I'm not totally new to cranking in bikes, but I want to upgrade the crankset on mine and am seeing that they all indicate what size bb spindle length they are compatible with. I understand the importance of spindle length on the alignment with the cassette, but why does it matter, when it comes to the crankset itself? Thanks!

Line axis being proper reduces premature wearing.
side loading
chain sucking
shifting efficiency
clearance concerns

Different cranksets sit in different places on the taper of the spindle. You would think that all would sit say 14mmm onto the taper so any two cranksets would sit it the same place with the same chainline. One would think two countries separated by 12 miles of water would use the same bottom bracket threading. (English and French. They cannot even agree which direction to thread the cups. One would think the bicycle industry would get itself together and create standards. (Scratches head.) But they have! Dozens of them! There are standards to fit any possible application(/nationality/units system/you name it).

rant, rant. But you get the point.

And then there is the fact that square taper cranksets tend to sit progressively further in as they get re-assembled. Sometimes so far that the crank starts to hit the chainstay and therefore now requiring a longer spindle (or shims or the bottom bracket cup needs a washer to space it out).

Ben

To expand on the previous response a bit: Cranks can have different offsets of chainrings and arms relative to the spindle hole. Those are chosen by considering the engineering implications of those offsets, along with frame clearances, ankle clearance, and chainline. Why some companies choose one package of offsets or another is a complicated question that has to do with the above, plus manufacturing, materials, aesthetics, etc. Pretty hard to cover in a forum post.

Latest stuff eliminated the choice .. tube spindle fixed to crank arms.

Spindle too short, and the chainring hits the chainstay.

Spindle too long, and chainline offset makes shifting suffer.

BITD, you could make small adjustments to "Q" (lateral pedal spacing) with different length spindles. Petite riders might like lower "Q", while duck-footed galoots (like me) might be more comfortable with more "Q".

As Bob says- that's gone by the wayside with modern two-piece cranks.

I've been tempted to increase Q just for the duck (non webbed) feet. Unsure if it's even possible based on my reading-google-lbs-searching.

You could opt for pedal spacers instead of changing your bottom bracket and affecting chainline. Pedal spacers would even give you a little more clearance between your heel and the crank arm.

Aka; Knee Savers... I used a set on a bike tour that promised to be wet... West Ireland..

moved the pedal out so my shoe-covers cleared the crank-arms.



https://www.bikemania.biz/knee-savers...mbent-bikes.ht

I honestly had forgotten all about the pedal spacers being an option. Thanks!

the link yields a 404 error.

I see SS & chrome moly as an option. Pretty sure one will gull & the other would eventually rust.

Is that a west coast thing? Actually (assuming the crankset and spindle are compatible) they sit in the same place. What is (may be) different is the distance from this point to where the rings and pedal threads end up from crank to crank.

Semantics. For a given chainline, different cranks have their tapers in different places or for a given taper, different cranks have different chainlines. Same thing. I don't care. To place different cranks in the right place for chainline and to not hit chainstays and chains, different BB spindles are often required to get those two cranks to do the same thing.

Ben

Back in the olden days, crank arms were designed to be at a right angle to the bottom bracket spindle. That required a bottom bracket spindle in the 120's to space the chainring out far enough to miss the frame's chain stay.

Modern square taper cranksets have the crank arms angled or offset outward a skosh. Some are offset a little more than others. The good news is that they allow much shorter bottom bracket spindles to be used. The bad news is that now you have to match the bottom bracket with the crankset.