Gears. Put simply, they’re levers, multiplying small amounts of effort into much bigger results. Think of your pushbike – you can shift gears to allow you to continue crawling up ever steeper hills without suddenly needing superhuman thighs. When going downhill, you can use the gears to stop your legs spinning round helter-skelter. The right gears will make sure you get the most from your exertions, which is important when you only have a finite energy store – such as a Formula E battery.
Formula E’s first generation of racing cars all featured a five-speed gearbox, made by Hewland to a spec set by motor producer McLaren. The second generation of powertrain, however, features a wide range of solutions, with everything from a single gear to the full five on show. Gearboxes are a mature technology, so why the diversity in choice? And, given that the electric motor is able to deliver peak torque from nearly zero revs, why bother with one at all?
“Simply put, using a gearbox helps you to extract the maximum efficiency possible from your chosen electric motor and to stay at full power for as long as possible at all road speeds,” says Vin Patel, chief engineer at Mahindra Racing. The Indian team has elected to use a four-speed ’box in season two, dropping a gear from the first season unit. “Motor efficiency varies greatly depending on its architecture and design philosophy. This mainly translates as variations of efficiency at different motor speeds.”
In the second season, different motors are in use, with a twin-unit configuration found at DS Virgin Racing and NextEV TCR and single motors from varying suppliers employed elsewhere in the pit lane. Mahindra Racing’s M2Electro uses a revised and updated version of the McLaren unit used in season one, developed in partnership with McLaren Applied Technologies. Altering the specification of key components of the motor, improving materials used and revising software controls resulted in a 20% increase in peak torque and a 15% increase in rpm.
“Some motors might rev up to 25,000rpm while others only reach 9,000rpm,” Patel goes on. “They will be delivering very different torque bands, which need to be translated into extracting maximum permitted power from the battery. Using a gearbox helps you keep the motor in the most efficient operating range possible.”
There’s another fundamental reason why Formula E cars use a gearbox: because the rules say so. “The regulations stipulate that the motor has to deliver drive to both wheels through a single mechanical transmission, so you need at least one gear set to accomplish this final drive,” Patel confirms.
The design timeline for the second season gearbox was extremely tight and had to be wrapped up before the first season was even halfway through, which posed significant challenges. “We had to begin specification and ratio selection in January 2015, having only completed three races!” says Patel. “We had no idea about the requirements for the rest of the tracks that season but we still had to design a gearbox which would cope with them in season two.”
That mystery factor influenced Mahindra to take an evolutionary rather than revolutionary route for its M2Electro gearbox, choosing to refine the first season unit with Hewland instead of going for something very new (which could have effectively been an expensive gamble).
“The gearbox is arguably the longest lead item,” says Patel. “It’s a complicated product with many intricate and time-consuming areas of design. In all, we took 18 weeks to develop our M2Electro unit, which was a relatively quick design and development timeline.”
The result is a four-speed gearbox with a heat-treated magnesium casing and precision ground high grade steel ratios. “The gears have superfinished coatings for better durability and lower friction,” explains Patel. “In season one, you were allowed to get into the gearbox and differential to service the internals. In season two, the gearbox was sealed the night before Beijing. You have a ‘joker’ – a spare unit that you can swap if you run into problems, but there are penalties associated with opening up the gearbox.”
The M2Electro continues to use an electro-pneumatic gear shift mechanism, controlled by the driver using steering-wheel mounted paddles. “We’re using a carbon fibre compressed air reservoir in place of the first season compressor to reduce sapping powertrain energy,” Patel says.
Despite gearbox technology being well understood, Formula E’s regenerative braking characteristics are very different from other racing cars and necessitated quite some thought. “Performance gearboxes are usually designed to take loads in one direction – forward drive,” says Patel. “Regenerative braking introduces significant loading in the opposite direction, especially in the instantaneous transition from 170kW driven power in race mode to 100kW regen power when entering braking zones.”
A reprofiled crown wheel and pinion has helped address the issue, along with software adaptations designed to smooth transitions. The casing has also been resigned to cope with the unique pressures of Formula E racing, where the cars car often brush the walls lining the sport’s narrow street circuits. In the first half of season one, this led to many wrecked gearboxes; a crude (and weighty) strut was added to alleviate the issue in the second half of the season. In season two, these stresses are now known factors and have been engineered into the product from the beginning (and, as drivers have got more used to the characteristics of the cars, they’re doing a better job at avoiding contact with the concrete).
Current championship leader Renault went for a two-gear design for season two, with an impressive but pricey carbon fibre motor bellhousing and gearbox casing. ABT is running second in the teams’ standings at time of writing, with a three-speed configuration. Dragon Racing is in third, with four gears, while the single-geared, twin-motor DS Virgin Racing car is in fourth, ahead of Mahindra Racing. Does that suggest that fewer gears are better?
“For season three, we’ve revolutionised our approach,” notes Patel. “We’ve changed product partners and re-engineered our design philosophy. The gearbox is potentially the single biggest performance differentiator in Formula E due to the efficiency gains that can be made. In season two, we took a small step forward from season one, making improvements in ratio selection, weight and strength while leaning heavily on the base design of the original unit. From our research, manufacturers like Renault took a much bigger step. We now know what we’re aiming for in season three.”
The team’s third season car, the M3Electro, has successful passed its powertrain crash tests and is set to begin track testing before the Berlin 2016 race.
M2 Electro: gearbox summary
- Length: 678mm
- Width: 312mm
- Height: 296mm
- Weight: classified (estimated 30kg to 40kg)
- Materials: heat-treated magnesium casing, high grade steel gears
- Gears: four
- Servicing: sealed for the season