Vehicle seat and shock-absorbing material

A vehicle seat reduces the amount of vibrations from the vehicles engine or riding surface felt by the rider, without sacrificing the comfort and fitness of the seat. The seat desirably includes a shock-absorbing member layered with a urethane cushion member. The shock-absorbing material comprises a viscoelastic material using a super-soft urethane elastomer as a matrix resin with a low density filler of resinous microballoons. The percent weight ratio between the resinous microballoons and the resin matrix advantageously ranges from 1% to 5%. In order to provide adequate damping without presenting too hard of a feel, the viscoelastic material desirable has a normal storage modulus (E.sub.1) ranging between 0.0628 MPa and 0.234 MPa, and a normal loss modulus (E.sub.2) ranging between 0.0171 MPa and 0.131 MPa.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to shock-absorbing material, and more 
particularly to shock-absorbing material comprising a viscoelastic 
urethane elastomer which is used in a vehicle seat. 
2. Description of Related Art 
Vehicle seats, especially motorcycle vehicle seats, are generally 
constructed of a urethane foam cushion covered with a synthetic seat skin 
and secured on a bottom seat plate. Vibrations from the motorcycle 
generated by its engine, or by irregularities of a road surface, are 
transmitted to the motorcycle seat during operation. The urethane cushion 
in the seat lessens the amount of vibrations transmitted to the rider, 
especially to the rider's buttocks. 
Although the urethane cushion softens the ride, urethane foam typically 
does not meaningfully isolate the rider from road and engine vibrations. 
Prior seats therefore have included a shock-absorbing member of a 
viscoelastic urethane elastomer together with a urethane cushion to 
provide even greater vibration absorption in the seat. The shock-absorbing 
member has a greater hardness and spring constant than the urethane 
cushion in order to provide greater vibration absorption. 
Although the shock-absorbing member sufficiently reduces vibrations in the 
seat, the increased hardness of the shock-absorbing member creates a hard 
and uncomfortable seat, especially when riding for long periods of time. 
An abrupt and uncomfortable stiffness change also is felt at the boundary 
between the harder shock-absorbing member and the softer urethane cushion. 
Shock-absorbing members made of a super-soft urethane elastomer have been 
incorporated into seats in order to provide the necessary amount of shock 
absorption without an uncomfortable level of hardness. However, 
shock-absorbing members constructed of this type of material have such 
high adhesive properties that they are difficult to handle during seat 
assembly. 
SUMMARY OF THE INVENTION 
A need therefore exists for a shock-absorbing material which reduces the 
amount of vibrations felt by the rider through the seat without 
sacrificing the comfort and fitness of the seat. 
The present invention includes the recognition that the viscoelastic 
material of the shock-absorber member in the seat should have a sufficient 
spring constant that it absorbs and reduces vibrations transmitted to the 
rider. The viscoelastic material, however, should have a normal storage 
modulus and a normal loss modulus selected to provide the seat with a 
degree of comfort that prior vehicle seats have failed to achieve. 
Thus, in accordance with an aspect of the present invention, an improved 
vehicle seat is provided. The vehicle seat comprises a seat base and a 
seat cushion arrangement positioned on the seat base. The seat cushion 
arrangement includes a cushion member and a shock-absorbing member. The 
seat skin surface surrounds the cushion arrangement. The shock-absorbing 
member comprises a viscoelastic material which includes a urethane 
elastomer matrix containing resinous microballoons. 
An additional aspect of the present invention involves a shock-absorbing 
material comprises a viscoelastic urethane elastomer resin matrix 
containing a plurality of resinous microballoons. In a preferred 
embodiment, the shock-absorbing material has a normal storage modulus 
which is not less than 0.0628 MPa and is not greater than 0.234 MPa. A 
normal loss modulus of the material desirably is not less than 0.0171 MPa 
and is not greater than 0.131 MPa. A percentage weight ratio of the 
resinous microballoons to the viscoelastic urethane elastomer matrix 
advantageously is not less than 1% and is not greater than 5%.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 illustrates a vehicle seat which is configured in accordance with 
the preferred embodiment of the present invention. The seat is applied to 
a motorcycle body; however, it is understood that the present vehicle seat 
can be used with other types of vehicles, such as, for example, personal 
watercrafts, snowmobiles, all-terrain vehicles, bicycles and the like. 
The seat includes a shock-absorbing member comprising a novel viscoelastic 
material which is easy to handle and which effectively dampens vibrations 
without significantly diminishing the comfort of the seat. As used herein, 
"viscoelastic" means a material which has both viscous and elastic 
properties. The present viscoelastic material is particularly well suited 
for use with the vehicle seat. Those skilled in the art, however, will 
appreciate that the present viscoelastic material can be used in other 
applications. For instance, a hand grip, a foot pad, a back rest or the 
like can include a shock-absorbing member formed of the present 
viscoelastic material. 
