Making a legendary motorcycle like the Sportster even better is one tough assignment. Discover how engineers met the challenge, making the bike’s ride much smoother without crimping its style:

How do you improve upon a much beloved and respected product? The answer is by staying true to its signature style while taking its performance to the next level. At least that was the case when engineers were faced with the challenge of redesigning Harley-Davidson’s Sportster, the oldest and one of the most venerated motorcycle model lines in the business. They responded by drastically re-engineering the Sportster while maintaining its well-known look. “We changed everything without changing a thing,” Bill Davidson, director of marketing in motorcycle planning for Harley-Davidson, Inc. and a great grandson of founder, William A. Davidson, tells Design News.

First of all, Harley-Davidson engineers set out to make the Sportster’s ride as smooth as possible. Heading the effort was engineer Brian Ruffert, who kicked off the project by integrating what the engineers had learned from a previous test ride: the trick to taming vibration is separating the engine from the chassis. But this represented a huge challenge for the engineering team because the Sportster has featured a rigid mount design—in which chassis and engine were securely connected—since its 1957 market debut. What’s more, this configuration contributed significantly to the bike’s structural integrity.

To address the issue of vibration, the engineering team used rubber mounts to isolate the engine from the frame. They placed these engine mounts—which are roughly the size of hockey pucks—at the crankcase, between the crankcase casting and the frame of the bike. Meanwhile, to maintain the bike’s structural rigidity, they mounted tie links—dog-bone-shaped devices—between the crankcase and frame. These links provided a direct rigid connection between the two, preventing the engine from moving from side to side. Thus, the team was able to significantly lessen the transmission of vibration by employing the rubber mounts and to preserve the bike’s structural integrity by adding the tie links.

Deciding where to place the rubber mounts and the tie links was a challenge, however. To determine their optimal locations, the engineers conducted months of physical testing on frames and linkages. Moreover, they performed several finite element analyses on the engine and frame utilizing UGS PLM Solutions’ I-DEAS Master Series software. Finally, they determined that the best spot to put the mounts was at the front and back of the engine’s crankcase, between it and the frame, while they found that the tie links were most effective in three places—low on the engine, higher near the cylinder heads and at the back, close to the rubber mounts.

Adding the mounts and tie links necessitated the development of special casting for the engine’s crankcase. The engineers worked with the company’s manufacturing group on machining the castings. Then they collaborated with a crankcase castings supplier to refine the castings and later conferred with the assembly team to ensure that every part could be placed on the transfer line while still facilitating easy assembly. The development of special casting also provided powertrain engineers with the chance to fine-tune nearly every other component of the engine, such as the cylinder heads, connecting rods, valves, valve springs, pistons and gaskets.

And the improvements to the bike did not end there. The engineers also made it more ergonomic, upgrading seating and changing the position of hand grips and foot pegs. They decreased the hand grip diameter from 1.25 inches to 1.125 inches, a change that the company says accommodates smaller riders while being imperceptible to bigger riders. Additionally, they chose a new brake manufacturer to reduce the lever force that has to be applied to the front brake and altered the gearing to lessen the spring rates of the clutch spring pack.

The engineers’ meticulous efforts have paid off. The drastically redesigned Sportster, which rode into the market last fall, is a hit with fans. According to the engineers, the motorcycle’s unveiling last summer as part of Harley-Davidson’s 100th anniversary drew warm response from fans, reviewers and dealers, who appreciated its classic look and marveled at its smooth ride—even as they cranked up the speed to 100 mph.

Source:

Good Vibrations
Design News, February 23, 2004
www.designnews.com/article/CA380721?stt=001&pubdate=02%2F23%2F04