Originally Posted by
Gresp15C
Last fall I discovered that my single-speed has a seriously "out of true" crank. I haven't had a chance to diagnose it more closely, and it doesn't look like it's been bent or the square tapered hole damaged. And the spindle runs true. So it's a mystery, but the chainring sloshes from side to side by about 1/8 inch. It's off the bike for now, as I had a spare.
The problem isn't that rare, and has become more common with the expansion of BBs to all sorts of makers beyond the original few that sold cranks also.
What I often see is a spindle shaving one side of the square tape as the crank is pressed tight. It takes very little to drive the system off center to a degree that's very obvious at the chainring. If you go back and look at the spindles produced by the likes of Shimano and Campagnolo, you'll see a machined lead taper to ensure that there's no sharp edges. However many other brands skip this (IMO) critical step and produce spindles where the taper runs to the end, leaving a sharp corner.
Skis have the tips curved up so they float on the snow. Can you imagine skiing if the flat boittom went all the way to the front.