Epoxy bonding can be a quick means of throwing together structural parts when you need a fast prototype.
John Nelson Smirneos
Sometimes the only way of convincing companies to take on a project is to show them a working prototype. This was the case with an innovative, mountain-bike suspension system. Though it had a European patent, bike makers wouldn't take the design seriously without seeing it work with their own eyes.
The situation called for a fast prototype. Stereolithography and similar techniques can whip out component models quickly. But they are no help, of course, for structural members such as aluminum bike frames that have high stress loads at their connection points.
Moreover, the prototype bike frame would have to withstand rigorous off-road testing. The main problem was in attaching a free-floating suspension arm and machined aluminum hardware to the frame. The attachment method had to be strong enough to handle the high loads that an ordinary production bike would see.
An epoxy lamination technique fit the bill. The process has a proven track record in boat building and is a common choice for custom bike designers. Perhaps most notable of these is Damon Rinard who has been racing and building bikes since 1978.
This technique is something to keep in mind, for instance, when a fast but strong prototype is in order. The sequence of events surrounding the bike frame project serves as an example of how it can be utilized.
The suspension arm and bike frame are built around foam cores covered with epoxy laminates made from unidirectional, bidirectional, and twill carbon-fiber cloth. This prototype employed a large safety margin. Rinard recommends 17 to 19 layers of carbon fiber. But the frame used 30 carbon-fiber plies and three Kevlar layers laid up in three groups of 10 carbons followed by one of Kevlar. The design also incorporates unidirectional Kevlar tape to beef up the assembly in other high-stress areas where the aluminum parts are imbedded or "locked" inside the resulting composite.
All components are bonded with a two-part epoxy resin system from Gougeon Brothers Inc., Bay City, Mich. (www.westsystem.com). The West System Brand epoxy bonds to fiberglass, wood, metal, fabrics, and other composite materials and is easily modified for a wide range of coating and adhesive applications and fully cures at room temperature. It is used for construction and repairs requiring superior moisture resistance and high strength. It also forms a strong bond with the epoxy resin used in the construction of the carbon-fiber/Kevlar laminates.
The goal of this prototype was to prove that a suspension-equipped bike could climb better than a hardtail (no rear-wheel suspension) and be easier to ride on technically challenging terrain. To date, the prototype has predominately been ridden on single track and dirt roads in excess of 7,000 kilometers (4,350 miles).
Putting it together
The suspension hardware was machined from Fortal aluminum from Pechery Rhenalu, Paris (www.pechiney.com). The alloy is similar to 7075 and has breaking and elastic limits of 580 and 510 MPa, respectively and a Brinell hardness of 180. The aluminum machines easily and is a common choice of European aerospace designers.
The first step consists of sandblasting the aluminum components before assembly. Sandblasting the contact surfaces of the aluminum removed residual contamination from milling operations and abraded the surfaces to give the epoxy a better grip.
Next, the aluminum was treated with an anticorrosion coating. Care was in order, however, that the parts were quickly coated after sandblasting. Otherwise atmospheric exposure forms a thin oxide film on the aluminum surface. A quick touch-up with a wire brush, however, removes the oxide layer if the coating can't be applied soon after sandblasting.
The coating called Nubian from Brava srl, Ceranesi, Italy (www.brava.it), isolates the metal from the carbon fiber and prevents a corrosive galvanic reaction from taking place between the two materials. (Another method for preventing galvanic reactions would be to sandwich a layer of fiberglass between the aluminum and the carbon fiber.)
All the aluminum parts were designed to "lock" inside the carbon material that wraps and adhesively bonds around them. Some of the parts have slots or holes machined into them. This lets the Kevlar tapes pass through the part and will help boost the laminates structurally as well as making it easier to wrap the tapes flush around the parts.
