[Retric]

Yea, but the wheel is more than just the rims. Play around here http://www.wheelbuilder.com/wheel-weight-calculator.html. The Rim is generally a large fraction of total wheel weight, but both spokes and Hub's are important.

On a bike the total weight also includes the fork and parts of the suspension. Not sure about wheelchairs, but I suspect deforming the center of the wheel still easily cuts unsprung mass in half.