20/06/2019
NEWS STORY
Brakes are high priority for F1 teams. If they are not working optimally or a driver uses them incorrectly, it can prove costly - both for lap time and track position.
On the other hand, making sure that you maximise the stopping potential of the car and tailor the brake settings to specific corner characteristics can improve your performance significantly.
Braking is the first element in a Formula One car's cornering phase. If the car isn't slowed down at the right point and with the right pressure on the pedal, it will compromise the remaining phases - hitting the apex, taking the right line, carrying the optimum speed through the corner, getting the power down on exit and completing a clean run to the next turn. This can have a major impact on a driver's lap time.
Similar to a road car, the brakes on a Formula One car work on all four wheels. When the driver steps on the brake pedal, it compresses two master brake cylinders - one for the front wheels and one for the rear - which generate fluid pressure.
At the front, the system is very straightforward. The fluid pressure is delivered directly to the front brake callipers. Inside each calliper, six pistons clamp pads against the disc - and it is this friction that slows the car down. At the rear, things are a lot more complex...
At the rear, the wheels can be decelerated by three separate sources: friction from the brakes, resistance from the spinning engine - so called "engine braking" - and finally, electrical braking that results from harvesting energy by the hybrid electric motor - the MGU-K (Motor Generator Unit - Kinetic).
Although the driver can adjust each of these effects independently on his steering wheel, when he presses the brake pedal, the three systems act in concert via the Brake By Wire (BBW) system to provide the driver the overall retardation he has requested.
When the driver presses the pedal, the fluid pressure he generates in the rear braking circuit is picked up by an electronic pressure sensor. The signal from this sensor represents the overall rear braking demand from the driver and is passed to the Electronic Control Unit (ECU) where it is turned into a series of commands to brake the rear of the car. The harder the driver presses, the larger the signal; the larger the signal the more aggressively the ECU will send out demands to the three rear systems (the brake callipers, the engine braking and the MGU-K) to provide the retardation requested. The ECU distributes its efforts to the three system according to the manner that the team has set up the car, modified by the way that the driver has adjusted the switch settings on the steering wheel.
Compared with the front, at first glance the rear system looks byzantine in its complexity. Why would we ever want to use a hydraulic master cylinder to generate pressure to be picked up by a pressure sensor simply to generate an electronic demand signal to an ECU? Why would we not do it much more simply by measuring the position of the brake pedal to get an electronic demand signal? Why would we ever want to have a rear braking system that arbitrates between three separate systems when we could simply use conventional brakes like a normal car?
The answers to these questions fall in two camps: safety and performance. Once you have committed to using a Brake By Wire system to control the rear braking, you need to ensure that there is a safe backup in the event that the system fails. This is why teams go to all the trouble of using the driver's pedal action to produce hydraulic pressure in a brake line. If the BBW system ever fails (and there is a set of sensors and a computer routine continuously devoted to checking its integrity), then it is immediately bypassed, and the pressure generated by the driver's foot is passed directly to the rear brake callipers just like in a normal car. The more interesting question is why does a Brake By Wire System offer the performance to justify such complexity - the answer to this is worthy of a section of its own.
If you spend any time with a driver listening to them talk about the car, one of the most common terms you will hear them refer to is "braking stability". Drivers, of course, want brakes that have good "bite" (sharp initial deceleration when they stamp on the pedal), they want strong deceleration without any "fade" (more of this later), they want good "feel" (a sort of predictable response, push harder = stop more, push less = stop less), but above all, they want good braking stability.
Unlike bite, fade and feel, something that all drivers will be able to relate to, it is much harder to understand what a driver means when they talk about "braking stability". This is because we do not use brakes in the way that racing drivers use brakes.
When we use the brakes in a road car, we are generally pointed in a straight line, and we rarely push them hard enough to make the tyres come close to activating the anti-lock brake system. Racing drivers operate in a very different place. Every time they press the brakes they want to slow down as rapidly as the car will permit them. This means that they push the brakes right up to the point where the tyres will lock up - every corner, every lap. Furthermore, they don't only press the brakes when the car is in a straight line. They push them from the end of the straight, they keep pushing them as they are turning in, and they only finally come off the brakes right at the apex of the corner, an instant before they start to apply the power for the exit. Throughout this entire time, the brakes are almost as important as the steering wheel for controlling the direction that the car is pointed.
