Acceleration sensitive shock absorber

A twin tube shock absorber has concentric inner and outer tubes connected to the wheel of a vehicle. A piston in the inner tube is connected to the chassis of the vehicle and divides the interior of the inner tube into upper and lower chambers. Fluid passes between these chambers and an annular reservoir between the tubes for absorbing shock in a conventional manner. An orifice is provided through the sidewall of the lower portion of the inner tube and is surrounded by a lower sleeve, which normally keeps the orifice closed. Upon upward acceleration of the tubes, the sleeve essentially remains fixed in space. The relative movement between the sleeve and tubes aligns a passage through the sleeve with the orifice and permits fluid to flow from the lower chamber below the piston to the annular reservoir. This provides a softer shock absorber when high upward acceleration is felt by the tubes of the shock absorber and a stiffer shock absorber during times when there is no large upward acceleration. A similar orifice and sleeve are provided at the upper end of the inner tube for permitting fluid to flow from the upper chamber into the reservoir during rapid downward acceleration of the tubes. The orifices are spaced apart from the ends of the inner tube so that when the piston passes an orifice, the characteristics of the shock may change from soft to stiff before the end of the stroke of the piston.

BACKGROUND 
This invention relates to vehicle shock absorbers typically mounted between 
the wheels and frame of an automobile, truck, motorcycle, etc. The 
invention relates to a shock absorber with damping characteristics that 
change depending on the acceleration of parts of the shock absorber. 
This application is related to U.S. patent application Ser. No. 07/798,036, 
filed Nov. 20, 1991. 
Hydraulic shock absorbers are universally employed in automotive vehicles. 
Each wheel of the vehicle is coupled to the frame by a spring so that 
bumps or dips in the road are not transmitted directly to the passengers 
or vehicle lead. A spring alone, however, would still give a rough ride. 
Shock absorbers are therefore mounted in parallel with the springs to damp 
the accelerations applied to the frame from the wheel. There is a long 
history of shock absorber development to obtain desired characteristics of 
passenger ride, comfort, handling for steering, road traction and the 
like. 
Most shock absorbers are designed to have a certain operating 
characteristic or load-velocity curve which is a compromise of the 
characteristics desired for a variety of road conditions. The 
characteristics suitable for driving on relatively smooth road may, 
however, be inappropriate where the vehicle wheels may encounter short 
range bumps or dips. Such conditions are not limited to vehicles like 
those used on off-road terrain, but also include ordinary passenger and 
freight vehicles which may unexpectedly encounter chuck holes, speed bumps 
or foreign objects on the roadway. 
Efforts have therefore been made to provide shock absorbers with variable 
characteristics in response to varying road conditions. Attempts to have 
been made to have computer controlled shock absorbers employing sensors, 
solid state computers which are preprogrammed, and electrically operated 
valves controlled by the computer. As with any computer, its output is no 
better than its input and even though the speed of response is indeed 
amazing, the overall system response, including that of the mechanical 
valves, determines the actual speed of adjustment. 
A vehicle traveling at about 100 km/h advances over the ground 
approximately 16 centimeters in five milliseconds. A computer controlled 
system which has an overall response time in that range cannot provide 
effective compensation for surface conditions based on any remote sensing 
of either present or anticipated road conditions. The acceleration of the 
wheel upon hitting a bump or dip may be over before the system can 
adequately respond. Furthermore, by directing optical or other sensors 
ahead of the vehicle, the sensors incur problems of dust, rain or other 
conditions which may mask the true road condition and prevent effective 
computer controlled compensation. 
Another approach is to employ acceleration responsive valves which require 
no computer or manual adjustment. A number of acceleration responsive 
shock absorbers have been proposed which internally respond to 
accelerations for providing automatic adjustment of orifices in the shock 
absorbers. Many of these systems are limited since they provide no 
reduction in rebound resistance or resistance to extension of the shock 
absorber in the presence of terrain defects, particularly, sharp dips. 
There are two principal types of shock absorbers employed on automotive 
vehicles. One of them comprises a single tube or cylinder with a piston in 
the cylinder. A variety of valves and orifices are used for controlling 
flow of hydraulic shock absorber fluid from one end of the cylinder to the 
other end through the piston. A variety of acceleration sensitive 
adjustments have been proposed for this type of shock absorber. 
