Noun : the underframe of a bus.    From Dictionary.com. 

The seat can also be removed from the underframe for use as a pouffe.    From Wordnik.com.  [The Drops Chair by Camilla Hounsell Halvorsen] Reference

The underframe is provided with spring axle boxes, and also with spring buffers and drawbars.    From Wordnik.com.  [Scientific American Supplement, No. 1157, March 5, 1898] Reference

For gages of 20 inches and upward the motors can be mounted on springs and attached to the running axles inside of the wagon underframe.    From Wordnik.com.  [Scientific American Supplement, No. 1157, March 5, 1898] Reference

Then, returning to the bed, I clasped it by its underframe and, with a grunt of effort, dragged it towards the center of the room until the loosened board was fully exposed.    From Wordnik.com.  [Nevermore] Reference

The seat is not permanently fixed to the underframe, but the inner ring in the seat fits over the outer ring of the underframe so that the chair can be tilted into the desired position by the user.    From Wordnik.com.  [The Drops Chair by Camilla Hounsell Halvorsen] Reference

Kathy contacted the show about the "underframe of a boxcar in our basement," she recalls.    From Wordnik.com.  [Denver Post: News: Breaking: Local] Reference

Its place was then supplied by other and better vehicles, — though they were no other than old stage-coach bodies purchased by the company, and each mounted upon an underframe with flange-wheels.    From Wordnik.com.  [Lives of the Engineers The Locomotive. George and Robert Stephenson] Reference

As further endorsement of the standard V8 Vantage underframe, the active safety systems are directly carried over to the Vantage GT4, including: dynamic stability control, traction control, ABS and electronic brakeforce distribution, providing reassurance and enhanced control in slippery conditions.    From Wordnik.com.  [Jalopnik] Reference