This section will discuss how the power from the two sources is delivered to the wheels. Like other subsystems, many components were borrowed from the NA Miata. Other parts were designed to interface these components with the engine and motor.
Uprights and Wheel Bearings
These parts were directly borrowed from a ’94 Miata. The four uprights purchased online each came with their own wheel bearings already mounted. The rear spindles have splines to adapt with driveshafts, of course. These won’t be modified either.
Drive Axles and Differential
These too come straight out of a Miata. The differential is the regular, open differential from a ’94 with a 4.10 final drive. The design requirements of the vehicle demanded a higher final drive ratio, but the 4.10 was very common and cheap. So, the decision was made to introduce additional ratios via the intermediate shaft.
The drive axles are aftermarket Miata parts. Unlike the half shafts used in the ’94 and ’95 Miatas, the axles chosen are a lighter one-piece design. These were purchased new, since used CV joints are likely to be worn out.
All of these parts were purchased from different sources online, so care was taken to get components from the same generation vehicle. The parts were delivered and all fit well. The shafts’s splines fit into the differential and wheel hubs just fine.
Intermediate Shaft
Connecting the engine, motor, and differential presented a few design difficulties. The first had to do with the direction of rotation of the engine. The engine we selected had counterclockwise rotation when viewed head-on. For forward vehicle motion, the input shaft to the differential has to rotate clockwise, when viewed from the front of the vehicle. This means that either the engine must be positioned with its shaft pointed aft or the a gear system must be employed to switch the rotation. The first option is desired, since designing a gear box is complex and expensive, but having the engine’s output shaft point directly into the differential would position the engine between the passengers. So, an intermediate shaft was designed to move the engine 36 inches back, behind the drivers.
The intermediate shaft (orange in above picture) combines the power from the engine and motor and transmits it to the final drive. The engine’s output shaft has on it the primary clutch (blue) of a continuously variable transmission. The secondary clutch (also blue) sits on the intermediate shaft. The motor (not pictured) is linked to the intermediate shaft via a size 50H chain drive. Finally, the intermediate shaft connects down to the differential via another set of sprockets and a chain.
The use of this chain drive system solves the second design challenge of the drivetrain. It is desired to have the maximum speeds of the motor and engine occur at the same vehicle speed, thereby using the powerplants most effectively. The motor maxes out at 8000 rpm, while the engine can rev to 4750. The desired top speed of the vehicle is 75 mph.
Working backwards from the wheels, through the differential, to the engine, the ratio between the intermediate shaft and the differential input shaft was selected to be 30/22. At 75 mph, this equates to the intermediate shaft spinning at 6101 rpm and the engine at 4758 rpm. A 21/18 ratio was chosen for the chain between the motor and intermediate shaft, putting the motor at 7118 rpm at 75 mph.
Continuously Variable Transmission
A CVT commonly used in ATVs and snowmobiles was chosen to connect the engine to the intermediate shaft. This transmission utilizes primary and secondary clutches to vary the ratio of a belt drive. The selected primary clutch is a Comet 94C, and the secondary is a Comet 90D. Together, these clutches provide a low ratio of 3.49 and a high of 0.78.
At idle, the primary clutch is so separated that the belt is loose, and the engine is effectively decoupled. At the engagement rpm of the CVT, the primary first grips the belt and links the engine to the drivetrain. The CVT holds the low ratio of 3.49 until the shift rpm is reached. The ratio then decreases as vehicle speed increases, holding the engine speed nearly constant near the shift rpm. After the high ratio of 0.78 is attained, the engine speed increases up to redline. Ideally, the shift rpm of the CVT matches the rpm of maximum power of the engine, maximizing performance. The primary clutch has adjustable weights and springs that allow tuning of the engagement and shift rpm. Our engine reaches max power (37 HP) at 3900 rpm, so the CVT will be tuned to set hold the engine as near to there as possible.
Fatigue Analysis
Since the majority of the drivetrain components were pulled from a more powerful production vehicle, their durability can be trusted. The intermediate shaft, however, requires analysis. A free body diagram was drawn using the forces caused by simultaneous max power outputs from the engine and motor. These values along with the geometry of the shaft equate to an alternating bending moment of 2556 in*lb and a midrange torque of 3792 in*lb. The Goodman fatigue stress equations used with 1045 steel produce the following fatigue factors of safety for different shaft diameters.
| Shaft diameter | Safety Factor | Cost |
| 1.0 in | 0.37 | $42 |
| 1.25 in | 0.72 | $55 |
| 1.50 in | 1.25 | $66 |
| 1.75 in | 1.98 | $78 |
The 1.50 inch shaft was chosen, since it was the smallest with a fatigue factor of safety greater than one, leading to infinite life in these conditions.
The bearings chosen have a rated load of 6,535 lbs at 1,000,000 cycles. The two bearings on the intermediate shaft see a maximum resultant load of 1535 lbs. This gives a life of 320 hours at full power at 4000 rpm. Since full power loads are infrequent, this life is sufficient.
PH-571 