so i just finished building up my first fixed gear, and im noticing that if i try to accelerate very quickly or skid, my cog is slipping. I can't figure out how to get the cog and lockring tight enough so that this doesnt happen (the hubs are new and definitely not stripped). I tightened the cog down with a chainwhip and held it there while i tightened the lockring down with the proper spanner. my only difficulty is that the lockring spanner doesn't fit on really snugly when the chainwhip is on the cog, because the chain gets in the way a little bit. any suggestions? i thought about that rotafix method, but the chain is then still in the way as im trying to tighten the lockring.