This article was originally published in the October 1998 issue of Motorcycle Cruiser.

After 20 years of clay modeling for the automotive and aerospace industries, Stephen Stringer opened his own company nine years ago. Despite its name, Alternative Automotive Design offers design services to many different industries for projects ranging from his custom Royal Star, Virago 535, and trailer-and-truck combo, to vehicle design and prototyping (like his role in the Cirrus composite aircraft). While some modeling projects require Stringer to match drawings and design specifications, clay modeling can be used—like in his personal project—as a means of sculpting the object from scratch.

Although Denny Berg (Motorcycle Cruiser August '97) constructed Eleganza's fenders out of carbon fiber, the brake lights were shaped to the fender with Bondo.
If the fender (left) were damaged, the entire work would need to be duplicated. Stringer's fender (right) could be completely recreated using the original molds.
Dean Groover

Although the process has become more complex with the advent of composite materials (such as carbon fiber and Kevlar fabrics), clay modeling prototypes have changed little since the early ’30s when Harley Earl (a head of GM design), had a brainstorm while playing with a piece of clay he pulled out of a river bed. Earl contacted Chavant, a manufacturer of modeling clays, and a new automotive design method was born. Today, Chavant remains the industry standard in automotive clay modeling. Clay’s non-drying nature makes it perfect for design, since sections of a vehicle may go through many changes before a final look is decided on. At room temperature the clay remains firm; but with the application of heat, the clay returns to a more malleable state. Scraps can simply be warmed and reformed into blocks for reuse—a boon to designers who don’t have the deep pockets major manufacturers do.

(Top) The composite fiber cloth is placed inside the mold. The inner portion of the luggage compartment can be seen in the shape of the mold. (Bottom) The hardened inner fender slips out of the mold. It will be cleaned up and mounted on the bike so that the outer fender can be modeled.Fran Kuhn

The rear fender of Stringer’s Royal Star provides a perfect example of how clay modeling, in conjunction with composites, can provide customizers with the ultimate flexibility in design that yields an end product of working molds that produce strong, lightweight parts. Stringer made a mold of the OE fender then altered it to frame the inner portion of the storage compartments, and a composite fender was produced with the inner walls. This composite fender was mounted to the bike, and the shape of the outer fender/compartment doors was roughed out using polystyrene board. Once the polystyrene armature was shaved down, the entire part was shellacked to give the clay a surface to stick to. The clay was then attached to the fender and sculpted until the determination of the final shape.

clay modeling
Two phases of clay modeling. The left side shows the rough construction with polystyrene board; the right side shows the clay modeling in progress.Stephen Stringer

Before the mold could be made, the surface of the clay needed to be as smooth as possible. Utilizing metal shims with progressively thinner profiles, the surface imperfections were removed in a process similar to sanding with finer grains of sandpaper prior to painting. The fender was removed from the bike, and a gypsum-mold was made of the whole shebang.

The production of the actual fender from the mold didn’t differ much from making any part from fiberglass or carbon fiber. The mold was lined with gray gelcoat. Once the gelcoat tacked up, a layer of 10-ounce carbon cloth was applied, followed by a final layer of carbon fiber and Kevlar®. Before the epoxy resin had a chance to dry, the whole mold was placed inside a vacuum bag to force the composite fabrics and gelcoat to take the shape of the mold.

clay model
Viewed from the front, the inner fender, the clay outer fender, and compartment door are ready to cast the second half of the female mold.Stephen Stringer

Once the composite inner fender and outer fenders were out of the molds, the fender portion of the outer fender was cut off, leaving the compartment door to be mounted to the inner fender. Voilà—built-in hard luggage! Stringer estimated that the entire fender assembly, complete with the compartments weighs only three and one-half pounds. Less than the stock fender alone. The flexibility of the design, its light weight, strength of the composite materials, and the ease of duplication should make clay modeling and composites a more common customizer’s tool in the not-too-distant future. In fact, we predict that the carbon-fiber look that swept through sportbikes will appear on cruising muscle bikes.