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I think the title is self explanatory: why wouldn't either one or the other be enough?

After removing the bolt, you still need a specific tool to pull off the crank arm from the spindle, because the crank's teeth are holding to the spindle so strong, that you cannot just remove it by hand (so it is even much less probable that the arm falls off by itself, even without the bolt). What is the bolt for then?

Alternatively, the bolt already presses the crank arm to the spindle very strongly, why do we need them to be engaged with the extra teeth then? And due to this also a specific tool to remove the crank arm is now required, whereas with the bolt only a standard tool (either a hex wrench or a socket wrench) would be sufficient. (What's even worse, you risk ruining the thread of the crank arm, if you are not careful enough with this tool.)

To be clear: I know there must be a good reason to have both, I just want to better understand what this good reason is.

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The teeth (splines) on ISIS, octalink etc. or the square taper on older spindles do indeed grip the crank arm pretty tightly once it's been forced on hard. But what forces it on hard? The bolt. But the loading on a crank arm is really high. You're stamping on it with a significant fraction of your bodyweight a few hundred times per km. As a crankset might stay on for around 10 000km, you're stepping on it a few million times. A lot can go wrong in that, and painfully if the crank falls off when riding. This (and associated, potential expensive damage to the bike) is why I don;t suggest to do the experiment.

Consider 2 cases:

  1. If you had a round spindle and a single bolt, the torque you'd exert when pedalling would tend to rotate the crank arm after the spindle (or around the bolt depending on the exact construction). You'd also have trouble getting the cranks perfectly positions at 180° to each other. Note that I use single-bolt hardware quite a lot in work. There the torque is far lower (finger vs leg pressure) and the loading far less frequent. They still move annoyingly, sometimes even when torqued up fairly hard for their much smaller size).
  2. If you just had a press-fit, no bolt, you'd be vulnerable to loosening with cyclic loading, temperature changes, and sideways forces. So the bolt secures the splines (just as a cassette lockring does)

If you've ever removed a crank that's just been fitted, you'll find it's far easier than when it's been on for some time (though you're still likely to need to use a crank puller or at least hit the other side). The metal sticks together over time. We use grease to reduce but not eliminate this.

I've damaged the crank arm threads with a crank puller, but I was being stupid (left the octalink adaptor on when pulling a square taper crank, so it didn't engage all the threads, and wasn't pushing against the spindle. It's very annoying but not the end of the world. Gear pullers, leverage against the BB lockrings and hitting the other side with a mallet (and a piece of wood in between) will recover from such silly mistakes.

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  • If the bolt has been tightened with the specified torque it would still require the specific tool to pull it off the spindle. Because the first 50 or so km may loosen it the slightest bit, it is strongly recommended to check the bolt.
    – Carel
    Feb 27 at 18:52
  • @Carel I take it you're referring to the penultimate paragraph? Even if you have to use the same tool, you don't have to work nearly as hard to do so than with a crank that's been on for a year, even with square taper. Still less for octalink, where I did use a carpenter's mallet to remove a freshly fitted crank because I'd mislaid my crank puller. Definitely check after a bit of riding.
    – Chris H
    Feb 27 at 20:06
  • It would be possible to use one or other, but the weight of the required components would increase. BB and crank sets have been though a very large number of iterations over the last 2 decades, current iteration is very different and more refined than the interference fit of older generations BB's/cranks.
    – mattnz
    Mar 1 at 1:18

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