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This question got me wondering about self-extracting cranks: What bottom brackets are compatible with Miche Evo Max cranksets (self-extracting type)?

Where I've seen them on cartridge bottom brackets, I take it for granted that the crank bolt tightens the crank onto the splined axle and helps hold it there (i.e. 40Nm torque).

But, how is the crank held onto the axle, are the splines tapered and holding everything together (once it's on, it's on), or is it reliant on the crank bolt keeping it together?

Secondly, with external bottom brackets (or press-fit), how is bearing preload managed for self-extracting cranks? Seems to me that once it's on, it's on, it won't go on any more! if the spacing between the cranks is off just a bit, I don't see how it gets adjusted (maybe spring washers?)

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Designs that use self-extractors and either a splined or lobed interface between the crank and spindle all feature a press fit at that interface to prevent any small movement that would otherwise cause damage. Otherwise you wouldn't need a self-extractor; remove the bolt and your crank would fall off. They're also reliant on the crank bolt being present and torqued properly to prevent them from coming loose.

Preload is managed on such cranks in a couple different ways. A wavy washer or an elastomer can be used to establish preload automatically. These have have the downside that BB width tolerance has a direct impact on how frictiony or loose the bearings end up being. The SRAM/Truvativ GXP design pinches the left bearing and allows the spindle to float through the right one. And then there are the cranks with adjustable preload rings, which are good at allowing you to always have a perfect adjustment but by the same token add a maintenance point.

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  • Lovely answer. Seems to me that GXP is easy to install (no washers to lose) and gives fairly automatic preload (just tighten up the crank bolt). Does this bring any particular disadvantages? – Swifty Jul 28 '18 at 10:18
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    Good question, seems like it probably depends on who you ask. There are a lot of questions when it comes to bearing design and production for bikes that people will give different spiels on but I'm fairly convinced you have to be a bearing engineer to get to the heart of. The floating bearing thing is used in some applications to give amazing service life. GXP bottom brackets last okay but aren't really ahead of the curve there. The design allows some dynamic movement between the spindle and the right bearing, which can cause noise and wear in theory, but again GXP doesn't stand out here. – Nathan Knutson Jul 28 '18 at 16:58

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