During this manoeuvre, if things are going well for the driver, then the car is held with all four wheels right at the very limit of skidding but without the car deviating from the driving line that the driver wants to follow through the corner. If things are going slightly less well, then the front wheels might start to slide a little more than the rear wheels, giving the driver a lazy, understeering car that will not turn. If things are going less well still, then it might be the rear wheels that slide more than the front wheels, and if they slide too much, then the car will start to spin.
For this reason, the driver cares enormously how much braking happens on the front wheels compared to the rear wheels. If the car is unstable, and wanting to spin on corner entry, then you probably need to ask less of the rear brakes, and more of the front. If the car is lazy and understeery, then you would do the opposite.
Furthermore, as the corner progresses from initial braking to turn-in to corner apex, the driver wants different things. As the car starts to turn in, the car can often have a natural tendency to oversteer which is progressively replaced by understeer as the apex approaches. This tendency can be counteracted to some extent by a clever braking system which would ask less of the rear brakes on turn in (to stabilize the car) and then ask progressively more of the rear brakes (compared to the front) as the apex approaches. This clever process is called Brake Migration - a dynamic change of the brake balance as a function of the brake pressure.
It is this cleverness that the Brake By Wire system provides. Guided by the rotary switch settings that the driver has made on their steering wheel, the BBW system juggles the braking input of the three main actuators (the callipers, the engine and the MGU-K) to provide the driver with a smooth, predictable shape to the rear braking action that allows him to keep the car at limit of adhesion (without any anti-lock brakes) while steering the car through the braking phase of the corner.
The drivers really have to stamp on the brakes with every application, almost standing up in the car to do so. On road cars, servo-assisted brake systems multiply the pressure you apply to the master cylinder but the regulations in Formula One demand that the braking force has to be generated by the driver alone. They need very strong legs to do this, but they do get some help from the violence of the braking manoeuvre itself. The cars decelerate at around 5G (compared to the 1G we might see during an emergency brake in our road cars). At this deceleration, their leg will weigh approximately 100kg, and the weight of the leg on the brake pedal provides its own form of servo-assistance to help them - the harder they press, the more the car slows, the more it slows, the more their leg weighs which helps them to press harder. What is remarkable is that in the midst of all this , while pressing the pedal with well over 100kg of force, the driver is required to modulate his effort on the pedal with all the delicacy of a concert pianist in order to coax the car through the corner at the very limit of what the tyres will permit - it is a delightful contrast of violence and gentleness.
The perfect spot to hit the brakes in an F1 car will depend on a lot of variables - fuel loads, tyre compound, the amount of degradation on the tyre and the levels of management a driver is doing. Therefore, in the race, this will constantly be shifting, as fuel burns off and tyres wear down, with drivers having to be reactive.
Qualifying is less variable owing to similar fuel loads and fresh tyres, meaning the braking points remain largely the same. Drivers ramp up the braking as the weekend goes on, using practice to really find the limit. They'll start off conservatively, before pushing deeper and deeper into the corner until they find the right marker to begin applying the brake pedal.
Undoubtedly the trickiest moment of the weekend is braking for the first corner on lap one. First, drivers don't receive a whole lot of opportunity to drive on track on Sunday before the race begins. This means that drivers have to base the crucial decision of when to brake into Turn 1 on a rough estimate of the day's grip levels which they form on the laps to grid and the formation lap. Second, despite the usual weaving to warm the tyres on the formation lap and the fact that tyre blankets are fitted to the car as long as possible, brakes and tyres are still not at the optimal temperatures at the start of the race, making it harder to judge just how much stopping potential they'll provide. Finally, the field is bunched up at the start of the race, with all cars vying for the same patch of track. So, drivers need to react to many different elements - making sure they get into the right slipstream, predicting what rivals will be doing, estimating grip levels, ensuring they don't brake too early and lose positions, but also taking any opportunity to gain places.
