ELECTRIC CAR ADAPTORS by Shari Prange c. 1993 Shari Prange A chain is only as strong as its weakest link, and so it is with an electric car's drivetrain. All of the horsepower and torque needed to move the car down the road must pass through the adaptor on its way to the transmission and wheels. This is no place for half measures. The adaptor needs to be designed, machined, and installed precisely. This does not mean laying the transmission face down on a piece of cardboard and tracing the outline and holes with a pencil to make a pattern. A professional adaptor manufacturer will measure and machine each bolt or dowel hole, as well as the thickness of the plate itself, to within a few thousands of an inch tolerance. This kind of accuracy assures a secure fit between motor, adaptor, and transmission. Any sloppiness in fit will eventually cause poor performance and could damage the motor or transmission. What is an Adaptor? Okay, let's look at what an adaptor is. Pay attention, there'll be a quiz later. There are two sections to an adaptor: the plate that mounts the motor to the periphery of the transmission, and the hub that connects the motor shaft to the flywheel and physically transmits the power. The plate can be made of aluminum in order to save weight. It should be no less than 5/8 inch thick for adequate rigidity. The plate may actually come in two parts, as long as they are precision machined to fit together. At Electro Automotive, we use a 5/8 inch thick transmission profile plate and a motor ring that varies from 5/8 inch to 2 3/4 inches in thickness. There are two reasons for this design. The first is weight and economy. There is a minimum thickness needed, which is determined by the space needed for the hub. If the profile plate were this thick, it would be very heavy and expensive. The ring, which is the diameter of the motor rather than of the transmission, is much more affordable and light. The second reason is that it allows the car to be upgraded to a newer or larger motor by simply changing the ring, instead of the entire adaptor. Adaptor Hubs in the Naked City The other section of the adaptor is the hub. This connects the motor shaft to the flywheel. This must be made of steel, not aluminum. The motor shaft has a square steel key in it that fits into a slot in the hub. If the hub is soft aluminum, that steel key will eventually egg out the slot and start cutting into the hub like a machine tool bit. This is not a pretty sight, especially at 50 mph. There are a million styles of adaptor hubs in the naked city, (twenty points if you can identify that TV show reference) and most of them are wrong. All the power of that motor passes through a little hub the size of a hockey puck. This is not the place to skimp. One style holds itself in place with a setscrew through the side of the hub. Another uses a flat washer and a bolt in the end of the output shaft of the motor. These both have the same problem: threads loosen. It doesn't matter how much Loctite you use, it will eventually work loose, and you'll be a pedestrian. A third style is the shrink fit. In this type, the hub is heated to very high temperatures, then slipped over the shaft and allowed to cool and shrink into place. This will give a good secure connection. Unfortunately, it's too secure. If you ever need to remove that hub, you've got a problem. The last style is a taperlock. This, like the plate, comes in two pieces. One is a cone-shaped bushing that slides over the motor shaft. It also slides inside the second piece, a hub that mounts to the original flywheel. The cone has a slit through it, and starts out a little bigger than the hole it fits into, so it won't go in all the way. As screws are tightened to pull it in, it compresses the slit and squeezes the rotor shaft. It's easy to install, but once it's in place, the only way it will come off is with a special pulling tool. Now, class, who can tell me which style is the right one? The taperlock? Good, fifty points! This type of connection is the standard for industrial power shaft connections. Special Numbers There are some numbers that need special attention here. The first is what we call the "magic number." On the gas or diesel car, this is the distance between the back of the motor and the farthest flat surface of the flywheel. This distance needs to be duplicated when installing the flywheel on the electric motor. The second number is a measurement of torque. The hub is attached to the flywheel using the original bolts. These are special hardened bolts. When installing them, "use the Force, Luke." Specifically, use a torque wrench and the torque numbers in the manufacturer's service manual. In order to get the leverage you will need to tighten these bolts, the electric motor needs to be immobilized. One way to do that is by using one or two very muscular friends. A better way is to strap the motor to the workbench. This can be done by installing sturdy eyebolts in the workbench on either side of the motor. Then pad the top of the motor with shop towels, and run a length of chain between the eyebolts and across the motor. Tighten the chain with a turnbuckle until the motor doesn't move. Flywheel Still with me? Okay, let's move on. We discussed earlier why we use a transmission and clutch at all (see "Electric Car Conversion," Home Power #32), and why it needs to be a manual transmission (see "Choosing a Conversion Car," Home Power #31). Now let's talk a little about the flywheel. The flywheel has a safety function. If you were to over-rev the motor with no load (flooring the throttle with the car out of gear, for example), the motor could "run away" and self- destruct. The flywheel provides just enough load to protect against that. Some people suggest lightening the flywheel to improve drivetrain efficiency. There is some advantage to this. The most benefit would come from removing the ring gear, which is no longer needed. The spinning flywheel teeth actually create some drag through air turbulence. This modification is not really necessary for the average conversion, though. It's something for electric vehicle hotrodders to play with, since they don't have carburetors anymore. There are some reasons not to alter the flywheel. Too much lightening will dangerously weaken the flywheel, and efficiency gains are minor. Also, it may be impossible to find a local machine shop capable of doing this specialized work. The flywheel is part of the clutch assembly. When you are installing the adaptor, this is the time to replace the clutch pressure plate, disc, and release bearing with new parts. If you do it right the first time, you won't have to change the clutch again until you replace the motor brushes at 80,000 miles. After the Fact Once you have your motor attached to the transmission with a good quality adaptor, and the entire assembly installed in the car, it's time to take a break, pat yourself on the back, and have a cold drink. You've earned it. Access Shari Prange, Electro Automotive, POB 1113, Felton, CA 95018- 1113 ù 408-429-1989