Air damping system
With the new rear strut for the HP2 Enduro BMW is yet again presenting a world innovation in motorcycle chassis design. With the new air damping system, air exclusively undertakes both suspension and damping functions - such a gas spring damper system provides a weight advantage of nearly 2 kg compared to conventional struts with steel helical spring and oil damping: the innovative component weighs only 2.3 kg in total.
The basic layout of the air damping system features analogies to conventional struts: a piston travels in a cylinder, but displaces instead of oil air that flows from one chamber into the other. Damping is achieved when plate valves throttle this air flow; suspension is obtained when the trapped air is compressed.
Air as a medium provides the optimal conditions for use in a spring damper system.
- Under high load (when the suspension is highly deflected) and at high temperatures (over longer use) the pressure in the system rises, causing a natural progression of the spring rate: the higher the pressure in the chamber, the greater the force needed for further compression. This physical behaviour therefore ensures good overload protection that improves under higher loads.
- Also the increasing viscosity of air causes under high load a natural progression of damping at rising temperatures, but without the risk of overheating as with conventional struts.
- Precisely matching the internal flow system and the gap throttles gives rise to frequency-independent and frequency-selective air damping: the rear wheel ideally follows all unevenness, also e.g. over very short and fast excitations (corrugated metal track). The optimal ground contact in every situation means in turn the ideal transfer of forces during acceleration and greater braking safety.
- The system's smaller unsprung masses also exert a positive effect on the strut's response characteristics and rear wheel traction.
On the outside the air damping system consists of an aluminium cylinder that forms the actual strut with two air chambers and gastight rubber bellows that completely seals the strut including the piston rod. The system is therefore protected against soiling, facilitating maintenance and preventing e.g. dust from abrading the piston's O ring. The strut is filled with ambient air through a normal Auto valve; a high pressure hand pump and pressure gauge are included with the product. Varying the filling pressure in the strut means that the motorcycle can be quite easily tuned to various loading states or the seat height adjusted. A bubble on the frame indicates the motorcycle's normal position as an adjustment aid. In addition, the only other adjusting facility is a hand wheel that varies the damping characteristics between the two variants Sport and Competition by opening up a small bypass between the air chambers.