Originally Posted by strongbad
If the scale(s) are accurate, and the bike is level, the weight measured with a scale under each wheel then added, or a scale under one wheel at a time then added, is accurate. The key thing is that the bike must be level.
Experimental aircraft builders get weight and balance info this way sometimes.
That's what I did.... just used a bathroom scale that sits about 2" off the ground under one tire, put a 2x4 under the other wheel, then switch and add.