Tightening up the crankset onto square taper BB.

[fossyant]

Most likely the chainset isn't manufactured very well, and the rings aren't completely true. I don't have any issues on my various shimano and SRAM cranksets - these all run completely true. The only ones that can rub, is on my daughter's bike as it's a pressed steel chainset - it doesn't run true.

 

[Ajax Bay]

The suggestion is that if both the spindle and crankset have slight misalignments, rotation by 90 degrees might find a 'best' position where the misalignments cancel one another out, to an extent.
This has the associated benefit of evening out wear on the chainring(s) which will be greater at a specific angle to the crank (about 70-110 degrees) as the greatest force is applied to the pedals, twice each cycle.
Bit windy here!

 

[raleighnut]

I asked on bicycles.stackexchange, got a great answer about manufacturing tolerances: https://bicycles.stackexchange.com/a/82894/46010

Yep sums it up nicely.

 

[Twilkes]

The suggestion is that if both the spindle and crankset have slight misalignments, rotation by 90 degrees might find a 'best' position where the misalignments cancel one another out, to an extent.
This has the associated benefit of evening out wear on the chainring(s) which will be greater at a specific angle to the crank (about 70-110 degrees) as the greatest force is applied to the pedals, twice each cycle.
Bit windy here!

That's interesting, I would never ever have thought about rotating chainrings to even out wear, but now you've written it down it's obvious, even if it's just once to move the 'dead spots' around.

 

[raleighnut]

That's interesting, I would never ever have thought about rotating chainrings to even out wear, but now you've written it down it's obvious, even if it's just once to move the 'dead spots' around.

Rotating the crankset also evens out wear on the BB spindle, that is assuming you are rebuilding/regreasing the same spindle setup.

 

[Ajax Bay]

I don't think rotating is worth doing for that. And if you think about it, you don't (well I don't) check whether you put them the same you took 'em off. So I guess there's inadvertent swapping around.

 

[Twilkes]

It's my wife's bike and I've never worked on square taper before. Rotated 90 degrees and that seemed to sort the problem, at least no discernible chainring wobble that I could detect with the chain off, using the derailleur as a sight; but also refitted in the original position which also seemed to resolve the problem, so maybe it was just fitted incorrectly at some point in the past.

Took the opportunity to dismantle and clean the crank and chainrings and they fit flat against a flat surface so all good. Hopefully all's still good once everything's dried and back on the bike again!