The Aprilia RSV4R and Yamaha YZF-R1 share the same bore, stroke and mission statement, but
This month's comparison between the Aprilia RSV4R and Yamaha YZF-R1 provides an opportunity to reflect on motorcycle performance on several levels, from the street to the racetrack and all the way to World Superbike and MotoGP.
In 2009, the R1 was king of the hill. It took Motorcyclist's Motorcycle of the Year award and Ben Spies rode one to the World Superbike Championship after a close-fought battle with Ducati. Yamaha asserts that the R1 is kin to its '09 MotoGP World Championship-winning YZR-M1, and there are good reasons to make that claim. The R1 doesn't share a single part with the M1, but its general layout and crossplane crankshaft share important DNA.
The RSV4 can't yet claim an SBK title-and Aprilia's announced intention of putting a similar bike in MotoGP after the series returns to a 1000cc limit in 2012 is only talk at this point-but the bike has won races and some European magazines' shootouts and is definitely being taken seriously. It's hard not to take it seriously when, in a first-year effort with a new bike and team, Aprilia beat every World Superbike competitor except one Yamaha and two Ducatis.
If we look at MotoGP, V4s dominate, with Ducati, Honda and Suzuki fielding such machines. Now that Kawasaki has dropped out of the series, only Yamaha is left to fly the inline-four flag. In SBK, the opposite situation holds, with 1000cc inline-fours from BMW, Honda, Kawasaki, Suzuki and Yamaha making up the majority. Aprilia is the lone V4. (I haven't forgotten the 1200cc V-twins of Ducati and, someday, KTM, but we'll stick to the 1000cc fours here).
A V4 and an inline-four of the same displacement-especially those with the same bore and stroke (78.0 x 52.3mm) like the R1 and RSV4-have essentially the same power potential. Actual power will depend on many factors, but neither type is inherently inferior or superior.
When it comes to engine weight, the V4 has a slight potential advantage over the inline-four. A V needs two cam drives to the inline's one, but the V's crankshaft is shorter. The V4 has only three main bearings to the inline-four's five, but each is bigger. Weight, like power, depends on many design factors. The RSV4 is indeed lighter than the R1, and the engine may be lighter, but short of completely disassembling and weighing the components, it's hard to know.
Weight distribution favors the inline-four if the designer wants more weight on the front wheel, but it favors the V4 if he wants that weight more centralized. Mass centralization looks better for the V4 if you consider engine mass alone, but its more complex exhaust has to share part of that central location.
Making the mass-centralization equation more complex is the fact that the fuel load of an inline-four can be placed behind the airbox and intakes, right down to the top of the transmission, while on the V4 the rear cylinder and its exhaust occupy some of that space. On the MotoGP Honda and Suzuki, some fuel is put under the seat, but this space is shared with the exhaust.
Aerodynamically, the narrow V4 would seem to have an advantage, but the aero package of any bike is dominated by the size of the rider. The width of the fairing is designed to get the rider's body out of the airflow, and that width covers the current inline-fours so that their width relative to the V4 is not an aerodynamic hindrance.
I've been trying to pick a theoretical winner between the V4 and the inline-four, and I don't think it can be done. Either type can be developed into a world-class motorcycle, or languish as an also-ran. As they say, the devil is in the details. By winning both the MotoGP and World Superbike Championships in '09, Yamaha has shown that the inline-four is far from being an impediment to progress. By posting an extraordinary debut season, Aprilia has shown the V4 to have deep potential. It's too close to call.