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For someone who's just beginning their journey into the world of electric vehicles, the sheer amount of information about power output can be confusing and frustrating. This is particularly true when comparing different vehicles and deciding which to buy.
Here’s a short guide that should help you in your journey to understand watt matters and watt doesn’t.
What the Watt?
In electric vehicles, power is usually measured in Watts (W) as opposed to horsepower (hp). If you want to compare Watts to horsepower, 746W = 1hp. Like gas vehicles, motor power can be measured as a peak rating (which can only be sustained for a few seconds) or at continuous rating (which can be sustained indefinitely). These ratings can be very different, so make sure you’re comparing apples to apples when exploring options. Trusty Trucker is capable of outputting a continuous 3,750W, or in other words, 5hp.
Torque versus speed
Torque x speed = power. This means that when a motor produces power, a machine (like a motorbike) can convert that power into either torque or speed, but not both at the same time. Huh? First, everyone is familiar with speed, but maybe not torque. Torque is the force of rotation. It’s what either gives you the convenience of rapid acceleration or the ability to muscle up steep slopes with heavy loads.
For a given amount of power, you can either have high torque or high speed, but not both at the same time. Intuitively, you can drive up a steep slope at low speed or drive on a flat surface at high speed, but you can’t drive up a steep slope at high speed.
Gas-powered vehicles use transmissions that allow the vehicle to change gear ratios while moving. This way, the power from their motor can be used for high torque at low speeds as well as high speed once the vehicle is moving.
One of the primary reasons for this is that petrol motors can only produce useful power and efficiency in a narrow RPM range. Nobody wants to drive down the highway with their motor revving at 15,000RPM. A transmission allows the motor to stay in its happy place (2000-3000RPM) while varying the amount of torque and speed delivered to the wheels.
Electric motors are wonderful because they 1) produce a lot of torque at low RPM and 2) are happy operating at high RPM. This means that many electric vehicles do not use a transmission. Instead, a vehicle designer first choses a motor with a certain power output, then selects a gear ratio that prioritizes top speed or torque.
In the case of Trusty Trucker, the choice was easy. As an off-road vehicle, a high top speed is almost useless. Except on wide, open dirt roads, there would be few occasions when it would be safe or desirable to travel faster than our chosen max speed of 30mph (45kph). This means that with a relatively low power output (3750W / 5HP), Trusty is able to climb very steep slopes while carrying very heavy loads. That's why an electric powertrain is perfect for an off-road utility vehicle; there’s no need for a large motor or a transmission.
Hub-drive versus mid-drive motors
Now, for anyone in the market for an electric motorbike, there’s one final important point: Where is the motor located? Many electric two-wheelers use a hub-drive design, where the motor is inside one or both wheels and rotates at the same speed as the wheels. Remember when I said that a designer will choose a gear ratio to prioritize either high speed or high torque? With hub motors, the designer doesn’t get this choice - there's no ratio. The motor and the wheel are a single unit and rotate together.
There are three reasons why we believe hub-drive motors are the wrong choice for off-road vehicles:
Electric motors (like petrol motors) produce more power at higher RPM. In theory, you could design a powerful motorbike using a very small motor operating at 30,000RPM. However, such speeds would require complex gear reduction to be useful at a reasonable wheel speed of 200RPM. For reference, a hub motor operating at 30,000RPM would be propelling a motorbike faster than the speed of sound… if it were even moving at all. Without gear reduction converting some of that motor speed into torque, there would not be enough torque to even get the vehicle rolling.
Electric motors can be designed to operate at low RPM while producing lots of torque. However, such motors are large and heavy. Unfortunately for the makers of hub motors, this is the only option. These hub motors struggle to perform on steep slopes, and their heft can make for a jarring ride. As a result, hub motors are generally used for on-road bikes where the motors can operate at the higher speeds associated with riding on smooth surfaces.
It’s notable that some hub motors do have gear reduction built into them. They generally allow the motor to rotate 5x faster than the wheel. These gear reduction systems improve performance but reduce the motor size by consuming space. This means that the gains, though valuable, are somewhat reduced. Meanwhile, cost increases significantly.
A mid-drive motor, like the one you can find in Trusty Trucker, avoids all three of these problems. Additionally, it’s:
Small in size, while still producing significant power.
Protected from the elements.
Isolated from strong vibrations caused by trail riding.
Actively cooled with a fan to allow for sustained power output.
Finally, it’s much easier to remove wheels without hub motors, so you can easily transport or repair Trusty Trucker.
Wrapping up
In this post, I've tried to address the most important aspects of motor power output while skipping over many of the finer details. Still, it should be enough to understand why we chose a high-RPM, 5HP mid-drive motor with gear reduction for Trusty Trucker. With our design, Trusty Trucker has:
Hill-conquering torque;
a compact, highly-efficient powertrain;
low weight;
the ability to haul heavy loads;
and active motor cooling for continuous performance.