By Bill Bryant, F65627
March 2004
Produced by General Motors from 1973 through 1978, the innovative GMC Motorhome gained a following that is still as strong “” if not stronger “” today. This three-part series explores the fascinating history of a motorhome still considered by many to have been far ahead of its time. Last month, Part 1, discussed how the GMC Motorhome was conceptualized and developed. This month’s installment covers completion of the design, building of prototypes, and the beginning of testing.
Design of the GMC Motorhome continued to evolve in the two main areas of styling and chassis, with the Design Center concentrating on the external and interior designs. At one time, there were 12 designers working with sketches and 1/8-scale (called A-scale) clay models. Three or four of these clay models were made, each with distinct design characteristics, and each refining its shape closer to the final form.
Since only one side of a model could be viewed at a time, the first scale models were sculpted with two different designs, one on each side. Thus, a single clay model could be used to evaluate two different designs at the same time. The final A-scale model, however, would be viewed as a completed single design.
Once these models were completed, evaluated, and approved, full-sized drawings were made using 1/4-inch tape to outline the front, rear, and side designs. These drawings would guide the designers in the next stage: a full-size clay model.
An important part in developing the shape of this motorhome was to determine just how efficient it would be moving through the air. While the design looked “clean,” tests would determine whether in fact it was. GM used various methods to determine this in its vehicle designs. One method was to put ink spots on a model and place the model in a wind tunnel to watch the directional flow of the ink. In another test method, small ribbons were fastened to the surface of a test vehicle and a camera car was driven alongside the vehicle to photograph the directional flow of the ribbons. While either of these methods may have been used, what is known is that a 1/16-scale model was built for the purpose of determining the coefficient of drag, known as CD. A mahogany block was modeled to the designed shape, and a fiberglass model was then made from this master. Holes were drilled about one inch apart over the fiberglass body, and flush-mounted, hollow tubes were installed and connected to pressure-measurement devices. Time was purchased at the Guggenheim wind tunnel facility in California, and the test was performed. Was the shape clean? You bet! The CD was .310, better than that era’s Corvette, which was .312.
A great deal of clay is required for a 26-foot motorhome, probably the largest clay model GM ever made. The clay had to be heated in an oven to 150 degrees Fahrenheit to soften it so that it could be applied to the substrate, which in this case was Styrofoam. It was then scraped and shaped by the modelers. The clay buildup continued until the dimensions of the design were achieved. This work was done in the basement of the Tech Center in an area without a well-controlled environment, and without air-conditioning. Each morning the room had to be brought up to temperature so that the clay could be worked. Once the shape was completed, the clay surface was “polished” with a sponge and cold water. This full-size clay model, now identified as RV-26, was finished with a silver-blue film of Di-Noc, replicating the painted surface of a vehicle.
Once the full-size clay model was completed, plaster casts were made of it. Dimensional drawings of these casts were made for tooling and for building the early fiberglass parts for the first prototype bodies. The full-sized clay model existed for only about a month, after which it began to sag. Since it was no longer needed, it was destroyed.
Pictures of the scale models, the sketches and drawings, and finally the full-size clay model are most interesting. The evolution from the earliest designs, with pronounced fender flares, wraparound rear windows and taillights, and other eye-pleasing shapes, drifted toward a still pleasing but more “manufacturable at reasonable cost” design in the end.
Changes made to the clay model after being “finalized” were the incorporation of the parking lights below the headlights within the headlight bezel area and the use of four Suburban (production) taillights. The quad taillight idea, however, was quickly discarded as being too cluttered. Designer Mike Lathers proposed a couple of other features that didn’t make the cut; these included a crown (curve) in the rear window glass, which would have minimized reflections caused by a large window surface. Another of Mike’s novel ideas was to accent the rear suspension, instead of hiding it (a polished, plated, and painted suspension assembly, much like Harley-Davidson does with a motorcycle). The view of a “flashy” suspension in action would have been an impressive sight. Even though some of the suggested features didn’t materialize, I have yet to meet a person who doesn’t think that the GMC Motorhome is arguably the nicest design to ever come down the RV pike. When I look at the earliest renderings, I usually have a big smile on my face, thinking of what might have been.
