The Spitfire Mk IV got a revised suspension that was different in several ways.
The rear spring was composed of 4 leaves instead of 7. The lower leaf was flat
but the others had a slight notch upward in the center. Since they were held
by a plate that exerted a force downward, the upper three leaves could actually
pivot over this notch with minimal resistance, thus giving no roll resistance.
The spring and mount looked a lot like:
_n_________n_
==========|====^====|=========== ...
==========|====^====|============ ...
==========|====^====|============== ...
==========|=========|=============== ...
|-----------|
| (diff) |
Only the lower leaf added roll resistance. (Hint, do *not* add an external
"sway"-bar to the rear!)
The net effect was to decrease the rear roll stiffness but increase the single-
wheel spring rate. Another significant change was that the *front* "sway"-bar
was increased to compensate for the loss of roll stiffness. (Hint: Check
those front chassis "swar"-bar mounts. They are stressed more heavily in the
later cars.)
In 1973, the 1500 was introduced (in the US, but not until 1975 in Europe), and
it got rear axles longer by 1 inch.
In general the "swing-spring" did work, and the longer axles helped a bit more.
A note about spinning and rollover, though: It is possible to make almost any
car spin or handle with a loose tail, even just by poor tire pressures. B-L
recommended 21/26 for F/R pressures on the 1500, just to keep the rear stuck.
Also, any decceleration, whether by brakes or trailing-throttle, will reduce
the rear downloading, and thus the rear traction, while increasing the front's.
Trailing-throttle is generally worse because it further uses ("uses up") the
rear tires' available traction with decceleration forces.
To a first approximation, rollover is a simple matter of loading geometry. If
the roll torque is more than the stability emparted by gravity acting on a
wide-based (i.e. the track width) platform, then the car will roll. This is
true even for solid-axle cars, but a swing-axle car has the added disadvantage
that if the wheel does tuck under, the platform's width decreases! The simple
forces are thus:
lateral force L
<--------* (Center Of Mass, assuming that front and rear are alike)
a / |
/ b | - gravity G
/ |
W V
W is the wheel. If L * sin(a) > G * sin(b), then the car will go over! Any
second-order suspension-change effects will modify this slightly. Consider the
limiting case of a car that slides sideways into a curb! This is nothing more
that the lateral force going very high, since the tire can't ride up over the
curb. This is guaranteed to roll almost any car if it hits hard enough.
What is different *today* is that tires are much stickier! This makes L go up
but it doesn't affect G or the angles at all. The stickier your tires, the
more easily your car will roll. All cars are designed with some safety factor,
but older designers did not anticipate today's autocross-special tires that can
generate a coefficient of friction of over 1.0. When you lower the suspension
or increase the track, you are changing the angles a and b in your favor. But
if you put super-gumballs on your older car with no other changes, you are
shaving away at that margin of stability. If other bad factors like a rough
course, poor driving, swing-axles effects, and other vehicle dynamics features
line up against you, you just might run out of that margin. (OOOoooops!)
Drive carefully,
Jim
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