Ian Macky writes >
>The bar was in an AddCo box as foretold. It's 1 7/8" thick,
This is a rather large diameter. The really stiff front bar in my Midget was
7/8" diameter. I will have to guess that both the "bottom of the U" part and
the arms are quite long.
A sway bar acts stiffer when:
1 The "bottom of the U" section is shorter.
2 The effective length of the lever arms is shorter. (This is the most common
method used to make adjustable bars, by allowing the link to be moved
to a different position on the lever arm.)
3 The bar is larger in diameter. (A small change in diameter causes a large
change in stiffness.)
4 The material is more resistant to torque. (I have no knowledge of how much
variation is possible.)
5 The links are connected to the suspension closer to the tire patches.
6 The links have less compliance.
>The bar is only supposed to resist inward and outward forces (cornering
>forces that want to rotate the wheel off-vertical).
The wheel goes off vertical (this is called a positive change in *camber*) due
to body roll acting on the suspension geometry. Instead of changing the
suspension geometry, a sway bar limits camber change by limiting body roll.
Compliance in the suspension bushings can also allow an additional positive
change in camber, but this is out of the realm of sway bars.
>It should not resist up-down forces (normal vertical suspension travel),
>and does so by twisting in its two body-mounted bushings,
It acts this way when both wheels are moving together (speed bumps, dips).
>while having
>enough slop in its end-link mounts to allow the bar end position to
>rotate unrestricted (within its pretty small operational range).
>But how are the lateral forces transmitted from the end link to the bar-end?
The "slop" in the end links is to allow for the normal lateral or longitudinal
movement of the suspension as it goes through its vertical travels. Often,
the movements combine to form an arc, but the sway bar only cares about the
vertical component. The links allow the arc of the sway bar to be matched to
the different arc of the suspension. You see the same thing in the links used
in lever-type shock absorbers (dampers) on British cars.
>In the original gear, the end-link shaft has an extra sleeve that the
>two rubber bushings fit onto, so it's the sleeve that actually bears
>on the rod end. But, why are they such different sizes? The hole in
>the bar end is twice as big as the sleeve... ???
The sleeve around the link shaft usually butts up against a large washer which
supports the rubber bushing which fits into that large hole in the sway bar.
There is a second rubber bushing on the other side held by a similar washer,
which in turn is held by the nut on the link shaft. So the hole in the swaybar
is not relevant to the sleeve diameter, but to the dimensions of the step cut
in the bushings. This whole system is a tradeoff of bar stiffness (losses in
the bushings) for link rotation. The way to skip out on the tradeoff is to use
spherical ("Heim" or "rose") joints. They eliminate the compromise, but can be
considered harsh for street use. I loved them on my Midget, but comfort came
long after cornering on my priority list.
>Someone explain all this please! Is there a FAQ?
I do have the article I wrote in THE RIGHT LINE last February. If you have
seen it, you can cut out now, there is nothing else new in this message.
--------------------------
Sway Bars by Phil Ethier
Here's the short explanation of how a sway bar (anti-roll bar is a more
accurate description) works: "The sway bar pulls up on the inside wheel."
That's it.
As the outside wheel is pushed up into the body by cornering forces, the sway
bar rotates up. The inside end of the sway bar then tries to pull up the
inside wheel. The stiffer the sway bar, the harder it tries to lift the inside
wheel. This works AGAINST the inside spring which was HELPING the car to roll.
The force of the sway bar is split evenly between opposing the inside spring
and helping the outside spring.
The problem with this in many rear-drive cars is that the rear bar can lift the
inside rear wheel enough to cause severe wheelspin exiting corners. This is
not to your advantage. If an autocrosser with a rear-drive car wants to reduce
inside wheelspin, there has choices to make. A limited slip differential is a
good choice if you happen to have the money to buy one and they are legal in
your class. Or, you can fiddle with the roll stiffness on either end.
The torsional rigidity of the chassis will transmit the forces from front to
rear assuming you have any rigidity in your chassis. Imagine what would happen
to a "Flexible Flyer" Oz-built Capri convert if you put a very stiff bar at one
end. Probably pop the doors open.
If you want to reduce wheelspin and reduce body roll, stiffen the FRONT sway
bar. This will try to lift the inside front wheel more. The resulting
reduction in body roll will lift the inside rear wheel less. Unfortunately,
this also throws more load to the outside front tire, which can overload it,
causing more understeer. If the car oversteers too much in the first place,
this is a happy effect.
Similarly, softening the rear bar will cause it to lift the inside rear wheel
less. This will not reduce the overall body roll, and may increase it. It
also puts more strain on the outside front wheel, so Dreaded Understeer may
return.
On many cars, understeer is reduced by more roll stiffness because the
unfavorable effects of camber change are greater in the front suspension than
in the rear. Adding more negative camber in front helps. If you still have
gobs of understeer, and are still lifting the rear wheel, you are stuck behind
the eight ball. Some cars seem to be able to walk the line in this, but some
rear-drive cars which have a lot of forward weight bias cannot be tuned this
way and must have a locked or limited-slip differential to work at all.
Otherwise, you wind up with a car that understeers AND spins the inside rear
wheel!
Formula Vees and rear-engine Porsches go around on three wheels, but they have
a lot of REAR weight bias. Vees often use NO roll stiffness at the rear and
get it all from the front, trying to lift the inside front wheel. Nose-heavy
rear-drive cars just use the wrong three wheels.
Most front-drive cars lift the rear wheel. This is not a big problem unless
you go to hell with the gag and roll it.
--------------------------
Phil Ethier, THE RIGHT LINE, 672 Orleans Street, Saint Paul, MN 55107-2676
h (612) 224-3105 w (612) 298-5324 phile@pwcs.stpaul.gov
"The workingman's GT-40" - Anthony Colin Bruce Chapman
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