Toe-out change preventive swing-arm vehicle suspension

A swing-arm type suspension having a suspension arm for suspending a road wheel on a vehicle body. A lateral link is also provided for restricting yawing movement of the suspension arm and thereby for restricting toe-angle change. The suspension arm is connected to the vehicle body by bifurcated legs via elastic bushings. The suspension arm is adapted to roll in response to rolling movement about a camber axis to change camber. The actual camber axis of the suspension arm is so arranged as to reduce toe-out change in accordance with the rolling movement of the suspension arm to cause positive camber.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a swing-arm type vehicle 
suspension, such as a trailing arm or semi-trailing arm suspension, which 
can provide both good drivability and good cornering stability. More 
particularly, the invention relates to a swing-arm type suspension which 
prevents occurrence of toe-out change in compliance or roll steering, 
especially in roll steering which causes the camber to change in the 
positive direction. 
It is well known that good drivability and higher cornering force can be 
obtained by providing weak understeer characteristics for a vehicle 
suspension. One of the essential features for providing understeer 
characteristics is to keep the toe angle in the toe-in direction when 
subjected to a cornering moment. Otherwise, toe-out change may provide an 
oversteer characteristic for the vehicle suspension which can adversely 
affect drivability and cornering stability. This toe-out change in the 
vehicle suspension may occur in compliance steering or roll steering as a 
result of changes of the suspension geometry and deformation of the 
suspension bushings. 
In order to prevent the occurance of toe-out change in a vehicle 
suspension, European Published Application No. 0070025, filed by the 
assignee of the present application, discloses a swing-arm type suspension 
having a laterally extending assist link for resisting external lateral 
forces. 
The present invention is generally directed to an improvement of the 
foregoing swing-arm type suspension described in Published European 
Application No. 0070025. More particularly, the invention concerns a 
swing-arm type suspension which eliminates the tendency for toe-out change 
to occur in roll steering which brings the vehicle suspension into 
positive camber. This camber change occurs as a suspension arm in the 
swing-arm suspension is rotated about an axis which is inclined with 
respect to the longitudinal axis of the vehicle, which rotational axis of 
the suspension arm will be referred to hereinafter as the "camber axis". 
In the swing-arm type suspension, especially in a trailing arm or 
semi-trailing arm suspension, the rotation of the suspension arm about the 
camber axis affects the toe angle and causes toe angle change due to 
lateral displacement of the suspension arm. If the axis of a wheel spindle 
mounted on the suspension arm is displaced upwardly and rearwardly by the 
rotation of the suspension arm about camber axis, toe-out change will 
occur. If the lateral displacement of the wheel spindle upon the 
occurrence of positive camber changes can be prevented, toe-out change can 
be prevented. 
On the other hand, the toe angle does not change even though camber 
changes, when the camber axis lies parallel to the rolling direction of 
the road wheel. Therefore, to prevent the road wheel from changing toe 
angle, the camber axis should be as nearly parallel as possible to the 
rolling direction of the road wheel. In a trailing arm or semi-trailing 
arm suspension, the longitudinal central axis of the suspension arm is 
generally shifted or inclined outwardly toward the rear with respect to 
the transverse axis of the road wheel so that a relatively short spindle 
may be used to mount the road wheel. It is impractical to shift the rear 
end of the suspension arm inwardly so that the suspension arm has a camber 
axis substantially parallel to the rolling direction of the road wheel, 
since this would require a longer spindle which, to have enough strength, 
would undesirably increase the suspension arm weight. Therefore, it is 
impractical provide a suspension arm which is substantially parallel to 
the rolling direction of the road wheel. 
Unless otherwise specified, references herein to the laterally outward and 
laterally inward directions are intended to refer to the directions away 
from, or toward, the longitudinal axis of the vehicle, respectively. 
The term "rolling direction" of a road wheel is intended to refer to the 
direction established by a horizontal line through the center of the wheel 
perpendicular to the rotational axis of the wheel. 
SUMMARY OF THE INVENTION 
Accordingly it is the object of the present invention to provide and 
improved swing arm vehicle suspension for a wheeled vehicle. 
Another object of the invention is to provide a swing arm vehicle 
suspension which facilitates improved drivability and good cornering 
stability for a vehicle equipped therewith. 
It is also an object of the present invention to provide a swing arm 
vehicle suspension which exhibits less tendency for toe-out change to 
occur when the wheel camber changes in the positive direction. 
