I've converted an old peugeot to singlespeed--eventually it'll be fixed, when I can afford a new wheel, etc. Until then, I've taken off the freewheel w/cogs and replaced it with a 16t BMX freewheel. I shortened the chain and, using the original outer cog on the crankset, put together what looked like a beautiful 52-16 (16-52?). I was so psyched. I took it out to test ride immediately.
With each downstroke on the cranks, the rear wheel was somehow pushed out of whack. The whole wheel was pushed into the left rear fork with the pressure on the pedals and with the wheel at that angle, the chain even seemed to skip teeth on the rear cog.
When not pedaling, the rear wheel is good. The chainline is straight and the chain sits well on the teeth of the BMX freewheel. Is there any way to stabilize the rear wheel? Is something else going on? Let me know what you think/what I'm doing wrong or if this has happened to anyone else. Thanks for any suggestions or ideas.