With reference to FIG. 1, a motorcycle 50, to which the vehicle seat is 
attached, includes frame 52. At the front of the frame 52 a fuel tank 54 
is provided and at the rear of the frame 52 the motorcycle seat 56 is 
supported by a seat frame 58. Also at the rear of the motorcycle body 
frame 52, a rear wheel 60 is connected to the body frame 52 by a 
shock-absorber 62 and by a rear swing arm (not shown) extending from the 
frame 52. 
In the illustrated embodiment, the seat 56 is a tandem-type seat with a 
front seat portion 64 on which an operating rider sits in a straddle 
fashion. A rear seat portion 66, or tandem seat, is provided at the rear 
of the seat 56 on which a passenger rider sits in a similar fashion. The 
seat of the present invention is not limited to a tandem-type seat and it 
will be apparent to those skilled in the art that other types of seat 
designs for any number of riders can incorporate some or all of the 
aspects of the present invention. 
With reference to FIGS. 1 and 2, the seat 56 includes a seat base 68 that 
is mounted on the seat frame 58 of the motorcycle 50. The seat base 68 
desirably has a rigid construction and is made of plastic or sheet metal. 
An elastic cushion member 70 of urethane foam is supported by the seat 
base 68. A viscoelastic member 72, or shock-absorbing member, is placed 
between the seat base 68 and cushion member 70. The shock-absorbing member 
72 and the cushion member 70 form a seat cushion or seat cushion 
arrangement 74. Although in the illustrated embodiment the seat cushion 74 
includes the cushion member 70 and shock-absorbing member 72 arranged 
together in layers, other arrangements of the seat cushion are possible 
while incorporating the advantages of the present invention. Several 
additional vehicle seat designs are described below. The seat cushion 74 
is covered with a seat skin 76 desirably constructed of a synthetic 
material, such as, for example, PVC leather, and a sheet of wooly nylon as 
a lining. The seat skin 76 is affixed to the peripheral portion of the 
seat base 68 in order to hold the seat cushion arrangement 74 together. 
The shock-absorbing member 72 of the vehicle seat 56 desirably includes a 
novel viscoelastic material, as mentioned above. The present viscoelastic 
material uses a urethane elastomer as the matrix resin of the foam 
material and contains a low density filler to increase its viscosity. The 
urethane elastomer advantageously has an Ascar C hardness of not more than 
15 measured by a durometer for soft materials, such as that available 
commercially from Kobunshi Keiki in K.K., Japan. 
Any one of a number of super-soft urethane elastomers (i.e., urethane 
elastomers which have an Ascar C hardness of not more than 15) can be used 
as the matrix resin of the present viscoelastic material. For instance, 
the super-soft viscoelastic urethane elastomer can comprise a mixture of: 
a trifunctional polyol and difunctional polyol mixture (polyether polyol 
and polyester polyol); trially isocyanurate; MDI 
(diphenylmethane-4,4'-diisocyanate)/TDI (tolyene diisocyanate) 
pre-polymer; Bi-catalyst; Sn-catalyst; and phthalic acid plasticizer. 
Other viscoelastic urethane elastomers, such as, for example, the urethane 
resin disclosed in Japanese patent publication Hei 3-17848 (Applicant: 
IIDA Sangyo K.K.), the super soft elastomer composition disclosed in 
Japanese unexamined patent publication Hei 3-3217 (Applicant: K.K. Nippon 
Automation, et al.), and the viscoelastic urethane elastomer matrix resin 
sold under the trade name ELASCOAT and made by Polyurethane Kasei K.K., 
can be used as the matrix resin of the present viscoelastic material. 
The low density filler mixed in the matrix resin desirably comprises a 
plurality of resinous microballoons. Each resinous microballoon includes a 
vinylidenechloride resin (vinylidenechloride acrylonitrile copolymer) 
shell (average grain size of 40-60 micrometers, grain diameter ranging 
from 10-100 micrometers) filled with butane gas. Such resinous 
microballoons are commercially available under the trade name EXPANCEL DE 
manufactured by EXPANCELL AB of Sweden. 
The resinous microballoons desirably are mixed with the super-soft urethane 
elastomer in a sufficient quantity such that the resinous microballoons 
constitute between 1% and 5% of the weight of the super-soft urethane 
elastomer. It has been found that if the mixing ratio (measured as a 
percentage of weight) of resinous microballoons to super-soft urethane 
elastomer is less than 1%, the mixture will separate into two layers after 
molding the viscoelastic material. If the mixing ratio is greater than 5% 
(as measured as a percentage of weight) the mixture will include an 
excessive amount of air due to the high viscosity of the material during 
molding. The ratio of microballoons to the super-soft urethane elastomer 
therefore should be between 1% and 5% by weight. 
The present viscoelastic material including the desired ratio of resinous 
microballoons to super-soft urethane elastomer advantageously has a lower 
adhesive property than super-soft urethane elastomer itself. It therefore 
is easier to handle during the seat assembly process. The material also is 
lightweight (e.g., has a specific gravity equal to about 0.62) and has a 
small impact resilience factor compared to other viscoelastic materials. 
