Caspro-GMC Performance Parts & Accessories for the GMC Motorhome

Stability and Comfort at Highway Speeds

These are the two most troublesome items for GMC owners. To some, highway driving is tiring, unpleasant, difficult to tolerate, & the incurable nature of the vehicle.
This is not true.
A GMC can be driven comfortably & securely at 70 mph, provided the chassis is in correct adjustment, and some refinements to the basic design are made. Most GMC's have been maladjusted over the years, & suffer from a multitude of errors.
The following is a list of items which must be done, to achieve excellent results.

1 - The vehicle MUST be weighed & balanced. Record the individual weight on each front wheel, & each rear pair (bogey) of rear wheels, & the total vehicle weight The vehicle must be level when being weighed. Good police/trucker scales usually have the necessary equipment, & are often willing to be helpful. When properly adjusted, side to side front weight should be within 75 lbs; 100 is tolerable, & 25 is attainable. The rear weight should be within 100 lbs side to side, 150 is tolerable, & 50 is attainable, (on most GMC's). The rear suspension air valve links may need adjustment.

2 - The suspension heights should be checked, BUT the overriding criteria must be #1 above. Therefore, when balanced the vehicle may not be level; perhaps it will be 1/4" low on one corner or side. This is positively OK. Making it level will degrade the handling & road manners.

3 - The alignment must be correct on all 6 wheels, in relation to each other, & to the centerline of the vehicle.
Ideal settings are:
Front wheels: 0 to 1/4 degree positive camber; as much positive caster as possible & equal on both wheels. This means 2 1/2º positive (minimum) to 3 1/2º if possible. Toe-in must be a total of 1/8" positive (NOT negative).
Rear Wheels: 0 to 1/4º positive camber on each wheel; 1/8" total positive toe-in on each pair of wheels. (there is no such thing as caster on the rear wheels). If these settings cannot be obtained, then usually one or more suspension arms are bent. Obtain a print-out of the suspension settings.

4 - All mechanical components must be in good condition with no excessive wear. Upper ball joints seldom ever wear out. Lower ball joints are often unnecessarily replaced -- up to 1/8" wear/vertical movement is acceptable (Read the GMC manual !). Lower control arm bushings usually look bad externally, but they are just fine internally -- usually good for at least 200K miles. If the lower ball joints are replaced, they must be stitch-welded -- repeat, they MUST be welded to the control arms. If totally relied upon, the supplied 1/4" bolts will fail, as surely as you are reading this (= loss of steering, at least !). Tie rod ends are prone to wear, & should be replaced about every 100K miles. The idler arm bushings must be tight, & the arm mounting bolt torqued to 150 lb -ft. The relay arm bushings must be tight with only .005" end/vertical clearance; the thru-bolt must be tightened to 250 - 300 lb ft. The rear suspension arm bushings must be checked for wear & adjustment.

5 - Disconnect the drag link to accomplish 6, 7, & 8, below.

6 - The transverse steering arm MUST be centered in the chassis, L. to R.

7 - The tie rods must be equal in length (within 1/4"), in the straight ahead position.

8 - The steering gearbox must be on-center in the straight-ahead position.

9 - If the drag link will not easily connect the drop arm & the relay arm with a max of 1/8 to 1/4" movement of the arm, then install an adjustable drag link (these were standard on early '73 - '74 models, & available as a spare part).

10 - The steering box must be in good condition. There is no such thing as a rebuilt box, even though some are marketed as such. For best results, install a Caspro Precision Steering box, which is internally greatly improved, & 100% new (ask somebody who owns one). See detailed info on this website.

11 - The coupling shaft must be in perfect condition. For best results, install a Caspro Coupling Shaft. The same comments apply as above.

12 - When 5, 6, 7, & 8 are correct, disconnect the steering wheel & reconnect with the spokes horizontal (if necessary).

13 - Install 16" Alcoa alloy wheels, with Alcoa nuts torqued to 125 lb ft., & Alcoa hubcaps (NOT the fake ones). Use 245/75 x 16 radial tires inflated to 60 psi. As of this writing, Bridgestone R273 245/75 x 16 SWP II tires work best, after testing all major brands.

14 - Install Caspro shock absorbers. They are specifically designed for the GMC. Install all 6; just installing the fronts is a waste of time & $. See detailed info on our Shock Absorbers page.

15 - Install a Caspro Front Stabilizer bar. See detailed info on this site

16 - Install Caspro Twin Rear Stabilizer bars. See detailed info on this site.

17 - Install a Caspro Steering Control. See detailed info on this site.

18 - Weigh the vehicle again to check all work.

A GMC will steer & handle incredibly well, if all of the above recommendations are followed.

Find the necessary parts and detailed information at our Suspension page