Doc, Larry, and all -
Yes, the swing spring was implemented to minimise the inside wheel tuck.
But how did it achieve this? Think about the difference in the systems
- the fixed spring resists both wheels travelling in the same direction
(thus supporting the car), and also resists one wheel travelling up and
one travelling down (thus resisting roll). The swing-spring resists
both wheels travelling up, but offers minimal resistance to one wheel
travelling up and one travelling down, so little roll resistance.
Wheel tuck occurs when the inside wheel is unloaded in hard cornering.
By giving the rear suspension very little roll resistance, the weight at
the rear of the car must be supported almost equally by both rear wheels
- if there was more vertical force on one wheel than the other, the
imbalance in force would just rotate the spring about its pivot on the
diff until the forces were equal (if the lowest leaf weren't fixed, i.e.
it was a fully swinging spring, you could be certain that the contact
force between each rear wheel and the road would be pretty much
identical).
If you fit an anti-roll bar, what will that do? Well, on hard
cornering, the car will roll (a bit at least), the outside wheel will
move up in the wheel-well. The anti-roll bar exists to resist the
wheels being at different heights (relative to the chassis), so it will
push down on the outside wheel and up on the inside wheel. Hmm, pushing
up on the inside wheel, reducing it's contact force with the ground -
that's what caused the wheel tuck in the first place, wasn't it? That's
what we're trying to avoid, right?
OK, that may have got a bit convoluted. A somewhat simpler way of
envisaging it - with a swing-spring, if one wheel goes up, the spring
swings and pushes the other wheel down. An anti-roll bar works by, if
one wheel goes up, it pushes the other wheel up. An anti-roll bar
directly counter-acts what the swing-spring is trying to do.
Any attempt to add roll stiffness to the rear of a swing-spring car will
reduce the effectiveness of the swing-spring at preventing wheel tuck.
I will read attentively and open-mindedly to any reasoned explanation of
why I am wrong! Like I said, I'm not a suspension expert by any means,
but I should temper that with being a professional mechanical engineer,
who has thought a lot and read some about how the suspension on my
little (and sadly long-departed) Spitfire works.
Richard Gosling
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