These and other objects of the invention are achieved by providing a swing 
arm vehicle suspension for a wheeled vehicle comprising a suspension arm, 
wheel means for rotatably mounting a road wheel oriented in a desired 
rolling direction on one end of the suspension arm, pivot means for 
pivotably securing the other end of the suspension arm to a vehicle body, 
the pivot means permitting vertical pitching movement of the suspension 
arm about a pitching axis and resiliently permitting lateral yawing 
movement of the suspension arm and rotational movement of the suspension 
arm about a camber axis, the suspension arm having a theoretical camber 
axis extending along a line through said one end of the suspension arm and 
through the midpoint of the pivot means, the thoretical axis forming an 
angle with respect to the desired rolling direction of the road wheel, 
means interposed between the suspension arm and the vehicle body for 
retarding vertical pitching movement of the suspension arm, means 
connected to the suspension arm and the vehicle body for preventing 
lateral yawing movement of the suspension arm, and means for establishing 
an actual camber axis for the suspension arm which is more nearly parallel 
to the desired rolling direction of the road wheel than is the theoretical 
camber axis; whereby the tendency is reduced for toe-out change of the 
orientation of the road wheel to take place when rotational movement of 
the suspension arm and consequent camber change of the road wheel occur. 
According to one preferred embodiment of the invention, the actual camber 
axis of the suspension arm is established by shifting the rolling center 
on the pitching axis outwardly from its theoretical location at the 
midpoint of the pivot means. To shift the rolling center outwardly, the 
rigidities of the bushing assemblies provided at the pivot end of the 
suspension arm are varied so that the outward bushing assembly is more 
rigid than the inward bushing assembly. By shifting the rolling center 
outwardly, the angle between the camber axis and the rolling direction of 
the road wheel becomes smaller, ie they become more nearly parallel. 
According to another preferred embodiment of the invention, the actual 
camber axis is established by shifting the pivot point between the rear 
end of the suspension arm and the lateral yaw preventing means inwardly. 
For this purpose, a laterally inwardly extending bracket fixedly joined to 
the rear end of the suspension arm is provided to shift the pivot point. 
In further prefered embodiments of the invention, the forward end of the 
suspension arm is bifurcated and is attached to the vehicle body by the 
pivot means, and the rearward end of the suspension arm is shifted 
laterally outwardly with respect to the forward end; the pitching movement 
retarding means comprises a shock absorber, and the yawing movement 
preventing means comprises a lateral member having one end pivotably 
attached to the rear end of the suspension arm and the other end secured 
to the vehicle body. Also, in a particularly preferred embodiment the 
pivot means comprises two resilient bushings which are co-axially aligned 
with the pivot axis of the pivot means; one of said bushings being 
disposed in a laterally outward position and the other of said bushings 
being disposed in a laterally inward position, and the outward bushing 
having a greater rigidity than the inward bushing. In yet another 
particularly prefered embodiment of the present invention, a laterally 
inwardly extending bracket is fixedly joined to the rear end of the 
suspension arm, and the yaw preventing means is pivotably joined to the 
inward end of the bracket.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, particularly to FIG. 1, there is illustrated 
the general construction of one preferred embodiment of a semi-trailing 
arm vehicle suspension according to the present invention. The 
semi-trailing arm vehicle suspension generally comprises a suspension arm 
10 including an arm body 12 and bifurcated legs 14 and 16 extending from 
the front end of the arm body. Leg 14 has a cylindrical end section 18 at 
the front end thereof with a bushing assembly 20 received therein. 
Likewise, leg 16 has a cylindrical end section 22 at the front end thereof 
for receiving a bushing assembly 24. Bushing assemblies 20 and 24 
generally comprise inner tubes 26 and 28, outer tubes 30 and 32, and 
elastic rubber bushings 34 and 36, respectively. The bushing assemblies 20 
and 24 are disposed in the cylindrical end sections 18 and 22 coaxially to 
the latter. The axes of the bushings are in alignment and constitute a 
pivot axis 38 about which the suspension arm may rotate vertically. 
The suspension arm 10 is rotatably mounted on a vehicle body cross member 
(not shown) with brackets via the bushing assemblies 20 and 24 so that it 
can move vertically about the pivot axis 38. The bushing assemblies 20 and 
24 further allow lateral displacement and rotational movement of the 
suspension arm 10 by elastic deformation thereof. 
