Aprilia’s off-road twin caused quite a stir when it was first presented to the public and motorcycle media. The design was characterised by solutions in complete contrast with conventional technology, and was totally unlike the single cylinder machines that then dominated motocross, enduro and supermotard racing. The idea that drove the development of Aprilia’s new off-roader was the desire to create a twin cylinder machine of similar size and weight to a single, but with far better performance. The new bike was also to have excellent ground clearance and be as narrow as possible. To cut a long story short, anybody sitting on the new Aprilia should not be able to feel any difference between it and a classical single, apart from its performance. The need to keep overall dimensions down therefore played a fundamental role in the design process. The V angle between the cylinders, for example, was set at 77° to make room for the engine’s auxiliary equipment. In theory, this means that the crank cannot be perfectly balanced, but on a machine like the SXV or RXV, designed and made exclusively for competition use, this was not a primary consideration. Another important fact is that the 4.5 engine (450 cc) and the 5.5 engine (550 cc) both use the same crankcases. The crankshaft of the higher displacement unit therefore requires extremely compact flyweights. Tiny pads of mallory (a tungsten alloy of extremely high density, used to balance rotating parts when there is no room to fit large counterweights) were used to provide the necessary balancing.
The step up from 450 to 550 cc has been achieved by increasing stroke (from 49.5 to 55 mm) and also increasing bore (from 76 to 80 mm). This has meant completely new pistons, cylinders, con-rods and, of course, completely different valve timing. Only the heads themselves remain identical. Ultra-short con-rods have been used to limit the height of the cylinders, even though this creates a high thrust between the piston skirt and the cylinder. This factor too can be overlooked given the fact that these engines are made specifically for competition use. After all, the SXV and RXV are thoroughbred racing machines and are built without compromise. The models homologated for road use were derived from them at a later date. Owners of Aprilia’s new off-road twins therefore benefit from machines that are ‘tame’ enough to ride on the public highways but still deliver fantastic performance thanks to the very latest motorcycle technology (Aprilia also supplies a range of dedicated accessories to modify road-going models for competition use). Also to keep overall dimensions down, the valve gear is driven by two chains, one for the front and one for the rear cylinder. These chains are driven by a layshaft in the V between the two cylinders, which is itself driven from the crank. This solution has made it possible to keep the diameter of the camshaft pulleys down, and this in turn has meant that the valve covers can be compact too. The necessary reduction in revolutions is achieved instead by the gear that drives the layshaft off the crank. Both cylinders have just one cam each, even though they are four valve heads. The two inlet valves are operated directly by spring cups while the two exhaust valves are operated by a single rocker arm with two fingers, driven in turn by the central lobe of the cam and a small intermediate roller. Because the valves are made from titanium, the seats and guides have to be made from a special material to resist wear.
Even the lubrication system is unusual. The engine features a dry sump with separate gearbox lubrication. A double stage oil pump draws oil from the bottom of the crankcase and pumps it up to an external oil tank located behind the steering head. From here, the pump feeds oil under pressure into the crank and to the top of the cylinder heads where it lubricates the camshaft, valve cups and rockers. The oil lines between the pump and the tank are embedded in the left hand side crankcase and flywheel cover. This not only provides a high degree of component integration but also contributes to a reduction in overall dimensions.
The gearbox is lubricated in such way that the oil level is kept high in the gearbox itself but low in the clutch casing. This limits the power loss that occurs when a clutch runs in a deep oil bath. Having two separate lubrication systems for engine and gearbox is of fundamental importance. Because bushes are used for the crankshaft journals and big end, it is essential for the oil supply to remain perfectly clean. Gearbox oil can easily become contaminated with particles of clutch plate that can cause rapid wear in bushes. The new Aprilia off-road engine also features quite a complex oil fume recovery circuit. Fumes escape from the crankcase through a small hole. This is shielded by the starter motor drive gear which separates out the bigger oil droplets. The remaining fumes then pass along a pipe into the oil tank behind the steering head. The oil tank itself is specially designed, with internal walls to prevent the oil sloshing around (as it otherwise would on an off-road bike), and to ensure a constant supply of oil to the pump. Once the oil fumes have released all remaining droplets back into the oil, they are fed into the filter box, from where they are sucked back into the engine via a small sponge filter which removes all remaining traces of lubricant.
The new Aprilia twin is fuelled by an electronic injection system. The throttle bodies, made by Dell’Orto, are miniature works of art. The ECU for the fuel injection, which also controls the ignition, is made by Walbro. Their website offers a selection of mappings, that can be downloaded into the ECU using a special diagnostic instrument. The fuel supply to the two injectors is a return-less system, so a fuel pressure regulator is installed on the fuel pump delivery line inside the fuel tank. It goes without saying that Aprilia’s twin cylinder off-road engine far excels alternative single cylinder powerplants in terms of performance.
Because it can rev higher and more easily, a twin develops more power than a single of equivalent displacement. It therefore stands to reason that the new Aprilia engine is at a significant advantage in supermotard racing. A lot of development remains to be done to perfect it for motocross and enduro use, where more low down grunt is needed, but the Aprilia twin is still a young project. Design work only began in October 2002 and the engine ran for the first time in August 2003. So there is still a lot of scope for further development. The Aprilia design team are currently working on specific valve timing diagrams, pistons (to change the compression ratio), and exhaust and inlet systems for the various speciality competitions in which the new bikes are likely to compete. We can be confident that the right level of performance will soon be achieved, with either more top end power or more low end muscle. Neither the 4.5 nor the 5.5 engine is equipped with a kick start, but an automatic (centrifugal) valve lift device is nevertheless provided to reduce compression on start-up. This has allowed Aprilia to fit a small, lighter starter motor that does not need to produce a high torque output. Finally, another factor that we cannot afford to ignore is that a twin cylinder engine produces far less exhaust noise than a single, because the shock waves from the two cylinders tend to balance each other out in the silencer expansion chamber, dramatically cutting noise emissions. Noise reduction is becoming an increasingly important issue, since noisy bikes can be a major source of annoyance in the countryside, and the authorities are actively cracking down on vehicle noise in general.
Just like the engine, the frames of the RXV and SXV also feature unique designs. A mixed tubular trellis frame with aluminium side plates was adopted for a specific purpose. Maximum rigidity is achieved at the side plates, while in the remaining areas of the frame, rigidity can be varied quite easily and at little cost, simply by modifying the thickness of the trellis tubes. It is therefore a simple matter to adapt the frame and to obtain different dynamics without having to interfere with any of the major parts. The holes in the aluminium side plates that hold the steel trellis tubes are also highly unusual. The first section of the hole is conical to stop the aluminium plate from coming into direct contact with the steel tube. This prevents flexing in the steel tube from causing cracks in the aluminium. The tubes are then held in two other sections of the hole with an increasingly tight interference fit. The frame is first hot-assembled, using a special glue, then the ends of the steel tube are rectified and secured to the plates with screws. The frames produced with this advanced manufacturing technique have proved extremely reliable under competition conditions, when the fixing screws are actually omitted as a further weight-saving measure. Even the swingarms of the RXV and SXV are impressive: the side members are made from an aluminium box section (formed by welding together pressed profiles), while the pivot section is made in cast aluminium.
