kingbolt

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A vertical bolt that joins the body of a wagon or other vehicle to its front axle and usually acts as a pivot.

from The Century Dictionary.

• A large bolt connecting the fore part of a carriage with the fore axle. The axle rotates about it as a joint “when the carriage is turned.
• A large bolt which passes through the truck and body-bolsters and center-plates of a carbody and the center of a truck.
• Also kiny-pin.
• noun An iron rod in a roof- or bridge-truss, used in place of a king-post to prevent the inclined members from allowing an increase of the distance between the tie and the compression pieces.

from the GNU version of the Collaborative International Dictionary of English.

• noun A vertical iron bolt, by which the forward axle and wheels of a vehicle or the trucks of a railroad car are connected with the other parts.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The main bolt of a structure, especially the bolt in a motor vehicle that links the chassis to the axle, providing the steering pivot.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun bolt that provides a steering joint in a motor vehicle