BMW's new range of 650 singles

The springs and dampers are all specially tuned for offroad use, meeting the highest demands in every respect. As a function of his own weight and personal preferences, the truly ambitious Enduro rider is even able to fine-tune the dampers on their inbound and rebound strokes, the adjustment mechanisms offering predetermined rest points for smooth, consistent adjustment.

Again in the interest of minimum weight, the wheel axle measuring 20 millimetres in diameter is drilled hollow on both the front and rear wheel. The rear wheel suspension also allows various adjustments and set-ups, albeit of a very different character: Like the BMW HP2 Enduro, the BMW G 650 Xchallenge features the BMW Motorrad Air Damping System, a special spring/damper system operating exclusively with air. In its configuration and functions, the Air Damping System is the same as on the HP2 Enduro and has merely been adjusted in terms of weight and its running requirements.

The BMW Motorrad Air Damping System comes with a piston inside, similar to the configuration in conventional spring struts. Instead of hydraulic fluid, however, this system forces out air through plate valves into a second chamber, the damping effect being achieved by throttling the flow of air.

Since gas is compressible, the air within the system can act as a kind of spring replacing the usual steel spring and thus offering the advantages already lauded on the BMW HP 2 Enduro:

• “Natural” progression of spring rates under high load (with growing pressure within the system). • Reliable and smooth operation minimising the risk of the suspension giving way (physical law of gases: 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 effect. • Resistant to overheating (no temperature-induced weakening of the damping effect under high loads). • Simple option to adjust the dampers to the load carried. • Low weight of the entire system and its components. • Individual adjustment and set-up of the suspension matched to the rider’s weight and the load carried by means of a water level presenting the normal position and trim of the motorcycle.

The reduction in unsprung masses also improves spring response and traction on the rear wheel. To the outside, the spring strut is sealed hermetically and is therefore protected from contamination, with any losses caused by leakage being set off by filling in air through a valve.

The system adjusts very easily and conveniently to different load conditions simply by varying the pressure filling. And to pump up the system while travelling, the rider has the option to use a hand-operated high-pressure pump fitted beneath the seat of the G 650 Xchallenge.

A feature unique to this air spring damper system is the choice of frequencyrelated damping achieved by varying the inner flow system together with the throttle openings. And an important effect achieved in this way is much better traction of the rear wheel on bumpy, undulating surfaces: The damping effect can be appropriately adjusted in the frequency ranges typically encountered on the rear wheel swing arm as a result of bumpy surfaces, the wheel thus following any such bumps in an ideal motion, maintaining optimum ground contact at all times. The result is even better grip and traction when accelerating all-out, as well as extra safety when applying the brakes.

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