Formula One: Brake-by-wire – How it works

In the closing laps of the Bahrain Grand Prix, both Mercedes W06s suffered near-simultaneous brake-by-wire failures, the cause of which was mainly attributed to set-up changes to the cars of Lewis Hamilton and Nico Rosberg which put an extra strain on the brakes and compromised the entire system.

F1i
by F1i
26th April 2015

article:26th April 2015

While F1 cars have used fly-by-wire for throttle input since 1992, the good old brake pedal remained until very recently mechanically linked to the master cylinders.
While F1 cars have used fly-by-wire for throttle input since 1992, the good old brake pedal remained until very recently mechanically linked to the master cylinders.

In the closing laps of the Bahrain Grand Prix, both Mercedes W06s suffered near-simultaneous brake-by-wire failures, the cause of which was mainly attributed to set-up changes to the cars of Lewis Hamilton and Nico Rosberg which put an extra strain on the brakes and compromised the entire system.

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F1i takes a closer look at the technical intricacies of the brake-by-wire concept and how it operates.

Along with the introduction by the FIA in 2014 of the new 1.6-litre turbocharged V6 'green' engines, the technical regulations also included a new rule allowing teams to use electronic brake assistance on the rear wheels for the very first time.

The purpose of this evolution found its reason in the considerable increase in harvesting demands of the energy recovery system (ERS) as well as its increased performance. 

While F1 cars have used fly-by-wire for throttle input since 1992, the good old brake pedal remained until very recently mechanically linked to the master cylinders and, ultimately, to the front and rear brake calipers.

The front brakes still operate this way, but the rear brakes are now electronically controlled in a manner which assesses how much brake pressure a driver has called for in order to slow the car in a consistent way while ensuring the maximum energy harvest for the ERS.

Energy recovery impact

A car's brake pedal is connected to two master cylinders, one which is linked to the front calipers and the other to the rear.

The master cylinders simply convert non-hydraulic pressure, typically induced by a drivers' foot,  into hydraulic pressure.

The incompressible hydraulic fluid is distributed to the calipers by Aeroquip piping, exerts pressure on a cylinder which then pushes the brake pads towards the carbon fibre disc in order to slow a car down.

In 2009, a kinetic energy recovery system (KERS) was added to F1's conventional braking system.

It featured a device connected directly to the drive shaft through a motor-generator that, under braking, driven by the same shaft, converted kinetic energy into electrical energy.

Until  2013, the KERS system allowed drivers 6.7 seconds of power from the 60kW unit which offered 400kJ of usable storage.

A dial on the steering wheel enabled the driver to set the rate at which the KERS unit harvested and stored its energy.

A new ERS system introduced in 2014, which now goes by the name "MGU-K" (for kinetic motor generator unit), doubled the energy harvesting potential, with power transmission now amounting to 120 kW, while a maximum of 2MJ per lap can be transferred to the energy store (or battery).

Because of this considerable increase in performance, and the subsequent increase and variations in reverse torque in the rear axle (what is commonly known as engine braking), it was essential that the engineers install a system to compensate for the powerful effect the MGU-K has on brake balance and braking stability.

Ultimately, this was the reason for the FIA allowing the adoption of an electronically controlled brake balance in order to ensure the system did not destabilise a car with a sudden balance shift, and help it remain as consistent and stable as possible.

How it works

The rear brake system includes sensors, two hydraulic pumps (instead of one) and an ECU, a self-contained engine management system and data logger manufactured by McLaren Electronics.

Basically, when the brake pedal is pressed, the control unit uses information from a sensor fitted to the rear master-cylinder to determine how much braking force each rear caliper needs.

Relevant information such as the pressure a driver is applying to the brake pedal, and the level of energy harvest the driver is requesting under braking to charge the battery are also sent to and processed by the ECU.

The unit will then combine the two signals, and the rear-brake pressure needed to achieve the front-to-rear brake balance requested by the driver will be defined.

Pressure is generated by a hydraulic pump and managed by an ERS brake pressure reducing valve mounted near the rear of the car on the pipework that would normally feed the brake fluid to the rear calipers.

In short, the additional braking capacity of the MGU-K produced by the reverse torque effect on the drive shaft (in order to recover a certain amount of energy) will be compensated, in a split second, by bleeding excess hydraulic fluid back to the fluid tank which will subsequently decrease the amount of pressure applied to the rear calipers, thus appropriately balancing the conventional brakes with the ERS braking.

In the early stages of braking, the calipers do the main work but progressively, pressure is modulated according to a series of criteria: the quantity of energy generated by the MGU-K (limited to 2MJ per lap) and how much needs to be stored, brake temperatures, and the front-to-rear brake balance required by the driver in order to insure a stable ride during the slow down phase.

The influx of real time information to the ECU is incessant, while the brake pressure reducing valve works overtime, continuously opening and shutting to maintain the varying pressure and give a stable front-to-rear brake balance. Managing the complex series of procedures in a reliable way and, especially, getting the feel right for the driver were major headaches for most of F1's teams back in 2014.

Rod Nelson: "Its key to the mapping and the brake setup that when you come off the brakes there is no residual force that may give a little bit of instability or a lock up. Some drivers are very, very sensitive to this."

“The driver needs to have a good feeling of retardation versus pressure that is not steppy or moves around, it has to stay the same," explains Williams Chief Test Engineer Rod Nelson.

"He can adjust the bias forwards or rearwards as in the past but we are also balancing how much energy he uses from the rears with how much we are trying to recover.

Its key to the mapping and the brake setup that when you come off the brakes there is no residual force that may give a little bit of instability or a lock up. Some drivers are very, very sensitive to this."

Losing weight

With so much braking now being done by the torque reversal upon the rear axle of the ERS, several teams have reduced the size of their brake discs – mainly for weight and aerodynamic benefit.

In most cases, diameter and thickness has been reduced by 3mm than the maximum allowable 28mm.

Six-pot calipers have been replaced by four-pots at Mercedes, Red Bull, McLaren and Sauber. (And some engineers have even suggested that it would be possible to run no rear brake discs at all if the regulations allowed it, with all the braking done by the ERS!).

In the event of a failure of the MGU-K unit a back-up hydraulic system will intervene.

But the downsizing of the carbon fiber discs provides the system with a limited efficiency, as was witnessed in Sakhir last Sunday, but also in Canada last year, a track which is particularly demanding on brakes and where Lewis Hamilton suffered a brake disc failure when the MGU-K unit ceased to function following an electrical break down.

In the end, the arrival of the brake-by-wire system means that only two actions remain in the mechanical control of the driver: steering the front wheels and the pressure applied to the front brakes. Every other system on a F1 car is now 'drive-by-wire'.

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