View Single Post
Old 02-05-2011, 09:41 AM   #4
G(/)S ... what else!
StephenB's Avatar
Joined: Aug 2005
Location: Ontario
Oddometer: 2,046
Please proceed with caution, no guarantees for any of the following being correct or applicable for your application:

Stiffer spring yes, but not for the angle. May I elaborate:

Applicable lever law (thanks Wikipedia):
The force applied (at end points of the lever) is proportional to the ratio of the length of the lever arm measured between the fulcrum (pivoting point) and application point of the force applied at each end of the lever.
Mathematically, this is expressed by M = Fd, where F is the force, d is the perpendicular distance between the force and the fulcrum, and M is the turning force known as the moment or torque.

Translated to a rear swingarm (I am totally simplifying): the distance between the (swingarm) pivot point and the moving mass (final drive) is extended, hence the loads (torque and perpendicular forces) around the pivot point is increased -> hence more damping (stiffer spring) for the same amount of spring action (=comfort) is required. The increased perpendicular forces on the pivot pin are also the reason why at the same time, you may have to strengthen the frame tube around the swingarm pivot point.

Using rudimentary calculation methods (just to get an idea) for my 30mm extension I calculated the perpendicular force at the swingarm pivot point to increase by about 30%. Since the Paralever GS (with a longer swingarm than the G/S) uses the same frame, I took my chances and did not reinforce the area around the swingarm pivot pin. At the same time, the 30mm extension also requires a 17% higher spring rate of the rear shock, because of the change of the levers and dynamics. So my spring is a 95kg/mm as opposed to the 85kg/mm which would be the correct one for a 220lbs rider.

My university lecture days are way back in the past, this is only a rudimentary analysis and should in no way being taken as gospel. But I think I got the right idea and proportions. My road tests including sections of the TAT on a loaded bike seem to prove me right though.

Some of the above is fact, some is fiction, some is my personal imagination and some is just simple truth. [me]

... i'm not touring around the world, but neither are most of the guys i see running overloaded spam cans ... [bmwblake]

Stephen Bottcher
Ontario, Canada

'72 R75/5 The Blues
StephenB is offline   Reply With Quote