Racecar Engineering – September 2019

SEPTEMBER 2019 http://www.racecar-engineering.com 67

Figure2: Thrustcurvesfora 5-speedgearbox.Bluelineillustratesthemaximumspeedof thecardictatedbydrag

Figure3: Thrustcurvesfora 6-speedgearbox.Notethattheredareasaremuchsmallercomparedto Figure 2

Figure 4: Peak torque curve of an electric motor. Note gradual decline as the unit becomes power limited

‘Therefore, the aim is to use the number of ratios

that get the thrust curves to maximise the space

underneath the power curve as this means

that the maximum amount of power is getting

transmitted to the ground. First gear is a large

reduction and so transmits a large amount of

torque, which is why the tractive effort is off

the chart. By the time you get to fifth gear the

peaky shape of the thrust curves have levelled

out. The red sections highlight areas under

the power curve where you are not getting

the maximum tractive effort available, which

therefore presents an opportunity.’

Figure 3 shows the thrust curves for a

6-speed gearbox and as you can see, although

still evident, the red areas are much smaller.

Therefore, in the case of this IC-engined racecar,

switching to a 6-speed gearbox maximises

the amount of power transmitted to the track;

improving performance compared to a 5-speed

gearbox. However, this also comes with the

compromise of not only having to carry more

weight around, but also having more gearshifts

for the driver to make, which introduces torque-

interrupts during acceleration.

However, a higher number of gears is

counter-intuitive in Formula E because the

peak torque curve for an electric motor is

a completely different shape to that of an

engine, as illustrated by Figure 4. Peak torque

is instantaneous and then continuous for

approximately 50 per cent of the speed range,

gradually decreasing throughout the rest of the

rpm range as the motor becomes power limited.

If we then plot the thrust curves for the season

one 5-speed gearbox, we get Figure 5.

Sparked into life

The first season of Formula E ran from 2014

to 2015 and was effectively a spec series. The

Gen1 car (STR01) produced by Spark Racing

technologies featured a standard 28kWh

battery from Williams Advanced Engineering.

A higher number of

gears is counter-

intuitive in Formula E

because the peak

torque curve for

an electric motor

is a completely

different shape to

that of an IC engine