Well. . .since you asked. . .:-)
I'll try not to illustrate without getting too technical. . .and this is
certainly NOT a merits of one system or another topic. It's just an
explanation of the workings of a particular design - hopefully it will
illustrate, in simpler to understand terms, how the magical, mystical,
Swing spring works
First off, in this swing spring design the entire spring is not bolted
solidly to the differential housing. the bottom leaf of the assy is the
ONLY spring portion that contributes to any roll stiffness. The other
spring leaves, for the most part within the confines of the design, are
free to pivot about a central axis. In this case the bolt that goes
through the housing or clamp the sits atop the diff which effectively
houses the top leaves. If you look at the construction, you'll see that
the spring leaves are indeed not rigidly fixed. This ability to pivot
eliminates any possibility for this spring portion to resist the car from
rolling on this axis. In other words, for an example, if you were to
remove the main leaf (which is contributing to roll stiffness by virtue of
it's solid mounting configuration) then removed the front springs on the
car (and supported it centrally on a pivot under the front cross member for
this discussion), the vehicle would roll to one side or the other, because
nothing is trying to keep it in a level, roll position even though there is
indeed a spring mounted on the rear. Now, if you were (able) to grab each
rear corner of the car to keep it from rolling and try to push straight up
and down on the rear it would resist this motion because it's still a
spring after all, but since the motion is produced in a vertical axis, the
load applied equally on the ends and transferring this load to the center
pivot, and not a roll one, it resists this motion just as any other spring
would. It's operation is no different that an old playground
teeter-totter. If both individuals try to go in the same direction at the
same time it's impossible, but if they go in opposite directions, no
problem. In this way the swing spring allows full springing action in a
vertical axis (to support the vehicle), but contributes little in its roll
axis. As Joe pointed out a larger front roll bar was added to compensate
for the reduced roll stiffness in the rear. REMEMBER, roll is not
necessarily a bad thing as far as handling is concerned. Only when it
contributes to loss of adhesion does it become undesirable. It is however
a little disconcerting to one if there is excessive roll while cornering
giving one the feeling the car is about to tip over :-)
I'll not get into the many complicated reasons why, but suffice it to say
that with the swing axle design in the Spitfire, one way to prevent the
dreaded rear wheel tuck under during hard cornering transitions is to
reduce the REAR roll stiffness. The swing spring was a design that did
exactly that
Barry Schwartz (San Diego) bschwart@pacbell.net
72 PI, V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (long term project)
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