So what IS brake by wire?

This article is from Racecar Engineering.

When the driver hits the brake it is not just the carbon brake discs and pads that slows the car down on a 2014 car, the energy recover system also does a significant amount too, rather like engine braking but a much stronger effect. This means that the drivers left pedal (F1 cars have no clutch pedal) is no longer linked directly to the rear brakes instead it is linked to a computer which then controls the rear brakes. The front brakes continue to operate in the same way as they always have done.

The main reason for this is that rules say that the car is only allowed to recover a certain amount of energy per lap from the rear brakes (it does not recover from the fronts), and there is only a finite amount of energy that can be stored in the battery. When either of these limits is reached the ERS stops recovering energy and the braking effect is lost and the traditional brakes take over. But for the driver it is important to retain the brake feeling otherwise when he hits the pedal he is never quite sure what will happen. If you imagine driving down a steep hill in a low gear using the engine braking alone to slow you down, then suddenly that braking effect stops it makes the car almost impossible to drive smoothly. The semi active Brake By Wire system should stop that from happening and automatically balance the conventional brakes with the ERS braking. But getting to work properly is a problem currently for a number of teams including Lotus “the biggest problems are how the chassis works with the power unit and how the energy recovery system works. So there are some inconsistencies there which are making it very difficult for the driver to predict what he is going to get when he arrives at the corner” Technical Director Nick Chester admitted. “So the system is not doing exactly the same thing every time and that is disturbing the driver and losing us a lot of time.”

Getting the feel right for drivers is a major headache for some teams as well as making the systems reliable. At Melbourne Brake By Wire issues directly lead to a few off track moments for some drivers.
“You just take the hydraulic inputs that the FIA specify and work with an electronically controlled hydraulic link to the caliper, at the same time you have some redundancy in there so if you have a failure it should revert to a manual brake circuit” Toro Rosso Technical Director James Key explains. “You have to account for any failure mode you can think of both mechanically and in software. Its bit like a differential or a clutch, but the tricky bit is mapping it well.”

Mapping the systems is an area where some teams, notably those Renault runners who lost track time at the Jerez and Bahrain 1 tests, will be struggling in terms of time. “Brake by wire is a massive for us in 2014, you have control system mapping, driver mapping to get him comfortable, you have state of charge control, making sure the battery topped up at the right time and temperature and vibration and that is just one system” Williams Chief Test Engineer Rod Nelson explains. “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. 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.

We can model the brakes on the simulator and that is what we have done, but they are not straightforward as there is a thermal effect, the amount of stopping power the brakes have depends on the temperature of the brake so that's an input we need to understand. We set a recovery target for each lap, so whatever a driver does not put in the MGU does. We have had issues with losing brake by wire and the driver ends up on his own. The pedal has a very different feel when that happens it is much softer than you expect it to be. More significantly the brake bias shifts substantially, so if you come into a corner with a Brake By Wire failure then you are going to get a wake up call, it gets them thinking.”

It also create a challenge for the caliper manufacturers like Alcon, AP, Brembo and Akebono who have to develop control systems to aid the braking effort at the rear, negating the need for the driver to constantly alter the brake bias, and also contributing in preventing rear lock-up.

The arrival of Brake By Wire in F1 means that now the only things the driver now controls mechanically are the steering angle of the front wheels and the pressure applied to the front brakes. Every other system on the car is now drive by wire.