The suspension arm 10 supports a wheel spindle 43 on the outer peripheral 
side of the arm body 12. A wheel hub and a road wheel 44 are mounted on 
wheel spindle 43 in substantially parallel relationship to the 
longitudinal vehicle axis. At the opposite side of the arm body, the 
suspension arm 10 is secured to the lower end of a suspension strut 46 of 
a shock absorber assembly 48 via a bracket 50. Suspension strut 46 
encloses a shock absorber from which a thrusting piston rod 52 extends 
upwardly from the top of the suspension strut. The top end of the piston 
rod 52 is secured to a vehicle body (not shown) with a fastening nut 54. A 
suspension coil spring 56 is wound around the piston rod 52 with opposite 
ends of the spring seated on upper and lower spring seats 58 and 60, 
respectively. Upper spring seat 58 is secured to the piston rod 52 
adjacent the top of the rod and engages a mounting insulator 62 inserted 
between the vehicle body and the upper spring seat. Lower spring seat 60 
is secured to the suspension strut 46. The shock absorber assembly 48 thus 
provides a damping force to absorb vertical forces which cause relative 
vertical displacement between the vehicle body and the wheel axle. 
Suspension arm 10 is further pivotably connected to the outer end of a 
lateral link or member 64 at the rear end of the arm body 12. The other 
end of the lateral member 64 is pivotably connected to the vehicle body 
via bracket 66. The lateral link 64 has substantially cylindrical end 
sections 68 and 70, respectively, oriented generally transverse to the 
axis of the lateral link. Each cylindrical end section 68 and 70 receives 
a bushing assembly (not shown) therein. This lateral link 64 provides 
resistance against lateral forces applied to the road wheel 44, which 
lateral forces otherwise cause yawing movement and toe-out change. 
As can be best seen from FIGS. 4(a) and 4(b) the theoretical rolling center 
designated by point P.sub.2 about which rotational rolling movement of the 
suspension arm 10 theoretically occurs to cause a camber change is located 
at the dimensional center of the pivot arm 10 between the bushing 
assemblies 20 and 24. The theoretical camber axis passes through the 
pitching axis midpoint P.sub.2 of the pivot arm 10 and through the pivot 
point 76 between the suspension arm 10 and the lateral member or link 64. 
To place the road wheel at an appropriate position in the wheel well of 
the vehicle body and to provide sufficient strength for the spindle for 
mounting the road wheel, it is generally necessary to shift or incline the 
rear end of the suspension arm 10 outwardly so that it projects into the 
wheel well of the vehicle body. Accordingly, the pivot point 76 between 
the rear end of the suspension arm 10 and the lateral link 64 is shifted 
outwardly so that the lateral distance between pivot point 76 and the 
longitudinal axis of the vehicle body is greater than the lateral distance 
between the midpoint P.sub.2 of the pivot means and the vehicle 
longitudinal axis. As a result, the theoretical camber axis is tilted or 
inclined outwardly toward the rear in relation to the rolling direction of 
the road wheel, which is substantially parallel to the longitudinal 
vehicle axis. 
If the suspension arm is rotated about the theoretical camber axis, the 
rotation of the suspension arm 10 causes relative lateral displacement of 
the front and rear ends of the road wheel, which in turn produces a toe 
angle change. If the theoretical camber axis is tilted outwardly toward 
the rear, the toe angle change resulting from a camber change in a 
direction to cause positive camber will occur in the toe-out direction. 
In the illustrated embodiment of the present invention, the angle of 
inclination of the actual camber axis with respect to the rolling 
direction of the wheel is reduced by providing different resilient 
characteristics or hardness for the bushing assemblies 20 and 24. By this 
means the angle of inclination the actual camber axis is reduced compared 
to the theoretical axis, without requiring any change in the suspension 
arm construction or use of a longer wheel spindle. 
FIGS. 2 and 3 show respectively the bushing assemblies 20 and 24. As 
previously mentioned, the bushing assembly 20 generally comprises inner 
and outer tubes 26 and 30 and annular rubber bushing 34. The rubber 
bushing 34 is formed with annular or circumferential grooves 35 in the 
axis ends thereof. Similarly, the bushing assembly 24 comprises inner and 
outer tubes 28 and 32 and annular rubber bushing 36 which is formed with 
annular or circumferential grooves 37 in each axial end thereof. 
The depths of the grooves 35 and 37 differ from each other so that the 
depth D.sub.2 of the grooves 37 in rubber bushing 36 is deeper than the 
depth D.sub.1 of the grooves 35 in rubber bushing 34. As a result, the 
rigidity of rubber bushing 34 is higher than that of rubber bushing 36. 
Alternatively, the grooves 35 in bushing 34 may be omitted to further 
increase the rigidity thereof. The rigidity of outward bushing 34 may also 
be made higher than that of inward bushing 36 by forming it of resilient 
material having a lower modulus of elasticity. 
