I tend to agree with Joe on this one.
Although Barry puts up a convincing argument, the swing spring balances on the
mid point (I think we all agree). It will reach an equilibrium when the weight
applied on each end is equal. If one end is slightly weaker, spacers worn, or
deformed due to heavier use (e.g. driver side), it will deflect a greater
distance until the applied force is equal to the opposite end. You would then
end up with one side lower than the other.
To illustrate this, take this to the extreme. Consider one side of the pivot to
be totally rigid (e.g. steel) and the other flexible (e.g. plastic). With no
weight applied, it would still balance on a pivot point (assume each side is
equal weight). Add equal weights to each end, and the arms will balance out but
one will be significantly lower than the other. One problem with this example is
that the arm with the greater deflection (plastic) will also have a shorter
moment-arm due to a decreased distance from the weighted point to the pivot.
Since the deflection in the rear spring is so small, this shouldn't be a factor
in real life.
Now for the engineering types, let's have some of the math with spring rates and
moment arms and all such stuff included in an equation. Or not...
Vic Whitmore
76 Spitfire
Thornhill, Ontario
Joe Curry wrote:
>
> Barry,
> The Swing Spring is weaker than its predecessor (the fixed spring) and
> because of this, and the fact that it flops from side to side, it IS the
> major cause of most sags in late Spits.
>
> Take a look at early Spits and see how many (that have not been
> converted to swing springs) show evidence of this sag.
>
> It's my opinion, based entirely upon comparison of the two types of
> suspension that the swing spring is indeed the cause. It has fewer
> leaves and therefore when the car has weight continuously applied
> unevenly (driver only) it will sag long before an equivalent fixed
> spring model.
>
> Compounding this of course is the greater negative camber of the longer
> axles which seems to me to make the applied weight rest more on the
> spring.
>
> Regards,
> Joe
>
> Barry Schwartz wrote:
> >
> > >"My Spit (like so many others) has a definite lean toward the driver side
>of
> > >the car.
> > ***********************************
> > I've read several listers complaining about one corner of the car sitting
> > lower than the other over the past few weeks, and the general consensus is
> > to blame the Swing Spring. I can't see how the rear spring, in a Spitfire
> > with a SWING SPRING, can CAUSE this problem. I feel that this poor spring
> > is getting a bad rap, and I believe that it is undeserved. My own opinion
> > is it is a very CLEVER and elegant solution, given the constraints at the
> > time to solve a potentially serious problem. Overall rear end sag from a
> > worn spring, yes and this is evident with any well worn rear spring, but
> > side loading is virtually impossible by it's very design. It may not
> > counter an exiting leaning/listing condition by something else that is
> > causing the lean, and therefore, these vehicles may be more sensitive to
> > the problem but there is no roll or side to side loading with a swing
> > spring except from the main leaf itself, and that would be a very small
> > amount indeed. It's conceivable that a bad main leaf (however unlikely)
> > could cause a slight list, but I contend it would be very slight. The
> > other spring leaves are, if all is functioning correctly, and in good
> > order, FREE to rock about the diff center, and do nothing as far as biasing
> > the car on it long axis. You could cut one of the leaves off one side
> > (provided it was not the main one) and all it would do is cause the entire
> > rear end to sag a little more. The way this spring is designed it CAN'T
> > cause a "roll" bias, because the action is "divided" between the two
> > wheels. If one were to imagine, for illustrative purposes, that you had no
> > front wheels or roll bar on the car. Place a pivot under the front cross
> > member directly at its center to support the weight at the front end, but
> > allowing it to rock about that pivot. The only thing now keeping the car
> > level, with respect to roll, would be the main rear leaf itself, and if
> > just that spring leaf was removed, (and all others were to remain in place)
> > then it would roll over to one side or the other "pivoting" as it were on
> > the center bolt in the spring clamp assy. That's the very premise of this
> > design. Now you can see, that the remaining spring leaves do NOTHING to
> > cause the car to lean or list in one direction or the other. So I say
> > (for what it's worth) if your car is experiencing a tendency to lean to one
> > side or another, look elsewhere (possibly the rubber pad p/n 149191 UNDER
> > the clamp if you must look at the rear), not at the Swing spring-
> >
> > 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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