Finally a subject about which I know a bit. :-)
On 20 Sep 2003 at 10:08, scotts junk wrote:
> How exactly does the swing spring used on the late model Spits
> and GT6 Mk III improve the handling.
If you don't mind, I'll address your other points first...
> I've seen in numerous magazine articles that it lowered the
> roll center, but from my limited understanding of swing
> axle geometry this appears to be a load of marketing hooey
I'm not sure I'd call it marketing hooey but you're right that the
swing spring doesn't change the geometry. It can't, simply because
it doesn't change the pivot points or arm lengths or anything else
that affects the kinematics of the suspension. (Kinematics means how
it moves regardless of any forces present.) Thus it doesn't change
the roll center. The basic idea is correct but the words are wrong.
> The reason for the question stems from Kas Kastner's remark in the MK 2 GT6
> tuning manual that swapping in a swing spring will improve handling in a
> roto flex car.
Never known anyone who did this but it seems reasonable. It might
help for the same reason that helps the swing axle cars, but the
benefit shouldn't be near as great.
The swing spring simply lowers (by roughly a factor of 4, as I
remember it) the roll stiffness at the rear. To regain the lost roll
stiffness, the diameter of the front sway bar was increased.
Tha basic problem with the original swing axle suspension was this.
Swing axle geometry places the roll center at the rear somewhere near
the center of the diff. Also the height of the center of mass at the
rear is fairly close to the center of the diff too. (Note, the diff
is roughly centered with the frame vertically.) So there is almost
no roll torque (i.e. roll couple) at the rear. At the front though,
an unequal-length double A-arm suspension places the roll center
below road height. (It would be at road height for equal-length
arms.) In contrast, the height of the center of mass will be
somewhere near the center of the engine, possibly a bit lower if you
take the frame weight into account. Since the engine is taller than
the diff, the c.o.m. height is certainly higher at the front. Higher
c.o.m. and lower roll center means a longer lever arm. Combined with
the much greater mass of the engine, you get a roll couple at the
front waaaay bigger than at the rear.
One way to understand what this means is to imagine the car sliced in
half vertically, separating front from rear, then re-attached with a
longitudinal shaft though the center that allows the two ends to
rotate w.r.t. each other on a longitudinal axis. In other words,
imagine a car that behaves normally except that the front and rear
are each allowed to roll independently as much or as little as it
"wants to" in a corner. Suppose their "preferred" roll angles were
different, with the front "wanting" to lean over more than the rear.
When you re-connect those halves together by glueing the frame and
floor panels back together (since in a real car they were never
cut!), the rear will be pulling the front back toward level, and more
importantly, the front will be pulling the rear further over.
Technically, this will redistribute the weight among the wheels,
increasing the load on the outside rear and inside front, and
reducing it on the inside rear and outside front. In a nutshell, re-
balancing the roll stiffness will add weight to one diagonal and take
it off the other. Balanced roll stiffness will make each end of the
car "want to" roll the same amount.
As we saw above looking at the roll heights and c.o.m. heights, the
front of any car with double A-arm suspension wants to roll a lot, so
it needs a lot of roll stiffness. But the swing axle wants to roll
almost none. In the original design, the rear roll stiffness was way
too big for its roll couple, and much of the car's weight was
transfered to the insider-front/outside-rear diagonal in a turn.
[We're almost done.] Balanced roll stiffness won't necessarily give
the best overall cornering. For that, you want the most evenly
distributed weight across all tires. Since the higher c.o.m. at the
front means the front's static weight transfer to the outside wheel
will be greater than the rear's, the front has the poorer L-R
distribution. So the front is the car's limiting factor for maximum
lateral g's, causing understeer. A "too stiff" rear will shift the
weight to the inside front, improving the front's maximum at the
expense of the rear's. So the stiff rear is a good thing.
But swing axles have another problem. In a corner, the lateral force
plus gravity give a net force vector on the car's body. Normally
that vector falls inside the pivot point of the tire on the road
surface when applied at the c.o.m., which is why the car doesn't roll
over. But it also operates on the kinematics of the suspension,
operating at the point where the tire hits the road. As seen at that
point, it has a vertical component from the car's weight and a
lateral component from cornering. With a swing axle, it can rotate
the axle at its u-joint and push the suspension up or down in its
range of travel. The net torque of this vector around a vertical
axis at the u-joint determines the ride height. If it projects above
the u-joint, it pushes the wheel up. But when you back off in a
turn, the suspension drops down as the vertical component is reduced.
If this vector ever falls below the u-joint, it will torque the axle
downward and cause tuckunder. The lower rear stiffness from the
swing spring "fixes" this by allowing the outside wheel to stay
higher in its range of motion during a turn, so that the vector never
falls below the u-joint. In this sense, it does allow for a lower
ride height at the rear *while in a turn*. But a more concise way to
put it is that the swing spring lowers the rear roll stiffness.
Sorry about the length. It's a complicated issue! I've probably
muddied it even further.
--
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire, '70 GT6+
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