EV Gear Reduction and Driveline Setup

Unlike internal combustion engine (ICE) vehicles, the vast majority of EVs on the road today do not have a multi-speed transmission. Almost every OEM electric vehicle currently produced, including Teslas, BMWs, Rivians, and the Ford Mach E, use a single speed transmission that provides gear reduction from the EV motor(s) to the driveline or axle. The fact that EVs can operate effectively with a simple single-speed transmission is a sign of an extremely capable and efficient electric powerplant. Electric motors provide consistent torque throughout a massive RPM band, all in a small and power-dense package.

Gear reduction uses mechanical advantage to trade speed for torque. An electrical motor may spin at 8,000 rpm and put out 200 lb-ft of torque without gear reduction. However, if you mate that motor to a single speed gear box with a gear ratio of 2:1, the output of the system will be 4,000 rpm and 400 lb-ft of torque. Total power output is the product of speed and torque, so power is not reduced in the system, but is merely applied in a different way.

Now, while many EVs use gear reduction, you can also maintain an existing manual transmission, use a two-speed transmission, or explore a number of other options. That’s why the focus of this article will be, what is gear reduction… and… do I need it in my EV?

While gear reduction is an excellent way to take advantage of an electric motor’s potential, it’s not right for every build. There are a few factors you will want to consider when determining if gear reduction is right for your build.

• How will the gear reduction affect the top speed of your vehicle?
• Will gear reduction fit within your project’s budget?
• Do you have the space to forgo gear reduction?
• How will the efficiency of your daily drive be affected by gear reduction?
• Do you want a smooth/effortless driving experience or a more interactive driving experience (using a manual transmission)?

Let’s dive into each question, below.

01. How will the gear reduction affect the top speed of your vehicle?

Depending on your electric motor’s max RPM, your tire size, and differential gear ratio, the top speed of your vehicle will change. If the gearing is too high or low in your differential, a torque box may not lower the final drive ratio enough or may lower the drive ratio too much. Use the calculator at legacyev.com/pages/top-speed-calculator to find if a torque box is right for you.

02. Can your build afford gear reduction (or afford to skip on it)?

If you plan to use a manual transmission for your build, you will want to make sure your transmission can handle the torque of your new motor. You will also need to source adapter plates, new flywheels, and clutch plates. In some cases, you might even need to source an entirely new transmission for the vehicle. Comparatively, a torque box does add anywhere from around $3,000 to $5,500 in cost to your build.

03. Do you have space to forgo gear reduction?

Gear reduction takes up ___% of the space on an average transmission, allowing more room for batteries. If your build is planning on using a large battery pack and space is tight, gear reduction will add much needed room for more batteries.

If your build is planning on using a large battery pack and space is tight, gear reduction will add much needed room for more batteries.

04. How will the efficiency of your daily drive be affected by gear reduction?

EV motors have an optimal operating efficiency for their individual RPM band, as you can see in the efficiency curve for a Hyper 9 motor in the figure here. Depending on the gearing in your EV, optimal efficiency will occur at different speeds. To find the optimal operating efficiency you can use the maximum RPM efficiency of your motor in the top speed calculator below to find what speed is best for efficiency in your EV.

05. Do you want a smooth/ effortless driving experience or a more interactive driving experience (using a  manual transmission)?

If you plan to use a manual transmission you may allow for a more interactive driving experience, but you will also want to ensure you are comfortable locking gears that you will no longer need (often 1st and/or 4th). Often when building a manual EV, the motor can spin through first gear so quickly it becomes only useful for doing burnouts. This can create a somewhat clunky driving experience but for those that enjoy a more interactive drive, it may be the right fit.

Building an EV may seem like a complex undertaking at first glance, but once you understand new technology, it’s actually much simpler than its combustion counterpart.

Building an EV may seem like a complex undertaking at first glance, but once you understand new technology, it’s actually much simpler than its combustion counterpart. Understanding whether gear reduction is beneficial to your build is just one small step towards successfully building your own EV. To understand how to install your own Torque Box or simply understand more on the intricacies of your own EV build make sure to check out the next issue of EV builders guide and read more about Legacy EV’s certified technician program on pg. ___ of this issue.