Modern General Motors trucks use front torsion bar springs for the front suspension, usually because of space requirements of the 4WD system. These torsion bars are adjustable, and have an adjustment bolt that can raise or lower the active spring rate. The spring rate for one bar should match the other, and it is recommended to keep the bars even with each other. The average backyard mechanic can adjust the rates of GMC torsion bars in about thirty minutes.
Raise the truck with the floor jack to relieve the weight from the suspension. Place the floor jack head underneath a frame rail and pump the lever until the wheel is in the air. It is not necessary to remove the wheel. Letting the weight off of the torsion bar will allow the bolt to turn with less resistance, but it is possible to turn the bolt without lifting the truck.
Locate the torsion bar adjustment bolt, nestled into the torsion bar frame mount. It can be seen by looking directly up at the mount while on the ground. Turn this bolt clockwise to raise the spring rate, and counterclockwise to lower it. Turning it 1/2 of a turn from the starting point will raise the truck about 1/4 inch from bumper to ground. It is not recommended that the bolt be turned more than four or five times, as this spring rate is too high and could damage other components.
Lower the truck from the floor jack by turning the pressure screw counterclockwise. Repeat the entire procedure on the opposite torsion bar adjustment bolt. be sure to note how many times each bolt is turned, to keep the front end even. The increased spring rate will also stiffen the suspension, and jolts from taking the truck off road will be substantially harder.
