on wheels vs portable

on wheels

prep
  • Used other than figuratively or idiomatically: see on, wheels. (Having wheels affixed to the bottom, and so transportable; synonym of wheeled). 

  • Mobile; able to be moved; especially, synonym of on the move. 

  • To a large degree, excessively. 

portable

noun
  • A trailer or other movable structure used for classes at a school. 

  • A hand-held video gaming device. 

adj
  • Of software: able to be run on multiple hardware or operating systems. 

  • Able to be carried or easily moved. 

  • Of a program: able to be copied directly from one machine to another without the use of an installer. 

  • Able to be transferred from one organization to another. 

How often have the words on wheels and portable occurred in a corpus of books? (source: Google Ngram Viewer )