GMC Western States

 Tech Center Number 6 December 1993

With all the recent talk about tires and tire wear at the past three Roundups, we thought it best to provide some basic terminology and diagrams of the GMC front end and steering system. The GMC uses a parallelogram steering system. The system gets its name because all of the components move in parallel directions. They keep the front wheels pointed not only in the desired direction, but also in correct relationship with each other. The components of the steering system are illustrated below. The system has common pivot points on one side of the vehicle connected to corresponding points on the other side of the vehicle. When one component moves, it forces all of the other components to move in the same or parallel direction.

The components of the steering system consist of:

Pitman arm. The Pitman arm is connected to the steering gear by a steering sector shaft. As the steering wheel is turned by the driver, the pitman arm swivels back and forth, changing the rotating motion of the steering wheel into a side-to-side motion.

Relay rod. Often called a "center link", the relay rod is a rigid steel bar that moves the other steering components back and forth. One end of the rod is connected to the Pitman arm and the other end is supported by the idler arm.

Idler arm. The idler arm holds up the relay rod and moves in the same direction or parallel to the Pitman arm. The other end of the idler arm is connected to the frame.

Tie rod. ends and sleeves. The tie rods are the connecting steel bars between the steering arms on the spindle and relay rod. Ball and socket type joints at both ends allow swivel action to occur. Tie rod ends are specially designed to join the tie rod with the steering arm. The ends are threaded on the rod end to permit adjustments by the sleeves. By rotating the sleeve, the rod is moved in or out, shortening or lengthening the entire rod assembly.

Normal wear and operation of the Motorhome can cause suspension and steering parts to change their relationship to one another. Bumps, holes in the road and running into curbs can cause parts to bend slightly, again changing their relationship. In order to find what parts are not within specification, a thorough inspection must be made of the coach. Part of the inspection will be visual while special equipment is necessary to complete the remainder of the inspection.

The following items should be checked before the front end is aligned.

Tires. Adjust tire pressure to the amount the tire manufacturer recommends. Check the tires for excessive or abnormal wear. This can be a clue to what is wrong with the coach. Also, be sure that the same type and size tire are on the same axle.

Ride height. A very important measurement which must be taken during the pre-alignment inspection. This is the distance between various locations on the coach and the ground. As with curb weights, manufacturers set the alignment specifications with the coach at a specific ride. The height must match that specification for a proper alignment.

Wheel bearing adjustment. Lift the front end off the ground grab the wheel at the top and bottom and wiggle, there should be no play felt. The applies to side-to-side play as well. GMC bearings cannot be adjusted, if they are excessively loose they must be replaced.

Idler arm. Wear of the idler arm is caused by contaminants such as oil, dirt and rust. An idler arm which is in poor condition can cause abnormal tire wear as a result of toe change, as well as steering wander and excessive free play in the steering wheel.

Relay rod. The relay rod should be inspected to see if it is bent or if the tie rod, idler arm or Pitman arm stud holes are loose. As with the idler arm, any defects found will require replacement of the rod.

Alignment. There are five different alignment angles which must be checked When inspecting for correct alignment. The angles are Caster, Camber, Steering axis inclination, Toe-in/out and Toe-out-on-turns. Of the five angles, only three are adjustable, Caster, Camber and Toe-in/out, the latter angle causes the most severe and the quickest tire wear problems when incorrectly set.


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