Chainwheel sorting, Where to start?

[presta]

No it does not. @presta is dealing in less than a mm possible differentials here.

BB lengths vary by a lot more than a millimetre. If you have a chainset designed for a shortish BB mounted on a longer BB it will increase the chainline, and that may amount to enough to run out of adjustment range on the front derailleur.

If the three rings of a triple are equally spaced then clearly the middle ring is is the 'centre' from which to measure.

Yes, but the three rings aren't necessarily equally spaced, which is why the measurement is defined in terms of the midpoint between the outer rings and not the centre of the middle ring. That difference is small though.

 

[Ajax Bay]

BB lengths vary by a lot more than a millimetre. If you have a chainset designed for a shortish BB mounted on a longer BB it will increase the chainline, and that may amount to enough to run out of adjustment range on the front derailleur.

Yes, but the three rings aren't necessarily equally spaced, which is why the measurement is defined in terms of the midpoint between the outer rings and not the centre of the middle ring. That difference is small though.

Of course spindle lengths of ST BBs vary and chainline matters.
@presta: "the centre of the chainset (defined as halfway between the large and small rings, which is not necessarily the centre of the middle ring)."
@DaveReading asked "If it [the centre of the chainset] isn't [middle ring centre], does it cause shifting issues ?"
I asserted that the difference between the centre of a middle ring and halfway (the mid-point) between the inner and large rings of a triple will be <1mm. As you say "that difference is small". Hence my answer to Dave: 'no'.
Given your special knowledge, I invited you to share with us which triples aren't equally spaced: we're waiting.

 

[presta]

Given your special knowledge, I invited you to share with us which triples aren't equally spaced: we're waiting.

It came from a Shimano document, but being as it was 15 years or more ago, I don't recall which one.

 

[Ajax Bay]

Sheldon on chainline (Shimano uses "middle ring" in its triple (road and MTB) specs)
https://sheldonbrown.com/chainline.html

 

[Andy in Germany]

Thanks for the suggestions; I'll probably get onto that next week as the boxes of cranksets are beginning to overflow, again. A lot of the extra information has been useful as well, not least that I'm not alone in having to resort to trial and error to figure which crankset works with which part.
Still, we're already a long way ahead of when I started, and had to explain that saving cranksets is a great thing, but you really need to make sure the crank arms are stored with them or there really isn't much point...

 

[Ajax Bay]

Thanks for the suggestions; I'll probably get onto that next week as the boxes of cranksets are beginning to overflow, again. A lot of the extra information has been useful as well, not least that I'm not alone in having to resort to trial and error to figure which crankset works with which part.
Still, we're already a long way ahead of when I started, and had to explain that saving cranksets is a great thing, but you really need to make sure the crank arms are stored with them or there really isn't much point...

Your OP and your subsequent comments allows an ontological approach to this for your maximum benefit: establish the most important discriminating criterion.
The most important category (for your purpose, imho) is whether the chainset is square taper (ST) or 'other' because most of the bikes will already have BBs fitted.
Then, for all the ST chainsets, the chainline which each chainset will achieve on a set length of BB spindle can be determined, as @presta noted, by torquing each chainset (RHS) on a benchmark BB (I suggest 127.5mm). Categorise them by the derived chainline dimension.
Although it'd be good to wire the LH crank to the rest of it, matching doesn't really matter, except aesthetically, so all the 'spare' LH cranks can merely be arranged by crank length. Once you've got a crankset (per chainline) chosen, if it's without its matching LH crank, all you need is one the same length.
HTH

 

[andrew_s]

Although it'd be good to wire the LH crank to the rest of it, matching doesn't really matter, except aesthetically, so all the 'spare' LH cranks can merely be arranged by crank length. Once you've got a crankset (per chainline) chosen, if it's without its matching LH crank, all you need is one the same length.

It's best to keep matching left and right cranks together, as LH cranks aren't always interchangeable.

a) some cranks have the square hole square to the crank, and some have it diagonal to the crank. If you end up with one of each, the left and right cranks will be at an angle of 135° to each other, rather than the more normal 180°.
b) square taper BBs are generally more or less symmetrical, with similar projection on left and right sides. If you use a short BB to match the RH crank, a LH crank from a chainset that uses a long BB will likely foul the LH chainstay.

This is especially annoying if you just went out and bought a replacement for a broken LH crank, only worrying about the length.