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Are Bolted Spindle and Spyder Cranks less Durable for Daily Use?

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Are Bolted Spindle and Spyder Cranks less Durable for Daily Use?

Old 11-22-20, 01:38 AM
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SuperPershing
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Are Bolted Spindle and Spyder Cranks less Durable for Daily Use?

Are Bolted Spindle and Spyder Cranks less Durable for Daily Use? I have heard the Spyder of the Rotor 3D Fixed Gear Cranks break. And are fixed Spindle and Crankarm a good choice for bombing the Streets(And Racing of course)?


Example of a Broken Rotor from my Friend


The Crank Im Planning to Buy, It has a similar design to it

And compared to this one, The Fixed Crankarm and Spindle. They're more Durable Right?
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Old 11-22-20, 09:59 PM
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Old 11-23-20, 04:27 AM
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Originally Posted by SuperPershing View Post
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Forum is not a live chat. No need to do that.

As for the question, first to note that I'm not a mechanical engineer (though I suspect that people who approve and marked many cycling-related designs aren't either) and hope those will correct me if/where I'm wrong. Now that's out of the way:

There is nothing really inherently wrong with bolted on chainrings. It boils down how well designed (and manufactured, including quality control) the cranks are.

When it comes to that: Rotor, as well as many other manufacturers (including, but not limited to Shimano) make their modern cranks as if they are not loaded through pedals. As your pictures show, crank is rather thin in one cross-section, and wide in the other. Which would probably make more sense if you stepped directly on the crank. But, since the pedaling force is done over pedals, those cranks experience most of their load in a bit of a bending direction, that is tryng to sort of "twist" the crank (if I've explained that correctly). Most extreme case is when a pedal is down (at 6 o'clock) and you are weighing it with your entire weight. That exerts a lot of bending force, and modern cranks aren't designed to be strong in resisting that kind of force (which is exerted over their most narrow cross-section).

For such loads, a better profile would be more square (closer to round) than a rectangle that has one side (the one going from bicycle, outwards) much longer than the other (the one in line with the bicycle's longitudinal axis).

It looks sleak, "aero", and quite beefy - when looking from the side at least. But it is not what I'd call a good design.

Another place where cranks often fail is at the pedal-to-crank interface. Solution has been tried by late Jobst Brandt decades ago, but no one seems to care. A bit more info on that:
Pedal to crank interface problem - and solution.

Third place where cranks ususally break is between the spider ("arm"?) that holds the chainring(s) that is just behind the crank (as it turns - trailing it). Sheldon Brown's site has an article (written by Jobst Brandt) that explains it (Cracking/Breaking cranks):
"The trailing spider leg adjacent to the crank generally has a thin web that connects it to the more rigid shaft of the crank. Stress is concentrated at this web, while the three preceding legs are more flexible. Spider-leg cracks are relatively benign because they are easily seen and rarely progress to failure."

I'm not sure if this problem is more easily solved using modern 4-arm spider design (more space between the crank and the spider amrs).

Either way - all these failures can be noted in time, when they are just small, hairline cracks (they don't grow much wider, just longer, until the cranks "suddenly" fail). Wipe, inspect, every week if you are a heavy rider.
None of these failures have anything to do with the way chainrings are attached to the crank, so no need to worry about that.

Are there any manufacturers who are willing to build stuff to be durable, robust, and safe for that matter? Not that I know of, in spite of their marketing. Just inspect regularly and "don't ride (too) hard" (if we can't at least laugh, what else can we do? ).

Last edited by Bike Gremlin; 11-23-20 at 04:30 AM.
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