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[GMCnet] Why was GMC setup to ride low in the rear? [message #139749]

Thu, 18 August 2011 00:19

USAussie

Messages: 15912
Registered: July 2007
Location: Sydney, Australia

Karma: 6

Senior Member

G'day,

I'm still trying to figure out why the GMC was setup with the rear end lower
than the front. Why didn't they set it up level?

It has been suggested that:

1) To get more caster

Comment: GMC could have located the mounting tabs for the upper and lower
control arms wherever they wanted to position the upper and lower ball
joints to get whatever caster they wanted; why would the locate them to
require running with the rear lower?

2) To make the fuel run into the rear tank

Comment: Quite possibly

3) Front to rear weight balance

Comment: GAWR = 4,500 lbs Front Wheels
GAWR = 8,000 lbs Rear Wheels

Comment: Running the GMC with the rear lower than the front would increase
the weight carried on the rear would it not? Why not run the GMC level?

4) Angle of attack (AoA)

Comment: I wonder how much having the rear end lower by 1 7/16" as measured
at the height measurement slots would affect AoA.

I took some more measurements the other day when I was at the workshop.
Here's a summary of all I have:

1) 13 1/8" = front ride height setting at 0" tolerance

2) 11 11/16" = rear ride height setting at 0" tolerance

3) 1 7/16" = front to rear drop at center of slots at 0" tolerance

4) 24 3/8" = center of front height adjustment slot to front axle center

5) 160" = center of the front axle to the center of the two rear wheels
(wheelbase)

6) 56 3/8" = center of rear height adjustment slot to the center of the two
rear wheels

7) 240 3/4" distance between center of front and rear adjustment slots (24
3/8" + 160" + 56 3/8")

8) 274" = end to end length of chrome trim piece on 1975 GMC Avion

9) 46 1/2" = distance from the center of the front ride height slot to the
front end of the beltline chrome trim piece

10) 34" = distance from the center of the rear ride height slot to the rear
end of the beltline chrome trim piece

I can take the measurements but my math skills are not good enough anymore
to figger the following out:

A) Using the measurements above what is the angle of the drop front to rear?

B) Using the measurements above what is the measurement of the drop at the
ends of the chrome trim piece?

It has been noted that with the front and rear ride height set as above the
front axles are straight and level with the ground with 225/75-16R tires
fitted.

I'm not sure but I think it has also been noted here that with the front and
rear ride height set correctly you can draw a straight line through the
center of the middle axle - front bogie pivot point - rear bogie pivot point
- rear axle; is that correct? I can't check it myself as The Blue Streak is
up on jack stands.

If anything I have stated is incorrect, illogical or my math incorrect
PLEASE correct it!

Regards,
Rob M.

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Regards, Rob M. (USAussie) The Pedantic Mechanic Sydney, Australia '75 Avion - AUS - The Blue Streak TZE365V100428 '75 Avion - USA - Double Trouble TZE365V100426