I'm able to, using my hands alone, wobble my crank arm about 1/8" to the left and right. I can only guess what this means for my riding performance, but I imagine it explains why when I pedal really hard my chain scrapes my front derailleur no matter how I fine tune it (it's non-indexed).
Does this mean that the bottom bracket isn't adjusted properly? Or, worse, that the BB is shot? My setup is about 12 years old.
Salsa Las Cruces, Burley R&R and a boat load of others.
If both arms move it is the BB. If it has been that way for a while you probably need a new one. If only the gear side crank arm moves maybe all you have to do is tighten that side. Does the crank us a cotter pin or 14or15 mm center bolt? Avoid riding it untill you can get it fixed.