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[GMCnet] Re: Centering the steering box [message #363619 is a reply to message #363611] Thu, 22 April 2021 14:19 Go to previous messageGo to previous message
Ken Henderson is currently offline  Ken Henderson   United States
Messages: 8726
Registered: March 2004
Location: Americus, GA
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Senior Member
Larry,

Thinking that there would be a good drawing to satisfy John L's
suggestion to familiarize yourself with GM front suspensions, I check
the web. NOPE! I found not one drawing worth referencing. So, my best
suggestion is to jack up the front end and safely block it, then crawl
under there on your back.

Once under the front of the coach, the arrangement and purpose of most of
the suspension will become quite clear -- and even the common names
assigned to most things may become obvious (here, I'll use what I know as
the common names for components; the manuals don't always use the same
terms): The wheels are mounted on bearings which run in "knuckles".
Those knuckles are attached to "Upper" and "Lower A-arms" through
"ball joints" which allow the knuckles to move vertically and to rotate
about their near-vertical axes. The A-arms are mounted at their wide ends
through "bushings" to the frame, restricting their movement to rotation
about their horizontal axes. As those A-arms pivot about their bushings,
they keep the knuckles attached to their outer ends approximately
vertical.

Since the knuckles can rotate about their vertical axes, rotating the
wheels with them, that rotation must be controlled. That control is
supplied by "tie rods", which are attached to arms on the rear side of each
knuckle via more ball joints, called "tie rod ends". Those tie rods, and
the knuckle arms to which they attach, are positioned about 1/2 way of the
vertical height of the knuckles (and, therefore, the distance between the
upper and lower A-arms). The lengths of those tie rods is approximately
the same as that of the A-arms (or the length one would require if it were
at that 1/2 height). This arrangement of A-arms and tie rods minimizes the
steering geometry variation as the suspension move up and down.

Almost coincidental to this discussion are the torsion bars which provide
the "springs" for the front suspension: They are hefty round spring steel
shafts which are hex-keyed into the lower A-arms at their forward ends.
Their aft ends are hex-keyed into adjustable "pork chops" mounted inside a
frame crossmember. Those "pork chops" have adjustment screws which enable
one to set the ride height -- but NOT by merely turning the screws --
they'll probably strip their threads if special torsion bar adjusting tools
are not used to relieve the weight from them.

Now, to the latest of your understanding problems: The left and right tie
rods must be connected together so that they synchronize the rotation of
the front wheels. So, the inner end of each of the tie rods is attached,
via tie rod ends, to the "Center Tie Rod" which runs horizontally behind
the "front" (main, under-engine) crossmember. That large, non-adjustable,
member is kept in its appointed location from all movement except
length-wise because it is suspended via rotation-only bushings from two
"arms" -- the "Relay Arm" and the "Idler Arm". The Relay Arm, at the top
left of the mail crossmember "relays" steering signals from the power
steering box's "Pitman Arm" to the tie rod. In addition to its pivot
point, it has two attachment arms, one for the Center Tie Rod and one for
the "Drag Link", which connects to the Pitman Arm. The Idler Arm is
symmetrically located on the top right of the main crossmember merely to
maintain the Tie Rod's proper position. It rotates about its pivot and its
single arm attaches to the Center Tie Rod. The Center Tie Rod arms on the
Relay Arm and the Idler Arm should be parallel to the side frame members
when the steering is set dead ahead.

There are a number of adjustable components: The Ride Height adjustment by
the torsion bars has been mentioned. The wheels' Caster and Camber are
adjusted by eccentric mounts at the inner pivots of the upper A-arms. The
toe-in/toe-out of the wheels is set by the adjustable-length Left and Right
Tie Rods. Those adjustments also affect the centering of the steering if
they are not equally adjusted. The final adjustment is not present on all
coaches: Early (and modified) coaches have a Drag Link between the
steering box's Pitman Arm and the Relay Arm which is adjustable in length
to allow minor adjustment of the steering center; later coaches came with
an non-adjustable Drag Link.

There are a LOT of minor details not mentioned here, but hopefully this
will help you understand suspension discussions better.

Ken H.


On Thu, Apr 22, 2021 at 12:19 PM wrote:

> RF_Burns wrote on Wed, 21 April 2021 09:39
>> Open the engine cover and look down at the position of the relay arm and
> idler arm. They should be pointing straight back, parallel with the main
>> ladder frame. If not, then the link between the tie-rods out to wheels
> is off of its center point of movement. Use the tie-rod adjustment sleeves
>> to get the relay arm and idler arm in the center of their swing
> (straight back, parallel with the chassis frame). Turn the sleeves a little
> bit on
>> each side, alternating side to side and in the opposite direction so you
> do not move the front wheel position. You only want to shift the connecting
>> rod to the center of its movement by setting the relay arm and idler arm
> point straight back.the center of its movement swing.
>
> Bruce - what are the relay arm and idler arms? Any chance of a diagram or
> photo of these things? I need to get them "straight back and parallel with
> the main ladder frame(?)" Main ladder frame? What's that? The coach frame?
>
> Like I said, my steering vocabulary is that of a 3 year old ..HELP!!
>
> Larry
> --
> Larry - Victoria BC -
>
> 1977 Palm Beach VIN TZE167V101295 - 39,000 miles, PO said everything
> working but forgot the word NOT. New wiper blades, New SS exhaust system ..
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> GMCnet mailing list
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach

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