Maximum temperatures for the brake discs can reach 1,000 degrees C or more. The carbon discs can easily handle these individual peak temperatures; however, high temperatures over a prolonged period can create some issues. Cooling mainly takes place on straights, when the car is travelling at high speeds, allowing for a lot of air to pass through the brake ducts. On a track like Monaco, for example, cooling the brakes can become a real problem despite the relatively low speeds as there are a lot of corners and thus a lot of braking with only very short straights in between.
The brakes need air passing through the brake ducts and out through the uprights to cool them down. There are over 1,000 holes drilled into the sides of the brake disc to maximise the surface area and therefore the cooling potential. While those holes help to drop the temperatures significantly when the car is going down the straight, they also mean that the discs reach higher temperatures because the thermal mass of the disc is lower.
The issue with a brake disc that is too hot is that you experience a phenomenon known as 'fade'. Fade means that there is not enough mutual friction between the pads and discs, rendering the brakes much less effective to slow the car. And it's not just the brake performance that suffers from high brake temperatures: the dispersed heat from the brakes also needs to be managed, as it exits around the wheels and tyres, which work in their own temperature windows to achieve peak performance. The brake duct brings in air to cool the temperatures down, but this also impacts aerodynamic performance. The bigger the air duct, the worse the impact on aero performance, but the more cooling that is gained. So, a balance must be found in order to provide the right level of cooling while also not negatively impacting the aerodynamic flow around the brakes. The brakes can run as low as 200°C. If the brakes are too cold, there isn't enough bite or initial grip to slow the car down. So, temperature management is a decisive factor in the performance of the brakes on an F1 car and getting them in the right window is crucial. This is particularly tough at key points in the race weekend like the race start or a Safety Car restart. F1 drivers will therefore oftentimes swerve around when they follow the Safety Car or press the BW ("Brake Warming") button on their steering wheels which lets them override the brake balance.
Circuits can pose challenges for different reasons when it comes to braking. Somewhere like Baku features a lot of twisty, lengthy sections of corners, where speeds are relatively low. So, the brakes can't be cooled as much. The long start/finish straight provides a welcome chance for the brakes to reduce in temperature, but they are then potentially too cold for the heavy stop of Turn 1. Monaco is tough owing to the fact the car speeds are so low, therefore less air is going through the ducts to cool the brakes down. It's also a relentless track for corners, with very few straights. So, brakes can get very hot. Somewhere like Canada also punishes the brakes, as there are continuous long straights and heavy stops, which puts the braking system through its paces and can lead to higher wear rates or higher temperatures than at other circuits.
Lock-ups are a relatively common phenomenon. They happen when too much force is applied to the brakes, causing the disc to stop or rotate slower than the car's motion. The tyre then scrubs along the surface of the track, sometimes creating white smoke. While you can see this happen relatively often in F1, lock-ups have become very rare in the road car world. There are two reasons for that. Aerodynamics play a major role in Formula One and mean that the faster an F1 car goes, the more downforce it creates. When the downforce increases, so does the grip level - which means that the cars have more stopping potential at high speeds than they have at low speeds. This also means that the grip level constantly changes while the car is slowing down. It would be relatively difficult to lock the wheels when the car is going 300 km/h; however, it is much easier to do at speeds below 100 km/h.
Drivers therefore typically hit the brake pedal harder when entering a braking zone as this is when the car has its maximum stopping potential, before easing off as they get towards the turning phase to try and avoid a lock-up.
But there's another reason why F1 cars lock up more often than road cars: modern road cars are all equipped with anti-lock braking systems (ABS); however, the regulations in F1 don't permit ABS. The introduction of ABS is generally considered one of the most important safety innovations in the automotive world that helped to dramatically reduce accident numbers as it means that the car will still react to steering input even in an emergency braking condition. Mercedes-Benz played an important role in its inception. In 1970, the company showcased the first generation of anti-lock braking system for passenger cars, commercial trucks and buses; eight years later the S Class was the first production car to offer second generation electronic four-wheel multi-channel ABS.
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