Page 4 - Combined_39_OCR

W. P. Wright 4 February 18, 1970

To achieve a transmission path which will attenuate the track induced vibration felt by
the driver, a very soft suspension is required, i.e., > 1. Treating the driver
and seat as a simple spring mass system, we have — - —r = 2 where 3 Hz. is
the forcing frequency and l05 Hz is the undamped natural frequency of the torsion bar
seat properly adjusted for the driver’s weight. From the preceding sketch, it is
observed that the response associated with an ~ ratio of 2 is only three tenths of the
input vibration; hence, a significant reduction in the vibration environment is antici­
pated during periods of high centrifugal force loading. The relationship between
frequency and mass in the equation - /k/M will not however, be satisfied at
low centrigugal force loading and the system damping due to the shock absorber in
the seat will become the primary vibration attenuator.

One notable disadvantage to this vibration isolating approach is the seat dis­
placement associated with a low valued spring constant, k. For the application being
considered herein this would necessitate a displacement of approximately six inches
for the combined driver weight and centrifugal loading with an additional double
amplitude displacement of 2 inches for the vibration environment. This problem can
be partially overcome by using a chain or similar tension type tie to restrict the up­
ward motion of the seat at a position about 1” to 1.5” above the equilibrium position
under the combined drivers weight and centrifugal force.

A second approach to modification of the Bostrom seat has been investigated
but no steps have been taken toward implementation. This method entails the use of
elastomer type shock mounts manufactured by Lord Manufacturing Company. By
utilizing the shear properties of this elastomer, a small spring constant with a damp­
ing ratio of 0.2 is achieved. Two methods of utilizing these mounts have been con­
sidered; (1) as the primary spring and (2) as an overload spring on the seat as it is
manufactured. As an overload spring, these mounts would offer the advantage of
being able to increase the spring constant during periods of centrifugal loading with­
out losing the advantage of small k when only the drivers weight is present. Still
another approach to the problem is available in the Bostrom Viking air seat which
would provide additional suspension stiffness by increasing the air pressure in the
cylinder supporting the seat.

Other methods investigated but discarded were the use of leaf and coil springs.

J. E . Serocki

JES:ck

cc: H. Bader, Jr.
G. Martin