The development of Corvette racecars is a fascinating topic. Zora Arkus-Duntov was always pushing for more and GM brass was always reining him in. While precluded from being directly involved in racing, Duntov worked to provide Corvette racers with his “racer kit” options that offered improved suspension and brakes to go along with the hottest engine available.
Through the C1 generation, the under-designed Corvette chassis structure was adequate thanks to additional bracing and other small improvements. The C2 perimeter frame and chassis was a huge improvement, more than adequate for racing activity. Things didn’t get shaky until the arrival of the L88 package, combined with the latest wide racing tires.
The first sign of deficiency didn’t show up until the arrival of the very successful 1968 L88 Owens-Corning Fiberglas Corvette. Years later in an interview Tony DeLorenzo disclosed that after every long-distance, 12 or 24-hour race the car’s frame had to be replaced!
In photos of their car taken in the late hours of the long distance races, the car looks unusually low. That wasn’t by design, it was the frame sagging as a result of the stress of speed, high g-loads, big-block torque and wide racing tires. The only thing added to a Corvette racecar’s structure was a roll bar for driver protection!
Tony DeLorenzo and Jerry Thompson took their Corvette to Logghe Stamping Company in suburban Detroit, Michigan for a proper tube-frame roll cage and bracing to supplement the stock frame. One of the Owens-Corning Fiberglas Corvettes went on to win 22 straight wins! By 1971 upstart racer John Greenwood broke the DeLorenzo/Thompson hold and took Corvette racecars to the next level.
Greenwood understood the necessity of a super rigid structure so that the suspension could be accurately tuned for each track. Greenwood went on to become one of the all-time great Corvette legends. By the time John was building the wide-body cars the structural enhancements were so extensive that a Hot Rod Magazine editor commented, “Is there any Corvette left in this car?” By the end of the ‘70s, factory Corvette frames were out, and the all-tube frame, Trans-Am cars were in.
Unless seriously crashed, old racecars soldier on. Greenwood’s iconic Stars and Stripes Corvette, raced in 1971 and featured on the cover of Karl Ludvigsen’s book, “CORVETTE: America’s Star Spangled Sports Car” was sold to Mike Murray at the end of the 1971 season was raced through mid 1973. Corvette racer Phil Currin became the next owner in 1974.
Attempting to lighten the car, Currin used a hole-saw and Swiss-cheesed the frame and front suspension control arms. In an article published by Wayne Ellwood, Phil admitted that he might have over done it with the hole-saw. While the process did remove some weight, it seriously compromised the structure, creating a racecar chassis that was quite flexible. At an IMSA race in California, Currin crashed the car, effectively ending the old Greenwood racecar’s career.
Lance Smith, a Corvette and Greenwood historian and restoration specialist, bought the racecar from Currin for two reasons. First, the car represents one of the winningest cars in IMSA history, and second, this was John Greenwood’s most victorious racecar, having racked up four major endurance races in one season.
Unlike today’s Corvette Racing Team, Greenwood’s cars, although “professional grade” were considered “amateur status” compared to the factory Porsche, BMW and other teams. In 1971, Greenwood’s Corvette won the following races: The 12 Hours of Sebring, March 20, 1971, the Donneybrooke National, May 30, 1971, the MIS 4 Hour Enduro on July 4, 1971, and the FIA Watkins Glen 6 Hours on July 24, 1971.
When Smith bought the car, it was wearing a Greenwood wide-body kit and the frame had additional side tubing for driver protection. Smith’s plan was to restore the car to its 1971 Sebring “Stars and Stripes” #48 configuration, with the Currin Swiss Cheese treatment, by summer 2016.
For comparison, lets take a look at a Kevin Mackay’s 1969 L88 “See-Thru Corvette” chassis, the restored Greenwood “Swiss Cheese” chassis, and a modern all-aluminum chassis. Kevin Mackay has carved a special nitch with his “unique creations.” His 1967 Drivable 427/435 Chassis and his magnificent 1969 See-Thru L88 Corvette are very popular attractions at Corvette shows.