For instance, the present viscoelastic material can have an impact 
resilience factor equal to about 8%. The present viscoelastic material 
also experiences minimal changes in hardness with a given change in 
temperature as compared with other viscoelastic materials. 
With an increasing percentage of resinous microballoons in the viscoelastic 
material, a loss factor of the material increases. The loss factor of the 
material represents the ratio between a normal loss modulus (E.sub.2) of 
the material and a storage modulus (E.sub.1) of the material. As used 
herein, "loss modulus" means the intrinsic property of the material which 
is proportional to the energy loss under a given deflection. "Storage 
modulus" means the intrinsic property of the material which is 
proportional to elastic deformation of the material. FIG. 3 illustrates 
the relationship between the loss factor and the weight percentage of the 
microballoons relative to the matrix resin. The loss factor of the 
viscoelastic material increases in value with an increase in the 
percentage of the microballoons by weight in the viscoelastic material. 
Experiments have shown that a vibration transmission factor in resonance of 
the viscoelastic material decreases with an increase in the loss factor of 
the material. As used herein, the "vibrational transmission factor in 
resonance" means a factor or constant that is proportional to the 
percentage of vibrational energy transmitted through the viscoelastic 
material when vibrated at a resonant frequency. The presence of the 
resinous microballoons in the matrix resin--which provides the present 
viscoelastic material with a larger loss factor than super-soft urethane 
elastomer by itself--consequently reduces the vibration transmission 
factor of the present viscoelastic material. The following test results 
illustrate this point. 
Vibrational tests were conducted to compare the vibration transmission 
characteristics of a cushion with the present viscoelastic material with 
resinous microballoon against a cushion made of only urethane foam. 
Vibration transmission factors were compared. The viscoelastic material 
used in the tests was a viscoelastic urethane elastomer (sold under the 
trade name Orotex U 1000, or U 1003, made by IIDA SANGYO K.K.) and 
included 3% resinous microballoon (sold under the trade name EXPANCEL DE 
made by EXPANCEL AB of Sweden). 
The vibrational tests were run for the present vehicle seat which included 
the shock-absorbing member made of the present viscoelastic material. FIG. 
4 illustrates the configuration of the present vehicle seat used in the 
vibrational tests. Each seat was coupled to a vibration testing machine 
and a rider sat on the seat in the manner shown in FIG. 5. 
A sinusoidal vibration of 1-10 Hz (.+-.3 mm amplitude in the vertical 
direction) was applied to the seats to simulate vibrations experienced by 
a rider in the seats under actual road conditions. Acceleration was 
measured by accelerometers placed below the seat and between the seat and 
the rider's buttocks. A vibrational transmission factor was obtained by 
calculating the ratio of the acceleration values measured. 
The graph in FIG. 6 shows the results of the vibrational testing. The 
vibration transmission factor of the seats of the present invention is 
reduced by 36% at resonant frequency and by 13% at approximately 6 Hz as 
compared to a seat made of urethane foam only. Because the resonant 
frequency of a human rider's abdomen is approximately between 4-8 Hz, the 
seat of the present invention also dampens resonance in a rider's abdomen. 
The combination of the super-soft urethane elastomer and the resinous 
microballoons also give the present viscoelastic material a reduced 
hardness. This is because the intrinsic normal storage modulus (E.sub.1) 
and normal loss modulus (E.sub.2) of the viscoelastic material 
advantageously fall within desired ranges for these properties. 
Through empirical work, it was determined that in order to optimize the 
comfort of the seat while effectively damping the transmission of road and 
engine vibrations through the seat, the viscoelastic material of the 
shock-absorbing member in the seat should have a normal storage modulus 
(E.sub.1) and a normal loss modulus (E.sub.2) falling within the following 
ranges: 
EQU 0.0628 MPa&lt;E.sub.1 &lt;0.234 MPa 
EQU 0.0171 MPa&lt;E.sub.2 &lt;0.131 MPa 
As used herein, the unit MPa (i.e., Mega-Pascal) means 0.101972 kg.sub.f 
/mm.sup.2. The following elaborates on the empirical tests conducted to 
determine the desired ranges of the normal storage modulus (E.sub.1) and 
the normal loss modulus (E.sub.2) of the viscoelastic material for 
optimizing vibration absorption (i.e., damping) and comfort in the vehicle 
seat, especially for riding for long durations of time. 
Values for storage spring constant, viscosity coefficient, apparent (i.e., 
empirically measured) normal storage modulus (G.sub.1) and apparent (i.e., 
empirically measured) normal loss modulus (G.sub.2) were determined in 
accordance with "Test 1" of Japanese Industrial Standard (JIS) K 6394-1976 
for various viscoelastic materials identified as specimens A through G. 
The results are shown in Table 1 below. 
