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The Iron Duke Resource Site |
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Welcome to my newly
redesigned Fiero 2M4 resource site. The original site recorded over
12000 visits, hopefully this one will also prove as useful.
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Vehicle
Dynamics 101:
The
basic theories of vehicle dynamics (handling) are often a mystery due to the
complex scientific principles involved. In this article and the one
which follows I will try to explain these principals in plain language.
We
begin by clarifying the point which sets the Fiero apart from other American
production cars, it is a mid engined car. Mid engine designs put the
engine within the wheelbase, usually behind the driver (some front engine cars
can qualify, such as the Panoz Lemans racer). This is the preferred
layout of racing cars since it results in a low polar
moment of inertia,
let me explain:
If
you have ever watched a figure skater in a spin you may notice that as he
pulls his arms in towards his body he spins faster, arms out, he slows down.
What the skater is doing is changing his polar moment of inertia, lowering it
as he brings his arms in and raising it as he pushes them back out.
Concentrating the mass at the centre allows for faster rotation, thus a
quicker reacting car.
The
Fiero has a slightly rearward weight balance and a low polar moment of
inertia, these factors make it a responsive car but also a less stable car.
Chevrolet in the C5 Corvette placed the engine at one end and the transmission
at the other to get a 50-50 weight distribution and a high polar moment,
although this makes the car slightly less responsive it makes it more stable
at high speeds. Ferrari has gone the same route with the 550.
POLAR MOMENT OF INERTIA DOES NOT DEAL DIRECTLY WITH UNDERSTEER OR OVERSTEER,
it effects the transition time or response, the C5 Corvette and Ferrari
550 are extremely capable cars which have quick responses because of careful
attention to detail in suspension and very large tires, the high polar moment
makes them inherently stable in a straight line. A Ferrari 355, which has a
low polar moment due to the mid engine, has "quicker reflexes" but
this also translates into a trickier "sneeze factor" (you sneeze,
where do you end up?). Many modern mid engine cars have much longer
wheelbases than in the past (note the Ferrari 360), although packaging
mechanicals and people is a big factor here, the longer wheelbase adds a
degree of stability without numbing the reflexes of these machines. To
summarize, a low polar moment adds quick responses but an increased tendency
to spin, a high polar moment slows responses but adds stability (reduces the
tendency to spin).
Understeer,
oversteer, neutral, these terms are used to describe the cornering behaviour
of an automobile. You may have noted all of the references to how strongly the
stock Fiero understeers. These terms refer to slip angles.
"What
are slip angles?" is usually the first question to come up. This is
the angle between the tire tread centreline and the actual direction the tire
is travelling. This tire slip is what causes a moving car to change direction.
NOTE:
Motorcycles
corner due to an effect called precession caused by extreme camber angles when
a bike is leaned into a turn, the same reason why a rolling coin will travel
in circles as it tips over. This makes motorcycle handling and
motorcycle tire technology a very different study from the automobile.
If
the slip angles of the front and rear tires are equal we call the result
neutral steer, the car goes around a circle with no change in steering input.
If
the slip angle is greater for the front tires we get understeer, more steering
input is required to stay on a circle. We say the car pushes or plows, which
aptly describes the stock Fiero. Most FWD sedans behave this way because
of their very high forward weight bias and the fact that the front wheels are
called upon to steer and motivate the car. This is a very "safe"
cornering attitude since a spin is not likely.
If
the rear slip angle is greater we have oversteer, steering input must be
reduced to stay on the circle. Steady state oversteer makes a car
difficult to drive since it is always on the verge of a spin, a very
unforgiving setup.
PLEASE
NOTE: These descriptions are of steady state cornering, any change in speed
during a corner will upset this balance and result in what can be called
induced effects. For example, the Fiero will tend to oversteer if the speed is
suddenly reduced in a corner (such as releasing the throttle or using the
brakes). Since this is throttle or brake induced oversteer it is a different
issue from the items above and is more related to driving style, weight
transfer and suspension design than it is to slip angles. Driver inputs
can completely change any car's behaviour and therefore has little to do with
our topic.
Ira
Crummey
Neutral
to slight understeer is the preferred all around setup since it is forgiving
but fast and controllable. This balance is effected by weight
distribution, spring rates, anti-roll bars and tire sizes.
To
be Continued (next
page)........
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