First, because it's one of the primary reasons for not going straight to a recycled VW cr Opel or something, let's consider weight and aerodynamics. The form for minimum aerodynamic drag for any speed is well known. The lowest drag in the speed range we're talking about would have a relatively blunt, rounded leading edge and a sharp trailing edge. A design for 55 MPH is not going to look like a formula racer, even going from 55 to 70 changes the shape somewhat, when you get to 150 it starts to look like an Indianapolis car. The VW Beetle isn't too bad at 45 MPH, the Porshe Spider isn't bad at 90 MPH The Pontiac Trans A.M. with spoilers and all isn't bad at 100. Probably the worst is a pickup truck.
One of the most significant improvements that can be made is in weight reduction. With a direct dollars to pounds relationship, not only to keep the vehicle moving but also as construction costs, a very careful design is justified. We have become accustomed to thinking of a 2000 pound car as about the minimum weight; and, given the economics of the manufacturing system used, it is close to it. Therefore, other systems should be considered, for instance, light aircraft construction methods. Aluminum monocoque construction has become standard practice with the major aircraft manufacturers. The economics for small production runs is favorable for this type of construction. Steel tube and fabric construction is suitable for smaller production runs, or one-of-a-kind prototypes and is very adequate from a strength, costs, or weight viewpoint. Some might object to these methods from a feeling of being less protected in an accident, however, the reverse is true. The typical light aircraft construction will protect the passengers in situations similar to hitting a brick wall at 50 MPH.
In the average automobile the rolling resistance and air resistance are about equal at 40 MPH, the air resistance increases as the square of the air speed while rolling resistance is in direct proportion. By reducing weight we have reduced rolling resistance a like proportion. By decreasing tire flexing, rolling resistance can be further reduced. Since we aren't obsessed with acceleration, and have much less weight to stop, much less tire surface in contact with the road is required. In other words, We can use a much thinner and much harder tire. This also follows the plan of less weight and drag. There are several ready-made options here, motor cycle tires, for instance. When it comes to wheels to mount the tires on we have less choice. Standard motor cycle wheels would seem ideal, but they are not capable of resisting very much side load. On a motorcycle they don't have to. The newer disk and solid-spoke motorcycle wheels, or side-car wheels, are adequate.
By optimizing the controllable factors, like weight and drag, we can use much smaller and more economical drive components. lf we were converting a VW or something we would have transmissions and differentials and so forth to use. The average gear train between the engine and wheels will take as much horsepower to keep moving as we're talking about to move the whole system, and weighs as much as our whole propulsion system should. For starting the vehicle we need lots of torque and almost no RPM, for cruising we need lots of RPM and very little torque. The conventional answer is a clutch and transmission between the engine and the wheels.
The electric motors we're going to use stop when the car does, so, we don't need a clutch. Electric motors can have many of the characteristics we need, so maybe we really don't need a transmission with different gear ratios.
With 3.5 X 14 tires at 60 MPH the wheels are doing about 960 RPM. Electric motors rated at 1200 RPM are common, which would give us about 75 MPH. Low RPM direct current electric motors are not unusual, they weigh and cost more than the same horsepower motor with a higher RPM, but gears also weigh and cost. The torque and RPM relationships of mechanical power have approximate corollaries with electrical power, as volts and amperes. Although it's not completely true it's somewhat helpful to think of watts as power, volts as RPM, and amps as torque. Our low RPM DC motor Could be many voltages, and, since amps determine wire and switch and contact size we have to decide and compromise. Component availability will also be a determining consideration. Batteries can be sized by deciding the time period we wish to average the power requirements over, or the excess power desirable for hill climbing, or the distance traveled on batteries alone. Unlike the pure electric vehicle, we won't have to carry the equivalent of the vehicle weight in batteries. The voltage of the system will be an important design consideration, so we need to make some preliminary assumptions.
The ideal vehicle starts to shape up with some realistic specifications. Two seats, luggage compartment, Lets say a drag reduction and rolling resistance reduction of 70%, steel tube and fabric body, motorcycle tires and modified wheels. This may be optimistic, but we're talking about ''ideal'' here; If we have to compromise let's do it for economics or utility, not at the design stage.
A lot of numbers and assumptions and computations and compromises later, I won't go into detail, we decided on 2 Honeywell 24 volt permanent magnet motors, and 2, 12 volt heavy duty batteries. A seperate motor on each rear wheel eliminates drive shaft and differential problems and simplifies electrical design. The 2 batteries are adequate for average conditions.