It is often forgotten that beneath every '58 to '64 Chevy fullsize car there lurks one of the most unusual and innovative frame designs in U.S. automotive history. Instead of the traditional perimeter-style steel frame used under competing body-on-frame cars (and the outgoing '57 Chevy), for 1958, GM unveiled a novel X-frame consisting of wishbone-shaped end sections-to locate the front and rear suspensions-with a rigid tubular driveshaft tunnel joining them together. The goal of the new frame was to allow the bolt-on body shell to be mounted as much as 4 inches lower to accentuate the lower, longer, wider styling trend of the day. Let's review some X-framed Chevys courtesy of Desert Valley Auto Parts (dvap.com; 800/905-8024).
This stack of Chevy frames provides an excellent opportunity to compare the '58 X-frame (top of stack) with the perimeter frame from a '57 Chevy. Chevy called it the Safety-Girder frame, but its complete lack of side rails couldn't have been much help in a T-bone crash. By contrast, the '57 frame (bottom) offers passengers more side impact protection. GM gave X-frame cars thicker rocker panels and extra under-body bracing to compensate.
This cool '58 Biscayne two-door post is 2 inches lower than a comparable '57 Bel Air thanks to its X-frame. The even sleeker first-year '58 Impala had a unique Cadillac-inspired greenhouse and was a full 4 inches lower than a '57 Bel Air. All X-frame Chevys have two-piece driveshaft and coil spring rear suspension (except for some '58s built with the leak-prone Level Air pneumatic suspension). We like everything about this six-cylinder stripper except the mill. We'd go for a stroked 348.
All-new batwing styling arrived for 1959, but the X-frame remained. Next time you see a '59 Chevy, notice how the floorpan buckets are 2 inches lower than the rocker panels. By mounting the seats lower, the passengers sit closer to the road and a lower roof line results. The clues tell us this first-year '59 El Camino was a six-cylinder, three-speed stick workhorse. Forget the incorrect rumors about four-speed '57 Bel Air Fuelies-1959 was the first year for the optional four-speed stick in fullsize Chevys.
The X-frame's final year was 1964. Space-age fins and Motorama dream car styling gave way to more conservative designs like this sweet '64 Impala. Despite the two-piece driveshaft and coil spring rear suspension, the X-frame proved to be a very effective dragstrip contender. You'll find one under every four-speed, dual-quad Positraction 409 ever built. A more conventional perimeter frame arrived under '65 fullsize models as GM worked to further isolate the body shell from road noise and suspension harmonics. The X-frame's days were over.
* Though GM designed the X-frame, the company was supplied by A.O. Smith, a huge OE contract house.
* X-frames were also used under most Buick, Olds, Pontiac, and Cadillac fullsize passenger-car models during the 1958 to 1964 period.