What is a Gear Ratio of an E-Axle?

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Editorial Team - everything PE

May 10, 2024

Gear Ratio refers to the ratio of the rotational speeds of the first and final gears in a gear train or any two meshing gears. In other words, it is the ratio of the number of teeth on one gear to the number of teeth on another gear in a mechanical system. It quantifies how many times one gear has to turn for another gear to turn once, influencing torque, speed, and efficiency in mechanical systems like vehicles. This ratio determines how much the torque is multiplied or the speed is reduced between the first and final gears. Gear ratio is crucial in automotive systems where it influences torque, speed, and overall efficiency of a vehicle.

Calculation of Gear Ratio

In a gear set, the gear that provides the power is called the driver or input gear. The second gear which is turned by the driver gear is called the driven or output gear. The size of the gear is not important, a small gear can drive a big gear or a big gear can drive a small gear. 

Gear ratio is calculated by dividing the number of teeth on the driven gear (output) by the number of teeth on the driving gear (input).

The driving gear will turn two times for each revolution of the driven gear. This results in a speed reduction and a torque increase. The speed of the output will be two times slower than the input speed but, the output torque will be twice that of input torque. The higher the ratio number, the lower the gear ratio. A 10:1 ratio is higher numerically, but, in terms of the speed of the driven gear, it is a lower ratio than 8:1.

Gear Mechanism in Electric Vehicles

The gear mechanism in electric vehicles (EVs) differs significantly from traditional internal combustion engine vehicles. EVs usually have a single-speed transmission, also known as a gear reduction unit, which is designed to optimize the performance and efficiency of the electric motor. The key aspects of the EV gear mechanism include:

  • Single-Speed Transmission: Most electric vehicles are equipped with a single-speed transmission that efficiently transfers power from the electric motor to the wheels without the need for multiple gears.
  • Efficiency and Performance: Electric motors in EVs operate efficiently across a wide range of revolutions per minute (RPMs), generating instant torque even at low speeds, eliminating the need for multiple gears to maintain performance and efficiency.
  • High RPM Range: Electric motors in EVs can rev at much higher rates compared to internal combustion engines, with some reaching up to 10,000 to 20,000 RPM, making a single-gear ratio sufficient for acceleration and top speed.
  • Optimal Power Band: Unlike internal combustion engines that require gearing to stay within an optimal power band, electric motors do not have such limitations, operating efficiently across a broad RPM range.
  • Benefits of Single Gear: Having a single gear in EVs offers benefits such as efficiency, reduced weight, simplicity, and durability, contributing to improved range and performance.

Gear Ratio of an E-axle Vs Traditional Axle

Gear Ratio of an E-axle
Gear Ratio of a Traditional Axle

E-axles are designed for electric vehicles (EVs) that have higher gear ratios compared to traditional axles to optimize torque delivery and efficiency.

Traditional axles are used in internal combustion engine vehicles that usually have lower gear ratios to balance torque and speed requirements.

E-axles of EVs have gear ratios like 1:10 or 1:12.5, providing more torque multiplication for improved acceleration and performance.

The gear ratios in traditional axles are optimized for the characteristics of combustion engines, considering factors like power band and fuel efficiency.

The gear ratios in e-axles are tailored to suit the characteristics of electric motors, focusing on maximizing efficiency and power delivery.

Traditional axles have gear ratios that vary based on the vehicle's intended use, balancing acceleration, top speed, and fuel economy.

Impact of Gear Ratio on Electric Vehicle Performance

Top Speed Calculation: The top speed of an electric vehicle is limited by the maximum rotating speed of the motor. Dividing this speed by the gear ratio determines the rotational speed of the wheels, which, when multiplied by the circumference of the tires, gives the theoretical maximum speed of the vehicle.

where 

  • RPM is the revolutions per minute of the motor
  • Tire circumference is the circumference of the vehicle’s tire

Rotational Speed: A lower gear ratio results in a higher rotational speed of the wheels, allowing the vehicle to reach higher speeds. Conversely, a higher gear ratio reduces the rotational speed and limits the top speed of the vehicle.

Efficiency and Complexity: Electric motors can reach high speeds efficiently, which enables them to operate with a single gear and still achieve high speeds. This eliminates the need for a transmission, reducing the complexity and weight of the vehicle.

Advancements in EV Technology: As advancements in electric vehicle technology continue, top speeds are on the rise, allowing certain luxury electric cars to achieve velocities that were traditionally linked with internal combustion engine vehicles. This development removes a former constraint for consumers contemplating the transition to electric vehicles.