TABLE 1 
______________________________________ 
Test 1 
Apparent 
Apparent 
Normal Normal 
Storage Storage Loss 
Spring Viscosity Modulus Modulus 
Constant Coefficient 
(G.sub.1) 
(G.sub.2) 
Specimens 
N/mm N - sec/mm MPa MPa 
______________________________________ 
A 41.2 0.147 0.105 0.016 
B 85.9 0.824 0.219 0.092 
C 270 2.58 0.687 0.288 
D 224 2.32 0.570 0.260 
E 386 6.34 0.984 0.710 
F 588 7.78 1.50 0.870 
G 349 6.09 0.889 0.682 
______________________________________ 
Values for rebound angle (H) and transmitted acceleration (a) also were 
evaluated for each of the viscoelastic materials (specimens A-G) in "Test 
2" and the results are shown in the Table 2 below. 
TABLE 2 
______________________________________ 
Test 2 
Rebound Transmitted 
Angle Acceleration 
(H) (a) 
Specimens C..degree. 
G 
______________________________________ 
A 11.9 5.82 
B 8.1 11.1 
C 5.4 20.2 
D 4.0 22.5 
E 1.4 33.5 
F 1.2 35.0 
G 3.4 24.3 
______________________________________ 
Test 2 involved the following experiments; however, it is understood that 
the rebound angle (H) and the transmitted acceleration (A) can be 
determined in a variety of different ways which will be readily apparent 
to those skilled in the art. 
The rebound angle (H) and the transmitted acceleration (a) were determined 
using the test device 78 shown in FIG. 7. The test device 78 included a 
base 80 and a swing arm 82 that was pivotally mounted to the base 80 by a 
pivot shaft 84. The construction of the test device allowed for 
up-and-down swinging movement of a metal weight 86 mounted at the end of 
the swing arm 82. The swing arm 82 had a length of 30 mm measured from the 
pivot shaft 84 to the weight 86. 
The test device 78 included at least two weights 86. The weights 86 had 
different shapes. One weight 86 had a semi-spherical shape with a diameter 
of 30 mm, and the other weight 86 had a disc shape with a diameter of 50 
mm. Both weights 86, however, had the same mass (M): 200 g. The weights 86 
removably attached to the swing arm 82 so as to be interchanged. 
A support 90 of the test device 78 supported a test piece 89 of each 
specimen under the weight 86 and over a load cell 88. When properly 
positioned on the support 90, the upper surface of the test piece 89 rests 
in the same horizontal plane 92 as the pivot axis of the pivot shaft 84. 
Each viscoelastic test piece 89 (specimens A through G) had a diameter of 
about 100 mm and a thickness of about 20 mm. The test piece also had a 
measurable mass (m). 
The load cell 88 was provided between the base 80 and the support 90. The 
load cell sensed the percussive load applied by the weight 86 and 
transmitted through the test piece 89 when the weight 86 struck the 
viscoelastic test piece 89, as described below. 
When measuring rebound angle (H) of the viscoelastic test piece 89, the 
viscoelastic test piece 89 was provided on the support 90 and the 
semi-spherical weight 86 was attached to the swing arm 82. The swing arm 
82 was then pivoted upward to an angle of .alpha.=30.degree. with the 
imaginary horizontal plane 92. From this position, the weight 86 was 
dropped freely and struck the viscoelastic test piece 89. A laser 
displacement meter 94 provided on the base 80 measured the maximum rebound 
angle (H) of the swing arm 82 after rebounding from striking the test 
piece 92. 
Measuring the transmitted acceleration (a) of the viscoelastic material was 
performed in a similar manner using the test device 78. Each viscoelastic 
test piece 89 (specimens A through G) was placed on the support 90 and the 
disc weight 86 was attached to the end of the swing arm 82. The swing arm 
82 was again pivoted upward until it reached an angle of 
.alpha.=30.degree. with the horizontal line 92. From this position, the 
weight 86 was dropped freely and struck the viscoelastic test piece 89. 
The load cell 88 measured the applied percussive force transmitted through 
the test piece 89 and generated an output signal (F.sub.o) indicative of 
the load. FIG. 8 illustrates a graph of the output signal from the load 
cell 88 over time. The transmitted acceleration (a) for each test piece 89 
of viscoelastic material (specimens A through G) was calculated from the 
following transmitted acceleration equation, knowing the applied load 
(F.sub.o), the mass of the weight (M), and the mass of the test piece (m): 
EQU A=F.sub.o (M+m) 
A sensory evaluation test for several of the viscoelastic specimens (A 
through E) also was made to evaluate a rider's comfort in terms of 
hardness and vibration damping. Table 3 sets forth the test results. 
TABLE 3 
______________________________________ 
Riding Test 
Specimens 
Evaluation 
______________________________________ 
A Poor 
B Fair 
C Good 
D Good 
E Good 
______________________________________ 
The relationship between the apparent normal loss modulus (G2) for the 
viscoelastic test pieces (specimens A-E) in Table 1 and the rebound angle 
(H) for the same viscoelastic test pieces (specimens A-E) from Table 2 is 
plotted on the graph illustrated in FIG. 9, with the X-axis on a natural 
log scale. The graph illustrates that the rebound angle (H) of the 
viscoelastic material is a function of the apparent normal loss modulus 
(G.sub.2) of the material. This relationship can be expressed by the 
following equation derived from the linear fit of the plotted points: 
EQU H=1.097-2.508 (natural log of G.sub.2) (1) 
Taken together, Table 3 and the graph illustrated in FIG. 9 reveal that in 
order to provide a fairly comfortable ride, the rebound angle (H) 
desirably is no greater than 8.1.degree.. This was determined because 
specimen B provided fair comfort to the rider and specimens C through E 
provided good comfort. Specimen B has a measured rebound angle of 
8.1.degree.. 
