Corvette Chassis War – Greenwood L88 vs C6 Z06/ZR1

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.

The rolling chassis was also on display at the Amelia Island 2016 Concours D’Elegance event the week before the 2016 12 Hours of Sebring race. Lance Smith told me that the car was 98-percent complete and that he didn’t want to show the car incomplete, so he decided to just show the rolling chassis. The car should be completed by summer 2016. Photo – K. Scott Teeters The rolling chassis was also on display at the Amelia Island 2016 Concours D’Elegance event the week before the 2016 12 Hours of Sebring race. Lance Smith told me that the car was 98-percent complete and that he didn’t want to show the car incomplete, so he decided to just show the rolling chassis. The car should be completed by summer

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.

This is Kevin Mackay’s beautiful “See-Thru Corvette L88.” To the right, you can see the black frame. The green part is the separate factory birdcage. Mackay’s unique creation is fully functional and drivable! Photo – K. Scott Teeters

This is Kevin Mackay’s beautiful “See-Thru Corvette L88.” To the right, you can see the black frame. The green part is the separate factory birdcage. Mackay’s unique creation is fully functional and drivable! Photo – K. Scott Teeters

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!

You can clearly see how the green birdcage forms a quasi-roll bar and provides the structure for the door hinges and A-pillar. This display piece of art is captivating – something you can look at for hours and hours. Mackay also has a See-Thru 1965 big-block, and is planning to make a similar See-Thru C1 Corvette. Photo – K. Scott Teeters

You can clearly see how the green birdcage forms a quasi-roll bar and provides the structure for the door hinges and A-pillar. This display piece of art is captivating – something you can look at for hours and hours. Mackay also has a See-Thru 1965 big-block, and is planning to make a similar See-Thru C1 Corvette. Photo – K. Scott Teeters

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.

Here’s what the car looked like, minus the Greenwood-style wide-body when Lance Smith bought the car. The Swiss Cheese treatment was performed by Corvette racer, “Fast Phil” Currin from Gainesville, Florida. The objective was to lighten the car, but Phil admitted that he over-did-it with the hole-saw. Photo – Lance Smith

Here’s what the car looked like, minus the Greenwood-style wide-body when Lance Smith bought the car. The Swiss Cheese treatment was performed by Corvette racer, “Fast Phil” Currin from Gainesville, Florida. The objective was to lighten the car, but Phil admitted that he over-did-it with the hole-saw. Photo – Lance Smith

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.

Note the horizontal tube connecting the front and rear sections of the roll cage. This was additional protection for the driver and added a little stiffness to the overall chassis. This was added after Greenwood sold the car. Smith removed this and took it back to the original configuration that you can see in the above photo. Photo – Lance Smith

Note the horizontal tube connecting the front and rear sections of the roll cage. This was additional protection for the driver and added a little stiffness to the overall chassis. This was added after Greenwood sold the car. Smith removed this and took it back to the original configuration that you can see in the above photo. Photo – Lance Smith

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.

Holes were drilled on the inside, outside and top side of the production frame. The engine cradle also got the Swiss Cheese treatment. Note the transverse bar added above the cradle, this was to add stiffness between the shock towers. Photo – K. Scott Teeters

Holes were drilled on the inside, outside and top side of the production frame. The engine cradle also got the Swiss Cheese treatment. Note the transverse bar added above the cradle, this was to add stiffness between the shock towers. Photo – K. Scott Teeters

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.

When Fast Phil Currin was finished with his lightening project, even the upper control A-arms were drilled. This seriously weakened the A-arms so Smith installed unaltered production A-arms. Note the triangulation tubing that ties the roll cage to the front of the frame. Photo – K. Scott Teeters

When Fast Phil Currin was finished with his lightening project, even the upper control A-arms were drilled. This seriously weakened the A-arms so Smith installed unaltered production A-arms. Note the triangulation tubing that ties the roll cage to the front of the frame. Photo – K. Scott Teeters

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.

The big-block engine is slightly set back. Note that even the transverse cross member of the frame was Swiss Cheesed. The period-correct Goodyear racing tires are about the same size as the tires used on production C4 Corvettes! Photo – K. Scott Teeters

The big-block engine is slightly set back. Note that even the transverse cross member of the frame was Swiss Cheesed. The period-correct Goodyear racing tires are about the same size as the tires used on production C4 Corvettes! Photo – K. Scott Teeters

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.