The other principal type of shock absorber has twin tubes. In this type of 
shock absorber there are concentric tubes with a piston in the inner tube. 
The annulus between the inner and outer tubes serves as a reservoir for 
hydraulic fluid and may serve as a pressure accumulator. Sometimes such 
shock absorbers may be connected to an external pressure accumulator. No 
efforts are known with respect to providing an acceleration sensitive twin 
tube shock absorber. 
It is also desirable that such an acceleration sensitive shock absorber 
have essentially instantaneous changes in characteristics when subjected 
to acceleration in either compression or extension. It is also desirable 
that the shock absorber include means for assuring a change of 
characteristics near the end of the stroke of the shock absorber so that 
the piston does not "bottom" against a hard metal end. 
BRIEF SUMMARY OF THE INVENTION 
There is, therefore, provided in practice of this invention according to a 
presently preferred embodiment an acceleration sensitive shock absorber 
having an outer tube and an inner tube fixed within the outer tube to 
define an annular reservoir between the tubes for shock absorber fluid. A 
piston is sealed within the inner tube and connected to a shaft which 
extends out of the shock absorber. The outer tube is connected to a 
portion of the vehicle, generally the wheel, and the shaft is connected to 
another portion of the vehicle, generally the chassis. The piston divides 
the inner tube into an upper chamber and a lower chamber and means are 
provided for passing shock absorber fluid between the two chambers. A 
normally closed valve is located in the annular reservoir for opening and 
increasing the flow of fluid from one of the chambers into the reservoir 
in the event of a vertical acceleration of the tubes at a rate exceeding a 
preselected magnitude. 
Preferably the valve comprises at least one orifice through the side wall 
of the inner tube and a movable sleeve surrounding the tube in the portion 
containing the orifice for opening the orifice upon acceleration of the 
tubes. 
A row of orifices at differing distances from the end of the tube may be 
used. As the piston passes such an orifice or orifices, flow through the 
orifice is stopped. Thus, when the acceleration exceeds some predetermined 
magnitude, the shock absorber has a soft or low resistance mid-portion of 
its stroke and a stiffer or higher resistance portion at the end before 
the piston strikes the end of the shock absorber.

DETAILED DESCRIPTION 
A representative shock absorber as illustrated semi-schematically in the 
drawings has an outer tube 10 sealed at its lower end by a lower end cap 
11 having a conventional fitting 12 for bolting the end cap to a the 
mounts for vehicle wheel 15. The upper end of the outer tube is sealed by 
an upper end cap 13. An inner tube 14 is also sealed to both the upper and 
lower end caps. This defines an annular fluid reservoir 16 between the 
inner and outer tubes. 
The shock absorber is illustrated in the drawings with the lower end cap at 
the bottom since this is the way of connecting the shock absorber to a 
vehicle. The fitting is bolted to the wheel assembly 15 in a conventional 
manner. 
A movable piston 17 is sealed in the inner tube, dividing its interior into 
an upper chamber 18 and lower chamber 19. The piston is connected to a 
shaft 21 which extends through the upper end cap and terminates in a 
fitting 22 which is used for bolting the shaft to a vehicle chassis 25, 
for example. In the drawings, various seals (such as a seal around the 
shaft), threaded connections and the like are not illustrated since they 
are not required for an understanding of this invention and are 
conventional features well known to those skilled in the art. 
The piston has an orifice 23 for metered fluid flow between the upper and 
lower chambers. In a typical embodiment, the piston also includes a 
disk-type check valve 24 for permitting fluid to flow from the lower 
chamber to the upper chamber, and preventing fluid flow from the upper 
chamber to the lower chamber. The piston and check valve are conventional 
in twin tube shock absorbers, although not all shock absorbers have such a 
check valve or they may use other types of check valves. 
The shock absorber may include three additional check valves to provide 
fluid flow between the annular reservoir between the tubes and the 
chambers in the inner tube. For example, in the upper end cap there may be 
a check valve 26 which permits fluid to flow from the annular reservoir 
into the upper chamber, and prevents flow from the upper chamber into the 
reservoir. 
What is commonly known as a foot valve 27 is provided in the lower end cap. 
The foot valve is a large-area check valve which permits fluid to flow 
from the reservoir into the lower chamber, and prevents flow from the 
lower chamber into the reservoir. In some embodiments of shock absorber, 
the foot valve is replaced by a fluid passage which meters flow in either 
direction between the reservoir and lower chamber. 
