Self extracting crank preload

Question

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?)

Answer 1

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.