(a) A down-in-rear frame angle must be subtracted from a positive caster specification.
(b)An up-in-rear frame angle must be added to a positive caster specification. (This is
the most common situation.)
(c) A down-in-rear frame angle must be added to a negative caster specification.
(d)An up-in-rear frame angle must be subtracted from a negative caster specification.
(You can make up the measuring tool by picking up a cheap plastic protractor, file a small
notch at the 0 center on the flat side. Then capture a knotted string in that notch with a
weight at the end of the string hanging down to indicate the degree of angle - not rocket
science. Of course, if your eyesight is as bad as mine, get a big protractor.) Also, note that
early versions of the P Chassis Service Manual had the words 'added' and 'subtracted' in a &
b above reversed and c & d above reversed. Their instructions were backward and incorrect.
As an example, let's assume you previously measured front ride height on both sides as 4
1/2”. The table says the CASTER should be set to 3 ¼ degrees. But, you then measured the
frame angle and found an up-in-rear angle of 3 degrees. (b) above says you need to add that
figure and you wind up with a setting of 6 ¼ degrees. That's the figure you should tell the
alignment shop to use for CASTER – in this example.
If you should measure the frame angle on both sides of the motorhome and come up with
one side positive angle and one side negative angle, stop and get some help. But, this would
be rather extreme.
Two more settings to go for the alignment: CAMBER and TOE-IN.
Here's the table from the same 1995 manual. Note that there is no line for “P-20, 30.”
Interesting.
Main Body Page 16 of 38 Total Pages
