GMC Motorhomes can be aligned using simple tools. Done carefully, many people report getting better results than alignment shops provided. While there are endless variations of tools and techniques, the procedure given below is the simplest, cheapest effective version I could derive. If your coach has a bent frame or suspension issues, it will probably not be sufficient. For a more complete alignment kit and recommendations on good but inexpensive digital levels and gauges, see the article by Jerry Work in the GMC Motorcoach section of his website. Special thanks to all the very helpful, knowledgeable people who contributed to this document.
If you have just installed the new front end, the adjustment cams should initially be set with the rear cams as close to the frame as possible, and the front cams approximately centered in their slot with the cam plates above the bolts. This is just a starting point, but should provide close to maximum caster and 0 camber.
Check that the tire pressure on all six wheels is correct. The water and fuel tanks should be filled to the levels at which you normally travel.Check that the front wheels are roughly parallel by measuring from a good tread at the front and rear of the wheels. Small changes in tie rod length have big consequences at the wheels, so they may be significantly out of alignment. If needed, adjust the tie rods as described below so you can drive the coach safely. You'll be checking and adjusting this more carefully later.
Drive the coach to settle the suspension and then park it on a level paved surface, preferably concrete. The wheels should be pointing straight ahead.
When doing an alignment, always check and adjust in this order: 1) ride height, 2) caster, 3) camber, 4) toe. Re-check all measurements after driving the coach. It can take many miles of driving before everything has settled properly. Since you want maximum caster, you can cheat a little by setting the cam bolts for maximum caster as described above and then worry about the other settings.
Check and adjust the ride height as described in the manual using the unloader tool. This should be done on a level concrete surface because asphalt surfaces are usually not flat enough to get an accurate reading. Block the rear of the coach to the correct height and release the air bag pressure to prevent the rear suspension from compensating for changes to the front suspension. The ride height should be 13 1/8" +/- 1/4" to the top of the slot located behind the front wheels, and 11 11/16" +/- 1/4" to the top of the slot behind the rear wheels. One inch of ride height equates to roughly six turns of the adjustment bolt.
Once you have figured out what change in height is needed, it is very helpful to jack up the front of the coach at the center frame member under the engine to take up most of the front end weight. This saves both you and the tool from having to work so hard against the weight of the coach. For safety, always be sure to put jack stands or blocks in place before getting under the coach. In order to keep the front weight balanced correctly, both adjustment bolts should be kept approximately the same number of turns so that the pork-chops are held at equal heights. If one pork-chop is higher than the other, the weight will shift to one side. This can dangerously overload the tires and cause handling problems even though the ride height may seem correct. Although uneven loading can also be a result of interior weight distribution or, very rarely, a weak torsion bar, one sign of an unbalanced front end is that the rear airbag on the side opposite the overloaded front tire consistently requires more air pressure than the other side. To keep the front weight balanced, whenever you adjust one pork-chop bolt up or down, count the number of turns and make the same adjustment to the other side.
Note: If you have non-stock wheels or tires, you may need to adjust the ride height to compensate for the difference in the radius of the wheels. Stock sizes are OEM wheels with 8.75/16.5 tires, or 16" wheels with LT225/75R16 tires. These are usually about 29.5 inches in diameter though there is variation between brands.
Alignment work is best done with the wheels resting on turn plates, but anything that allows the wheels to slide more easily will work. Some examples are metal or plastic plates with grease in between, sheet metal, floor tiles, several layers of plastic sheeting, or garbage bags with a little oil or water inside.
Center the steering by turning the steering wheel all the way to the left, then all the way to the right while counting turns. Divide by two and move the wheel back this number of turns.
Caster is defined by the relationship between the upper and lower ball joints. The lower ball joint is in a fixed position, so the only adjustment we have is to move the upper ball joint. The further to the rear the upper ball joint is set, the greater the caster. Camber is the amount the wheels tilt in or out at the top. We measure caster indirectly by measuring the change in camber when the wheels are turned to specific angles. For our coaches, we want as much caster as possible and it should ideally be the same on both sides, but the actual numbers are not very important.
