radius-bar

from The Century Dictionary.

• noun In a steam-engine, one of a pair of rods pivoted at one end and connected at the other with some concentrically moving part which it is necessary to keep at a definite distance from the pivot or center. Also called radius-rod and bridle-rod. See cuts under grasshopper-beam and paddle-wheel.
• noun A link or bar connected with the front truck of a locomotive so as to permit it to swing laterally enough to adjust itself to track-curves of varying radius without stress upon the frame.