BMW K 1300 GT - Manual (Page 24)

BMW
Media
Information
10/2008
Page 24
Paralever swing arm and optimised lightweight drive shaft.
Shaft drive is indispensable for a BMW motorcycle with its large engine – not
just for reasons of tradition, but primarily because of the well-known functional
benefi ts of a drive shaft.
The challenge in developing the drive shaft this time was to minimise the
greater unsprung masses versus chain drive to such an extent that the rider
would not even feel the difference.
An advantage was that the light and stiff drive shaft unit with its Paralever
swing arm featured for the time in the R 1200 GS in early 2004 and then
again in the K 1200 S, is ideally suited as a lightweight construction for the
K 1300 S.
The Paralever swing arm made of a high-strength cast aluminium alloy was
redesigned for the K 1300 S in order to make allowance for the change in
dimensions resulting from the upgraded ESA II electronic suspension around
the rear spring strut.
As before, the swing arm is particularly light since, in its design and
dimensions, it follows precisely the defi ned loads also on the K 1300 S.
Despite its low weight, the swing arm is stiffer than most conventional swing
arm constructions. Geometrically, it is set up for 90 per cent anti-dive,
the swing arm pivot point beneath the front universal joint on the drive shaft
helping to make the swing arm mount appropriately slender and allowing
installation of the footrests at a lower point.
The swing arm mount itself is positioned on the stiff main frame made up
at this point of a highly stable cast light-alloy structure. Forces acting on the
fi nal drive housing are guided above the swing arm, providing space for fi tting
the brake calliper below. The advantages, again, are a better thermal balance
and easier removal of the wheel whenever necessary.
The pivot point for the fi nal drive housing in the swing arm is beneath the
drive shaft axis, the overall system made up of six pivot points being set
up kinematically to avoid any effective changes in length on the drivetrain
throughout the entire range of spring travel. Hence, there is no need for
additional length and tolerance compensation.
On the K 1300 S the spring strut comes with even fi rmer suspension and
damping to provide even better feedback. Accordingly, the strut pivots via
a lever building up approximately 30 per cent progression near the pivot point,