Harley and Carley Team Up for Design Tag
Harley Davidson’s Softail family of motorcycles aren’t made out of plastic.
When Ron Hurth and the rest of the design team for the world-renown motorcycle designer remembered that fact, the door was opened for a metalcaster to design the oil tank for the new Harley Rocker motorcycle.
“[The oil tank] started out as a plastic inner tank with a cosmetic outer shield, but they couldn’t make it look good enough for the Softail standard, which incorporates metal for most components,” Hurth said. “The thing about Harley is that a lot of the stylists still want to stick with metal, so they didn’t like the way the plastic looked.”
Plus, the plastic tank would have required considerable one-time tooling costs, and doubts remained about its ability to withstand the high heats it would be exposed to on the body of the motorcycle.
Pouring A356 aluminum into semi-permanent molds, Carley Foundry Inc., Blaine, Minn., offered a solution to the majority of the problems presented by the plastic design, as well as a fabrication proposed immediately thereafter. The solution led them to the 2008 AFS Metal Casting of the Year award.
“What [semi-permanent mold] does, basically, is combine all the features of both the other tanks,” Hurth said. “You get heat resistance, the look of metal and a cost reduction from not having to assemble the internal parts. Everything is cast and machined into one piece, so it consolidated everything.”
A Pretty Piece
The Harley Rocker motorcycle has several kinds of visible metal pieces. Some provide the brawn—the drivetrain and structural supports—and others offer the beauty.
At first, Harley designers thought the oil tank could be lumped in with the stronger group, the parts that need only to be shot blasted to achieve the desired surface finish. After all, the oil tank is a mission critical part and hardly just a nice looking piece of jewelry. But because of its location and visibility, it later became clear that the oil tank would complement the other more decorative pieces nicely.
“We knew it had to be cosmetic and pressure tight,” said Randy Oehrlein, Carley’s vice president of engineering. “It had to be pretty, but a more effective part.”
The metalcaster couldn’t just blast the casting in the traditional manner and expect to achieve the fine surface finish it desired. To solve the problem, Carley employs a variant of a vibratory deburring process, which makes the critical cosmetic casting surfaces as smooth as the paneling on a luxury automobile.
Carley then subcontracts several different painting processes to give the smooth casting its color and sheen. For black, blue, pearl and red bikes, a wet spray is employed. Powder coat dresses up satin stainless bikes.
“We are responsible for the part as the Tier One supplier,” Oehrlein said. “Even though we don’t do machining and painting in-house, we were responsible for finding the sources. The multi-step manufacture of the assembly is controlled by Carley straight to the bike.”
More Than Pretty
The Harley Rocker’s oil tank had to do more than just look good. The success of the bike’s circulatory system depended on an effective part. To build such a part, Harley and Carley engaged in a vigorous game of design tag.
However, the team played a game of tag that is more familiar in the world of wrestling than in the schoolyard. The object is to tag your partner when you need help, rather than to make them “it.” And when the metal casting your team is working on wins Casting of the Year, you’ve defeated the other tag-team wrestlers.
Once the initial plastic design was rejected in-house by Harley engineers, they next considered a multi-piece assembly that involved an upper and lower die casting and a separate manifold. The design also required the use of gaskets. However, concerns remained that the assembly would leak—the oil level would reach above the split line between the two castings—and cost too much due to its elevated level of complexity.
That’s when Carley was first tagged in. “When they first asked us to take a look, it was almost like a challenge was thrown out,” Oehrlein said.
 Initially, the metalcaster proposed a one-piece aluminum design with a core opening at its top. To achieve the oil passageways needed within the casting, internal machining and threaded component assembly would occur through the cored opening. This assembly also necessitated internal bosses, creating a large isolated mass of aluminum. Keeping in mind the manufacturing issues and noting the way an oil tank works, the team soon threw that idea out of the ring.
Improving Circulation
The Rocker’s oil tank operates essentially like a heart, drawing oil in and then resupplying it to the rest of the motorcycles’ vital parts. In the initial design presented by Carley, though, the oil entering the tank was circulated in only a small section of the component before being redistributed.
Because of the limited size of the cored hole in that design, the oil channels could only be built in a small, pre-determined area.
“It was kind of like building a ship in a bottle,” Oehrlein said.
This created two problems for the motorcycle producer—considerable wasted space existed within the tank, and the hot oil built heat as it circulated in a closed loop. With the tank located close to the motorcycle operator’s leg, that heat would have posed an operational danger.
Fortunately, Carley tagged back in with the ability to cast the oil circulation tubes directly into the component. This reduced the isolated mass as well as solidification time in the remaining mass. With the space constraints of the core print opening eliminated, the tubes were constructed to span the length of the tank, and the oil could be circulated throughout. The hot oil from the engine now gets pumped into a location away from the rider’s leg, wraps around a corner and leaves from the other side.
Through another series of tagging in and out, Harley and Carley tackled numerous castability issues. They created ribs and bosses to retain the oil circulation tubes, added a riser to increase the feed and heat mass, added a cooling fin to enhance directional solidification, thickened certain design elements to aid in metal flow and feeding, and designed a waffle pattern on the component to increase surface area and enhance solidification.
“They selected us and said, ‘we have the best tentative mousetrap, now let’s work this down until everyone’s happy,’” Oehrlein said. ECS
|
