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| [GMCnet] Fwd: Front end string alignment [message #235180] |
Sun, 05 January 2014 17:46  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma:
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My documentation can't hold a candle to Jerry's, and my equipment may not
be as sophisticated, but IMHO, you can achieve professional-like results
with my jigs and a $50 Sears digital level/laser. My total investment is
<$100. The jigs are shown here:
http://www.gmcmhphotos.com/photos/g5634-wheel-alignment-jigs.html
One must have a reference plane which is perfectly parallel to that of the
wheel. That's easy to achieve with "T"s like shown. I made mine from 3/4"
square steel tubing; 1" would have been better because the magnets on the
level would have a better gripping surface. There are 3 easily controlled
critical parameters for the jigs:
1. The outer side of each T MUST be dead flat. I achieved that by
having an automotive machine shop make it so on their cylinder head belt
sander. That was free for me and shouldn't cost anyone very much.
2. The length of the upright of the T must be such that all 3 of the
rounded-tip legs seat on the same relative positions around the wheel rim.
My jigs were made for Alcoas; only the pin at the end of the T's upright
must move relative the capital of the T to fit 16.5" or other wheels.
3. The distance from the flat outer surface of the T to the rounded
ends of the 3 standoff pins must be EXACTLY the same for all 3 -- otherwise
the plane of the T will not be parallel to the plane of the wheel.
Adjusting the threaded pins and locking them in place is trivially easy.
Jerry places tips on the pins to protect the Alcoas' finish; I prefer hard
metal-to-metal contact and haven't noticeably damaged a wheel.
My crossbar, springs, and toggles to secure the jigs in place work well for
me. Bungee cords or other arrangements might work better for other people
or other wheels. The important thing is that the 3 pins MUST remain in
firm contact with consistent locations on the wheel throughout the front
alignment process.
It's pretty obvious how one measures Camber: Place the level on the level
horizontal work surface surface and zero the display. Move the level to
the vertical jig leg (a magnetic level is best) and calculate the
difference between 90* and the level's reading -- that's the Camber. If
you can't decide whether it's positive or negative, disregard all this and
find an alignment shop.
Caster is more complex, requiring turning the wheels right and left,
measuring Camber at those locations, and calculating the Caster. I devised
a simple way to measure the turn angle, as required for the precise
calculation of Caster. But frankly, I don't think it's worth the (or ANY)
trouble. We're restricted as to how much caster we can achieve on the GMC.
So I set the caster to maximum using the rear adjustment eccentric and
then adjust the camber using the front eccentric. That yields as much
caster as possible. Set the toe (see below) then drive the coach on a
smooth, level road. If there's no pull right or left, don't worry about
how much caster you have -- knowing is good only for bragging rights anyway.
If there is pull (and you're sure you've got the steering box centered and
the tires equally inflated), then you may want to compare Left Caster to
Right Caster -- but you still don't need to MEASURE the caster: Just
follow the Caster setting procedure but instead of measuring the wheel turn
angle, just turn the steering wheel one turn right from center, measure,
then turn one turn left from center and measure. Use those measurements in
the Caster calculation. Repeat on the opposite wheel. You won't learn
the exact casters, but you will know the difference between them. Make
appropriate adjustments and drive it again until you no longer have a
pulling problem. I've been very lucky, I suppose: I've never had to make
a Caster adjustment. I run all the Caster the 1-Ton with offset rear
bushings allows, how much that is, I don't know nor care.
Now for toe: Since the GMC's frame is "perfectly" rectangular for the
length of the side rails, if we use the laser in the Sears digital level,
we can set the wheels parallel to that frame to establish the baseline for
the steering linkage (including the steering box and linkage). With the
level attached to the crossbar of the jig, turn on the laser -- aimed aft.
Use a rectangular piece of rigid material as a target, placing it against
the frame rail as near the front of the straight side rail as feasible.
Mark the location of the laser dot on the target. Move the target as far
to the rear as possible and against the frame exactly as it was in the
front position. Adjust the tie rod on that side of the coach to move the
laser dot to the marked location. Iterate that procedure until there is no
need to adjust after moving the target. 2-3 cycles should suffice. When
both sides have been adjusted, the front wheels are parallel to the frame,
toe=0".
If you want other than 0" toe, follow this: Two square aluminum tubes 60"
long are are drilled for two 1/4-20 bolts each. With bolts and wing nuts,
I attach the tubes to the horizontal bars through holes which center the
tubes fore & aft. With a helper, measure between the fore and aft ends of
the tubes. BE SURE the 3 pins of each jig stay in contact with its wheel --
those long levers make it easy to distort the measurement plane. The
difference between the two measurement is twice the toe (because the tubes
are about twice the diameter of the wheels). Again, if you can't determine
whether it's toe-in or toe-out, you need professional help.
For rear toe wheels. two carefully assembled, magnetically attached
standoffs for string suffice. My design is shown at the URL. Camber
measurement is the same as at the front.
Follow Jerry's basic instructions but instead of his swivel plates, use
poly trash bags or similar, 3-4 layers thick, to allow the front wheels to
turn easily (you won't be able to drive onto them, but a couple of people
can push the GMC on what must be a hard level surface). DO NOT jack up the
wheels to get the "swivel plates" beneath the tires -- your measurements
will all be wrong because the coach WILL NOT settle back to its normal
height.
HTH Someone,
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI & EBL
www.gmcwipersetc.com
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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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