Look carefully and you’ll see the floor through the holes in the top of the frame, meaning that the bottom of the frame is open. Gobbs of horsepower and torque didn’t get along well with a frame as lightened as this. Photo – K. Scott Teeters

Look carefully and you’ll see the floor through the holes in the top of the frame, meaning that the bottom of the frame is open. Gobbs of horsepower and torque didn’t get along well with a frame as lightened as this. Photo – K. Scott Teeters

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.

Greenwood was on the right track with his triangulation work atop of the production Corvette frame. Only a few years before, the only addition to the frame was a roll bar for driver protection! Photo – K. Scott Teeters

Greenwood was on the right track with his triangulation work atop of the production Corvette frame. Only a few years before, the only addition to the frame was a roll bar for driver protection! Photo – K. Scott Teeters

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.

The rear section of the chassis needed some serious beefing up to handle hours and hours of abuse from the L88 torque-monster engine. More triangulation helped keep the frame from pretzeling. Photo – K. Scott Teeters

The rear section of the chassis needed some serious beefing up to handle hours and hours of abuse from the L88 torque-monster engine. More triangulation helped keep the frame from pretzeling. Photo – K. Scott Teeters

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.

When you remove all the extra tubing, the factory frame seems puny, especially when compared to the C6 all-aluminum frame. But in 1960 when Duntov’s engineers first started work on the C2 Corvette’s all-new structure, the frame we see here was more than adequate for a car what would have 360-horsepower and 352-lb-ft of torque. Photo – K. Scott Teeters

When you remove all the extra tubing, the factory frame seems puny, especially when compared to the C6 all-aluminum frame. But in 1960 when Duntov’s engineers first started work on the C2 Corvette’s all-new structure, the frame we see here was more than adequate for a car what would have 360-horsepower and 352-lb-ft of torque. Photo – K. Scott Teeters

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.

Here’s the rolling display chassis of the 2009 ZR1 Corvette. GM sure knows how to make display pieces. Photo – K. Scott Teeters

Here’s the rolling display chassis of the 2009 ZR1 Corvette. GM sure knows how to make display pieces. Photo – K. Scott Teeters

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.

15-C6-ZR1-Chassis-1BThe 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.

When the C6 Z06 made its debut, no one in the Corvette community was clamoring for an aluminum frame. Engineers used computer simulations to determine where on the frame the aluminum needed to be thicker or thinner. The completed chassis weighed 392-pounds, 110-pounds less than its steel counterpart and was 50-percent more rigid.

When the C6 Z06 made its debut, no one in the Corvette community was clamoring for an aluminum frame. Engineers used computer simulations to determine where on the frame the aluminum needed to be thicker or thinner. The completed chassis weighed 392-pounds, 110-pounds less than its steel counterpart and was 50-percent more rigid.

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.

The difference between a modern all-aluminum Corvette chassis and the C2/C3 chassis is like the difference between a 1946 RCA 630-TS 10-inch black & white TV and a 2016 LG Electronics 65-inch OLED TV. Night and Day! No comparison! Photo – K. Scott Teeters

The difference between a modern all-aluminum Corvette chassis and the C2/C3 chassis is like the difference between a 1946 RCA 630-TS 10-inch black & white TV and a 2016 LG Electronics 65-inch OLED TV. Night and Day! No comparison! Photo – K. Scott Teeters

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.

The seeds for a mostly aluminum Corvette were sown in 1957 with the Q-Corvette concept that used an all-aluminum, fuel-injected small-block with an aluminum-cased 4-speed transaxle. This was the early days of aluminum alloys for car parts. A structure such as that of the C6 Z06/ZR1 was nowhere near possible. Photo – K. Scott Teeters

The seeds for a mostly aluminum Corvette were sown in 1957 with the Q-Corvette concept that used an all-aluminum, fuel-injected small-block with an aluminum-cased 4-speed transaxle. This was the early days of aluminum alloys for car parts. A structure such as that of the C6 Z06/ZR1 was nowhere near possible. Photo – K. Scott Teeters

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.

The C2 perimeter frame and chassis was an improvement and adequate for racing activity. Things didn’t get shaky until the L88 package arrived, combined with the latest wide racing tires.

The C2 perimeter frame and chassis was an improvement and adequate for racing activity. Things didn’t get shaky until the L88 package arrived, combined with the latest wide racing tires.

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.

About the author

Scott Teeters

Scott Teeters has been a contributing artist and writer for Vette Magazine since 1976, and in 1997 launched his Vette Magazine monthly column, The Illustrated Corvette Series. His stories focus on the historical, technical, and people aspect of the world of Corvettes, off and on the racetrack. He contributes to Power Automedia as a freelance writer.
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