Without the fiberglass body, the Corvette looks more like a buggy than a badass street machine. If you look past the engine, exhaust system, interior, and door hardware, you can see the black steel frame and the green part.
There are two parts to the complete chassis; the frame and the birdcage. The birdcage is green because it was sprayed at the factory with zinc phosphate before priming and painting to prevent rusting.
Production birdcages were black, but by leaving it green, viewers can see that the birdcage is a separate component that provides the structure to attach the door hinges, windshield cowlings and frame. It also serves as a quasi-roll bar, and helps stiffen the frame/chassis. For street use, this is fine, but for a racecar with gobs of power and grip, it is very inadequate for endurance racing, as DeLorenzo and Johnson learned with their Owens-Corning Fiberglas racer.
Now lets look at the Greenwood rolling chassis. Racecars live hard lives and when no longer competitive they’re sold off, sometimes into oblivion. The drilling of holes into the structure of racecars was an old technique that’s no longer used.
The forward part of the bird cage is still there (with LOTS of holes), but the back part was replaced with a roll bar attached to be top of the frame, just in front of the rear wheels, and connected to the forward part of the roll cage with two connecting tubes, welded to tubes that parallel the A-pillar, then straight down to the factory frame.
There’s also a tube connecting the forward corner bend of the cage to the top of the frame, just behind the upper A-arms. Then there’s a transverse tube between the side frame rails, just over the front suspension cradle, and in front of the big-block engine.
As purchased from Currin, the driver’s side of the cage had a horizontal tube connecting the front and rear of the roll cage for additional bracing and driver’s side protection.
The passenger side had the same configuration when built by Greenwood. Smith restored the chassis back to the Greenwood configuration that did not have a horizontal driver’s side bar, but a bar angled down to the top of the frame for ease of entrance into the driver’s side.
It’s not often we get to see the bare guts of an old racecar. This is a wonderful example of early ‘70s sports racecar construction that’s similar to NASCAR cars of the day. The structure and suspension is essentially factory. The front upper and lower control arm, spindles, disc brakes, rear spring, control arms, and rear sway-bar are factory.
It is the roll cage and extensive triangulation that connects the front and rear parts of the overall frame that makes all the difference. That is, unless so many holes are drilled into the factory frame that the lower part of the overall structure is now compromised. I suppose you don’t know until you try.
Now lets look at the first all-aluminum chassis ever offered on a Corvette, the C6 Z06/ZR1 chassis. Compared to the Greenwood factory-based 1971 racecar, the modern chassis looks more like an aluminum-hulled howitzer. This is an extremely complicated chassis, but here’s what’s most obvious. The frame rails are massive compared to the C2/C3 frame.
The forward section of the birdcage is equally massive. Note the forward braces coming off the front of the birdcage – similar to the Greenwood racer. The cowling brace, A-pillar, roof, and B-pillar connect to the beefy rear section of the birdcage. And lastly, the rear section of the frame where the suspension is attached is big with a transverse bar that connects the side rails. Also note how slender the front and rear, upper and lower control bars are.
What’s not obvious is that every inch of the C6 Z07/ZR1 chassis structure is computer engineered for proper thickness. In areas there the material doesn’t need to be thick, it isn’t. In high stress areas, everything is thicker and stronger. Engineers didn’t just build a thick frame from stem-to-stern. Although the material is aluminum, engineers were still concerned with weight saving.
So, when we look back at the example of Mackay’s See-Thru 1969 L88 and look at the C6 Z06/ZR1 the difference is astonishing. Then factor in the all-aluminum LS7 or LS9 engine, aluminum transaxle, etc, we’re looking at a car that even the Greenwood racer would have struggled to keep up with at Nurburgring. And to push this conversation through to today, the base model Corvette rides on a chassis that is similar to, but lighter and better than the C6 Z06/ZR1 configuration. Makes me wonder what kind of jewel we’ll see when the C8 mid-engine Corvette arrives. I have no doubt it will be a mechanical wonder.