvanic corrosion
The problem with that is that that "damage" is actually alu of the part that disattaches as loose white powder, which at a mechanical connection, causes disconnection. So, that 'box section' should then just come loose from the alu crank parts, without the latter (glued together) separating from eachother.
Yet; that is apparently what does happen here. What is left: the glue itself, regardless the parts. Maybe they chosed a glue that didn't suit the usage.
Jameshow
Veteran
Not so....
"The crank arms are forged around a steel block which is later removed, to create a hollow crank arm.
But don’t think for a second that this equates to anything which feels less than rock solid; you’d need specialist equipment to register the slightest amount of flex on a set of Shimano Hollowtech crank arms."
https://www.cyclist.co.uk/reviews/shimano-105-r7000-groupset-review
The galvanic corrosion mechanism would produce a different failure, which would be seen in 105, Tiagra and Sora as well.
I've been involved in projects where we were using similar processes, and there are two main reasons why it is difficult to get right in mass production.
1. The adhesive is usually a two part system, which requires care to ensure that the correct proportions are used. Some systems are very demanding, and sometimes people get sloppy and end with substandard adhesive because of that. It can also happen that the factory decides that something cheaper and less demanding will be fine instead, and by the time you find out a lot of almost good enough but not quite parts are out there. DAMHIKT.
2. Surface preparation. A good bond usually requires particular surface features and cleanliness. A particular surface finish might be expensive, and again, the factory may decide that a visually similar but cheaper finish will do, until it doesn't.
As I said before, there's probably nothing wrong with the design, other than probably being quite demanding in manufacturing control.
OP
OP
Ajax Bay
Guru
- Location
- East Devon
The spindle is steel; the spider/crank material is aluminium. Add electrolyte (water with some salts) and the higher metal will corrode.
Jameshow
Veteran
How does that affect a bond rather than a weld? That's the interesting question?!
OP
OP
Ajax Bay
Guru
- Location
- East Devon
The inside of aluminium spider and crank deteriorates and that adversely affects the glue between the two aluminium parts which then separate under stress because the crank's strength depends on its box section. The weld is a weld.
I have no figures but SM reporting suggests that this failure is rarely reported in the left crank (which has the same section etc but no steel to provide the cathode). Of course it hasn't got all the spider interfaces that the right hand crank has either, so other failure mechanisms (see above) could be in play.
I have no figures but SM reporting suggests that this failure is rarely reported in the left crank (which has the same section etc but no steel to provide the cathode). Of course it hasn't got all the spider interfaces that the right hand crank has either, so other failure mechanisms (see above) could be in play.
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The "least noble" metal corrodes, alu is one of the least of all metal, the corrosion manifests as dissolving the alu, so one would expect the spindle to start having play inside the crank hole, but the crank, constructed of two longitudal parts, glued together, rips apart. If it was galvanic corrosion inside the two crank parts, that could serve as explanation, but not the spindle. I had had galvanic corrosion in my bike stand, an alu block with two bolt holes in it, between the frame tube and the stand, the bolts were A2-70 stainless, and my stand started to get play, needing further retensions all the time, until I threw the thing open, and saw the bolt holes full of white powder.
Nothing broke or so, just play, because that powder is "lost" material form the parts.
So it's like hard to see galvanic corrosion here as an explanation for the U parts separating.
The lost material / play actually makes the electric current stop flowing, or less contact time, decreasing or halting further corrosion, but play is not acceptable on a drive train. For ex, a tyre valve is also stainless, and if you look good, alot show some white dust traces around their basis, that's alu from the rim, but it does the hole grow in diameter so the problem stops, and the play of the valve in the hole is not a problem.
fossyant
Ride It Like You Stole It!
- Location
- South Manchester
Blimey folks, they were a bad design. I'm sure if you looked after them they'd be fine, but most folk get them covered in crap, salty road water etc etc and don't clean them, and the bonded design didn't hold out. It's a silly design for a 100g saving or so from a forged crank. The MTB cranks aren't hollow, nor are the older ones. I'd take older or generic cranks over these as they are much easier to keep clean, for a slight weight penalty that's not in a crucial area. I really object to being even more limited on chain rings - you need the OEM fancy ones per chainset or they look crap.
I don't like being limited when replacing wearable parts.
I don't like being limited when replacing wearable parts.
