Mechanical construction of drivetrains

29.3.2016 | author Petr Sucháček | translated by Mark Gurevich

As implied by it's name, our open wheeler is an all-wheel drive vehicle. Although it wasn’t always like this.

The first electric formula car, developed by the CTU's Faculty of Electrical Engineering, was driven by one electric motor with a maximum output power of 30 kW and an RPM value of 3000. The power was transmitted by a planetary gearbox (3:1 gear ratio) and a chain drive (1,3:1 gear ratio) to a limited slip differential. In spite the fact, that it was the first car, the aim was to construct a drivetrain from market-available and purchasable components. Then, in order to be able to carry the system, frame had to be quite complex. Resulting weight of the drivetrain was 39 kg, that without the frame, which, as a result of its complexity, would add to that ten more kilos.

The second and the third electric formula vehicles ran on a similar drivetrain system. These cars were driven by two electric motors, each responsible for traction of one rear wheel (with the help of electronic differential). In order to simplify the whole traction system, it was decided to use motors with low RPM and high torque without any further gear transfer. Therefore halfshafts were fastened directly on the output drive shaft of the motor through tripod joints.

We could afford to use this concept of a drivetrain thanks to our partnership with TG Drives, a company from Brno, which provided us with made-to-measure motors with required specifications. As a result we got 2 electric motors, each with a remarkable 35kW power output, maximum torque of 450 Nm and a weight of 20 kgs. Motors were connected together by back shells making them a one compact compound. Resulting weight of the powertrain featured 43 kg(40 kg for motors, 3 kg for halfshafts and tripod joints)

The further the development process of FSE.04x, our fourth formula car, proceeded the more concerned we became about the impact of increased power on the carriage. That is why we decided, that the car should be all-wheel drive. Along with that, as we watched other teams’ all-wheel wheel drive cars perform, we noticed their outstanding dynamic qualities, especially on low-friction conditions.

Then we decided to aim for maximum possible weight reduction. So, we again contacted TG Drives for help with the development of an electric motor for our new open wheeler. We agreed upon high RPM motors (max. 9000 RPM), with a significantly higher power output. As in previous models, two rear motors, with maximum power output of 2 x 23 kW and each 9 kg heavy, were placed in the back of the vehicle. Each motor recieved a custom spur gears gearbox with 5,3:1 gear ratio, with it’s output adapted for a tripod joint. Whole rear drivetrain system indicated then 25 kg.

Zadní motory, jejichž maximální výkon je 2 x 23 kW a hmotnost každého 9 kg, jsme umístili do zadní části rámu formule, podobně jako pohonné ústrojí předchozích elektroformulí. K těmto elektromotorům jsme navrhli vlastní převodovky s čelním ozubením s převodovým poměrem 5,3:1, jejichž výstup je přímo uzpůsoben pro tripodový kloub. Celá zástavba zadního pohonu pak váží 25 kg.

Front motors with an output power of 2 x 8 kW and a weight of 3,4 kg each, were on the contrary placed directly inside front wheels. This decision has proved itself as the simplest and most effective regarding the lack of installation space in the front of our formula car, as well as because of a considerable weight of constant-velocity joints transmitting power to the steering. The motor was secured inside steering knuckle, with an integrated planetary gearbox with a 5,3:1 gear ratio (similar to back axle in terms of max. RPM). The output of the planetary gearbox leads straight to the wheel hub that rotates the rim and the tire.

Motors were designed in such a way, that output power of each would correspond to the maximum momentum transfer on the axle its placed upon during acceleration. Hence, the powertrain weight indicated 30 kg and a total output power of 66 kW. With the car weighting approx. 200 kg that would allow to reach the load value of up to 1,1 g and to accelarate from 0 to 100 km/h in 3,8 s.

Each of front wheels was now driven by 16 kW motors. That significantly improved traction, dynamics and especially handling of the vehicle. The handling was supported by an electronic system that distributed torque to each wheel. This system simulated the function of certain differentials (front, rear and inter-axle) distributing torque to each wheel, according to settings set by the pilot. That means, in the event of extreme turns the car is able to turn almost on the spot.

This concept of a powertrain proved itself in such a good way, that, with minor adjustments, it wil be used for the construction of FSE.05.