Originally Posted by
Artmo
In a word "momentum." Mass x Velocity = Momentum (force)
The higher the mass, the higher the force against the wind (which is about 80% of the resistance to be overcome on a bike), so speed increases; then momentum (force) increases even more as the speed continues to increase.
It's very common to see heavier riders on solo bikes coast faster downhill, even when all have light bikes.
Coincidentally, I noticed yesterday that my max speed coasting down a bridge on my renovated 22 lb bike was about 4 mph faster than I usually do on my Scott, which weighs about 17 lb
Again, its rider position on the two bikes.
The 5lb weight difference explains .6MPH difference on a 8% grade, with an average weight rider, with the speed for the heavier bike being 40.8mph, and the lighter bike 40.2mph.
Aero differences of the equipment could also explain a few tenths mph. But to get the 4mph difference you've got to change rider position.
http://www.kreuzotter.de/english/espeed.htm
For the tandem the 14 pound difference makes a .9mph difference in speed. The 50lb tandem coasts down an 8% grade at 50.7mph. The 36 pound tandem at 49.8mph.