In parallel with the exterior work at the Tech Center, six designers were developing the interiors. The plywood buck mentioned earlier helped to define space and floor plan requirements. An important consideration was the loading of the vehicle, distributing the weight as evenly as possible. Considerable effort was made in evaluating competing interior features. At one point, it also was suggested that the Frigidaire Division might build RV appliances, stoves, and refrigerators. But with a limited market and very competitive pricing, Frigidaire decided there were better business arenas available.
House and Garden magazine was used as a consultant for the interior décor and exterior colors. This publication was the authority on the colors of that era. Ed Cole, GM president, suggested that bright colors should be used. One story “” confirmed by several people “” reported that during one meeting in which colors were discussed, Ed Cole pointed to the tie of chief engineer Wally Edwards and said, “One of the colors should be like Wally’s tie.” The tie was adorned with orange stripes; an orange color (bittersweet) was offered in 1973-74. Wally still has that tie.
The chassis design was continuing as well. The engine and drivetrain layout was progressing rapidly, since it required only minor changes to the existing Oldsmobile Toronado design. Ralph Merkle’s original 24-foot design had been stretched to 26 feet in order to make room for the interior furniture and appliances. This displeased Ralph, who had envisioned using many existing GM passenger vehicle components to keep the motorhome price affordable. Now additional length and weight were going to compromise the use of these parts, as well as the front-wheel traction.
Ralph Merkle related a story about the frame design. When the design was nearly completed, Ralph asked John Locklin for the final body design details, because frame and body designs had to be coordinated. However, the body design was not yet finalized. Since lead times for the frame subcontractors were critical, Ralph was forced to make his best guess. His guess was off by about 2 inches. That is why the rear frame cross member sticks out about 2 inches beyond the body. The frame was made of 1/8-inch-thick steel by Midland.
Ralph and John enjoyed some good-natured ribbing during this process. Ralph kidded John that his frame was holding up John’s body, and John would respond by saying it was his body holding up Ralph’s frame. They were probably both right.
The rear tandem-axle hydro-air suspension designed by GM’s Saginaw Division had been the key design element for this vehicle. It allowed for the low floor as well as a measured improvement in ride and handling. Another feature was less intrusion into the living area than dual wheels would have caused. Tooling costs for the hydro-air design had come in much higher than anticipated, however. A less costly design was requested as a backup, and work began on a replacement.
A full air spring was designed. A Goodyear model using a long cylindrical (air) spring, one piston, and one bag convolution had been found to have interference problems. A new full air spring design was started using the rolling lobe principle with two tapered pistons and a floating air spring between them. The tapered pistons kept the spring centered, and as the rubber spring rolled up and down the piston surface, the spring rate was changed. The first prototype spring was delivered by Firestone in March 1971 and became the production air spring with only a minor modification (slightly larger in diameter and slightly shorter in length).
In the Experimental Engineering area, five pre-test chassis were being built. The pre-test bodies would later be installed on four of them. The fifth chassis would be used for pictures and display. The completed units would be used for various purposes “” some for testing, some for pictures and shows. We will hear more about these later.
John Locklin, the body engineer, was spending most of his time at G.L. Bowen & Co., the job shop that was supplying the draftsmen charged with documenting the motorhome body. Measurements of the plaster casts made from the full-size clay model were translated into drawings to be used to make the dies, body structure, and skin. John’s background as an aeronautical engineer was showing up in many of the design features “” for example, the light weight of the aluminum and fiberglass body skin, the all-aluminum body framing, and the bonding of body panels with adhesives instead of rivets and screws. Another important design feature was the use of flat aluminum panels for body surfaces above the belt molding, including the roof. No body stampings were required, thus minimizing tooling and fabrication costs. John also had insisted on a flat driver’s compartment floor. In driving a competitor’s motorhome, he found that having to squeeze between the driver’s seat and the engine cover was uncomfortable and, in his mind, unnecessary.
