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Street Legal

Street Legal

Ford's GT40 concept car design team retained the look of the original racer (rear) in a modern technology package for a street performance car. They developed a production ready car in about seven months.

Ford created quite a stir at this year's North American International Auto Show in Detroit this past January when it unveiled its GT40 concept car. The curvy and aggressive looking car evoked the original GT40 endurance racer of the 1960s-a car that brought Ford back into international racing, and whipped Ferraris and Porsches for four years in a row. The reception was so positive that within six weeks of its unveiling, the company announced plans for limited production. Cars are to be available sometime in 2003 for the 100th anniversary of Ford-challenging the Viper and Corvette as king of the road.

Leading up to the debut was an intense effort involving automotive engineers and designers to develop this road car based on a successful racer-and do it in seven months. Requirements were deceptively simple-it had to go fast, handle well, and look good, while being true to the GT40's heritage. On top of those, the design also had to be readily translated into a production car if that decision was made.

Teamwork. The company turned to its Special Vehicle Team (SVT), and John Coletti, SVT chief engineer, along with its Living Legends studio, where creative designer Camilo Pardo headed formulating the car's look (see sidebar). Pardo's first cut at the car was less than successful. Sporting a short nose, smooth integrated surfaces, and aligned features typical of modern cars, the design "walked away from the original GT40 with its misalignments, swells, lumps, and bumps," he says. "It looked like a nice car, but the original is not a nice car, it is a mean car, an aggressive car." And as a racecar, every part of the original had a function, "very much like an aircraft." Pardo notes that sitting in the original is like sitting in an old fighter plane. What he wanted to achieve in the new car was akin to sitting in a new fighter.

The modular V8 engine uses Ford's widely developed MOD engine architecture that allows aftermarket engine equipment makers to have a large customer base on many automobile models, not just the GT40. Engineers endeavored to position the superchargers (top) and other components to minimize noise directed forward into the passenger cabin bulkhead, just ahead of the engine.

The team then evolved the design so the final concept is more in proportion to the original 1960's racer's lines-a foot and a half longer and 3.5 inches taller (the 40 in the name comes from the original height in inches). Pardo says they basically took the original shape and cleaned up and simplified its lines.

But obviously, no one is going to build merely a cleaned up version of a car and still incorporate old racing technology. "There's been 35 years of technology evolution since the original," says John Coletti, with benefits in powertrains (engine, transmission), chassis (handling packages and suspension tuning), and brakes, as well as emissions and fuel economy. Perhaps most important are the computer modeling techniques available to engineers. "These eliminate 90% of the black art, so we can use modeling to zero in on what is the right combination of hardware," he notes. As for significant tools Ford engineers use, Coletti says the company has developed much of its software internally. "We can do sophisticated chassis geometry modeling and finite element analysis for engines and powertrains." Such tools ensure "the technical integrity," as Coletti puts it, which, for example, was important for the GT40 back-end structure. Here the entire rear skin is a clamshell that pivots upward via hydraulic actuators, giving easy access to the aluminum V8 engine and transaxle mounted on the spaceframe structure.

The GT40's extruded aluminum spaceframe structure has a central backbone for rigidity. Composite panels form the body of the car.

In commercial packages, Ford has standardized on I-DEAS from EDS (Cincinnati, OH) for CAD, and Coletti cites ADAMS simulation software from Mechanical Dynamics (Ann Arbor, MI), a company recently purchased by MSC.

Software. "We use this to ensure keeping the tire patch on the ground. You want the suspension to work in such a way that the tires stay in contact with the ground as much as possible to keep the power on the road." And while the designers have slightly larger tires in the rear (19-inch diameter wheels, 10-inches wide, versus 18 x 8 inches up front) for an aggressive look, the resulting larger tire patches in the back also helped the engineers transfer power to the ground.

But even with new technology available for a GT40 package, why wouldn't an enthusiast merely go out and buy one of the replica cars that recreate the original almost exactly, or even acquire an original? Coletti points out that in the sixties, Ford sold some as private racers. But when it tried marketing a version as a streetcar (the Mark III with a 289-cu inch engine), a grand total of only seven were sold. Obviously there were features a racer would put up with that the public would not.

One problem was egress. "Cars with wide shoulders, like the original, put the structure outboard for stiffness," notes Coletti. This made the cabin window area small and resulted in the nearly foot-wide door sills that a driver or passenger had to step over, onto the seat, and then wriggle down to get into the seat-obviously not something conducive to picking up a date. The sills also contained the fuel and shift linkage to the rear mounted transaxle, and there were cutouts in the roof for the curved-in door panels to help improve access.