An apparent normal loss modulus (G.sub.2) of 0.061 MPa is obtained by 
inserting the rebound angle (H) of 8.1.degree. into Equation 1. It 
therefore can be deduced that when the apparent normal loss modulus 
(G.sub.2) of the viscoelastic material is greater than 0.061 MPa, i.e., 
EQU (G.sub.2)&gt;0.061 MPa, (2) 
the viscoelastic material sufficiently dampens vibrations and shocks. 
Further sensory evaluations were conducted on specimens B through G in 
order to determine the relationship between the apparent normal storage 
modulus (G.sub.1) for the viscoelastic test pieces (specimens B-G), see 
Table 2, and the comfort of the ride produced by vehicle seats including 
shock-absorbing members made of the various specimens B-G. That is, 
viscoelastic test pieces (B-G) were provided in respective cushions and 
tests were conducted for determining sensory evaluations in a rider's 
buttocks while riding on the seat. The test results are shown in the Table 
4. 
TABLE 4 
______________________________________ 
Riding Test 
Specimens 
Evaluation 
______________________________________ 
B Good 
C Good 
D Good 
E Poor 
F Poor 
G Fair 
______________________________________ 
The relationship between the apparent normal storage modulus (G.sub.1) for 
the viscoelastic test pieces (B-G), reported in Table 2, and the 
transmitted acceleration (a) for the same viscoelastic test pieces (B-G), 
as reported in Table 4, is plotted on the graph shown in FIG. 10, on a 
natural log scale. The graph of FIG. 10 illustrates that the transmitted 
acceleration (a) through the viscoelastic material is a function of the 
apparent normal storage modulus (G.sub.1) of the material. This 
relationship can be expressed by the following equation derived from a 
linear fit of the plotted points: 
EQU A=27.47.times.G.sub.1.sup.0.677 (3) 
Taken together, Table 4 and the graph illustrated in FIG. 10 reveal that in 
order to provide a fairly comfortable ride, the transmitted acceleration 
(a) desirably is not greater than 24.3 G. This was determined because 
specimen G provided fair comfort to the rider and specimens B through D 
provided good comfort. Specimen G has a measured transmitted acceleration 
of 24.3 G. 
An apparent normal storage modulus (G.sub.1) of 0.834 MPa is obtained by 
inserting a transmitted acceleration (a) of 24.3 G into Equation 3. It 
therefore can be deduced that when the apparent normal storage modulus 
(G.sub.1) of the viscoelastic material is less than 0.843 MPa, i.e., 
EQU (G.sub.2)&lt;0.834 MPa, (4) 
the viscoelastic material sufficiently dampens vibrations without overly 
stiffening the seat and causing the rider's buttocks to ache, especially 
when riding for long durations of time. 
The relationship between apparent normal storage modulus (G.sub.1) and the 
apparent normal loss modulus (G.sub.2) of the sample viscoelastic 
materials can be derived by plotting the apparent normal loss modulus 
(G.sub.2) of the materials against the apparent normal storage modulus 
(G.sub.1) of the materials on a natural log scale. The graph shown in FIG. 
11 illustrates this relationship which can be expressed by the following 
equation derived from a linear fit of the plotted points: 
EQU G.sub.2 =0.621.times.G.sub.1.sup.1.54 (5) 
After determining the lower end of the desirable range for the apparent 
normal loss modulus (G.sub.2), Equation 5 can be solved to determine the 
lower end of the desirable range for the apparent normal storage modulus 
(G.sub.1) . The apparent normal storage modulus (G.sub.1) equals 0.222 
MPa, where the apparent normal loss modulus (G.sub.2) equals 0.061 MPa. 
The desired range for the apparent normal storage modulus thus can be 
expressed as: 
EQU 0.222 MPa&lt;G.sub.1 &lt;0.834 MPa (6) 
Likewise, after determining the upper end of the desirable range of the 
apparent normal storage modulus (G.sub.1), Equation 5 can be solved to 
determine the upper end of the desirable range for the apparent normal 
loss modulus (G.sub.2). The apparent normal loss modulus (G.sub.2) equals 
0.469 MPa, where the apparent normal storage modulus (G.sub.1) equals 
0.834 MPa. The desired range for the apparent normal storage modulus thus 
can be expressed as: 
EQU 0.061 MPa&lt;G.sub.2 &lt;0.469 MPa (7) 
The measured apparent normal loss modulus (G.sub.2) and measured apparent 
normal storage modulus (G.sub.1) are dependent upon the shape of the test 
specimens. In order to derive the intrinsic properties of the material 
itself, the apparent moduli are normalized to account for the shape of the 
specimens. That is, the normal storage modulus (E.sub.1) and normal loss 
modulus (E.sub.2)--which are not shape dependent--are determined from the 
apparent moduli; i.e., the extrinsic quantity measured from the specific 
cylindrically shaped specimen. 
