THE GYRO EFFECTS OF
ROTATING MASSES IN THE SPARROW ELECTRIC TRICYCLE VEHICLE
The Sparrow Electric Vehicle
is a three-wheel electrically driven motorcycle with two front wheels for
steering and a single rear wheel for drive. It has five or six
significant gyroscopic masses, which if perturbed by an acceleration force, can
force the vehicle to tumble like a free body in a gyroscope frame at the
Exploratorium or an astronaut in free space.
In its current configuration, a torque to
one or two of those gyroscopic masses or a sudden acceleration causing the rear
wheel to lose traction will cause the vehicle to spin out and tumble. This
torque phenomenon is familiar to pilots who try to climb too quickly, causing
the plane to heel over, and this acceleration problem is familiar to
motorcyclists who spin out on their rear drive wheel.
Configuring the rotating masses of the
vehicleÕs drive train to cancel the lateral acceleration forces associated with
the drive train can probably solve this problem. A good
mechanical/aeronautical engineer or a physicist or anyone who has studied
conventional mechanics and can relate it to the dynamics of gyroscopes should
be able to analyze the problem and offer a number of solutions. Here are
some thoughts along those lines.
The Sparrow's gyroscopic forces are found
in the energy stored in the two front wheels for steering, the single rear
wheel for drive, a large belt driven pulley mounted to the left side of the
axle of the rear wheel to supply drive power, and the electric motor with an
internal rotor on a drive shaft mounted above transverse to the vehicle.
When the vehicle loses rear wheel lateral
traction, for whatever reason, the wheel slips sideways, the vehicle pitches
from the induced lateral torque on the single rear wheel and begins to tumble
until the energy stored in the gyroscopic flywheels of the vehicle is
dissipated or reaches a stable orientation. This can be experienced by
holding a spinning bicycle and twisting its axle, or holding an electric motor
while it accelerates under sudden application of power.
The forces on the Sparrow could be
observed and analyzed by mounting the Sparrow on a three-axis frame (like the
one at the Exploratorium) and testing the effects of the spinning masses in a
flat spin. If the front wheels are spinning in a flat spin, I believe
most or all of the gyroscopic lateral acceleration is canceled if the wheels
are exactly parallel and are both in contact with the ground, since the
acceleration on the wheels and the pressure on the ground counteract each
other. If the wheels are turned during a flat spin, then some imbalance
should be evident.
The lateral tilt caused by spinout on the
rear wheel is much more pronounced. The motor rotor, the pulley and the
rear wheel are mounted in tandem and have only a single ground contact point. They
all spin in the same direction. This combined huge spinning mass has
enough force in the stored energy in a spinout to cause violent reorientation. The
speed of the vehicle is almost irrelevant because the speed of the motor and
wheel increases when the rear wheel loses traction.
A possible solution is to reverse the
spin of the motor and possibly also the pulley relative to the spin of the
wheel. The counter rotating masses would tend to cancel one another during any
spinout and result in lesser instability. (Reversing the spin of the
pulley could be done through a reversing gearbox on the wheel drive shaft. There
may already be such gearboxes in such things as aircraft applications.) There
may also be reason for caution and further analysis. The torque of sudden
acceleration of the rotor of the motor on the frame might also be aggravating
or canceling the load change on the rear wheel. Further analysis is
needed.
Other solutions to be considered may
require major design changes:
Add a separate counter-rotating flywheel
Lengthen the wheelbase.
Add a fourth wheel in contact with the
ground and thus converting it into an automobile.
Install a limited slip drive wheel
coupling.
Lower the center of gravity.
Change to front wheel drive and minimize
spinning mass on the rear wheel.
Any or all of these design changes should
add to safety and stability, which is currently lacking.
Ken Allen
July 2000