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 vehicle’s 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