The relationship between the normal storage modulus (E.sub.1), the normal 
loss modulus (E.sub.2) and the shape factor (S) of the cylindrical test 
piece are shown below in Equations (8) and (9). 
EQU E.sub.1 =G.sub.1 /(1+1.645 S.sup.2) (8) 
EQU E.sub.2 =G.sub.2 /(1+1.645 S.sup.2) (9) 
The shape factor (S) is derived from the following equation: 
EQU S=d/4h 
where d is the diameter of the rubber section (D=100 mm) and h is the 
thickness of the test piece (h=20 mm). Thus, Equation (10) is obtained by 
inserting these values into the above equation: 
EQU S=1.25 (10) 
If Equation (10) is substituted into Equations (8) and (9), and Equations 
(8) and (9) are substituted into Equations (6) and (7), the following 
equations are obtained. 
EQU 0.0628 MPa&lt;E.sub.1 &lt;0.234 MPa (11) 
EQU 0.0171 MPa&lt;E.sub.2 0.131 MPa (12) 
The present viscoelastic material therefore desirably has a normal loss 
modulus (E.sub.2) and a normal storage modulus (E.sub.1) falling within 
the above recited ranges. The viscoelastic material with the desired 
moduli adequately absorbs road and engine vibrations while providing a 
comfortable ride to the rider, even over an extended riding period. The 
present vehicle seat with the novel viscoelastic material therefore 
reduces discomfort to and aching of the rider's buttocks due to extending 
riding periods. 
The combination of the present viscoelastic material--which includes a 
mixture of super-soft urethane elastomer and resinous microballoons--and a 
conventional urethane foam cushion member also provides a good feeling of 
fitness. As used herein, "fitness" means the degree to which the seat 
receives the rider's buttocks as a whole and the degree of deformation of 
the seat. A good fitness is considered to be where the seat firmly 
receives the rider's buttocks as a whole without large deformation, and 
comes into contact mildly with the tailbone of the rider, which typically 
protrudes with the rider positioned in the riding posture illustrated in 
FIG. 5. 
In order to quantify the comfort and fitness characteristics of the present 
vehicle seat, a number of tests on seats having shock absorbing members 
made of the following shock-absorbing materials were conducted: 
TABLE 5 
______________________________________ 
Specimens 
Material Description 
______________________________________ 
H Viscoelastic urethane elastomer matrix resin 
(OROTEX .TM.) mixed with resinous microballoons 
(EXPANCEL DE .TM.), 3% by weight 
I Conventional urethane foam 
J Urethane Foam 1 with low resilience factor 
(ZULEN 1 .TM.) 
K Urethane Foam 2 with low resilience factor 
(ZULEN 2 .TM.) 
L Urethane Foam 3 with low resilience factor 
(ZULEN 3 .TM.) 
M Shock-absorbing urethane foam (Poron .TM.) 
N Shock-absorbing urethane foam (Orotex U 
1003 .TM.) 
O Shock-absorbing urethane foam (SORBOTHANE .TM.) 
______________________________________ 
These tests determined a bench evaluation value F for the feeling of 
fitness, a spring constant ratio p between tension and compression, and 
the normal storage modulus measured in accordance with Japanese Industrial 
Standard K6394. Table 6 sets forth the results of these tests. 
TABLE 6 
______________________________________ 
Normal storage 
Specimens 
p value F value modulus (Mpa)* 
______________________________________ 
H 1.30 57.3 0.102 
I 0.66 15.7 0.0361 
J 1.21 19.8 0.493 
K 1.21 22.8 0.224 
L 1.11 23.1 0.0577 
M 0.964 16.2 
N 1.18 43.1 0.150 
O 1.29 67.4 0.378 
______________________________________ 
*The normal storage modulus is measured according to Japanese Industrial 
Standard K6394 for samples of 100 mm diameter and 20 mm thickness. 
&lt;Test condition 
Average strain 10% 
Strain amplitude 1% 
Note: 
1 Mpa in the table is equal to 0.101972 kgf/mm.sup.2. 
The bench evaluation value F for the feeling of fitness is the ratio 
between the forces required to deform the test piece by 5 mm first using a 
concentrated force and then using a force applied over a larger area. The 
following describes the specific procedures involved with this test. 
A tension and compression tester (shown in FIG. 12), which is available 
commercially under the trade name AGS 500A Autograph and manufactured by 
SHIMADZU CORP., was used to test the shock-absorbing material specimens 
for load values. A specimen was first loaded into the tester. Each 
specimen had a diameter of 100 mm and a thickness of 20 mm. 