Check the accuracy of a digital level by turning it around end for end on a level surface and re-checking the reading. It should produce consistent results. Some levels can be recalibrated.
To compare caster on the right and left sides, center the steering wheel and then turn it one complete turn to the left. This will turn the wheels approximately 20 degrees. Using a digital level or gauge, measure the camber of one wheel. You can calculate the angle changes, but it's much easier and less confusing to let the gauge do the work. Hold the gauge to the flat face of the wheel and push the zero button on the gauge. Keep track of exactly where you placed the gauge on the wheel so you can do it again. Now turn the steering wheel back to center and then one complete turn to the right. Measure the same wheel in the same place to get the change in camber. Repeat the procedure for the other wheel.
The measured change in camber is a fraction of caster and should be close to equal on both sides. On most coaches, the passenger (right) side has less maximum caster than the driver (left) side. You will need to reduce the side that has more caster to match the other side. How close they need to be is debated. Caster and camber will cause a pull toward the more negative set side of the vehicle, so you can use these settings to compensate for road crown or other conditions.
If you want to measure the actual caster instead of just the relative caster side to side, you'll need to be more exact in the angles you turned the wheels. Multiply the change in camber by a fixed amount according to the angle you used. For example, if you turned the wheels 20 degrees left and then 20 degrees past center to the right (40 degrees total), multiply the change in camber by 1.43. If you used 15 degrees, multiply by 1.91.
Camber and caster are both adjusted by moving the upper A-arm cam bolts in their slots. To maintain maximum caster, all camber adjustments should be made with the front cams only. To maintain a set caster however while adjusting camber, move both the front and rear cams an equal distance in the same direction. For example, to make the camber more positive while maintaining the same caster, move both the front and rear cam bolts outward from the frame by the same amount.
Adjust the wheels as close to plumb (0 camber) as possible, or very slightly negative (tilted in toward the coach at the top). If negative camber is used, the right side should be slightly more negative than the left. For example, left = 0, right = -1/4 to -1/2 degree, or left = -1/2, right = -1 degree. Once you've got the right settings, torque the cam nuts to 80 ft-lbs while holding the bolt head to keep the cams from moving.
Toe is determined by measuring how parallel the right and left wheels are to each other. Put a mark on the rear of each tire as high up as allows a tape measure to run under the coach without touching anything but the tires. You want as precise a mark as possible, so use a thin line on a piece of masking tape or the edge of a sharp tread and mark it clearly on both tires so you can easily find it again. It helps to measure starting at the 1-inch mark on a tape measure rather than the end since it's easier to see and usually more accurate. Measure the distance between the marks on the right and left tires. Also measure the distance from the mark to the ground. Roll the coach straight forward until the marks on the rear of the tires come to the front at the same height as they were in back. Measure the distance between the marks on the front of the tires. If the measurement is the same plus or minus 1/16th inch, the toe is correct.
If needed, adjust toe by loosening the clamps on both tie rods and turning the center sleeves. It doesn't take much to make big changes at the wheels. The right and left tie rod assemblies should both be adjusted so they are kept as close to the same length as is possible. Before you re-tighten the clamps, move the tie-rod end ball joints all the way in the same direction. This is to keep them from binding when in normal steering range. When adjusting the tie rod lengths, the gap in the clamps should NOT line up with the slot in the center tube. Torque the tie rod clamp nuts to 20 ft lbs.
Drive the coach and re-check and re-adjust as needed. This may take several miles and repetitions before everything settles into a stable configuration.
If after driving the coach you find your steering wheel is not centered, you can fix it easily if you have an adjustable drag link. If not, the tie rods can also be used to center the steering wheel. For details see http://www.bdub.net/center_steering_wheel.html .© Copyright 2011,2012,2013,2014,2015,2016 K. Bradley