BODY & SUSPENSION

ASC/McLaren increased speed and agility by improving the traction and lightening the load. The front suspension remains exactly the same as stock, however the rear axle is a different animal. The GNX has a ladder bar/panhard rod rear suspension instead of the standard control arm 4-link found on all other G-bodies:

 

The upper control arms are completely removed in favor of a ladder bar (16), to improve traction on street tires. It is attached to the passenger side of the aluminum alloy differential cover (5) that has a 60,000 psi yield strength. The front mount of the ladder bar attaches to a frame crossmember (13) about midway under the car, which also serves as a driveshaft loop.

During a brake stand in a rear wheel drive car, there is a tremendous amount of torque building against the staionary rear wheels. The torque load about the driveshaft has a tendancy to lift the left side of the car and squat the right side. The GNX ladder bar channels the rear axle twisting energy like a lever to apply upward lift at the midway frame cross member. The lift is applied slightly off-center to the right side, and thus equalizes the natural lift from the driveshaft torque to the left side. The result is an even rear-end lift of several inches. When you let the brake go, the rear drops evenly and the car charges straight. (See it in action in the GNXvsVette video )

Remember the upper control arms were removed to accomodate the ladder bar movement, so a panhard rod (20) and frame brace (2) are added to control lateral motion of the rear axle. These are located right behind the rear axle. In some of the GNX literature, the panhard rod is labeled incorrectly. The panhard rod goes from the left lower trailing arm mount (21) slightly upward to the right frame rail mount (11). Then, the frame brace rod goes back across (upward) to the other frame rail (1). This setup keeps the rear axle stiff during cornering.

Here is a picture of the less accurate diagram of the rear suspension that is from the GNX book. It may help you understand how things are positioned, but the mounts and labeling are incorrect. The panhard rod is missing and what is labeled "panhard rod" is actually the support brace. The ladder bar is pictured & mounted differently, and the driveshaft loop looks nothing like that. This was probably a pre-production exhibit.

Click for the "less accurate" rear diagram

Another key to improve handling and acceleration is to reduce weight. The GNX uses aluminum rear brake drums and bumper inserts which are good for a 40lb weight loss compared to stock.

The steel 15"x7" Grand National wheels are replaced with lighter 16 inch black "honeycomb" alloys (2-piece welded) and wear Good Year VR50 245/50/16 up front and 255/50/16 in back. Tires are readily available from a variety of manufacturers because they are the same size as the Corvette. The center cap is modified from the Trans Am GTA and the lug nuts use a screw-on plastic cap so they appear black.

The front and rear wheels were custom made for the GNX by JWC. The wheels are actually labeled "Rear Only" and "Front Only". They have specific offsets, which are different than the Grand National. The front wheels are stamped "-15" and the rears "-23", which is in millimeters. Here's a pic of the front:

This has been a topic of discussion, so I measured the total backspacing of the wheels: from the deck of inner hub to the outmost lip. The front is 3-15/16" and the rear is 3-9/16". The difference comes out to approx. 8mm which matches the difference in the offset spec.

Late 80's Trans Am GTA wheels look similar but are not a direct replacement. I don't think any of the aftermarket wheels are correct for the GNX either, because the rear track is wider with the factory wheels. For example, I tried the Classic Industries GTA repro wheels (fronts) and they rubbed my frame on both sides. They measure 4-7/16" backspacing hub to lip. I'll measure Kirban's repro wheels when I get a chance, but I do know they don't rub.

To accommodate the larger wheels, the fenders are cut and molded fender flares are added.

click on picture for full size image

To add further stability, a set of rubber body cushions are added in the "#5" bushing location between the frame and the body. This is approximately where the trunk meets the back seat. And finally, a stamped diagonal brace is installed behind the rear seat to further stabilize the body.

 

(my thanks to Preston Marshall for help describing the suspension)