Do the TIGHTEN UP
Foundational improvements on a C4 coupe's chassis
Ron Warren - August 01, 2011 09:00 AM
The complete kit includes the torque tubes and the brackets for both sides.
Close up of the brackets (front are to the left, rear to the right).
The front bracket installed.
The rear bracket installed.
A pre-production bar installed on a 1990 ZR-1. (Thanks to Mid America Motorworks for use of their lift at Bloomington Gold 2010.)
Corvette therapy; it is accomplished by either lowering the top on your Corvette convertible or removing the roof panel on the coupe (if so equipped). Point your machine in the direction of your favorite two-lane twisty road, crank up the tunes and drive on! What a great way to relieve the stress of a hectic work week.
That is, unless you own an ’84-’96 coupe. Then you get to experience the not-so-therapeutic cowl flex, body rattles and torsional steering shake that a rough, curvy country road induces on the fourth-generation cars. The C4 Corvette was originally designed to incorporate a T-bar instead of a targa roof. Lloyd Reuss, then Chevrolet’s general manager, made the decision to change to a targa roof after the program was well along. Dave McClellan, chief engineer during the C4 years, stated during the media introduction of the 1984 Corvette, “We’d already committed the pre-prototypes as T-roof, so that change (to targa) didn’t show up until the real prototype was done in 1981.” Dave explained that the planned T-bar wasn’t like the previous generation. “It was a one-piece roof panel on top of a T-bar that was inside. You couldn’t tell it was a T-bar until the roof panel was removed. The bar was a fixed, structural member.” Another drawback of the C4 design is the wide door sills that make it difficult to get in and out of these Corvettes. The large sill was a result of attempting to beef up the frame to make up for the loss of the T-bar.
What’s a C4 Corvette enthusiast to do if he wants to tighten up the chassis to have a better handling sports car? There are several solutions that many have tried in the past. In 1986, Chevrolet added a convertible to the lineup. In order to beef up the chassis, they added an X-brace, criss-crossing frame from the door hinge pillar to the opposite rear torque box. In addition, some bracing was added to the frame rails in front of the engine, under the engine crossmember, torque box reinforcement area and an additional bar across the top of the rear torque box from rail to rail behind the seats. The ride height was also increased 10 mm on the convertibles to maintain ground clearance with the X-brace. All 1986 through 1996 coupes have four holes drilled and tapped so that you can easily install the X-brace from a wrecked convertible.
You then have to drill holes and use self-tapping bolts to attach the rear of the brace to each torque box. Some aftermarket companies have developed their own X-style braces along with braces that attach in front of the engine and in the targa area. Some enthusiasts have designed their own frame stiffeners that tie the frame rails together.
Most of these designs restrict access to the driveline and exhaust systems. They also make it difficult to perform fluid changes on the automatic transmission as the bracing must be removed first. Used convertible X-braces are also becoming scarce in the various Corvette recycling yards around the world.
I recently installed the new “C4 No Flex” frame stiffener bar on my 1991 coupe. This system was designed and engineered by Corvette Hall of Fame inductee and former Corvette Assembly Plant employee Gordon Killebrew. In its press release, the National Corvette Museum refers to Gordon as the “Guru of the C4 Corvettes.” Gordon’s design is being manufactured and marketed by Jason Francis of Vette2Vette out of Streator, Illinois. The kit consists of two front brackets with eight bolts that attach to the factory-tapped holes found on the forward frame rails of all 1986 through 1996 Corvette coupes and convertibles. In addition, there are two rear brackets that mount to the rear torque box using the lower “dog bone” bolt. These brackets are tied together front to back by a torque tube with Heim joints that thread into the end of each bar along with jam nuts to lock them down. One end of the torque tube has lefthand thread and the other has righthand thread. After the kit is bolted to the car, then simply wrench on the hex-head that is designed into the front of the torque tube to pull the front and rear of the torque box together and prevent it from flexing.
I installed the C4 No Flex in my home garage by myself in about an hour. I drove the front wheels onto car ramps, jacked up the rear and placed jackstands under the rear shock mounts. You will need a torque wrench and a set of both metric and SAE sockets for the job. I measured the ground clearance of the system at its lowest point with a tape measure, and it was the same as that of the lowest portion of the stock exhaust system. Since my exhaust system appears to have no scrape marks on it, I assume that the C4 No Flex system will have no issues with ground clearance.
How does the car drive on the twisties?
In one sentence, my 110,000-mile 1991 coupe with the targa top both on and off drives like a showroom-new Corvette. On a strictly subjective basis, I found that there was no difference in whether I have the targa top on or off. Even with the top off, I am able to drive over a set of double railroad tracks near my home without any rattles or without slowing down. It handles the curvy roads on my favorite local Corvette therapy roads like a dream. I am highly pleased with the difference in how the car handles. I have owned two C4 convertibles and two C4 coupes over the years, and this car now out-handles them all.