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The bicycle component manufacturer Shimano makes crankarms named "Hollowtech".

Hollowtech crankarms are meant to be both strong and light. As the diagram below (source: Shimano) shows, the outer shell is made from an aluminum alloy and the inside is hollow.

hollowtech

On closer look it turns out that the cross-section of the crankarm is not made from a box section, but from two C-sections. (Source: bikeradar)

two c-sections

Another (older) style of Hollowtech consisted of a C-section closed off by a plate.

failure mode of hollowtech

The two parts are glued using epoxy. It's understood that epoxy, once it sets, is more than adequate for the task of carrying the weight of a cyclist on the crankarm, but even if epoxy does bind well with a (perhaps roughened) alloy, wouldn't the difference in the stress-strain curves of the two materials mean that when the alloy bends, it is the epoxy that will take all of the load? Since it's used here only as a binding agent, it would appear this is a recipe for failure. Do you understand how epoxy can reliably bind two parts of a crank arm, and can you explain it?

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    $\begingroup$ A hollow crank should be worse for aerodynamics than a solid one with the same strength (but higher weight). I don’t think it’s a hoax or marketing trick. Hollow crankarms (even if they are made from two or more parts) are a valid way to reduce weight while achieving the same strength. $\endgroup$
    – Michael
    Nov 8, 2022 at 15:25
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    $\begingroup$ This question seems to be fishing for supporting answers IMO. In my mind forces that would be required to damage hollow cranks are abnormal and would really have to be the result of impact, bad technique or plan miss-treatment, for that alone I would be more than happy to wreak a crank set then potentially ripping the BB out of the frame (being dramatic of course) $\endgroup$
    – Hursey
    Nov 9, 2022 at 3:45
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    $\begingroup$ "no one in construction manages to reach the torsional rigidity of a closed section" - 1) yes they do, and 2) most of the time they don't because they don't need to. For a building an I-Beam and a Box beam are both just as rigid. $\endgroup$ Nov 9, 2022 at 3:57
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    $\begingroup$ I'd be very careful with calling a product a "hoax". You may find yourself at the wrong end of a costly lawsuit. $\endgroup$ Nov 9, 2022 at 8:15
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    $\begingroup$ @cmaster - reinstate monica: or because epoxy or whatever else they used gave a better bang for the buck. Welding would sacrifices a lot of the strength from the forging process. $\endgroup$ Nov 9, 2022 at 8:31

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Agree with Vladimir. Note that the join between the two C-section halves can be created with a deliberate interference (force) fit involving the roughened surfaces in such a way that when they are driven together in a press, the pieces gall and seize. This means the parts are in a sense friction-welded together and thus the epoxy does not have to carry all the imposed loads.

Another thing to consider is that if the interference fit creates residual compressive stresses in the assembly, then all you have to do to prevent the stress on the glue line from ever becoming tensile (which is how it would fail in service) is to ensure that the magnitude of the residual compressive stress exceeds the maximum tensile service stress.

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  • $\begingroup$ Intriguing. You're somewhat upturning my thinking about loads in cycling. Normally the bike is the fluffy thing (~10kg) and the cyclist (being many times that) is relatively a monster. You're effectively saying this is a solution because in the grand scheme of things even the limit 110-kg cyclist is a fluffy thing compared to what a box-alloy crank can potentially handle. $\endgroup$
    – Sam7919
    Nov 19, 2022 at 15:14
  • $\begingroup$ But to be clear: there is one nagging point you do not address. Two geometrically identical beams, one made from the alloy and the other from epoxy, would have vastly different deflections under the same load. That's to be expected, for one is a metal, while the other is closer to glass. By the time a beam made from BOTH materials deflects, the alloy would be taking NONE of the load. The only remaining explanation is that the epoxy is merely a micro-scale glue. It basically shatters everywhere and only the alloy is load-bearing. Is that also your understanding? $\endgroup$
    – Sam7919
    Nov 19, 2022 at 15:14
  • $\begingroup$ @sam, there are many stressed airplane components held together with glue. There are bicycle frames that are glued together too. I do not know enough about how this crank arm was designed to comment further on it- NN $\endgroup$ Nov 20, 2022 at 1:28
  • $\begingroup$ Fair enough... Moving on... $\endgroup$
    – Sam7919
    Nov 20, 2022 at 3:39
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It does nothing for aerodynamics, the cavity makes the thing bigger and hence producing more drag (a very hypothetical reduction of Cd cannot change that). The point is simply the weight.

Perhaps it is also somewhat better to talk about stiffness rather than just strength (which may be an ambiguous term). You need the cranks to be as stiff as possible when doing sprints on high-end cranks.

Has the manufacturing process for R8000 series improved over R6800? Who knows, but the combination of two parts is still there and the failure is still possible. For example, https://yacf.co.uk/forum/index.php?topic=112701.0

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