Hi John,
Been awhile since I have seen the bathroom scales used. As I recall, you need the "big boy" scales (450#-500#), As Jim pointed out, there was some kind of tripod rig that sat on the scales. Perhaps you could make due with a wheel load checker http://www.speedwaymotors.com/DECO-Hydraulic-Wheel-Load-Checker-0-600-Lbs,32517.html about $190 from speedway motors while you save to buy a set of scales,
. Just sayin'. You want something that will give repeatable results.
Steve