Tips: Suspension
The following tips and information focus on how to optimize a race car
suspension. Because of the numerous types of suspension, we suggest
you read some excellent books
that cover this topic in much more detail. Depending on class rules,
these suggestions may or may not be valid. Always check your regulations.
General
Suspension Design Principles
Suspension
Design Tips
General
Suspension Design Principals
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Unsprung Weight
Unsprung weight is a measurement
of the weight of everything outboard of the wishbones or suspension
links, plus 1/2 of the weight of the wishbones or links and spring/shock.
It has a great effect on handling. The diagram below demonstrates why
unsprung weight is so important:
The more weight
outboard of the car, the more force bumps exert on the suspension (and
ultimately the chassis). This force must be dealt with using springs,
dampers and anti-roll bars (described below), and the more force, the
more difficult it is to keep the tire planted on the road. This is especially
true of lighter weight cars. In the example above, if the car weighs
1000 lbs, a 2G bump would result in a vertical force of 10% of the car's
weight. This will at the very least reduce the grip of the car, because
the weight of the car is what keeps the tire planted, and pushing a
car up into the air with that much force will inevitably reduce the
weight on the tire, and hence grip.
Tires
As the first point of contact
with the road, the tires work in conjunction with the suspension geometry
and weight transfer dynamics to provide grip. Many different types of
tires exist, but provided you are building for a specific class, you
can easily select a particularly good or popular tire.
The grip provided by a tire
is linked to the coefficient of friction (Cf) of the rubber compound
and to the tire's construction (Radial/bias). This coefficient indicates
the lateral grip the tire is capable of providing for a given weight
being placed on it. Racing slicks are very high Cf tires, in the range
of 1.0 or more. Street radials, on the other hand, rarely even approach
1.0. So what is in a number? If you were to place 500 lbs weight onto
each of four tires with a Cf of 1.0, you could expect 2000 lbs (actually
a little less) of lateral grip. Without aerodynamic aids to add to vehicle
weight, the car would almost achieve a 1G turn.
Wheels
Of course, the wheel is what
the tire mounts on. Wheels also come in a myriad of widths, sizes and
materials.
The primary types of wheels
used in racing are alloy and steel.
Alloy wheels can be constructed
to very minimal weights, as alloying materials such as aluminum and
magnesium can be used. They are also generally much more expensive than
their steel counterparts, but they also lack the dent resistance of
steel wheels. An alloy wheel, when struck by a curb will sometimes shatter,
and possibly worse, crack (only later to fly apart!). Nonetheless, for
most motorsports series, alloys are the choice.
Steel wheels can also be
constructed to amazingly low weights. Their cost is quite a bit less
than the alloys, due mostly to lower cost construction. Steel wheels
are deformable when struck, and will usually allow air to leak out of
the tire, as opposed to shattering. NASCAR, and the general stock car
scene use steel wheels due to the extreme forces encountered by 2 ton
cars.
Uprights (Wishbone suspension)/Knuckles
The upright or knuckle attaches
the wheel, brake rotor, hub, brake caliper and steering arm to the car
(of course, the wishbones and control arm(s) do the final attachment
to the chassis)
The upright or knuckle determines
the king-pin inclination, and the final camber, caster, and toe settings
of the wheel and tire. These various factors are demonstrated in the
diagram below.
Kingpin Inclination
determines steering feel to a great extent. In the front view
above, the red line on the right represents the center line of the tire/wheel.
The kingpin inclination is several degrees, the angle between the center
line and the line running through the upright or knuckle. The kingpin
inclination determines steering effort, and feedback.
Scrub radius is the
distance from the centerline of the tire/wheel to where the kingpin
line intersects with the road surface. The larger the distance, the
more effort is required to turn the wheel, as the wheel has to "scrub"
slightly to turn around the kingpin axis.
Camber is the angle
between vertical (perpendicular to a flat road surface) and the "lean"
of the tire/wheel. In the diagram above, negative camber of about 2
or 3 degrees is shown. Negative camber is often used to offset the normally
positive change in camber as the wheel moves up. The concept of camber
is simply to keep the tire contact patch as large as possible through
the complete range of suspension motion.
Toe-In/Out is a slight
steering angle that is preset into the suspension. Toe-in has the tires
pointing slightly toward the center of the car's front. Toe-out has
the cars pointing slightly away from the car. In the diagram above,
there is zero toe-in/out. Toe-in/out is used to offset the natural change
in toe position caused by braking and accelleration.
Caster is the angle
from vertical of the upright/knuckle, when viewing the wheel/tire from
the side. This angle is used to create a gyroscopic effect on steering.
This is easily demonstrated by turning the steering wheel in the car
and then letting go of the wheel (Do this in an empty parking lot!).
The caster causes the steering to correct itself back to straight ahead,
instead of turning, without the need for driver input.
As you can well imagine,
all these factors work together to produce a varying contact patch and
steering feel in the car. Software
exists for designing suspensions, and a computer makes it easy to see
changes and how they affect the contact patch of the tire.
Like the wheel and tire,
weight here plays an important part as well.
Wishbones/Control Arms
Wishbones and control arms
connect the previously mentioned upright or knuckle to the car chassis.
The wishbones or control arms (depending on suspension type) affect
the previously mentioned factors as well. Camber, castor, and toe are
all affected to some degree.
Essentially the wishbones
connect to the chassis with rod-ends or spherical bearings, allowing
the wishbones to pivot up and down with the wheel's movement and triangulating
the suspension to prevent the wheel from moving fore or aft of it's
designated position. Outboard, at the upright or knuckle, there are
two ball joints, one for each wishbone. See the diagram below for a
better visual representation:
The toe link, shown in blue
above, is attached to the steering rack at the front of the car, and
to the chassis at the rear. Toe adjustments are made by varying the
length of this link.