The modern GT40 road car's interior (top) captures the fighter-cockpit look and feel of the original race car (below) by using toggle switches and analog gauges (tachometer in the panel's center). The new configuration's structural backbone, forming the console, allows better passenger egress. Required comfort features include air conditioning and an audio system. The ignition key, toggles for the fuel pump and ignition, and a red starter button are sequenced racing-style to fire up the engine.

The GT40 concept design team corrected such shortcomings while keeping the look and feel of the original. Regarding the egress difficulty, the aluminum spaceframe of the new car has a high central backbone for stiffness. This moves the passenger positions outboard and the door sills are much narrower. There are still roof cutouts for easier entry and exit, but these were a unique feature desired to evoke the original car. The backbone also houses a Kevlar fuel cell more safely in the center of the car, encloses the coolant lines to the radiator, and forms the structure of the center console, according to Coletti.

The modular, 500-hp, aluminum V8 engine mounted just aft of the cabin provides more than adequate power. This engine architecture was introduced by Ford in the early nineties and so is common to other Ford engines, increasing parts availability and being easy for owners to work on, "while being a performance benchmark," notes Coletti. "With such widespread volume for the architecture, then the after-market will create performance and aesthetic parts for those engines."

While the GT40 will not be for everyone, Ford is hoping the technical and general interest shown in the car will help turn its economic fortunes around by having its glow be reflected in its other brands.

GT40 stats
Construction Aluminum spaceframe, unstressed composite body
Length 181.6 inches
Width 76.8 inches
Height 43.5 inches
Track width (front/rear) 64.4/65 inches
Wheelbase 106.7 inches
Engine 5.4-liter, double overhead cam, 32-valve, supercharged and intercooled MOD modular V8. Aluminum block and heads
Horsepower 500 @ 5,250 rpm
Torque 500 ft-lb @ 3,250 rpm
Transmission RBT 6-speed transaxle
Fuel capacity 28.4 gal of 91 octane

Headworks: While Ford engineers won't tell us the production car's target weight (since they want to better that number). They say the ballpark is less than 12 seconds to do a quarter mile and reach 120 mph. What is the approximate weight of the car?

To solve this problem involves a nonlinear equation that must normally be solved by a computerized integration program. However, some simplifying assumptions can be made in order to come up with a very loose estimate of the solution for the weight. A first order solution also requires estimates of several variables that add to the inaccuracy of the solution.

If you assume:

R = Radius of tires = 14.25 inches & /FONT >

r = Air density = 0.00238 slugs/cu ft (at sea level, 59F, 14.7 psia) & /FONT >

D = Aerodynamic drag coefficient = 0.33 & /FONT >

E = Mechanical efficiency = 0.82 & /FONT >

G = Overall gear ratio = 4.22:1 & /FONT >

T = Average torque during the run = 0.92 x 500 = 460 ft-lbs & /FONT >

A = projected frontal area = 43.5" x 76.8" = 23.2 sq ft. & /FONT >

Then you can calculate the engine generated tractive force at the driving wheels Fe,

Fe = T E G / R = 1,340 lbs & /FONT >

Fa = the average aerodynamic drag force is = 0.5 D A V2 /3 = 94 lbs

Where V = the speed attained over .25 mile = 120 mph = 176 fps & /FONT >

f = The average rolling resistance factor is = 0.01 x ( 1 + V / 294) = 0.0160 & /FONT >

Setting the work done by the engine tractive force, the rolling resistance, and the aerodynamic drag to the kinetic energy at the end of the .25 mile (1,320 feet), you get,

(Fe - Fa - f W) = 0.5 W V2 / g

g = the gravitational acceleration = 32.2 ft/sec2.

The values above can be plugged into this equation, and solved for W, giving:

W = 3,276 lbs

For another approach, engineers often use simple rules of thumb for making first approximations. One such technique is the "shadow area" method-taking the curb weight per square inch of planform area (length x width) for comparable vehicles and multiplying this by the planform area of the new design. Using published data for a Ferrari Modena 360 and a Porsche 911 yields an average factor of 0.235 lbs/inch2. Multiplying this by the approximate GT40 planform area (181.6 inches x 76.8 inches) results in:

W = 3,278 lbs

Larry Zirkle, P.E. forensic engineer and Rick DeMeis, Senior Technical Editor
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