The 1125R Helicon engine is Erik Buell's first liquid-cooled, non-Harley-Davidson powerplant since his original Barton-powered two-stroke RW750 roadracer in 1984. It was designed in collaboration with and built by BRP-Rotax in Austria and, as expected, incorporates plenty of forward-thinking technology. The 1125cc displacement is achieved via radically oversquare dimensions (103mm bore/67.5mm stroke), making room for big, 41.3mm intake and 35mm exhaust valves. Cylinders are splayed 72 degrees apart, a figure chosen because it provides a steep, 18-degree valve angle and the straightest possible flow path for the downdraft intake system, which is capped with dual 61mm throttle bodies managed by H-D's own DDFI 3 EFI technology. Dual overhead cams are operated by a combination chain/gear drive with a self-adjusting chain spinning the intake cam, which in turn drives the exhaust cam via gear-making for a more compact cylinder head, reducing engine weight and allowing the motor to be mounted farther forward in the chassis. Valves are actuated via finger followers (technology borrowed from Formula 1 racing) to reduce friction, provide quicker valve opening and virtually eliminate high-rpm valve float. To control vibration, the engine is fitted with no fewer than three balance shafts: two to cancel primary rotating imbalance and a third that cancels the rocking couple. Dry-sump oiling reduces internal windage losses, and oil is held in an integrated oil reservoir in the lower left side of the crankcase-not in the swingarm, as on the air-cooled XBs. A single large-volume muffler wraps around the bottom of the engine; careful tuning provides linear power delivery without use of an exhaust valve, and the muffler incorporates a Helmholtz chamber to reduce noise. All major engine components are unique to Buell, and the engine will not be shared with other brands or used to power any other recreational products.
Like any proper sportbike, the 1125R gets a stacked, close-ratio, six-speed gearbox complete with straight-cut gears operating on a sliding-dog mechanism for smooth, light shifting action. Attached to the output shaft is a compensated sprocket that essentially acts as a cush drive at the engine instead of at the rear wheel, another clever engineering solution that further centralizes mass and reduces unsprung weight.
Buell says the American-made "Intuitive Response" frame is its stiffest yet. The main spars of the fuel-bearing structure are even more massive than those of the XB, and that extra space is put to good use to hold an additional 1.8 gallons of fuel, raising total capacity from 3.8 to 5.6 gallons. The new frame has been optimized for torsional stiffness to better resist side loading in corners, aided by a solid-mounted engine that serves as a stressed chassis member. The swingarm mounts directly to the engine cases to minimize chassis width and create an even more rigid connection between the rear wheel and frame. The arm is also more than 2 inches longer than the XB's, increasing the wheelbase from 52 to 54.6 inches to better resist wheelies in light of an added 50-something horsepower. Front-end geometry remains identical to that of the XB with a steep 21 degrees of rake and 3.3 inches of trail.
The HVA (Hydraulic Vacuum Assist) clutch utilizes vacuum pressure generated within the intake manifolds to assist clutch action for reduced effort at the lever and to provide a "slipper" effect. At open throttle vacuum pressure in the intake manifold is low, which results in full clamping loads on the clutch plates to minimize slippage and maximize forward acceleration; at closed throttle the manifold pressure increases, leading to lower pressure on the clutch diaphragm and effectively reducing clutch clamping load, which lets the clutch slip slightly to reduce engine braking and rear-wheel chatter during aggressive downshifting.
The plus-sized, 47mm Showa fork-one of the fattest ever fitted to an OEM sportbike-was a necessary upgrade, as the slimmer, 43mm XB fork would be overpowered by the new, stiffer frame. The triple-clamps (handlebars are integrated into the upper clamp) have likewise been stiffened, and the fork is fully adjustable. The piggyback-reservoir shock is also sourced from Showa, and is likewise fully adjustable. The shock mounts horizontally to make room for the muffler below, and is acted on directly by the swingarm without benefit of a linkage, relying instead on a progressively wound spring.