[alexrex20]

my MTB frame has a 68mm Euro shell. i realize i need a Euro BB but i'm wondering what minimum spindle length is required. i know profile makes a 6in spindle, but i found some Profiles that come with a 5.7in spindle. is that long enough?

my frame is an AtomLab TrailKing btw

 

[sittingduck]

It's long enough, if you don't have to space your cranks out too far from the frame.
A small chainring helps (28 or smaller).
Your chain line can be a little tricky with a single speed hub, since the wider spacing tends to make you want to space the drive side arm out a bit.

 

[escapeartist]

I had one of the 5.7 spindles on my and it wasn't quite long enough. Once I got the arms spaced out enough to clear the chainstays there wasn't enough spindle left for me to feel comfortable riding.