Doing a build for my daughter. One way to keep costs down is to go for a square taper chainset – but is there an easy way to work out what length spindle I will need?
Or should I just wait for a cheap set of through axle cranks come up as I have a Shimano BB already….

In normal use (ie going for a ormal chainline) then the spindle length depends on the make/model of crank. If you’re lucky you’ll find it listed in Sheldon Brown’s database here: http://www.sheldonbrown.com/bbsize.html

If not then 113mm is the usual middle ground for an MTB triple.

Thanks – really helpful!

It’s quite simple but hard to explain. Fit the cranks with any bb, preferably the correct shell size. Take the chainline measurement and adjust the spindle length size to suit. Chainline sizes for double triple etc are googleable. Remember to take into account that you will need to double the adjustment to the length. Ie two millimetres chainline will require a four millimetre axle length adjustment.