Formula E cars use brake-by-wire on the rear axle to control braking with electronics instead of a simple direct hydraulic link from the pedal to the rear calipers. This matters because the rear electric motor can slow the car while converting kinetic energy back into electrical energy for the battery. The system must still give the driver stable, predictable braking at very high speeds.
It blends two braking methods: regenerative braking from the motor and friction braking from brake pads pressing on discs.
When the driver presses the brake pedal, sensors measure the requested deceleration and send that information to a control unit. The controller decides how much braking torque should come from motor regeneration and how much should come from hydraulic friction brakes. If the battery cannot accept more power, the tires are near their grip limit, or the car needs extra braking force, the system increases friction braking.
The engineering challenge is to recover as much energy as possible while keeping the rear wheels stable and the pedal feel consistent.
Key Facts
- Braking force creates a torque at the wheel: τ = F r
- Regenerative braking converts kinetic energy into electrical energy: E_k = 1/2 m v^2
- Mechanical power at the wheel is P = τω
- Braking power can also be estimated by P = Fv
- Total rear braking torque is blended: τ_total = τ_regen + τ_friction
- Maximum tire braking force is limited by grip: F_max = μN
Vocabulary
- Brake-by-wire
- A braking system that uses sensors, electronics, and actuators to control braking force instead of relying only on direct mechanical or hydraulic links.
- Regenerative braking
- A braking method in which an electric motor acts as a generator to slow the vehicle and send energy back to the battery.
- Friction brake
- A brake that slows a wheel by pressing pads against a disc and converting kinetic energy into heat.
- Braking torque
- The turning effect applied opposite the wheel rotation to slow the vehicle.
- Brake blending
- The controlled sharing of braking demand between regenerative braking and friction braking.
Common Mistakes to Avoid
- Assuming regenerative braking can always provide all the braking force, which is wrong because motor power, battery charge rate, speed, and tire grip all limit how much regen can be used.
- Forgetting that friction brakes still matter, which is wrong because they provide backup braking, extra peak braking force, and braking when regeneration is limited.
- Treating brake pedal position as the same thing as hydraulic pressure, which is wrong in brake-by-wire because the pedal is mainly an input signal interpreted by a controller.
- Ignoring tire grip when calculating braking, which is wrong because even a powerful brake system cannot create more deceleration than the tires can transmit to the track.
Practice Questions
- 1 A Formula E car has mass 900 kg and slows from 50 m/s to 30 m/s. How much kinetic energy is removed from the car?
- 2 A rear axle needs 1800 N m of total braking torque. If regenerative braking supplies 1200 N m, how much torque must the friction brakes supply?
- 3 Explain why a brake-by-wire controller may reduce regenerative braking and increase friction braking even when the driver presses the pedal by the same amount.