Roehr RV1250sc Supercharged All-American Superbike - The American Alternative

A Different Type Of Supercharger
When researching what blower to use on his V-Rod engine, Roehrich's automotive background (he is a former Porsche/Audi/VW master technician) was invaluable. "I considered all the options in choosing a blower," states Roehrich. "I first looked at a positive-displacement twin-rotor compressor, especially since there's a kit out there already for the V-Rod motor that's made in Australia by Sprintex. They were prepared to develop a dedicated system for my bike, but I did a lot of research before deciding not to go with that. That was partly because of its extra bulk; I didn't have a lot of room to play with, compared to a V-Rod installation. But the main reason was that, for every revolution, a positive-displacement compressor displaces a fixed amount of air. Because within a certain rev range the engine doesn't need as much air as the blower is delivering, you have to throttle the blower separately from the engine in order to bleed off the extra air, and satisfy the demands of each. That's very difficult to do, and it takes a lot of experimentation to get the balance just right, plus you're pretty much limited by the output of the supercharger in optimizing the torque curve. The Sprintex type usually displaces its maximum boost at around 3000 rpm, and just holds it there throughout the rev range. That's why the torque curve is so flat."

This led Roehr to the Rotrex centrifugal blower made in Denmark. A centrifugal blower is similar to a turbocharger's turbine setup; the faster it spins, the more air volume it moves for a given crankshaft revolution. But instead of using exhaust energy to spin the turbine, the Rotrex is driven directly off the crankshaft via a belt like any other supercharger, so there's no turbo lag. A centrifugal turbine, however, needs to spin at very high speed (up to 250,000 rpm) in order to move enough air, so gears are needed to multiply the engine's rpm to the turbine shaft rpm. This usually entails a lot of noise, bulk, weight, and mechanical wear issues, but the Rotrex's key is that it uses a planetary roller drive mechanism instead. Similar to a planetary gear set, the roller drive has a small shaft in the middle with three rollers orbiting around it. "The Rotrex uses a special traction fluid which, when it's squeezed between the rollers, coagulates and turns momentarily into a solid and then, when the pressure is released, it becomes liquid again," explains Roehrich. "Together with Rotrex's patented drive system, that fluid makes the supercharger very efficient, especially together with the small coolant radiator specifically for it that we've mounted in front to the right of the forks. For street use this probably isn't needed, but for track days and hard riding out on the highway, it's probably desirable, which is why I've fitted it." The blower belt is driven off the V-Rod engine's water pump shaft; the water pump is now remotely mounted and electrically driven.

Riding The Roehr
The chance to ride this American superbike came at the rural Blackhawk Farms racetrack in Wisconsin. Slinging a leg over the 1250sc's wide, well-padded seat without having to tiptoe, you discover balanced ergonomics that were obviously designed by someone who actually rides bikes. The Italian-made Barac mirrors don't vibrate and let you see more than just your elbows, and all the controls are smooth and precise in operation, with adjustable-reach levers for the front brake and the ultra light-action hydraulic clutch. The RV12050sc feels slim but spacious, with room to move around on the seat. The rather small four-gallon fuel tank (good for 90-100 miles with the bike ridden hard in street use, says Roehrich) is carried beneath and behind the single seat, with the stock V-Rod Delphi ECU and wiring harness mounted up front, along with twin injectors off the V-Rod Destroyer drag bike and engine mapping refined via Harley's RaceTuner software.

Thumb the starter button, and the Harley Revolution motor cranks to life, settling into a disappointingly quiet throb which even wide open at peak revs never becomes louder than a vague rumble from down below. You won't have to worry about waking your neighbors by firing it up early for a Sunday morning ride, and I guarantee that every single Roehr customer will purchase the Akrapovic aftermarket mufflers.

The Roehr is the polar opposite of the Vee Two Super Squalo in terms of power delivery. It's difficult to believe the Roehr is actually supercharged, thanks to the smooth, linear build of power that resembles a normally aspirated four-cylinder literbike in terms of performance, even with only five speeds in the smooth-shifting gearbox. Thanks to the traditional low-end grunt of the V-Rod engine, the Roehr still jumps off the line, yet the RV1250sc engine's flexibility meant I could lap Blackhawk Farms only using fourth gear. There's meaty acceleration from just 3000 rpm, but instead of peaking at a little over 7000 rpm, the supercharged Roehr keeps building power hard until you slam into the rev-limiter (the tacky stock V-Rod instruments are used, and the tiny tachometer is both hard to read and very inaccurate) at 9100 rpm and grab another gear.