GMC Greater Midwest Classics

Front End Alignment

By Bob Drewes

Not being satisfied with the driving habits of the GMC motorhome, after several front alignment sessions with a reputable alignment shop, I decided to fabricate some alignment brackets and investigate what were the settings on the front wheels, and how I could improve the road handling of the coach. The alignment shop always would comment on not being able to get quite enough positive caster. I fabricated two wheel units that fit on the Alcoa wheels, the two units being identical in size and accuracy. As shown, they have a flat steel plate fastened to the precision square tubing for the magnetic alignment gauge to cling to and the four stand-off rods that fit up to the flat machined surface of the Alcoa wheels. With these units, there is no need to remove the nut covers or the center cap. I used a sheet of poly that I taped to the floor and put a 1/4" thick steel plate on top of that for turn tables to put the front wheels on. The steel plate was lubricated with light grease that had powdered graphite mixed in for easy turning of the wheels. These alignment units are also an excellent way to check the alignment of the rear wheels, both for camber and for the position of the bogie assembly on the coach.

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I now have the tools to check what are the camber and caster readings, so I installed the wheel alignment units and put the alignment gauge on and checked the readings. The readings of the gauge showed a 1 degree positive caster on the right side, and less than 2 degree positive caster on the left side, far from an ideal caster setting. I soon found out that getting the desired camber reading was no problem, but did not have enough adjustment in the eccentric washers of the top control arms to get anywhere near the desired positive caster. The upper and lower control arm bushings were replaced 25 K miles ago, and the other components of the steering system are in good condition. What to do to get more positive caster? The answer is putting offset bushings in the upper control arms to give more range of caster and camber settings. I had a spare set of upper control arms, and purchased a set of Moog offset bushings #K7104 for the job. I fabricated a tool to push out the old bushings and to install the new offset bushings. The first photo below shows the tool I fabricated to remove/install the bushings. The second photo shows removing the old bushing. Note the split ring used to prevent the control arm sides from collapsing. This is very important to keep the control arm in it's original shape. The third photo shows the bushing being installed, note the use of the split ring again. The fourth photo shows the upper control arm for the right side with the old bushings removed. The fifth photo shows the Moog kit with the offset bushing and washers. If you put the offset bushing in the FRONT arm, the arrow must point to the ball joint. If you put the offset bushing in the REAR arm, the arrow must point away from the ball joint to increase the positive caster numbers. Disregard the instructions that came with the box of offset bushings. The last photo shows the bushings installed. I installed offset bushings in both the front and rear arms of the upper control arm, although I found that was not necessary, an offset bushing in the rear arm would have given enough caster adjustment.

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I now installed the upper control arms on the coach and set up the alignment units on the wheels and checked to see what I could get for positive caster. I found there was lots of range of adjustment now for both the caster and camber settings, enough range that one offset bushing for each control arm could possibly be all that would be necessary for the range of adjustment needed. The photo below of the alignment gauge shows 4 degree of positive caster (the center dial) on the gauge. The settings that I set the front end at was 3-1/2 degrees positive caster for both wheels, camber is less than 1/8 degree positive on the right wheel and 1/8 degree positive on the left wheel. The toe setting is set at "0"

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Now for the road test. It is hard to believe it is the same coach to drive, but when looking at the caster readings before the updated control arms were installed, those readings were far from being optimum for good road handling ability. Positive caster has a lot of influence on how the vehicle will hold it's position on the road, and the test drives I have done with the new settings sure does prove that. After a test drive of ten miles or so, I had to do some minor readjusting of the settings, as the new control arm bushings will seat in somewhat. This was done several times to check to see if the settings stayed as I wanted them.
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