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Chevrolet Corvette C4 Underbody Suspension Enhancements - Going Undercover

C4 Underbody Suspension Enhancements Made Easy

Chris Petris May 1, 2006
Corp_0605_16_z Chevrolet_corvette_c4_underbody_suspension Braces 2/1

One of the most common complaints with the '84-'96 Corvette has been body flex when the top has been removed. we've heard Corvette owners say they removed their top one time and one time only. They drove the car and never would drive it again sans the top because when the top is off, the frame has no reinforcement whatsoever. The only framemember is below the doors and whatever slight bit of reinforcement the door striker provides.

Depending on your locality, this can be a considerable issue. The farther north you go, the worse the rust and general condition of the roadways. Rust underneath the car can deteriorate the car's mounting points and find its way into the suspension bushings. Over time, this repeated pounding of the chassis creates squeaks and rattles that are hard to diagnose. the bond between the glass and top frame is also stressed. The frame/body flex affects the cornering stability of the car as well and, eventually, weakens the chassis as a unit. This fact was recognized when the convertibles were introduced in late-1986. GM fixed the problem by adding an X-frame to stiffen the convertible frame/body. In fact, all C4 convertibles received an underbody X-frame.

All Corvette coupes will benefit from the underbody bracing. We installed OE convertible top bracing on coupes to stiffen the chassis years ago. We found that late-'86 and newer Corvette coupes already had the holes in place at the front for the OE brace, making it somewhat simple to retrofit the brace to cars not originally equipped with them. During that time period, you had no other choice but to check the local swap meets hoping to find a complete convertible X-frame and hardware for installation in your coupe.

As time progressed, the OE X-braces became difficult to find. R&D Racing came up with their own Cross-frame bracing system. The R&D Racing Cross-frame has all the necessary pieces available to install their brace on the '84-'86 early cars. With this product, everyone can have the same rigid chassis that the Corvette should have had in the first place.

We decided to install the R&D Racing Cross-frame on a stroked high-horsepower '85 coupe that just might do some high-speed cornering. The installation is moderately difficult only because the pieces are somewhat awkward to handle, and a lot of drilling is required. We varied slightly from the directions when we installed the Cross-frame as an assembly instead of individual pieces. The theory was to check the positioning at all four corners before drilling. We can do this because we're working on a lift with jacks to hold the pieces in place while fitting them to the car. Working on jackstands is another issue you may want to talk a buddy into because you will probably need the help.

The Cross-frame uses Riv-nuts to hold the assembly in place, and they work well. Once the Riv-nuts are installed, they are in for good, barring any cross-threading issues. The Cross-frame is built from rectangular tubing that is much stiffer than the OE-stamped X-brace. The pieces fit well and are easy to remove if we need to do any underbody work at a later date.

Since we're trying to prevent flex, the next thing on the agenda is bracing the frame around the upper control arms. The upper control-arm frame sections take a pounding on rough road surfaces, thus changing alignment specifications over time. We found R&D Racing also has a Cross-brace available to alleviate this common ailment. The upper control arms are a long way from any factory bracing nearby and are literally hanging out in the breeze. Installing a Cross-brace strengthens this weak area by boxing in the control-arm mounting surfaces to prevent flexing.

Installing the Cross-brace is simple and quick. We suggest jacking the car up and letting the suspension hang free while installing the Cross-brace for a quick and easy installation. The '85 Corvette we were using had the engine and transmission out when we installed the pieces, so ride height would be high until the suspension settles. We decided to align the car after a few days so the chassis would be at typical ride height. Once these two products were installed, the '85 Corvette (with the top off) felt good, even over some pretty hairy bumps, and it now holds its alignment better than ever.

Parts ListCross-frame, '86 late-'96: $435Cross-brace: $220; additional cost for LT5 and modified LT1 and LT4 engines


R&D Racing Enterprises
Farmington, MO 63640

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