Shocker of an idea 22383

April 6, 1998 Design News

INSIDE STORY-MATERIALS/COVER STORY

Shocker of an idea

Retrofit rear-suspension system puts damper on mountain-bike costs

by Gary Chamberlain, Senior Editor

Beaverton, OR--Mountain-bike enthusiasts will love it. Design engineers with no mountain-bike experience will appreciate it. The it? An innovative add-on rear-suspension system designed to optimize a mountain-bike rider's power and performance, while offering added speed, control, and comfort.

"Shockster APS is the next significant advance in mountain-bike suspension technology," says Lester Erlston, president of BikeControl?, the Oregon firm that pioneered the just-introduced system. He adds: "For retailers, we have a new product for which there is a tremendous built-in demand and excellent profit margin. For consumers, it's a breakthrough solution both in terms of performance and price."

"This design is nothing short of revolutionary, since, prior to its introduction, the prevailing wisdom in the bike industry was that an add-on rear suspension was not achievable," adds Bob Laman, BikeControl's vice president, sales and marketing.

A key to making the system profitable for everyone concerned, according to Erlston, is that "until now, you had to buy a new bike to get full suspension. Our solution is not only affordable, but offers outstanding performance. All you have to do is install it on the mountain bike you already own."

Others seem to agree. After debuting the concept at last fall's Interbike trade show, Bicycle RetailerandIndustry News reported that the add-on garnered favorable comments from testers who liked its price and performance. "Shockster rode well under a variety of extreme conditions," Ray Teleng of Insane Cycles, told the trade journal. "Downhill fire roads were no problem. And climbing was a breeze. The overall ride was nice." In addition, Jim Rennie's Sports Letter called the Shockster "the hottest new product at Interbike."

No more pogo. Shockster's simple design addresses the disadvantage of many original-equipment, full-suspension bikes by eliminating the undesirable "pogo" effect (pedal-induced suspension movement common to many full-suspension bikes), while remaining fully active. And it weighs only 2 3/4 lbs.

Shockster mounts onto the rear dropouts and the rear brake bosses of most standard 26-inch-wheel mountain-bikes. However, 100% of the suspension load is borne by the bike's dropouts. The rear wheel and derailleur fit onto the system's dropouts, while the brakes mount on Shockster's cantilever bosses.

But its Shockster's U.S. and foreign patent-pending active pivot system that highlights the design. Not only does the system eliminate the pedal-induced suspension travel, but it allows 31/2 inches of fully active vertical movement.

Simply put, Shockster converts a "hardtail" bike (one with front-suspension only) into a full-suspension bike. Thus, a bike owner who already has a hardtail bike, but would like full-suspension, can upgrade it to meet this need at a reasonable cost. Typically, a new full-suspension bike retails for $1,500 and up.

Moreover, the system permits users to tune the suspension to their weight and riding preference. A preload dial with 11/2- inch preload at the bottom of the springs makes this possible. In addition, the user has a choice of normal or hard springs from which to select. The springs will accommodate riders who weigh anywhere from 100 to 250 lbs. Changing springs takes only a few minutes.

But what about the price, you ask? The rear-suspension system with coil springs and oil damping retails for $329. And it installs in about 30 minutes.

Mastering materials. As in any design, materials played a major role in making the system come alive. This was especially true in Shockster's case, considering it comes with a one-year warranty and must survive repeated rugged use.

Structural components, which include a U-brace, struts, and wheel dropouts, are made from high-pressure, die-cast aluminum alloy. Carbon-fiber, glass-fiber, and PTFE-filled thermoplastic composite injection moldings are used for the system's pivot points and sliding surfaces.

In considering which parts to produce from plastics and what plastics to use, the main issues revolved around durability, weatherability, low friction, and low wear. The BikeControl project team presented them to LNP Engineering Plastics (Exton, PA) technical personnel to identify the right materials.

For instance, a carbon-fiber reinforcement (Lubricompr RCL, a PTFE lubricated nylon 6/6) allowed sufficient strength in very small wall sections to reduce weight and create small design features without risk of failure. The material, specified for the system's trunnions, active pivot slots, slider strips, and sliders, gives the components maximum creep resistance, low wear, and low coefficient of friction.

"Lubricomp cut component costs and weight for BikeControl," reports Dan Schultz, LNP technical sales representative. "The material not only offers the parts added strength, but it resists abrasives, such as dirt and sand."

"The lubricity of the material, combined with its strength, allowed us to avoid the need for several extra parts, such as bearings or sliding surfaces, and its low-wear factor indicates the parts will have a long service life," adds BikeControl Design Engineer Michael D. Miles. "These attributes," he explains, "will reduce the need for replacement parts and direct customer support."

Miles cites an example. One of the part shapes involved a slot feature with a flange and wall combination that would ordinarily bow inward due to residual stress from molding. In anticipation of this happening, the part was designed to allow for the expected bow.

"To our surprise," Miles reveals, "carbon-reinforced nylon, because of its rigidity, did not bow. As a result, the feature had to be revised to be compatible with the material's characteristics."

For the Preload Knob (spring tension adjustment knob), BikeControl selected Vertonr, a chemically coupled, long-glass, fiber-reinforced, heat-stabilized polypropylene. The material had the right toughness, UV stability, and the ability to produce the required surface finish, Miles notes. Other key properties of the material include: high strength, stiffness, and notched Izod impact.

Miles sees more opportunities for weight savings and design optimization through the expanded use of plastics. However, the mountain-biking community, being accustomed metal systems, remains somewhat skeptical of structural bike components made of plastic, he explains. As confidence in the resins and designs grows, Miles feels confident that the use of engineering resins like the LNP materials will become more widespread.

Trials and tribulations. Erlston conceived of the add-on rear suspension system in 1995. Then, over the next two and one-half years, he and Tony Planck, BikeControl vice president of research and development, took the system through numerous design phases, prototyping, and testing. Miles, a private-practice mechanical engineer, joined the team in mid-1997 to finalize the design for tooling and production.

As with any new product, many early design iterations did not work. For example, a parallel linkage system with a single coil spring was prototyped, but then abandoned when it failed to deliver the desired performance.

Once these problems were overcome, the project moved from the bulky functional prototype that proved the operating principle, as well as the preliminary layouts developed by the inventors, to:

Refining the basic structure that would work best with the preferred fabrication processes (molding and die casting).

Prototyping of the final design began in October 1996. Production operations were finalized in February of this year, with the product hitting the market in March.

CAD considerations. The design team called on CoCreate SolidDesigner (Collins, CA) (v4.52), with utility enhancements and support from Visionary Design Systems (Sunnyvale, CA) (VDS Environment) to finalize the Shockster design. Running on a HP 715/100 workstation with an HCRX-8 graphics card and 160 MB RAM, the system served as a full solids modeling tool for the project. Originally based on the ACIS kernel from Spatial Technologies (Boulder, CO), it comes complete with dynamic pan/zoom/rotate of rendered models.

Models were exported in two forms:

IGES format as both trimmed parametric surfaces and wireframe.