An adjustable pressure relief valve 28 is arranged to permit fluid flow 
from the lower chamber into the annular reservoir and prevent reverse 
flow. The pressure relief valve has a rather high opening force, and is 
provided to release fluid displaced as the piston shaft moves downwardly. 
In principle, the shock absorber described to this point is conventional. 
It is merely exemplary of a twin tube shock absorber. Some of these 
features may be deleted in specific embodiments, and other features may be 
present which are commonly found in shock absorbers. For example, an 
external pressure accumulator may be connected to the annular reservoir 
for maintaining pressure in the shock absorber and accommodating fluid 
displaced by the piston. Likewise, a closed-cell foam (not illustrated) 
may be included in the reservoir to act as an internal pressure 
accumulator to accommodate pressure changes as the piston shaft displaces 
fluid. 
The shock absorber responds at zero or low acceleration as a conventional 
shock absorber. Thus, if the vehicle is forced downwardly by cornering or 
braking, for example, causing the piston to move downwardly relative to 
the tubes, fluid flows through the orifice 23 and check valve 24 in the 
piston from the lower chamber to the upper chamber. The pressure relief 
valve 28 opens to release fluid displaced by the shaft 21 entering the 
upper chamber. Restricted flow through the orifice and valves limits the 
rate of compression of the shock absorber. During compression fluid flows 
rather readily through the piston to avoid cavitation in the upper 
chamber. Metering of fluid flow through the pressure relief valve to limit 
movement of fluid displaced by the piston shaft provides the principal 
resistance to compression. 
When the compression is relieved and the vehicle rebounds, fluid flows from 
the reservoir through the foot valve into the lower chamber, and from the 
upper chamber into the lower chamber through the piston orifice. The 
rebound force on the piston comes from the vehicle spring (not shown), and 
in some cases from gas pressure, in a conventional manner. 
In the event the wheel extends from a mid-range position at a very low 
acceleration, such as when the road surface gradually drops away, the 
shock absorber expands in essentially the same way as when the vehicle is 
rebounding. 
Assume, however, that the wheel hits a bump or foreign object in the road. 
This drives the tubes upwardly with rapid acceleration. It is desirable 
under those circumstances to have the shock absorber suddenly become 
"softer" so that the impact of the bump is alleviated and less shock is 
transmitted to the body of the vehicle. For this purpose, a normally 
closed valve 31 opens for increasing the flow of fluid from the lower 
chamber 19 into the annular reservoir. 
This effect is illustrated in the longitudinal cross section of FIG. 2. A 
lower movable sleeve 31 in the reservoir surrounds the inner tube. The 
lower sleeve is supported by a spring 32 which at least supports the 
weight of the sleeve and holds it against a stop ring 33 at the top of the 
sleeve. When the tubes accelerate upwardly, the inertia of the lower 
sleeve leaves it essentially immovable in space, and the tubes move 
upwardly relative to the sleeve until the sleeve reaches the lower end cap 
in a position as illustrated in FIG. 2. 
When the sleeve reaches this position, a row of passages 34 through the 
sleeve align with a row of lower orifices 36 through the side wall of the 
inner tube. This alignment permits fluid to flow from the lower chamber 
into the annular reservoir, thereby permitting the tubes to travel faster 
relative to the piston. This permits the wheel to move quickly relative to 
the vehicle for alleviating the shock transmitted to the vehicle. 
When the wheel acceleration diminishes below a predetermined value, the 
spring causes the lower sleeve to return to its normally closed position 
against the stop, thereby closing the orifice and returning the shock 
absorber to its normally stiffer characteristic. Thus, in the event the 
wheel hits a raised bump or object in the roadway, the rapid acceleration 
causes the shock absorber to temporarily become soft for minimizing the 
shock transmitted to the vehicle body, while under normal road conditions 
the shock is relatively stiff for good handling. 
It may also be noted that the lower orifices 36 through the side wall of 
the inner tube are spaced above the bottom of the tube. This assures that 
in the event of an extreme compression of the shock absorber, its 
characteristic of being soft does not continue for the full stroke; but 
instead, when the piston passes the orifices, the shock absorber again 
becomes stiff before the end of the stroke is reached. In this embodiment, 
three orifices at different distances from the end of the stroke are used 
(with suitable passages through the lower sleeve) so that the shock 
absorber becomes progressively stiffer near the end of the stroke as less 
and less fluid can flow through the orifices. 
