Land vehicles – Suspension modification enacted during travel – Suspension geometry
Reexamination Certificate
1997-10-03
2001-08-28
Culbreth, Eric (Department: 3611)
Land vehicles
Suspension modification enacted during travel
Suspension geometry
C280S005500, C280S005520, C280S086751, C280S086757
Reexamination Certificate
active
06279920
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to land vehicles, and more particularly to dynamic adjustment of wheel camber of the suspension systems of motorized land vehicles. The inventor anticipates that primary application of the present invention will be for automobiles and trucks, but the present invention is also well suited to use in any vehicle having at least one set of transversely opposed wheels.
BACKGROUND ART
Modern vehicle suspension systems can be quite complex assemblies, adapting as they must from static conditions of the vehicle at rest (and even there with a range of possible loads) through dynamic conditions imposed in travel by road surface, road slope and pitch and turns, external forces like wind gusts, vehicle speed, load shifts, and all possible combinations of these. To adapt to such conditions most vehicle suspensions to date have employed characteristics like camber, caster, and toe which are set to particular values, the suspension locked in some manner to maintain these values, and then those characteristics not intentionally changed. This system of static suspension setup is an understandable attempt to simplify the complex mechanism used to address the dynamic environment encountered in driving.
The key characteristics of suspension systems are camber, toe, and caster.
For the following discussion camber is the most important, it refers to the vertical tilt of a wheel either toward or away from the vehicle center. On a vehicle having opposed transversely paired wheels, like an automobile, when a wheel is tilted top inward the wheel is said to have negative camber, and when it points top outward it is said to have positive camber. Similarly, changes in the camber of a wheel may be referred to as being more negative or more positive.
In contrast, toe is the horizontal tilt of paired wheels either together or apart. When the fronts of the wheels tilt inward the wheels are said to have toe-in, and when they point outward they are said to have toe-out. It should be noted that toe deals with a characteristic of wheel orientation which is both horizontal and longitudinal relative to the vehicle. Caster is the forward (negative caster) or rearward (positive caster) tilt of the steering axis of a wheel. For example, in most bicycles the front fork is almost always mounted tilted back, giving the front wheel positive caster. It should be noted that caster deals with a characteristic of wheel orientation which is both vertical and longitudinal relative to the vehicle. Finally, track is the separation between transversely separated wheels. Track is not usually discussed as a suspension setup characteristic, but it is important in the following discussion.
FIG. 1
(background art) depicts a vehicle
12
(in ghost outline) on a horizontal road
14
. The vehicle
12
has a conventional suspension system
16
which includes a spacing member
18
, which in actuality may be a more complex assembly than is shown. The spacing member
18
has a fixed horizontal displacement between its opposed ends
20
. At each end
20
a joint
22
is provided where the upper end of an arm
24
is attached. At the lower end of each arm
24
is a spindle
26
, upon which a wheel
28
is rotatably mounted (denoted as a left wheel
28
l
and a right wheel
28
r
in the figures; typically the wheels
28
will include tires, but the discussion herein will not generally treat these separately). In
FIG. 1
the wheels
28
are shown oriented to true vertical (i.e., zero camber). The vehicle
12
further includes a center of gravity “CG
30
,” which for the present discussion will always be assumed to be fixed at the transverse center of the vehicle
12
. A vertical center axis
32
is projected through the CG
30
to the road
14
, thereby dividing the overall track at the illustrated end of the vehicle
12
into a left track
34
l
and a right track
34
r
. The suspension system
16
is depicted simplistically here with components like springs, steering linkages, etc. omitted to facilitate clarity. Thus,
FIG. 1
depicts what has been considered proper wheel alignment and suspension setup during much of human history.
FIG. 2
(background art) illustrates the vehicle
12
at rest with the suspension system
16
set up in a conventional modem manner. The tops of the wheels
28
are tilted outward (somewhat exaggerated for illustration), away from the spacing member
18
. To emphasize this a pair of vertical side axie
36
, a pair of wheel axes
38
, and a pair of arcs
40
are provided to depict the angular separation of these.
FIG. 2
thus depicts positive camber. Today a slight amount positive camber is considered desirable by many manufacturers (e.g., Ford Motor company in most of its automobiles; but Daimler Benz is a counter example, using slightly negative setup camber in many Mercedes Benz automobiles). Slightly pre-loading camber away from zero in this manner is motivated by the modem use of flexible components like springs and inflated rubber tires, and the goal of maintaining the camber of the suspension system
16
within a useful range during vehicle
12
use, say, a range extending from slightly positive to zero to negative.
Many factors affect wheel
28
orientation when the vehicle
12
is moving, with some obvious examples having already been mentioned, such as passenger and cargo loading. However, less obvious factors must also be considered, such as the natural tendency of non-driving wheels
28
to spread outward at high speed.
FIG. 3
(background art) therefore illustrates the vehicle
12
and its suspension system
16
when engaged in typical straight forward motion at highway speed. The slightly positive camber of
FIG. 2
has now become slightly negative.
FIG. 4
(background art) illustrates the suspension system
16
as the vehicle
12
makes a hard unbanked turn to the right. A number of changes can be observed: the camber of the left wheel
28
l
is now positive, the camber of the right wheel
28
r
is somewhat more negative, and the left track
34
l
and the right track
34
r
are no longer equal. There are a number of factors that interact to bring about these changes. A typical lay person might say that the vehicle
12
is “leaning into the turn” and that the wheels
28
(i.e., the tires) are scrubbing the road.
FIG. 5
(background art) is a free body diagram depicting the forces present in the scenario depicted in FIG.
4
. At this point it is assumed that the vehicle
12
is not in such an extreme situation that it is skidding or has lifted a wheel
28
off the road
14
. The vehicle
12
is reduced here to three rigidly connected points: a left road contact point
42
l
, a right road contact point
42
r
, and the previously noted CG
30
. The vehicle
12
has weight (W), depicted by a vector vertically extending out of the CG
30
. Countering the weight (W) are a left normal force (N
l
) and a right normal force (N
r
), respectively depicted by vectors vertically extending out of the left road contact point
42
l
and the right road contact point
42
r
. The vehicle
12
also has a lateral force (L) effectively acting on the CG
30
(depicted by a vector horizontally extending out of the CG
30
). Countering the lateral force (L) are a left friction force (F
l
) and a right friction force (F
r
), respectively depicted by vectors horizontally extending out of the left road contact point
42
l
and the right road contact point
42
r
. Finally, resultant vectors are shown at all three points (
30
,
42
l
, and
42
r
).
Turning now also to
FIGS. 6
a-b,
these respectively depict close-ups of the left wheel
28
l
and the right wheel
28
r
as they contact the road
14
in the scenario of FIG.
4
. In particular, it should be noted that the road
14
surface contact of the left wheel
28
l
and the right wheel
28
r
are not the same, a left footprint
44
l
here is greater than a right footprint
44
r
. The net result is as depicted in
FIG. 5
, where the left friction force F
l
is shown with a longer vector than the right friction force F
r
be
Culbreth Eric
Oppenheimer Wolff & Donnelly LLP
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