Endre
(Andy) Bujtas
My tubular chassis uses the Ryane (now Held) engine cradle, which supports
an independent rear suspension. I'm also using a sway bar, custom made
for me by Sway-a-way (http://www.swayaway.com). The sway bar is a hollow
tube (1.125" OD) with a spring rate equivalent to a 22mm solid bar.
This suspension design will exhibit no bump steer since the upright is
held firmly in place by the upper control arm system - two parallel arms.
However, it could be susceptible to torque steer with a sufficiently powerful
motor because the upper control arm might twist under heavy load. I compensated
for this with a modification adding diagonal links or panhard rods.
The Ryane (Held) unit is designed to use the Wilwood 12 inch (Dynalite)
brake system.


General Rear Suspension Information
Sway Bars
Sway bars, or more accurately anti-roll bars, are used to add some roll
resistance to the car. But they are also used to control weight transfer
and thus, control the car's handling characteristics. The more weight
(force) you can put on the tires, the more friction with the ground, the
more grip you will have. Of course, having a full tire contact patch is
also desireable as well. When a car goes around a high-speed bend, the
body/chassis will tend to "roll" because of centrifugal force.
The anti-roll bar will add some resistance since as one side of the bar
is getting compressed (due to roll) the other side of the bar is tending
to follow the suspension on that side. The bar will twist, the amount
of which will depend on the stiffness of the bar itself. But in addition,
because the force going to the side of the car that is being compressed
is greater than the other side, the bar will tend to lift the wheels on
the side that is not being compressed. This causes an additional weight
(force) transfer from the raised side to the compressed side, giving the
car more grip. The stiffer the bar, the more weight is transferred because
the bar can resist the downward force of the springs on the raised side.
The key to handling, especially with mid-mounted engines, is in the selection
of the front-rear spring rates. Everything being equal, a larger anti-roll
bar up front will cause a weight shift to the front, reducing any oversteer.
Therefore, you do not want a particularly stiff or large roll bar in the
rear, but up front. The opposite would be true for front-engine cars.
Putting a large sway bar in the rear of a mid-engined car is asking for
trouble.
Brakes
The point about brakes is generally in the location where the caliper
is mounted. Understand that when the caliper "clamps" onto the
rotor (stopping), a torque is being generated that is transferred to the
chassis through the suspension. When looking at a chassis from the side
and the car is moving (say from left-right) the rotor is rotating clockwise.
If I clamp the rotor in front, the resulting torque will cause a tendency
of the chassis to raise giving more grip to the rear wheels. If I clamp
the rotor from the rear, the torque will cause the chassis to squat and
to reduce traction.
Designers tend to place the brake calipers as so: towards the front on
the rear suspension, and towards the rear for the front suspension. Or
both ends facing the passenger compartment. With some anti-dive geometry
up front, the car will tend to brake (sort of) level and not "nose"
down in the front so much.
Bump Steer
Bump steer occurs when the rear (or front) wheels toe (in or out) while
under bump. This is due because of the steering tie rod is moving up and
down along with the steering knuckle. This is particularly true for the
pre-88 Fieros since the rear suspension and cradle on those cars were
actually the front end of a Chevy Cavalier and as such, were originally
meant to steer. To prevent the rear wheels from turning, the Fiero engineers
replaced the steering tie rod with a toe adjustment arm attached to the
cradle. However, having this rod attached there and to the upright will
cause the wheels to turn slightly when the car hits a bump. The 88 Fiero
took care of that by redesigning the rear suspension from a MacPherson
strut to a Chapman strut design. The Chapman strut uses 2 parallel lower
control arms and thus, the wheels would not turn under bump conditions.
The way bump steer is minimized is through the proper setting of the front
wheel alignment (specifically the toe) and the location of the steering
rack. The rack location variable [in the equation], however, is fixed
so that there is not much you can do about it.
To correct the bump steer condition for those builders using the pre-88
rear suspension, you would either have to purchase a bump steer correction
kit or make something yourself. There are 3 ways that this can be done:
either by making a new lower control arm similar to the 88 Champan system,
or add a link to the upper end of the strut clamp that is bolted to the
upright. This link would be a diagonal rod that would be attached to the
chassis at a point such that it will allow the wheels to move up and down
as if it were not there, but would hold the wheel from turning. Or by
moving the toe arm point on the chassis higher or lower, depending on
how the car sits at full ride height. The object here is to have a very
slight downward angle to the upright so that under small bump deflections,
the toe rod would be almost level with the ground and not turn the wheels
any noticeable amount.
