Just some Theory
TELESCOPIC FORK WITH TRAVEL-DEPENDENT DAMPING
Front-wheel suspension on the HP2 is provided by a telescopic fork with 270 millimeters, or 10.63 inches of travel in upside-down construction, since this kind of spring travel cannot be provided sensibly by a conventional Telelever configuration. A special feature of the front-wheel fork is the travel-dependent damping, with the inbound and rebound stages being adjustable separately to a wide range of different settings. A further adjustable feature is the hydraulic system preventing the running gear from sagging down under extreme conditions.
The most outstanding feature in this system absolutely unique to BMW, is that damping in the inbound stage under pressure remains relatively independent of the degree of harshness, avoiding any sagging effect of the motorcycle. In other words, ride harshness and firm damping can be set to "hard" without making the inbound damping response in the main operating range of the damper significantly firmer or tauter.
With its fixed sleeve tube measuring 45 millimetres (or 1.77 inches) in diameter, the fork provides the stiffness required and at the same time allows maximum handlebar lock. The fixed sleeve tubes come with an extremely resistant special coating withstanding wear even better and more efficiently than a conventional surface coating based on titanium nitride.
REAR-WHEEL SUSPENSION: THE AIR/SPRING DAMPER SYSTEM
The new Paralever is a welded structure made up of high-strength, forged light-alloy shells. Together with the German specialist, Continental Automotive Systems, BMW's specialists have developed a spring/damper system running exclusively on air.
Weighing not quite 2.3 kilos or 5.1 lb, this air/spring damper system is approximately 2 kilos lighter than a conventional structure. The air spring strut also features a piston moving into the damper chamber.
In this case, however, the piston acts not on hydraulic fluid, but rather on air forced through plate valves into a second chamber. The damping effect is achieved by throttling the flow of air and with air being compressible, the air captured within the chamber is able to provide the necessary spring effect replacing a conventional steel spring.
Air is a medium with ideal characteristics for a spring damper system and offers a wide range of advantages:
* "Natural" progression of the spring rate under high load (with pressure in the system increasing).
* A high standard of security against the spring damper system suddenly giving way (according to the law of gas mixtures, pressure increases as a function of temperature).
* "Natural" progression of the damping effect under high loads (the viscosity of air increases as a function of temperature).
* Frequency-dependent, selective damping with automatic adjustment of the dampers to the load the motorcycle is currently carrying.
* Resistance to overheating (no temperature-induced decrease in damping under high loads.
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With unsprung masses being slightly lower than before, the response of the suspension and rear-wheel traction have been improved accordingly. In its structure, the air spring strut comes with three air chambers, one behind the other, and connected to one another by air ducts. An aluminum cylinder forms the two upper chambers separated from one another by a piston. Longitudinal movement of the separating piston compresses the air within the cylinder, the air itself acting as a kind of spring. At the same time, a defined volume of air flows into the other chamber via plate valves (slot throttles) and, through the throttling effect, damps any movements of the wheel.
The lower air chamber is formed by a gas-tight rolling gaiter made of rubber tissue. This gives the damper appropriate freedom in moving up and down and seals the spring strut to the outside, avoiding the need for a piston rod seal, which would merely increase friction within the system. The resistance built up by the rubber gaiter when rolling over a specially shaped cone supplements the progressive effect of the system.
To the outside the spring strut is fully sealed and air-tight. Any leakage occurring nevertheless can be compensated by filling in air through a valve. Such variation of air pressure from outside also allows simple adjustment of seat height simply by pumping in or discharging air as required. Inbound spring travel is reduced with the seat set to a lower position, meaning that this particular setting is more appropriate for a moderate style of riding.
Adjusting the system to different load conditions is again a very simple operation requiring only an appropriate change in air pressure. A perhaps minor, but nevertheless important and interesting feature, is the small "inspection balance" on the rear frame allowing the rider to read off the normal setting of the motorcycle in the interest of easier adjustment.
And to pump up the system while travelling, the HP2 naturally comes as standard with a manual high-pressure pump, complete with a pressure gauge also suitable for pumping up the tires.
A unique feature of the air/spring damper system is the option to vary the damping effect as a function of frequency achieved by carefully adapting the inner flow system together with the slot throttles to current requirements. A significant improvement of rear-wheel traction on undulating surfaces, the damping effect being specifically configured in the frequency bands of the rear-wheel swinging arm system, enabling the wheel to follow such an uneven surface in an ideal motion and maintain optimum ground contact.
A first in suspension, the total air shock
Whenever the springs sag through completely, as is often the case on long and stretched-out surface "waves" and under high loads, the "natural" adjustment of the air spring rates, together with frequency-specific damping, again acts against any excessive compression of the springs to prevent them from giving way too much.
The basic set-up of the dampers may also be adjusted by a hand wheel opening up a bypass in the damper. This allows adjustment in two stages between a more comfort-oriented roadgoing set-up and a firmer setting for rough terrain.
A significant advantage of the system also lies in the complete resistance of the spring strut to dirt and contamination from outside, thanks to the fully enclosed surface. This rules out even the slightest wear on the seals and guide units even in the finest desert sand.