It is also desirable that the shock absorber respond with different 
characteristics if a wheel drops away suddenly from the vehicle body. This 
may occur for example, if the wheel encounters a chuckhole, or it may 
occur immediately following shock absorber compression as the wheel passes 
over a bump or object in the road. It can be quite desirable under those 
conditions for the wheel to rapidly move toward the road surface with 
minimum inhibition by a stiff shock absorber since this enhances road 
traction. It is undesirable that the shock of such rapid wheel movement be 
transmitted to the vehicle, passengers or load. 
Normally closed valve means are also provided for changing the stiffness of 
the shock absorber upon rapid downward acceleration of the tubes. For this 
purpose, a movable upper sleeve 41 surrounds the inner tube near its upper 
end. A significant portion of the weight of the upper sleeve is supported 
by a low spring rate coil spring 42. During normal use of the shock 
absorber on a reasonably smooth road, the upper sleeve rests on a stop 
sleeve 43 inside the coil spring. The upper sleeve includes a radial fluid 
passage 44. 
In the event of rapid downward acceleration of the shock absorber tubes at 
more than a predetermined magnitude, the upper sleeve essentially remains 
fixed in space due to its inertia, and the tubes move away from the sleeve 
until the sleeve reaches the upper end cap as illustrated in FIG. 3. In 
this position, the passage through the upper sleeve aligns with an upper 
orifice 46 through the side wall of the inner tube near its upper end. 
This permits fluid to flow rapidly from the upper chamber 18 into the 
annular reservoir between the tubes. This relatively large orifice permits 
rapid extension of the shock absorber for outstanding rebound 
characteristics and accommodation of chuckholes, or the like, without 
transmitting significant shock loading to the vehicle. 
The spring 42 for the upper sleeve is optional. By supporting a significant 
portion of the weight of the sleeve, the sensitivity of the shock absorber 
to low magnitudes of acceleration is improved. It is found, however, that 
the upper sleeve valving for the extension stroke of the shock absorber 
works without a spring. The spring 32 supporting the lower sleeve for 
compression valving is important for maintaining the sleeve against the 
stop. 
The upper orifice is also spaced apart from the upper end cap so that the 
piston passes the upper orifice before the piston reaches the end of its 
stroke, thereby, increasing the stiffness near the end of the stroke. The 
distance of the orifice from the end of the stroke for extension can be 
appreciably less than for compression, since the violence of extension 
tends to be less than that of compression. 
It is often desirable during rebound or extension for the shock absorber to 
remain soft after acceleration has decreased below the magnitude that 
caused the orifice valve to open. The passages 44 through the upper sleeve 
are therefore angled downwardly and outwardly so that fluid flowing from 
the upper chamber into the reservoir is deflected downwardly and the 
sleeve is, therefore, biased upwardly. This tends to hold the orifice 
valve open after the acceleration has decreased beyond the magnitude that 
originally opens the valve and the valve remains open while fluid flow 
continues during shock absorber extension. Such an arrangement also 
assists in preventing "chatter" when the orifice is only partly open. 
Providing orifices through the side wall of the inner tube in positions 
where they may be closed by passage of the piston provides the shock 
absorber with the capability of being soft (under appropriate 
accelerations), in a mid-portion of the stroke and stiff near each end of 
the stroke, this capability minimizes the shock that may be transmitted to 
the vehicle upon "bottoming" of a shock absorber. This capability is not 
readily available in shock absorbers which have variable characteristics 
due to valving mounted in the piston. 
The actual characteristics desired in the shock absorber vary considerably 
depending on the application of the shock absorber. Thus, it will be 
apparent that different characteristics would be employed for shock 
absorbers used on motorcycles, light automobiles, heavy automobiles, light 
trucks and heavy trucks. There are a large number of design and 
dimensional changes which can be made in the shock absorber to accommodate 
these differing applications. Some of these are conventionally controlled 
in shock absorbers, such as orifice dimensions, check valve dimensions, 
check valve opening forces, piston diameter, piston stroke and the like. 