GMC Engineering was attempting to build the side window frames out of plastic. While windows with sharp corners were in vogue at the time, Mike Lathers felt this design resulted in stresses at the corners that could eventually cause cracks in the body skin. The rounded corner design reduced these stresses and provided a less dated look. The plastic frames couldn’t be made to work, however, and a multi-piece aluminum frame was put in production. A few years ago I asked a GMC engineer why GMC changed to Hehr windows in 1976, and he said the anodizing on the earlier windows didn’t match. He said they couldn’t build them, couldn’t seal them, and couldn’t service them. GMC initially planned for the window frames to be “bright.” At one of the design reviews, they hadn’t yet gotten around to plating the frames and they were painted black. Those reviewing the design liked the black frames better, and that settled that.
The large body dies required for the SMC (sheet molded compound) fiberglass panels were in place at the vendor, Engineered Molding Systems of Lancaster, Ohio. Lumps of SMC “dough” were strategically placed in the molds, and with heat and pressure, it took 12 minutes to process each panel. This cycle time was reduced somewhat as experience was gained with the curing process.
At GMC, the first hardware that we would recognize as a GMC Motorhome was being assembled. The chassis with the Toronado front-wheel-drive assembly was pretty well defined. The rear suspension was still the tandem-axle hydro-air suspension, and many components were still passenger car parts, such as the five-bolt wheels and hubs. In viewing the body hardware, things were a little different from how they had appeared in the clay model. The front end was nearly the same, but the sides had become vertical and straight. This particular vehicle was to become known as “straight sides,” not usually uttered in a complimentary tone. It turned out that GMC engineers thought the upper storage cabinets should be able to hold a 12-inch plate, and the tumble home (body side curve) design created by Mike Lathers didn’t allow for that. At the rear of the body, the large removable back panel had not yet been implemented.
When Mike Lathers got a look at straight sides, it is reported that he “went straight up in the air.” The design looked a lot like any other motorhome of the era. In selling the design to GM management, the point had been made that anybody could make an ordinary-looking motorhome; what GM needed was “style” to sell. And style is what they got. Mike lobbied for and won the argument to have his design reinstated. That was the last of “straight sides.”
GMC had decided to outsource the interior up-fitting. Bids were received from four manufacturers: Open Road, Sportscoach, Muntz Corp., and PRF Industries. Muntz Corp. had returned the most attractive bid, but they were located in California. GMC officials desired someone closer to Pontiac, Michigan. Although its bid was not the lowest, PRF Industries received the nod, since its location was to be 22 miles from the Pontiac plant, and further negotiations brought its bid close to GMC’s price point.
PRF Industries was owned by Peter R. Fink, the builder of Travco and Sightseer motorhomes. A new PRF division was formed to build the GMC interiors, Gemini Corp., located in Mount Clemens, Michigan. Rumor has it that Peter Fink named the new corporation “Gemini” because it was “GM & I.” Rumor also has it that his zodiac sign may have been Gemini; take your pick on which (or both) you want to believe.
The Gemini facility designed and built all of the furniture; assembled all of the modules; applied the Texolite laminate; and installed everything, bringing it in through the motorhome rear access opening. A number of start-up concerns had to be resolved. Between the motorhome body manufacturing tolerances and the interior module tolerances, which were affected by temperature and humidity, changes had to be implemented to make things fit. Every wooden part had to have a drawing and a GM part number. The complexity of 15 different floor plans and the many available options contributed to a complicated interior assembly process as well. Approvals from the many individual states as well as from the federal government had to be submitted and granted also. Gemini had a target of completing the interiors of 32 coaches per day; the best they achieved was closer to 20 per day.
