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.

• 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.
• 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.

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