Load values were measured for each sample. Each sample was first loaded 
into a pressure jig A, shown in FIG. 13, and a load value X was measured 
when deformation of the sample reached 5 mm. Each sample was then loaded 
into a pressure jig B, shown in FIG. 14, and a load value Y was measured 
when the deformation reached 5 mm. The bench evaluation test value (F) of 
the feeling of fitness for the seat was calculated by inserting the 
measured values into the following equation: 
EQU F=Y/X. (13) 
Using the same tension and compression tester shown in FIG. 12, each of the 
test samples was bonded between jigs C and D, shown in FIG. 15. The jigs C 
and D were also 30 mm in diameter. The spring constant (X.sub.1) of each 
sample was measured when compressed by 2 mm. The spring constant (X.sub.2) 
for each sample was measured when stretched by 2 mm. The ratio (P) of the 
spring constant in compression to the spring constant in tension was 
calculated by inserting the measured spring constants into the following 
equation: 
EQU P=X.sub.1 /X.sub.2. (14) 
Sensory evaluation tests of the seats using the same shock-absorbing 
materials as the bench tests described above were performed and fitness 
and shock absorption ratings were determined from the tests. Table 7 sets 
forth the results of these tests. 
TABLE 7 
______________________________________ 
Physical shock to 
Feeling of fitness 
rider's buttocks 
in sensory evaluation 
in sensory evaluation 
Specimens (feeling on the seat) 
(riding test) 
______________________________________ 
H Very Good Fair 
I Very Poor Very Good 
J Very Poor 
K Fair 
L Good Fair 
M Very Good Poor 
______________________________________ 
The relationship between the Fitness Value F and the Spring Constant Ratio 
p, reported in Table 7, for each of the shock-absorbing samples (specimens 
H-O), is plotted on the graph shown in FIG. 16. This relationship can be 
represented by a best curve fit of the plotted points. From the best curve 
fit and the results of the sensory tests reported in Table 7, it can be 
deduced that when a material has a spring constant ratio (p) in 
compression and tension greater than or equal to 1.1, the seat provides a 
good feeling of fitness. 
FIG. 17 shows the relationship between the normal storage modulus for each 
of the shock-absorbing materials (see Table 5) and the comfort values from 
physical shock to the rider's buttocks from the Table 7, where 1=very 
poor; 2=poor; 3=fair; 4=good; and 5=very good. A linear fit through the 
plotted points reveals that a shock-absorbing material with a normal 
storage modulus of 0.22 MPa does a fair job in absorbing shock to the 
rider's buttocks. This result, of course, corresponds with the results of 
the comfort test proved above which identified a desired range of the 
normal storage modulus (E.sub.1) as: 0.0628 MPa&lt;E.sub.1 &lt;0.234 MPa. 
Thus, from the test results given above, the tested embodiment of the 
present viscoelastic had a Spring Constant Ratio p of 1.30 and a normal 
storage modulus (E.sub.1) of 0.102 MPa. These values indicates the present 
vehicle seat with the viscoelastic material described as sample H above, 
will provide a good feeling of fitness and comfort to the rider with 
little physical shock discomfort in the rider's buttocks. 
As mentioned above, the present vehicle seat can take numerous forms other 
than the one described above, while embodying some or all of the 
principles of the present invention. For instance, FIG. 18 illustrates the 
shock-absorbing member as extending from side to side across the seat. The 
shock-absorbing member also can be layered on the upper surface of the 
cushion member, as shown in FIGS. 19-21, or can be assembled so as to be 
embedded in the cushion member of urethane foam, as shown in FIG. 22. 
If the shock-absorbing member 72 is interposed between the cushion member 
70 and the seat base 68, as with the embodiments shown in FIGS. 2 and 18, 
the cushion member 70 comes in more direct contact with a rider's 
buttocks. Because the cushion member 70 is softer and has a smaller normal 
storage modulus (E.sub.1) than the viscoelastic member 72, or 
shock-absorbing member, the upper surface of the seat 56 is deformed so as 
to fit the shape of the rider's buttocks when the rider sits on the seat 
56, thereby providing greater riding comfort, especially during riding for 
long time durations. 
If the shock-absorbing member 72 is layered on the upper surface of the 
cushion member 70, as shown in FIG. 19, greater vibration absorption and 
less vibration transmission will occur. When the shock-absorbing member 
72, or viscoelastic member 70, is arranged below the cushion member, the 
rebound angle (H) is 12.4.degree.. On the other hand, when the 
shock-absorbing member 72 is arranged over the upper surface of the 
cushion member, the rebound angle (H) is 9.76.degree.. Thus, rebound of 
the rider's buttocks on the upper surface of the seat due to vibration 
will be less when the shock-absorbing member 72 is arranged on the upper 
surface of the cushion member 70 compared to when the shock-absorbing 
member 72 is arranged below the cushion member 70. 
