Does an Electric Car need a Transmission?
“Does an electric car even need a transmission?”
That’s one common question I hear all the time. To find the answer, we first need to know what exactly a car transmission does and then compare that to both DIY electric car conversions and modern commercially-built electric cars.
To start with, let’s first define “Transmission”. In this case, I’m talking about the entire collection of parts that transmit the mechanical power from an engine (or electric motor) out to the driven wheels of a car. With that said, we can now examine what that system does, and how much of it could be eliminated in an electric vehicle.
MULTIPLE GEAR REDUCTION
The first thing we think of with a transmission is the multiple gears. An internal combustion engine only produces peak power in a narrow band of RPM range. To quickly accelerate, power up a hill, or pass on the freeway, the engine needs to be rev’d up to a certain speed. That needs to happen whether the car is at a stand-still or already traveling 65 MPH. Therefore, there needs to be some way to match the speed of the engine at power to the speed of the wheels. The multiple gear ratios of a standard transmission allow the driver to operate the vehicle at almost any speed, while still having the engine in its “power band”.
CLUTCH
The clutch allows the driver to COMPLETELY disconnect the engine from the wheels. This allows the vehicle to be at a stop WITHOUT stalling the engine. It also allows the operator to smoothly re-engage with an engine which must always be turning. Lastly, the clutch makes it quicker and easier to shift gears, especially with the rotating mass of a flywheel and other engine components. In an automatic transmission, a more complicated system of fluids provides a similar effect.
MOUNTING POINT
We don’t commonly think of it this way, but a transmission (specifically the bell-housing) provides one of several physical points of attachment between the engine and the vehicle. It helps hold the engine in place in the car.
POWER TRANSMITTING POINT
Likewise, the transmission also provides the point where the physical rotating power of the engine gets transferred to the rest of the driveline.
POWER SPLIT TO WHEELS
Most automobiles have two driven wheels. The power of the engine needs to be split and sent more or less evenly to both. This is done with a device called the differential. That allows one of the wheels to spin at a slightly different speed than the other – important when driving in anything other than a straight line. You probably recognize the differential as the “pumpkin” in the center of the rear axle of a pickup truck. In most front-wheel drive cars, the transmission and differential are combined as one physical unit, providing both gear reduction AND power split to both front wheels.
Now that we know what the transmission does, we can see whether or not an electric car needs one.
ELECTRIC VEHICLES:
Multiple Gear Reduction:
Generally, electric motors produce high torque at a much wider range of speeds than gas engines do. At a bare minimum, that at least means LESS shifting. Commercially-built electric cars use motors specifically designed for torque at a very wide range of speeds. However, they also tend to spin very fast. These motors are typically geared down to produce plenty of torque and appropriately match road speed. A gear ration in the neighborhood of 10:1 is not uncommon.
For home-built electric cars, the exact types of motors used vary considerably. Not all types of motors are as well suited for that wide range of torque the commercially-built cars provide. Because of that, it’s still useful to have more than one gear available. It’s not uncommon for a home-converted car to keep all the original gears of a manual transmission, but completely skip 1st and perhaps only use two or three of the total gears available.
Clutch:
Because most commercially-built electric cars only use a single gear, there’s no reason to shift in or out of it. It’s a fixed gear and therefore requires no clutch or the automatic transmission equivalent. This also saves space, weight, complexity, and moving parts.
On a home-built electric car, re-using a stock manual transmission, it’s a design choice whether to leave the clutch or not. There are advantages and disadvantages to both. Read more about that at Clutch vs Clutchless.
Mounting Point and Power Transmitting Point:
Even if you have an electric motor, it still needs to be mounted somewhere. It still needs to be physically connected to the vehicle. It still needs a place for the rotating power of the motor to connect to the driveline.
Commercially-built electric cars generally use dedicated, custom components in place of a transmission for the electric motor to attach to.
For DIY home-built electric cars, reusing the existing transmission is the easiest way to go (even if not using all the gears.) Anything else can become quite a lot of custom machining work.
Power Split to Wheels:
Any car, whether gas or electric, will still need to split the power between the two drive wheels. So, any electric car WILL STILL NEED A DIFFERENTIAL. Commercially-built electric cars will have a custom, dedicated differential. It is most commonly integral to a single-speed gear reduction.
