underframe

noun

Etymology: From under- + frame.

1. The supporting structure of a piece of furniture, a vehicle, etc.

   “The main fuel tank is slung from the underframe midway between the two bogies, and its capacity of 420 gal. supplies both the main engine and the train heating boiler.”

   “Each half-section of the battery weighs about eight tons, and the two underframes had to be strengthened to take this weight.”