The shock-absorbing member 72 may be embedded in the cushion member 70 of 
urethane foam, as shown in FIG. 22. By embedding the shock-absorbing 
member 72 in the cushion member 70, the vibration absorption and comfort 
advantages of the present vehicle seat are provided; additionally, the 
integrity of the shock-absorbing member 72 will not be as affected by heat 
or gasoline vapors from the engine or the ambient atmosphere. Also, if the 
shock-absorbing member 72 is embedded in the cushion member 70, as opposed 
to providing the shock-absorbing member 72 below the cushion member 70, 
the shock-absorbing member 72 will not have to be formed so that it will 
have to fit the irregularities of the upper surface of the seat base 68, 
thereby reducing manufacturing costs. 
Manufacturing the shock-absorbing member 72 in the cushion member 70 is 
done by forming the shock-absorbing member 72 and securing the 
shock-absorbing member 72 with pins 100 to a molding die 102 for molding 
the cushion member 70 of urethane foam, as shown in FIG. 23. A urethane 
stock solution is poured into the die, and the urethane foam cushion 
member 70 is formed around the shock-absorbing member 72, creating an 
embedded cushion arrangement 74, as shown in FIG. 22. 
In order to prevent discomfort from abrupt hardness changes between the 
cushion and shock-absorbing member boundaries, the shock-absorbing member 
72 also may have a gradually changing normal storage modulus, as shown by 
the embodiments illustrated in FIGS. 24-29. Arrows in the figures indicate 
the direction in which the normal storage modulus gradually decreases in 
the shock-absorbing member 72. The embodiments of the present vehicle seat 
in FIGS. 24-29 indicate that the normal storage modulus gradually changes 
in the longitudinal, transverse and vertical directions. The normal 
storage modulus, however, may gradually change in any direction in order 
to provide the comfort, vibrational absorption and avoidance of abrupt 
hardness change characteristic of the present vehicle seat. Ideally, the 
normal storage modulus of the shock-absorbing member 72 approaches the 
normal storage modulus of the adjacent cushion member 70, or the normal 
storage modulus of whatever material is adjacent to the shock-absorbing 
member 72. 
In FIG. 24, the shock-absorbing member 72 is arranged on the bottom side of 
the cushion member 70, and its normal storage modulus decreases toward the 
top face of the shock-absorbing member 72. In FIG. 25, the shock-absorbing 
member 72 is provided above the cushion member 70 with its normal storage 
modulus decreasing towards the bottom face of the shock-absorbing member 
72. In FIG. 26, the shock-absorbing member 72 is embedded within the 
cushion member 70 with its normal storage modulus decreasing towards the 
top face and the bottom face of the shock-absorbing member 72. 
In FIG. 27, the urethane foam cushion member 70 is arranged on both sides 
of the cushion arrangement 74 and the shock-absorbing member 72 occupies 
the center section of the cushion arrangement 74 from the top to the 
bottom of the cushion arrangement 74 between the cushion members 70. The 
normal storage modulus of the shock-absorbing member 72 decreases towards 
the top surface. 
Although in the foregoing embodiments, the normal storage modulus has 
varied in the vertical direction, it is possible to vary the normal 
storage modulus horizontally in the longitudinal or transverse directions 
of the seat, as shown in FIGS. 28 and 29. In these embodiments, the normal 
storage modulus in the center portion of the shock-absorbing member 72 is 
larger and decreases towards the side faces of the shock-absorbing member 
72, approaching the normal storage modulus of the adjacent cushion member 
70. 
Numerous methods exist for molding or forming the shock-absorbing member so 
that the normal storage modulus varies as described above. Several 
examples for forming the shock-absorbing member in order to vary the 
normal storage modulus will be described below; however, it is appreciated 
that various means to achieve varying a normal storage modulus are 
possible and will be readily apparent to those skilled in the art. 
The normal storage modulus may be varied by varying the density of the 
filler in the matrix resin through gravity or centrifugal force when 
molding the shock-absorbing member. For example, in the shock-absorbing 
member of the present invention, the normal storage modulus is higher in 
portions of the member where the resinous microballoon filler has a 
greater density in the viscoelastic urethane elastomer matrix, and the 
modulus decreases with decreasing resinous microballoon filler density. 
The normal storage modulus also can be varied by mixing a magnetic 
substance, such as iron powder, in the matrix resin when molding the 
shock-absorbing piece. A magnetic field may be used to distribute the 
magnetic powder in order to vary the normal storage modulus as desired. 
A matrix resin that may be used in injection molding machines may be 
injected in a manner so that different compositions that result in 
different hardnesses are injected in an orderly succession from soft 
material compositions to hard ones. 
The normal storage modulus may be varied by foaming the matrix resin so 
that the magnitude of the expansion ratio changes gradually. The matrix 
resin may be foamed and the expanded portion may be further impregnated 
with the resin so that the quantity of the impregnation changes gradually. 
Additionally, the modulus may be varied by changing the bridging density 
or the molecular weight of the matrix resin gradually, or by laminating 
thin materials with different hardnesses in an orderly fashion to form the 
shock-absorbing member. 
Although this invention has been described in terms of certain preferred 
embodiments, other embodiments apparent to those of ordinary skill in the 
art are also within the scope of this invention. Accordingly, the scope of 
the invention is intended to be defined only by the claims which follow.