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Cranks seating too far on spindle?

Old 04-14-21, 05:59 PM
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Shinkers
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Cranks seating too far on spindle?

Hello,

I've installed a set of FSA square taper cranks to a square taper spindle and the crank is covering up the flats of the spindle completely. In the past with other cranks and BB's I've been able to see the bottom portion of the flats going into the crank (hopefully that makes sense). The portion of the crank that mates to the spindle seems to be thicker than the Shimano cranks that I've used in the past.

I'm wondering what how likely it is to draw a crank too far onto the spindle and possibly split it at some point. I know this can be an issue with older, worn cranks that have been swapped several times or overtightened multiple times.

If I remove the cranks and look, I can see where the end of the spindle seems to be seating inside of the taper of the cranks so all should be fine, I've just never seen something like this before. I know that both of these tapers are JIS tapers.

Hopefully this makes sense. It's sort of hard to explain.

Thanks for any info!
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Old 04-14-21, 06:03 PM
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Old 04-14-21, 08:06 PM
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Could the bottom bracket be a Dura-Ace with a "low profile" J.I.S. taper? https://www.sheldonbrown.com/bbtaper.html
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Old 04-14-21, 08:32 PM
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Is there a hollow on the inside of the crank arm where we can't see the end of the taper of the spindle? Does the crank bolt seem to "bottom out" abruptly as you tighten it, or does it reach max torque gradually? I'm inclined to say that as long as the crank bolt isn't pulling the spindle all the way through the crank, the taper isn't bottoming out, and everything should be copacetic.

By the way, are we looking at a unicycle? If so, I'm not surprised that the machining of the tapers is a little different from Shimano's. I'll bet that one reason you're not seeing the ends of the tapers is because this axle is thinner than a Shimano BB spindle.
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Old 04-14-21, 08:57 PM
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This is a unicycle. There is a slight recess on the inside of the cranks but not much. As I torque the cranks it's definitely gradual. I'm also not surprised that the machining may be different but I am told that it's a JIS taper...

Thanks!
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Old 04-15-21, 11:58 AM
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There's a range of tolerance of manufactured products, like crank arm tapered holes and the "intended" BB axle's tapered ends. Being a taper of only 2 degrees a small amount of crank arm hole wear or out of tolerance of either the arm or axle of .002" results in the arm changing it's position on the axle by about .057" Or in metric for every .01mm of taper "wear" the arms slide up the taper by almost .3mm. So a minor amount of arm hole wear (expansion often due to poor draw bolt torque maintenance) becomes a large amount of the arm drawing further up the axle.

This is why it is suggested to not grease the tapers, to use a torque wrench to install and to further check after a few rides allow complete settling in of the fit. Of course mixing taper designs brings even more miss fit issues as mentioned.

Last tidbit- Unicycles are far more abusive to their crank arms as there's a constant back and forth stress cycle on the arm/axle fit due to the balancing/stalling efforts. The need to stay on top of (bad pun) the arm fit maintenance is greater then with a bicycle that sees lower loading and less back and forth forces. Andy
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