underrunner

noun3

1. •
   An additional runner added to the bottom of a sledge to protect the main runners in harsh conditions.

   • Less than half a mile to the south the party sighted an underrunner of a sledge, and assumed it to mark the location the Norwegians had assigned for 90°S. — 2002
2. •
   A runner (mechanical part intended to guide or aid something else to move) designed to be inserted underneath the item to be moved.

   • A triple tandem tow using underrunners. This method has been used for long ocean tows by tugs with a single-drum winch. — 1989
3. •
   The lower of a pair of grinding millstones.

   • FOR SALE AT A BARGAIN. […] Four run stones, Munson underrunners, 3 ft diameter. — 1890