American engineer Walter Roehrich thinks outside the norms-which is why his company is launching the world's first supercharged American superbike using the Porsche-designed V-twin engine from a Harley-Davidson V-Rod. This new all-American superbike joins the Australian-built Vee Two Super Squalo in bringing the supercharger system of forced induction to the two-wheel marketplace. But instead of the Super Squalo's format of converting an existing bike and clothing the result with streetfighter bodywork, Roehr is producing a complete from-the-ground-up product that-while also powered by a proprietary engine-also uses a different form of supercharging, housed in his own chassis design.
The Roehr RV1250sc builds on the promise of the prototype V-Roehr 1130 that I tested a year ago. That proof-of-concept model looked sharp, handled well, and had torque worthy of a sportcruiser, but with just 120 horsepower at the rear wheel in a package weighing 425 pounds dry, it simply lacked the requisite engine performance for a sportbike.
Roehrich solved this issue by attaching a Danish-made Rotrex centrifugal supercharger to the latest 5mm-overbore 1250cc version of the V-Rod motor, and modifying the chassis to suit. The outcome is substantially increased horsepower and torque, with a claimed 168 horsepower at 9100 rpm now on tap at the wheel, matched to 99.6 ft/lb of torque at 7600 rpm. All this is housed in a unique chassis that sports top-shelf equipment such as sleek carbon fibre bodywork, a single-sided swingarm, Ohlins suspension, Brembo radial brakes, forged aluminium Marchesini wheels, and Termignoni silencers. But the assemblage of parts doesn't come close to describing what a unique ride the RV1250sc really is.
Roehrich originally had planned to build a sportbike using the light and compact 936cc 60-degree V-twin from Swedish off-road manufacturer Highland, but when supplies for that engine dried up he went looking for another powerplant, and the 60-degree V-Rod engine fit the bill. "I went to the Harley dealership and took measurements of the V-Rod engine," says Roehrich. "Everyone said it was too heavy, it wasn't suitable for a sport bike, and it was too big. But I was optimistic I could get it to work. It's actually not quite as long as a Ducati motor, for instance. So I bought a used V-Rod, and took it from there."
Creating a compact, light sportbike powered by a cruiser engine represented an engineering challenge. "Motorcycles almost design themselves, to some extent, based on the engine you're using," says Roehrich. "I knew wheelbase would be an issue, because although the V-Rod motor is a 60-degree Vee, it's still long. But it's possible to put it in exactly the right spot in the chassis to optimize weight distribution, while still retaining a long enough swingarm for good traction."
Roehrich accomplished this by positioning the V-Rod engine (converted to chain final drive, thus dispensing with the heavy belt pulley and cover) as close to the front wheel as possible. There's still 6mm tire clearance at full bottom on the 125mm (4.9 in.) front suspension stroke, while yielding a handy 1422mm (56 in.) wheelbase-8mm less than a Ducati 1098. The fully adjustable 43mm TiN-coated Ohlins inverted fork is set at a 23.5-degree rake, with 89mm of trail, and a 54/46-per cent forward weight bias on a motorcycle weighing 430 pounds.
An unusual blend of materials is used to create the Roehr's hybrid chassis. Manufactured to Roehrich's design by Union, Illinois-based chassis specialists Framecrafters, the frame uses a pair of CNC-machined 6061 billet-aluminum swingarm pivot plates epoxy-bonded and bolted to rectangular-section 4130 chrome-moly steel main frame spars. Roehrich believes his design combines the rigidity of an aluminium deltabox frame with the responsiveness of a tubular steel chassis. "To provide space for the supercharger installation, I had to widen the frame an inch each side," he reveals. "I didn't want the supercharger to be obtrusive, to have the belt sticking up out of the frame, so I'm pleased with the way it turned out. It looks nice and clean." The single-sided swingarm is likewise a steel fabrication made from chrome-moly sheet, the same weight as a Ducati's comparable cast aluminium unit and working a fully adjustable Ohlins shock via a rising-rate link; wheel travel is 4.5 inches.
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.
Ironically, Harley-Davidson installed a very effective ramp-style slipper clutch on it's '08 V-Rod motor, and combined with the great power and feel from the two-pad/four-piston Brembo radial-mount brakes, the Roehr can be slowed controllably and effectively from speed. The sidestand is now tucked in properly so you don't risk lifting the rear wheel off the ground when you flick the bike over in left-handers, and the extra side grip afforded by the wider 190/55 rear tire (compared to the prototype's 180 rear) handled the RV1250sc's additional power without complaint. Roehrich didn't go with a 6.25-inch-wide rim out of concern for slowing the steering response, yet the front felt a little vague on turn in, which I put down to the fatter rear tire. Not so; turns out the steering head bearings were over-tightened, but it's a credit to how intrinsically right the Roehr is that it didn't feel too unwieldy swapping it from side to side in the Blackhawk chicanes. Still, it definitely wasn't as agile as the prototype, probably thanks to the extra weight of the Rotrex blower, although it didn't feel top heavy or want to fall on its side on turn-in, nor did it wobble and shake its head hitting a bump cranked over.
The compliant Ohlins suspension at either end could be dialed in ideally to iron out the many ripples in the rural racetrack surface, while providing acceptable ride quality in spite of the short rear wheel travel. It's a good chassis package, especially with the progressive link now fitted to the rear shock, with the pullrod that operates the linkage adjustable to either raise or lower the ride height. Feedback from the Pirelli Diablo Corsas was good from both ends of the bike.
The only real fly in the ointment is the lack of detail hardware. The aforementioned stock V-Rod instrument panel's tachometer issues are also accompanied by a hopelessly inaccurate fuel gauge, and other parts like the stock V-Rod footpegs don't seem appropriate on a $49,995 bike like this. One reason Bimota can get away with charging the same kind of money for comparable proprietary-engined Euro-exotica is because that company doesn't stint with gorgeous hand-carved hunks of hardware. Roehrich needs to get busy with the CNC machine or find a supplier who can, to add extra perceived value to his bike.
Nonetheless, the Roehr RV1250sc is a really promising package, and especially so with the extra performance from the supercharged engine it's now fitted with. Roehr's target customers will be different from those of other V-twin sportbike manufacturers, for its appeal lies in the way it combines its dynamic attributes with a degree of supercharged exclusivity. "We're looking first for dealers in the USA," declares Roehrich, "because we need to get our U.S. market sorted out first, then look at exports, especially with a low U.S. dollar. We've had huge interest from overseas, and we've taken several deposits from foreign buyers-indeed, two of the first bikes we'll build are headed for Australia. But it's hard to work around all the different technical requirements for each country, so they'll all be sold as race bikes only, and it'll be the responsibility of the customer to register them locally. It'll be nice to wave the Stars & Stripes around the globe with our all-American motorcycle-but I hope we can convince customers over here that we have something to offer which they can't get from anywhere else."
The American Way of delivering a performance package to the sportbike arena may end up pointing the direction for other, bigger companies to follow.