1/8" does it for us. It makes our robot rock back and forth about a degree. I can live with that. One thing we had a problem with last year though... when we welded our chassis, it curved upwards, so our middle wheel was more like 1/4" lower. When you're welding your chassis (if its welded) make sure to evenly weld it, dont do one side and flip it over to do the other, or your frame will banana like our 2005 bot