Race cars like the open wheel F1 and Indy cars do not have this problem
because the steer rack is located high and the tie rods are along the
same rotating plane as the upper control arms. That means as the wheels
go up and down, the steering tie rods move up and down on the same plane
as the upper control arms. Therefore, the tie rod movement does not force
the wheels to turn. This can't be realistically done for regular passenger
cars because: a) the engine up front will be in the way, or b) for mid/rear-engine
cars like the Diablo, the designers wanted a front trunk and a steering
rack mounted to prevent bump steer would eliminate any useable trunk space
up front.
For the front end, the Ryane/Held Slalom front suspension system compensates
for bump steer in their design. Their unit is bump-steer corrected, though
it does not totally eliminate it.
It has been suggested that bump steer could be corrected in the rear Fiero
suspension by relocating the toe rod from the chassis (or engine cradle)
to the lower control arm. But this will not work either and in reality,
will make it even worse. Why? Because the rear knuckle pivots (rotates)
in and out relative to the lower control arm due to the lower ball joint.
Worse how? Because the toe rod will have to be shorter than the original
rod, which will cause the rear wheels to toe faster and much more (s =
r * theta). The theoretical way to minimize bump steer in the pre-88 rear
suspension design (as it is) would be to have an infinitely long toe rod:)
But that is obviously impossible. Or make a longer toe rod such that the
left/right rods criss-cross so that the end point for the left side is
on the right and vice verse - if this is possible and the rods don't interfere
with anything else. Or they (the rods) can be joined at the car's center
line - if that is possible and still keep the down-angle of the tie rod
small enough. You want the angle of the tie rod going down to the knuckle
slightly (at ride height) so that when the wheel hits a bump, the rod
movement will cause the wheels to toe-in a small bit (NEVER TOE-OUT) and
reach level (zero angle) under normal bump conditions.
The Held bump steer correction kit (basic design idea) is the only way
to eliminate bump steer with essentially the same Fiero suspension setup.
It replaces the lower ball joint with 2 Heim joints and the upright is
held in place with the Heim joint pivot bar - preventing the knuckle from
turning or steering. But I'm not too keen on its overall design and have
some concerns if used with engines of significant power and torque. I
can easily see a couple of areas in this design that will bend out of
shape if someone dumped the clutch with a torquey V8 - causing permanent
misalignment or worse.
Torque Steer
Torque steer occurs when the rear suspension responds to the effects of
hard (or maybe moderate) acceleration. This probably will be especially
noticeable on Diablo builds using the Fiero rear suspension and having
torquey V8s powering them with those wide 335mm tires. What happens is
that under acceleration when the wheels turn one way, the upright will
go the other way. This is a result of Newton's 2nd law: a force or action
in one direction will cause an equal and opposite reaction. So that when
the upright shifts back it can cause the suspension to twist and the wheels
will tend to toe-out. Having the rear wheels toe-out under acceleration
will tend to make the car act "squirrelly" and is even more
dangerous in a high speed turn (e.g. severe oversteer). Torque steer is
generally not a problem with Hotchkiss setups (i.e. rear carrier-tube
differentials) since the wheels can't turn.
How do you handle this with a Fiero setup? With some difficulty. You
would either have to completely design a suspension for it or add some
longitudinal links like the Corvette rear suspension. The link that was
mentioned in the previous section for Bump Steer could aid in handling
torque steer. It would keep the upper end of the upright from rotating
back.
Squat
Squat occurs in almost every passenger car. Squat is when the chassis
dips or "squats" in the rear under acceleration. This has the
effect of losing traction and causing wheel hop. You see this occur in
the early Camaros where they only had a single leaf in their leaf springs.
Under acceleration, the car would squat, causing the spring to twist (called
wrap-around) making it ineffectual. The result was wheel hop and loss
of traction. Thus, the traction bar was born. The traction bar, being
attached to the differential, would rotate under acceleration, hit the
frame and transmit the torque to lift the chassis and increase the down
force on the rear wheels. Race cars (especially dragsters) have anti-squat
built into them. As a matter of fact, dragsters have so much anti-squat
that the entire rear of the car lifts up under acceleration. I've personally
seen a T-bodied Fuel Altered (AA/FA) actually lift completely OFF the
ground when the Christmas tree went green because it had so much anti-squat.
But it is difficult to incorporate anti-squat into independent rear suspensions.
The only thing you can really do is to increase the rear spring rate -
if you don't mind a stiff ride. But unless you plan on drag-racing your
build, squat should not be a big concern.
Andy |