As can be seen from FIGS. 4(a) and 4(b), this difference in bushing 
rigidity shifts the actual camber change center P.sub.1 on the horizontal 
pivot axis 38 toward the bushing assembly 20 from the midpoint P.sub.2 of 
the pivot axis 38. This will reduce the tilt angle .theta..sub.1 of the 
actual camber axis 72 which entends through the camber center P.sub.1 and 
through the pivot axis 76 between the suspension arm 10 and the lateral 
link 64, with respect to the rolling direction of the road wheel indicated 
by line 74. As a result of this reduction of the angle of inclination of 
the actual camber axis, the amount of toe angle change in the toe-out 
direction as the camber changes in the positive direction is geometrically 
reduced. 
The geometric function of the semi-trailing arm suspension of the 
illustrated embodiment will now be described with reference to FIGS. 4(a) 
and 4(b) which illustrate the change in the suspension geometry in 
response to an external lateral force F which causes a positive camber 
change. As shown in FIG. 4(a), the actual camber axis 72 of the suspension 
arm 10 extends through the camber change center P.sub.1 and the pivot axis 
76. By shifting the camber change center P.sub.1 from its theoretical 
location at P.sub.2, the tilt angle .theta..sub.1 of the actual camber 
axis 72 with respect to the rolling direction of the wheel is reduced to 
decrease the dimensional difference between the distance from the camber 
axis to point A on a horizontal wheel transverse axis 78 and the distance 
from the camber axis to point B on the horizontal wheel transverse axis. 
Points A and B are assumed to be located at the same distance from the 
rotational axis 80 of the road wheel. 
The external lateral force F is assumed sufficient to rotate the suspension 
arm 10 in the counterclockwise direction in FIG. 4(b) to vertically tilt 
the rotational axis 80 through an angle .gamma.. This suspension arm 
displacement changes the camber to a positive value. Due to the resulting 
shifting of the horizontal wheel transverse axis 78 inwardly, the points A 
and B are shifted respectively to A' and B', as shown in FIG. 4(a). Since 
the distances between the point A and the actual camber axis 72 and 
between the point B and the camber axis are approximately equal or not 
significantly different, the shifting distances .DELTA.X.sub.A and 
.DELTA.X.sub.B of the points A and B to A' and B' is not significantly 
different. Thus, the toe-out angle change is not so large as to adversely 
affect cornering stability or to cause significant power oversteering. 
According to the embodiment set forth the above, the angle of inclination 
between the actual camber axis and the rolling direction of the wheel may 
be sufficiently small so as to keep the toe angle of the road wheel at 
almost neutral even when the suspension arm rotates about the camber axis. 
Importantly, this advantageous result is achieved without any dimensional 
change in the overall dimensions or relative positions of the respective 
suspension components of the semi-trailing arm suspension. Thus, according 
to the invention, higher cornering force in roll steering to cause 
positive camber can be obtained by merely varying the resiliency or 
deformation ratio of the suspension bushings. 
Another preferred embodiment of the present invention is illustrated in 
FIGS. 5(a) and 5(b). In this embodiment, an inward tilt angle 
-.theta..sub.2 toward the rear of the vehicle is provided for the camber 
axis 72 of the suspension arm 10. This results in alteration of the 
dimensional relationship between the points A and B on the wheel 
transverse axis 78 with respect to the camber axis 72. Namely, the lateral 
distance between the point B and the camber axis 72 becomes larger than 
that between the point A and the camber axis. In this case, an increase in 
toe-in occurs in response to a camber change in the positive direction. 
To obtain the negative tilt angle -.theta..sub.2 in the camber axis 72, the 
vertical pivot axis 76 between the suspension arm 10 and the lateral link 
64 is shifted inwardly. For this, a laterally inwardly extending bracket 
84 having sufficient lateral length will be used to connect the suspension 
arm 10 and the lateral link or member 64 with the inwardly shifted pivot 
axis 76'. Bracket 84 is fixedly joined to the rear end of the suspension 
arm 10 and pivotably joined to lateral member 64. 
Although the foregoing alternate preferred embodiment is shown with the 
outwardly shifted camber change center P.sub.1 to cause the suspension 
geometry to have the camber axis tilted at a negative or inward angle, if 
desired the camber change center P.sub.1 could be maintained at the 
dimensional center P.sub.2 between the pivot bushings 20 and 24. 
The foregoing description has been set forth merely to illustrate the 
invention and is not intended to be limiting. Since modifications of the 
disclosed embodiments incorporating the spirit and substance of the 
invention may occur to persons skilled in the art, the scope of the 
invention is to be limited solely with respect to the appended claims and 
equivalents.