String Rear Wheel Alignment Check

Select a smooth level floor. Drive the coach to a straight stop. Check for proper tire inflation. Check for proper ride height and adjust to 11 11/16" rear and 13 1/8" front with coach empty weight on the wheels as shown in figure 70 of the maintenance manual. Newer coaches manuals show l3-1/8" Early model coaches show 12 7/8" front ride height. Jack up the coach and deflate the air bags. Check the wheel bearings and adjust if necessary by removing the cotter pin and tightening the spindle nut to 25 -30 ft lbs. torque. Then back off 1/2 turn and re-tighten finger tight. Replace with a new cotter pin. This is a good time to spin the wheels and listen for rough beatings. Check control arm end play at the end of the spindle. Lateral movement should be no more than .050" ( between 1/32" and 1/16")

Procure two 1" by 2" boards 100" long. Place the boards together and using a hack saw lightly cut slots in both ends 1" from the end of the boards. Measure and mark the center of the boards with a marks-a-lot. Using the marks-a-lot label one board "FRONT" and the other board "REAR". On the front board ONLY lightly cut another slot 3/8" INBOARD of the original slot on each end.

Clamp 2 short sections of 5" channel iron to the bottom of the front bumper. Clamp the front board with the center of the board aligned with the joint in the center of the front bumper.

Clamp the rear board directly to the lower edge of the rear bumper so that the mark you made in the center of the board aligns with the joint at the center of the rear bumper.

Stretch the string between the slots in the rear board to the OUTSDE slots in the front board on each side. You may find it necessary to remove the lug nut covers on your coach. Make sure that nothing interferes with the string.

Locate the ends of the main frame (the long straight channel). Measure from the channel to the string on each side of the rear of the coach and re-adjust the clamps until the measurement is the same on each side.

Follow the same procedure at the front of the coach, making sure the measurements are exact.

Move the string to the INSIDE slot on the board on both sides of the front bumper. You can now use the alignment bar to observe the proper alignment of each wheel by attaching 2 welders magnets to the center of the wheel between the lugs. Place a 3/4" thin wall square tool bar 31" long to the outer edge of the magnets. The bar should align with the string, RECORD the measurements on the forward and rear of the bar. Follow the same procedure for each wheel. RECORD each of them making sure you properly identify each wheel.

BE AWARE IT EASY TO BECOME CONFUSED. (Call toe in toe out etc.) I find it beneficial to write and draw these records on the floor adjacent to each wheel in addition to recording on paper.

Use a camber gauge to check for positive camber on each wheel. I use a Halibrand gauge that I bought from J.C. Whitney. RECORD the camber angle for each wheel. Remember positive camber is leaning out at the top. The 7525 manual calls for 0 to 1/2 degree positive camber. The 7725 manual calls for 0 to 1 degree positive camber. My target is 3/8 degree if I have 1/32" to 1/16" toe in. Please remember the weight of the coach must be on the wheels when camber is measured.

All of the above exercise is to determine what if anything needs to be done.

If the intermediate and trailing wheels on the same side are pointing in the same direction and are at the same camber angle, the adjustments can be made by adding or removing shims between the mounting bracket and the frame as shown in the manual and here.

Toe in: Jack up the coach. Deflate the air bags. Loosen the six bolts on the mounting bracket. Insert or remove vertical shims to obtain at front or rear of the mounting bracket. A .020" shim will change alignment about 1/16" at the wheel. Tighten 4 rail bolts to 65 to 85 ft. lbs torque. Tighten 2 cross-member bolts to 50 to 60 ft. lbs. torque. Lower coach re-inflate the air bags. Recheck the alignment.

Camber: Follow the same procedure using top shims. A .020" shim will usually change camber 1/8 degree.

Most alignment shops advocate the use of a port-a-power to bend the control arms while on the coach using chains to minimize frame bending. I know individuals who have successfully accomplished the procedure. I cannot say that I would choose this method as a bent frame or damaged cross-member is difficult to correct.

I prefer removing the entire assembly and support the assembly in a jig. Once the assembly is brought into proper alignment using the jig, it needs to be re-mounted and re-aligned using shims between the support bracket and frame as described above. The jig has aligned 30 coaches thus far. My coach has remained in alignment over 4 years by using a Wallace tree track. I also use all of Caspro's suspension hardware. On control arms that were severely bent, or if there was any doubt in my mind I magnefluxed the arm. Shot peening should be a part of the process.


I did this because I became disgusted with the quality of work available in my area.

Some of my personal observations are:

As little as 1/8" out of alignment can cause tire scuffing.

With radial tires negative camber acts like to toe out, positive camber acts like to toe in.

Traveling with one side as little as 1/2" lower than the other can cause a properly aligned coach to have negative camber on one side.

Wide tires seem to help rut chasing; however the benefit is greatly reduced if wheel alignment is not correct.

The GMC was designed for bias ply tires; radial tires came to us after the coach was in service. The wide rear track reaches the edge of the road before the front causing the coach to veer to the left. Rear track is 6.84" wider than the front.

Most severe bends occur to the right intermediate control arm (toe in along with excessive positive camber). Left trailing control arm usually is toed out. Most bent control arms are also out of camber.

It takes only 4,000 lbs. at the spindle to bend the control arm.

The entire assembly, including frame, flexes approximately 5/8 of a degree when the empty weight of the coach is placed on the wheels.

When mounting a tracking device care must be taken to properly align the device.

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Image 5   Shows a bogie in a straightening jig

Image 6   Another of a bogie in a straightening jig