With the prototypes now being assembled, and their purposes defined, their activities would begin. The first public showing would be at Transpo 72, the U.S. International Transportation Exposition near Washington, D.C. (Dulles), from May 27 through June 4, 1972. Many GM cars and trucks were on display there as well as a tan-colored 26-foot motorhome. The exterior was rather plain, with no stripes or trim. In a brochure it was described as “An experimental prototype of GMC Truck & Coach Division’s complete motorhome, to be marketed in early 1973.” The display was still labeled as a “Multi Purpose Vehicle,” although this term would not be used again, as all efforts were now focused on developing and producing a motorhome.
Other motorhome prototypes were undergoing testing. A 23-foot motorhome model named “Cape Cod” was at bench test. Programmed testing with replicated Belgian blocks (cobblestones) cycled the suspension with hydraulic rams. Door latches were slammed, windshield wipers were cycled, brake pedals were pumped, all through a determined number of test cycles. Coaches were also run for what seemed like endless hours at the Milford, Michigan, GM Proving Grounds. Hot-weather testing was carried out on the “Baker Grade,” an 18-mile-long hill in the deserts of Southern California. This hot-weather testing resulted in an additional fan blade being added; use of the seven-bladed fan was now dictated.
Modified passenger vehicle test criteria were used, consisting of proving grounds roads for 25,000 miles (regular durability) and 5,000 miles of Belgian blocks (accelerated durability). During the “figure 8″ road testing, an early failure was the five-bolt Toronado passenger car wheels. They were replaced with eight-bolt one-ton truck wheels. This also required replacing the hubs to match the eight-bolt pattern. Another failure was a cracked frame near the front cross member. The addition of a diagonal brace fixed this problem. The front suspension’s lower A-frame and lower ball joint were other areas that required upgrades to the passenger vehicle parts.
Crash tests were performed at 30 to 35 mph as well. From Pennsylvania dealers, GM had purchased new Buicks that had been in a flood. The side crash test aimed the Buick at the motorhome’s large side window area. The Buick’s bumper penetrated the sidewall and came to rest above the floor. New aluminum vertical reinforcements were added, and this problem was fixed. The rear crash test resulted in damage to the frame kick-up area and spare tire carrier. Changes in this area fixed this as well.
A rollover test was also performed. The proving grounds had a rollover ramp designed for just that purpose. This worked well for passenger-type vehicles, but long wheelbase vehicles usually had to be assisted. A forklift raised one side of the frame to tip it over, 1-1/2 to 2 turns. The damage was reported to be not all that serious: a toilet had come loose along with a few other interior pieces, and the body was distorted to one side.
Plans for the manufacture of the motorhome were moving along rapidly. Plant No. 3 in Pontiac, Michigan, had been outfitted with equipment and tooling. This was one of GM’s older plants, and it had a split-level floor plan. At the lower level, the chassis frame was collecting various subassemblies as it proceeded down the line: front and rear suspension, tanks, air lines, electrical wiring, and, finally, the engine and drivetrain.
The upper level of the plant was used for body assembly. Individual aluminum pieces of the frame were placed in manufacturing jigs, which assembled and aligned them; the pieces were then welded together as a unit. Three unique jigs were used: right side, left side, and the roof assembly. Next, these panels were assembled and welded together as the body cage. Separately, each of the body panels was very flexible. When welded together, they became a strong yet lightweight unit. Aluminum and fiberglass exterior body panels were fastened to this frame with a 3M adhesive. John Locklin had wanted to use an adhesive made by a small company called REM, but GMC wanted a larger, well-known supplier instead. This assembled body was then cleaned, prepped, and painted.
When completed, the body was lowered to the main floor, where it was mated with the assembled chassis.
To be continued … Next month, Bill describes the mass production, marketing, and, sadly, the demise of GMC Motorhome manufacturing.