In addition, in an acceleration sensitive shock absorber, one may vary the 
normally closed orifice sizes and locations for changing the 
characteristics of the shock absorber. The acceleration at which the 
characteristics change can be varied by changes in the weight of the 
sliding sleeves and the spring constants of the springs that support a 
significant portion of the weight of the sleeve. In fact, if desired, the 
characteristics of the shock absorber may be changed after manufacture by 
providing means for adjusting springs or orifice sizes from outside the 
shock absorber. 
It might be noted that the mass of the acceleration sensitive valve in a 
shock absorber as described herein can be appreciably greater than where 
the acceleration sensitive valve is located in the piston. In this 
arrangement, the sleeve can be relatively thick and long to obtain a 
desired mass, since there is ample space in the annular reservoir for 
accommodating the sleeve. Space is much more limited in the piston. Thus, 
any desired sensitivity may be obtained. 
The sensitivity may be affected by fluid drag on the movable sleeves, thus 
streamlining in the form of rounded ends 47 may be provided on one or more 
of the sleeves for minimizing this effect. 
An exemplary automotive shock absorber has a tube length in the order of 25 
cm and a diameter of about 5 cm. The diameter of the inner tube is about 3 
cm. In such an embodiment, a typical lower orifice comprises two rows of 
three drilled holes about 1.5 millimeters in diameter. The upper orifice 
is appreciably larger, and the collective area of several holes drilled 
through the side wall of the inner tube is equivalent to the cross section 
of a round hole about 12 millimeters in diameter. Several holes about 2.5 
millimeter diameter are typically used for virtually unrestricted fluid 
flow upon rapid acceleration. Larger orifices give a softer shock absorber 
and smaller holes give a stiffer shock absorber. 
The clearance between the sleeves and the inner tube should be sufficient 
that fluid shear in the clearance does not substantially retard the 
relative displacement between the tube and sleeve, yet the clearance must 
be small enough that the orifices are closed when the sleeve is over them. 
A clearance of about 130 microns appears to be sufficient. The sleeve 
should also be sufficiently long relative to the diameter of the tube that 
it will not cock and can move freely. 
It will be recognized that the shock absorber, illustrated herein, is 
semi-schematic and that some features are omitted for clarity. For 
example, the piston seal is omitted. A circumferential recess in the 
sleeve adjacent to the passages is also not shown. This is used as a fluid 
passage so that it is not necessary to keep the sleeve passages aligned 
with the orifices through the tube wall when the respective valves are 
open. 
It will also be apparent that the actual construction may differ from the 
illustration. For example, stop rings for the sleeves may not be practical 
in small size shock absorbers, since the wall thickness of the inner tube 
may be too thin for a groove to receive the stop ring. Thus, the stop ring 
33 and sleeve 43 may be in the form of shoulders on the outside of a 
machined tube. 
It will also be apparent that there may be many modifications, variations 
and embellishments of the shock absorber. As suggested above, some of the 
check valves may be omitted or replaced by passages in specific 
embodiments. The shape of the orifices through the side wall of the inner 
tube and the corresponding passages through the sleeves may be varied or 
chamfers provided so that the change between stiff and soft 
characteristics of the shock absorber change at a controlled rate. 
The interior of each sleeve may be provided with a circumferential groove 
so that the passages need not be aligned with the orifices through the 
wall of the tube. Instead of passages through the sleeves, the inside of 
the sleeve may be enlarged or counterbored nearer one end so that fluid 
can flow longitudinally between the sleeve and inner tube wall when the 
orifices are not covered by the closer fitting portion of the sleeve. In 
effect, this is a passage through the sleeve, but discharging at an end of 
the sleeve. Such a counterbore can also serve to direct fluid flow in a 
direction that keeps the normally closed valve in its open position. 
If desired, different degrees of stiffness may be provided for different 
acceleration rates. It may be desirable to have a somewhat soft shock 
absorber for mild accelerations and a considerably softer shock absorber 
for severe accelerations. Such performance may be achieved by employing a 
pair of concentric sleeves around the inner tube. One of the sleeves, has 
a suitable mass and spring so that it displaces a sufficient distance to 
open one set of orifices at a relatively lower magnitude of acceleration. 
The other sleeve has a sufficient mass and spring constant for opening 
another orifice at a higher acceleration. Thus, at low acceleration, fluid 
may flow through one orifice and at higher accelerations, may flow through 
both orifices for a softer effect. 
Since there are many such modifications and variations, which will be 
apparent to those skilled in the art, it is to be understood that the 
invention may be practiced, otherwise than as specification described.