On a common front-wheel drive DIY electric car, the fact that the differential is built into the transmission is the single biggest reason all by itself to keep the transmission. On a rear-wheel drive vehicle (such as a small pickup truck,) the differential is SEPARATE from the transmission. So, the transmission could be completely removed without effecting the differential. However, the electric motor still needs a physical point of mounting, and a way to transfer power to the driveline. A designer can build a custom cradle to mount the electric motor and then connect it to the driveshaft with a universal joint. The down-side of that is no gear reduction (beyond what is integral to the differential itself.) A DIY rear-wheel drive vehicle with NO TRANSMISSION typically requires a high current/high voltage motor for torque at low speeds and and appropriate top speed.
Typical Commercially-Built Electric Car
Does a modern electric car need a transmission? No, it doesn’t. It does still need much of what a transmission provides. The most common layout of a commercially-built electric car is a large single alternating-current motor directly connected to single-speed gear reduction and differential. AC motors can also be controlled to spin the opposite direction, so even reverse “gear” can be taken care of without need for a transmission and separate gear just to spin the opposite direction.
It should also be noted that since there is no shifting (it’s just a single gear,) that power and the ride is extremely smooth! It’s all one “whooooosh!” from zero to top speed!
Typical DIY Home-Built Electric Car
Without locating some sort of special dedicated single-speed gear reduction and differential unit, the easiest way to convert a car to electric is to REUSE the existing transmission. It provides a mounting point for the electric motor, a way to connect power to the driveline, provides gear reduction (whether one or many gears are used) and likely provides power splitting to the wheels as well. Depending on the exact motor used for the project, it may or may NOT be easy to reverse the direction of the motor. In the case of our 1996 “Electro-Metro” Geo Metro conversion, it was simpler to use reverse gear rather than adding more electrical components to be able to spin the other direction.
That said, we are finally starting to see more and more components available on the hobbyist market, including dedicated single-speed gear reduction and differential units. These units allow a builder a place to mount their motor and connect the half-shafts of the two drive wheels. It’s a single, simple component that provides everything a transmission in a gas car does, but more efficiently and with fewer moving parts.
EXCEPTIONS
Aren’t there any other ways around reusing a transmission or having a single-speed thing which is very much LIKE a transmission?
HUB MOTORS
Sure. The most commons questions are usually about HUB motors.
Hub motors are motors built directly into a wheel. They don’t need a drive-shaft, chain, or belt to connect to the wheel, which does make them simpler and saves some space. However, they add greatly to the weight of a wheel. (Specifically, the “un-sprung weight”) This causes difficulty in the handling of the vehicle and the motor itself will take a lot of road shock over its lifetime. Hub motors also have issues with how large and powerful they can be built, and often have poor performance at low speed/hill-climbing/high-torque-required situations. They are also limited in cooling. Hub motors have generally been air-cooled, whereas most commercial electric vehicle motors are liquid cooled. Hub motors aren’t anything new either. They were used in some of the earliest hybrids and even on NASA’s Lunar Rover on the moon! They’ve been popular on electric bicycles and even scooters. I’m sure we’ll continue to see advances in hub motors in the future and will see them more in use, although perhaps more in niche applications.
MOTORCYCLES
For a motorcycle, it’s easy to completely eliminate the transmission.
For starters, a bike will have either a chain or belt connecting the transmission to the rear wheel. That’s a convenient point of power connection. The electric motor can be mounted to the frame of the motorcycle with a simple custom mounting plate. Gear reduction can be taken care of by replacing the sprocket with one of a more appropriate size. Since there’s only one wheel, there’s no need for a differential. On many motorcycles, the engine and transmission are also in a combined case. It’s difficult to remove the one but keep the other.
In fact, MOST electric motorcycles completely eliminate the transmission, whether commercially-built or D.I.Y.!
Having no transmission also frees up space in the frame for more batteries. (Learn more about this D.I.Y. motorcycle by clicking here.)
A few electric motorcycles out there also make use of hub motors. That frees up even more space in the frame for batteries.
Moral of the story? Does an electric car need a transmission? No, it doesn’t. It still needs much of what a transmission provides, but that can be slimmed down into fewer parts, creating a simplified, low-maintenance driveline.
