Spring devices – Vehicle – Parallel depression
Reexamination Certificate
2001-08-09
2004-03-09
Schwartz, Christopher P. (Department: 3613)
Spring devices
Vehicle
Parallel depression
C267S190000, C267S222000, C267S224000, C267S225000, C280S124104, C280S124106, C280S124179
Reexamination Certificate
active
06702265
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a suspension system for a vehicle having four or more wheels.
The invention has been developed primarily for use in motor cars of various types and will be described hereinafter with reference to that application. However, it will be appreciated that the invention is not limited to that particular field of use.
BACKGROUND OF THE INVENTION
All vehicles that travel over, and support themselves from the ground, benefit from having a compliant suspension system. The suspension allows the vehicle and its contents to travel in a relatively smooth path while the suspension accommodates for the unevenness of the ground and keeps the wheels in contact with the road.
If a car is designed primarily for a comfortable “ride” it is provided with soft, long travel suspension springs to absorb the bumps on the road. However, when such a car drives around a corner at speed the body “rolls” outwards. This is uncomfortable for the passengers and can also reduce the precision with which the car can be placed on the road, thus impairing the “handling” of the car. On the other hand, a car designed primarily for good handling is usually provided with stiff, short travel springs which make the car feel more responsive to the driver's inputs, but also make the ride harsh over bumps and reduce the total “grip” of the tyres on the road. The choice between soft and stiff springing is often called the “ride-handling” compromise.
Some expressions with common but perhaps vague meanings are more closely defined below.
The term “car” will be used to cover cars, trucks, trailers, caravans, etc., generally with four wheels but possibly with more.
The “suspension” of the car refers to the mechanisms which connect the wheels to the “body” of the car (also called the “sprung-mass”) allowing the wheels to move in a predominantly vertical direction whilst also supporting the weight of the car. Most cars have a single “spring-damper” unit which controls this vertical movement of each wheel, while some cars have additional springs which are connected to more than one wheel.
A “spring” is any elastic element that will deflect by a predetermined and significant amount when a force is applied to it, and then return to its original length when the force is removed. A spring can be made of any of the common materials such as steel, rubber, fibreglass or compressed gas in a container. The ratio of applied force increment per deflection increment (measured in pounds-per-inch or newtons-per-meter) is called the “spring rate”. For most steel springs, the spring rate is constant and is determined by the size of the spring. However, due to leverages in the suspension mechanism, the deflection for a given load increment at the wheelprint, called the “wheel rate”, is usually different to the spring rate and can vary as the suspension deflects. This is termed “variable-rate” springing.
An uncontrolled spring-mass system will oscillate indefinitely when set in motion. It is the function of the “dampers” to minimise the unwanted oscillations. The main characteristics of dampers is that they only exert a force on the suspension when the suspension is moving (that is, moving up or down), and the force is always in the direction opposite to that of the suspension movement. The force exerted is therefore a function of the velocity of the suspension and is almost always of variable rate.
Any solid object can move with six “degrees of freedom” with respect to another solid object. For vehicles (of all types) it is usual to use a rectangular coordinate system fixed to the vehicle to describe the motion of the vehicle body with respect to the ground. The degrees of freedom are then usually named “forward/backward” for longitudinal motion of the body; “sideslip” for lateral motion of the body; “bounce” (or “heave”) for vertical motion of the body; “roll” for rotation around a longitudinal axis through the body; “pitch” for rotation about a lateral axis; and “yaw” for rotation about a vertical axis (see FIG.
1
). This assumes that the body is relatively rigid. Note also that for a car travelling on an essentially flat surface only bounce, pitch and roll motions will cause vertical movements of the suspension.
While the vehicle body moves with respect to the ground, the wheels can also move with respect to the body. This movement refers only to the relative motion between the body and the contact area between wheel and ground and not to wheel rotation, steering and so on. For the purpose of this discussion, only the one degree of freedom of motion that each “wheelprint” has along a predominantly vertical linear path will be considered. So, for a car with four wheels there are four degrees of freedom for the motion of the four wheelprints with respect to the body. To put it another way, it would be necessary to specify four separate parameters (one for each degree of freedom) to completely define the positions of all four wheelprints with respect to the body. The simplest way to do this is to specify the vertical position of each wheelprint with respect to the body (see FIG.
2
).
Another way of specifying the four wheelprint positions is shown in FIG.
3
and is called a “modal” description of the suspension. Each mode (or degree of freedom) involves the movement of all four wheels. The first three modes of “bounce”, “pitch” and “roll” are similar but opposite to the motions of the body with respect to ground. That is, as the body bounces down towards the ground, the wheelprints bounce up towards the body, and similarly for pitch and roll. These three modes are chosen because they can be directly related to the similar motions of the body.
Any combination of suspension bounce, pitch and roll will always keep all four wheelprints in a “flat” plane. However any stretch of road will, in practice, have some degree of unevenness. Small scale unevenness will be referred to as “bumps” while larger scale unevenness of the road surface will be called “twist”. Bumps are typically shorter in length than the car's wheelbase while twist of the road surface is a bump that is longer than the wheelbase. This unevenness will inevitably try to force one wheelprint out of the plane defined by the other three wheelprints. The fourth mode of suspension movement, shown in
FIG. 3
d
, is “twist” and is the only mode that allows all four wheelprints to stay in contact with uneven ground.
The units used to measure the modes in
FIGS. 3
a
to
3
d
are linear (for example, metres or inches). Angular units could be used for pitch, roll and twist. However, it is more convenient to use linear units for all the modes as it is then a simple matter of summation to calculate the heights of the individual wheelprints. The two sets of wheelprint positions shown in FIG.
2
and
FIG. 3
describe the same physical situation. It is seen that a single wheel bounce is made up of one quarter contributions of the four modes of bounce, pitch, roll and twist. Also for equal loading on all wheels the single wheel spring rate is given by the summation of one-sixteenth of each of the spring rates in bounce, pitch, roll and twist.
The four modes of suspension movement of bounce, pitch, roll and twist are introduced here because they are the key to understanding the operation of the suspension system of the present invention which will be described later.
Requirements of a “Good” Suspension
There are many aspects of a car's design that affect its dynamic behaviour. The following section considers the contribution of the suspension, as defined above, to handling, ride and grip.
Handling is a subjective concept. It can be described as the precision of feeling that the driver has for the reactions of the car in response to the driver's control inputs. The driver's main inputs to the car are via the accelerator, brake and steering wheel. Therefore the main reactions of the car that the driver wants to feel are a forwards acceleration, or force, in response to the accelerator, a backwards force from the brakes, and a yawin
Henricks Slavin & Holmes LLP
Kramer Devon
Schwartz Christopher P.
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