Inconsistent chain tension on single-speed conversion
I converted an old ten-speed to single and now the chain is alternately too loose and too tight. The problem seems to be related to the position of the cranks.
The chain is new, so stretch shouldn't be an issue. There's no obvious play in the BB or freewheel and the chainring appears to be running true both vertically and laterally. Unfortunatly I can't swap the chainring to to check against another 'cause the bolt circle isn't compatible with anything else I have handy.
Either your chainring is out of round, your bb spindle is imprecisely milled or your crank spider is a bit out. Probably a bit of all three. Just about every drivetrain component made will have this issue, and you only really notice when you go to singlespeed and dont have the derailer taking up slack. You can try and minimize the problem by rotating the ring on the spider or the crank on the bb spindle, but the only sure fire solution is to buy very expensive, high precision track parts like Sugino 75 bb, 75 cranks and zen ring. For most people this is unnecessary though ---just do the rotatey trick to try and minimize the problem, then set chain tension so it's good with the cranks at the tightest spot, and live with the variation.
DaBomb Single (urban), Cannondale fixed (road), Electra Straight 8 (beach)
Pretty much all not-great equipment (b/b +crank+chainring) equates to the tight/loose chain syndrome. Higher quality track-specific stuff seems to have higher tolerances and less likelihood of the tight/loose chain syndrome. Running a heavy chain helps somewhat, they run straight and true unless you're really torquing on it. Or upgrade your goodies. Dropping a chain at full tilt sprint sucks all kinds of ass though.
another thing to look for is the BB spindle being slightly bent. i've seen this a few times on loose ball style BB's. another thing is the ring being slightly off center on the bolts. taking them off and then carefully reseting the ring could fix it too (Sheldon brown describes this). just look it over real carefully to identify whats off.
a sugino 75 or other expensive track part is not the cure of all ills (sigh). i've set up plenty of single speeds with run-of-the-mill used components to not have tension problems. just make sure the parts you do use are solid and set up properly.