The best example illustrating the assembly process is the rear-wheel dropouts. Their contact area was wire brushed to remove any oxide build-up and coated with Nubian. Then a foam insert was placed in the dropout hole. The foam insert keeps the resin from pooling in this area and helps the carbon-fiber laminates lie flush against the metal during assembly.
Next, the parts were aligned in an assembly jig to keep the aluminum parts accurately positioned with relation to the suspension arm. The suspension arm's foam core at this point was already covered with two layers of carbon-fiber material laminated around it.
The carbon fiber is carefully wrapped around the aluminum's contact areas and pressed tightly around the suspension arm with the aid of some vinyl tape. The tape was wrapped around the assembly "sticky-side" out so its adhesive wouldn't adversely bond with the epoxy. After covering the structure, small holes must be punctured in the vinyl tape to let any excess epoxy drain from the assembly.
Care was in order when applying the first layer of carbon cloth to ensure it was done smoothly and evenly and didn't crease as it passed through the machined hole. After the epoxy cured, the vinyl tape was removed and the assembly sanded smooth.
The assembly must be cleaned and abraded prior to adhering the Kevlar tapes that will boost its structural integrity. The tapes prevent the carbon fiber from tearing under heavy loads. As before, vinyl tape can be used "sticky side" out to apply pressure to the Kevlar while the epoxy cures.
Once the epoxy sets the vinyl tape comes off and a final aesthetic layer of carbon-fiber fabric is epoxied over the Kevlar and sanded smooth -- it's much easier to sand carbon fiber than to sand smooth the Kevlar tapes. A set of stickers helped dress up the suspension arm before the structure was covered with three layers of varnish.
Most of the Fortal parts were "locked" in the carbon structure and secured with Kevlar tapes passing through machined slots and holes following this procedure. They included the dropouts, the suspension arm pivot plates (the parts that support the pivots of the suspension arms), and the subframe mount.
Other parts such as the headset tube and the suspension arm base were covered with 30 layers of carbon-fiber material and "wrapped" with the Kevlar tape. The shape of these parts let the carbon-fiber material "lock" naturally around them, without the need for additional slots or holes.
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No rock'n while you roll
Suspension-equipped bikes aren't a new invention. But historically they were often viewed as a novelty and weren't a popular choice for the average cyclist. The first hybrid BMX/road bikes (a.k.a. mountain bikes) equipped with suspension systems initially faced an uphill battle to gain public acceptance as well. Riders transitioning from traditional touring bikes didn't like the feel or the lack of response they got when riding the hybrid bikes. One reason was that the early suspension-equipped mountain bikes absorbed a lot of power -- as the suspension compressed under load with each downward pedal stroke the system would "biopace" or damp the muscular output or power exerted by the rider.
Additional energy was lost when a rider's pedal cadence rose. The rider's pedaling rhythm coupled with suspension compression can generate unwanted resonance or vibration. To combat this so-called "bobbing" effect, designers can stiffen the suspension. But this counteracts the suspension's ability to absorb energy as the bike travels over rough terrain.
The new prototype rear suspension addresses these difficulties. It is a full floating design with a sensitive suspension arm that has a rising rate of compression (i.e., the suspension stiffens in relation to acceleration effort). Depending on the riding position, whether standing or seated, it stiffens without locking. The system virtually eliminates energy-zapping bobbing and biopacing effects making for a smoother ride with less energy loss.
The prototype suspension system is designed to also boost rear-wheel traction with no loss to shock absorbency. And as the rider moves back and forward the suspension adjusts the frame relative to the front fork casters. This increases the bike's stability at high speeds of descent and lets the rider maneuver more easily during steep climbs as the rear end pushes the frame upwards and forward during an ascent.
Typical physical properties of West System Brand epoxy
|Unidirectional carbon fiber||200||10,000|
|The West System Brand epoxy has a tensile adhesion of 1,700 psi to 2024 aluminum which has been sandblasted and chemically prepped.|
|Typical values; not to be construed as specifications. Fiber properties, fiber orientation and lamination technique will affect results.|