Definition of "unsprung"

The locomotive is carried on two six-wheel bogies, the outer axles of each bogie being driven by series type d.c. motors, completely suspended to reduce the unsprung weight to a minimum.

1947 January and February, “Gas Turbine-Electric Locomotive for G.W.R.”, in Railway Magazine, page 26

Amongst the factors peculiar to electric m.u. traction are the heavy unsprung weight of the motors (generally, the greater the weight, the worse is the riding); […] .

1960 February, “Letters to the Editor”, in Trains Illustrated, page 125

He had an unsprung trailer for the job[.]

2000, Bob Foster, Birdum or Bust!, Henley Beach, SA: Seaview Press, page 169