equal chain tension?

I have just built up an old gazelle, but have found a strange problem with the chain tension. It seems when i have the drive side crank pointed upwards the chain is a bit loose, but when the crank is pointed down the chain is much tighter. I have checked the axle and it is not crooked, so what else could it be? My only guess is that the bottom bracket spindle is somehow not straight, or my chainring or cog is slightly elliptical. Anyone else have this problem?

Your chainring or crank spider are eccentric, just like almost all of them are.

Sheldon Brown has a method for seating the chainring properly but I haven’t tried that myself.

It’s not really a problem, unless it annoys you.

Hey thanks, i just read the sheldon brown article and will adjusting my chainring very soon.

as long as it’s not too extreme either way its fine.

after buying new cranks, getting a chain-ring cut and changing the bottom bracket I am still getting significant fluctuations in chain tension. I was about to explode and sell all my bikes when I managed to work out that the tension isn’t consistent in relation to crank position which to me means that it HAS to be uneven “stretch” in the chain, most likely caused by the really really tight spot on the old chain-ring/crank combo. much relief.

Is it consistent in relation to rear wheel position (eccentric cog)?

I haven’t checked but i’d be pretty surprised if it was the cog. Not hard to check though