Motorcycle Engine Belt Drive System Delivers Smooth, Quiet Reliability
S&S Cycle manufactures V-twin motorcycle engines and high quality components used by custom OE bike builders—American Ironhorse, Arlen Ness, Big Dog, Orange County Choppers, Swift, Victory, West Coast Choppers and the like.
In 2003, this Lacrosse, Wisconsin-based company with third generation owners took on a new challenge. It was simple, but far from easy. Design a new motorcycle engine that would meet higher EPA emissions standards, perform reliably, cost less to build, reduce component complexity and run quietly. Noisy, vibrating engines are perceived as poor in quality by builders and riders. The new S&S Cycle engine had to sing.
Roy Meyer, Principal Engineer for S&S Cycle, described the challenge this way: “When you start an ‘out of the box’ project, you can’t underestimate the difficulties…or the opportunities.”
The three-cam belt drive system (cutaway) operates one intake valve and two exhaust valves in the S&S Cycle X-Wedge engine.
S&S called their design the X-Wedge™ (cross-wedge) engine. In the cross-wedge combustion chamber, intake and exhaust ports align with the valve stem plane for greater efficiency and performance. The fuel injector mounts in the head for improved aim and more efficient combustion. But a real performance challenge for the 56° V-twin engine was the three-cam drive system.
Space was at a premium. Three large base-circle cams with drive components had to fit into a shoebox-sized compartment. Operating temperatures were high. So were torsional forces. Tolerances were tight. S&S engineers explored various drive designs: a chain drive, a combination belt/chain drive, high contact ratio gears. They went so far as to develop a new gear drive, but shelved it. Gear tolerances had to be too precise. Too loose and the gears rattle. Too tight and they whine. S&S sought a unique solution: a high performance belt drive system from Gates.
Roy Meyer said, "The onus was on Gates engineers to meet the tolerances we gave them. We depended on them to size the sprockets, figure out the belt length and develop the tensioner."
Gates engineers and the S&S development team began trading drawings. The first belt drive designs used a standard Gates Poly Chain® GT® Carbon® belt, the same type used to power the blowers on 7,000 hp NHRA Top Fuel dragsters. But soon it became evident that standard products wouldn’t do. Cam drive temperatures inside the small enclosure caused the standard belt to fail. Torsional spikes from the pistons firing and cam lobe interaction sheared off belt teeth. Then there was the challenge of designing a tensioner small enough to fit the space.
Gates engineers developed a custom test rig. They formulated and tested special polyurethane compounds for the body of the carbon fiber reinforced belt—ones that would resist oil and extreme heat. They developed a nylon facing for the belt teeth to prevent tooth shear. And they designed a tensioner with an off-center spring load to meet the space requirement. Drive tolerances were so precise that even the thickness of the coating on the sprockets was taken into account.
Iteration followed iteration. With each prototype the bar was raised and a new baseline established. Finally came the moment of truth. Gates engineers met with the S&S team to answer the ultimate question: Can it be done? The answer was, "We’ll find a way." And they did.
According to Scott Sjovall, VP of Product Development for S&S Cycle, "When you’re going out of the box on a project, you want a qualified partner who will take it to the end."
And the "end" is the S&S X-Wedge engine, which went into full production in February 2008. This low-vibration 117 cubic inch engine packs a lot of performance into a compact size, while exceeding the EPA Tier II emission standards for reduced hydrocarbon (HC) and nitrogen oxides (NOx) emissions that took effect in 2010. Plus, it’s 5dB quieter than its V-twin predecessors.
If you want to hear what Jay Leno says about his new bike with the S&S X-Wedge engine and Gates belt drive, visit Jay Leno’s Garage.
Gates is now helping S&S Cycle develop a line of drive train components to expand their market. What can Gates do for you? For help with your application, speak with a Gates product application engineer at 303-744-5800, or email ptpasupport@gates.com.
