V-Force
V-FORCE
Racing doesn’t just improve the breed for the Aprilia RSV4, racing is the breed
KEVIN CAMERON
APRILIA'S NEW 1000CC V-Four went straight to the front in World Superbike Racing this year, with MotoGP veteran Max Biaggi at the throttle. It evidently makes a difference that Aprilia has a ton ot GP and Superbike experience, because Biaggi was able to make trouble for series "heavies" Noriyuki Haga (Ducati 1198R) and Beii Spies ( \`amalia Y7 F~ R 1). I low S that for a lirst shot? It makes a person curious aS to just what's in the package. Now it can he told.
The first choice is engine architecture, because Aprihia's engineers know that wheel base has to he short for quick turning and that mass must be well-centralized to minimize resistance to turning (111(1 rollover. Melting the engine into spherical form would be ideal, but next-best is a V-Four. A 90-degree Vee would give perfect primary balance, but its length would extend the wheelbase or push crankcase weight too far aft. So, Aprilia closed it up, but not all the way to the 60 degrees of its RSV 1000 V-Twin Superbike. The lesson from that bike was “don’t crowd the intake system.” That made 65 degrees the choice-but also required a balance shaft to smooth out the vibes that resulted.
A chain drives each intake cam, and intake and exhaust cams are geared together in each of the two heads. A single spring controls each valve, driven by an inverted bucket tappet. Yes, MotoGP engines have gear-driven cams, but Aprilia is building a bike to sell. Have a heart.
The next big compromise to decide is bore and stroke. As the stroke is made shorter, piston stress at any given rpm is reduced, which is good if you need revs to make a lot of power. But as the stroke shrinks, the bore grows, and the combustion chamber spreads out so much that very careful design (and maybe more R&D time than you have!) is necessary to obtain fast combustion.
Leave the rad stuff to F-l, because when this isn’t done right, the engine either accelerates well or makes top-end power well, not both.
Aprilia chose 78.0 x 52.3mm for 999.6cc, or a bore/stroke ratio of 1.49, a good compromise that avoids territory where things get difficult. The production engine is claimed to make 177.5 horsepower at 12,500 rpm, measured at the crankshaft. At that engine speed, this translates to about 4300 piston feet per minute, or a peak piston acceleration of 5759 g, only about 3A of what you’ll find in the latest 600cc screamers. Good. Rev-limited rpm is 14,100. Peak torque claimed in the brochure is 115 Newtonmeters at 10,000 (the 2500 revs between there and peak hp means a good spread of pulling power), which translates in our system of cubits and bushels to 84.5 foot-pounds. And that means that the airflow and combustion in this engine are excellent at this point-a strokeaveraged net combustion pressure of 209 psi, or 91 percent of the highest number you might see in a pure racing engine. An excellent number.
The crankshaft has two crankpins (rods are sideby-side on each) set at 180 degrees. Such a crank needs only three main bearings rather than the five that an inline-Four requires.
Of course, this engine is liquid-cooled. Look at the compression ratio of 13.0:1. There have been air-cooled engines with that much compression, but they have been track-only tool-room jobs that often didn’t last the afternoon. With water to keep things cool and detonation safely at bay, this high compression becomes a routine tool for making a lot of torque. It’s clear this bike is designed to accelerate, because that’s what high compression is best at.
All engine covers are magnesium-only 65 percent of the weight of aluminum equivalents.
We see the hand of experience in the choice of materials for the four valves per cylinder. The intake valves are titanium, only 60 percent the weight of steel. The smaller exhausts are a Nimonic alloy, one of the highly heat-resistant materials originally developed for jet-engine turbine blades. This engine’s conservative designers expect it to serve with distinction in heavy going. Light valves and four per cylinder mean these poppets can follow the shortduration, high-lift cams that combine strong acceleration and good top-end power.
Each 48mm throttle body has two fuel injectors, one below the butterfly, one in showerhead position for high-speed, high-load operation. An ECU-controlled dual-length intake system is implemented. A fixedlength system always has an anti-resonance right at the rpm where you’d like to start your off-corner drive. The ram airbox recovers 85 percent of the possible pressure gain of about one-half a psi at 175 mph-just over 3.5 percent extra for free.
The exhaust is fitted with a computer-controlled butterfly valve, but unlike those on some other bikes, this one has no effect on power. It closes only when the engine is idling in neutral, then opens when the bike is put in gear and stays open. It is, in effect, simply a “good neighbor” valve that quiets the exhaust when you start the engine in the morning.
The FIM says no more than six speeds in racing, so that’s what the RSV4 has-but built in cassette-style for quick ratio changes. A -wk wet slipper clutch protects stability from the ill effects of engine braking.
Ride-by-wire? Engine mode switch? Get used to virtual power. When the rider turns the throttle, the Marelli engine-control system interprets throttle position as a specific torque demand, and it then continually moves the throttle butterflies appropriately to deliver only that torque. This means that the engine’s actual torque curve can have a whole Swiss Alps of tractionthreatening dips and bumps in it, but the system makes them disappear, trimming the bumps and filling in the dips by closing and opening.
Much of the beauty we admire in Aprilias is in their chassis. This one, with swingarm, weighs 33 pounds and is sculptural polished aluminum in perimeter style, welded from castings and formed sheet. Racers take notice!
This chassis’s middle name is “adjustability.” The steering head can be moved forward or back and altered in rake by inserts. The swingarm pivot can be raised or lowered to make the bike accelerate off turns rather than either squat down and push or jack up and spin. And engine height can be raised or lowered.
Suspension and brakes are from the international Superbike Tinkertoy setÖhlins and Brembo goodies. Notice that the fork bottoms holding axle and caliper are forged. This is no accident. Every time the rider stoppies, it’s as though giant hands were grasping these parts and twisting hard enough to lift the back wheel. Serious loads, in other words. Forgings are tough enough.
Fuel is carried mostly under the rider’s seat, near the same height as the overall eg so that differences in fuel load (which is 27 pounds, full) only minimally affect handling.
A ton of racing experience doesn’t just mean Aprilia people have a lot of old pit passes. It means they have learned how to get a lot of crucial details right to begin with.
