Formula 1 cars can come to a stop from 100
km/h in about 15 metres, which is almost a quarter of the braking distance of your average
road car. They can go from 300 km/h to a complete stop in under four seconds, pulling up to
6G of deceleration force. With such high speeds and tough corners, F1
cars need to be able to produce massive braking forces – not just for performance but for
safety as well. A driver needs to know the car will respond when they press the brake
pedal, and not cream into a wall or the back of a competitor.
Let’s look at the braking system of an F1 car as a whole before we dive into the individual
components. When the driver hits the brake pedal, it transmits
a force to two master cylinders. One cylinder controls rear braking and the other front
braking. Let’s focus on the front braking to start with as it’s much simpler.
The master cylinder acts on brake callipers which squeeze brake pads onto the brake discs
– this hard friction between the brake pads and the discs slows the car down.
Let’s take a closer look at those components then.
These master cylinders are filled with brake fluid – just a couple of hundred milliletres
worth. The fluid fills brake lines that run from these cylinders to the brake callipers,
acting as arteries of the braking system. Fluid is incompressible, so when the pedal
is pressed and the plunger is pushed into the cylinder the fluid immediately puts forces
on the other ends of the brake line. This is how hydraulic systems work.
The brake callipers are like clamshells around the brake discs and house brake pads within
each side of the shell. The hydraulics feed into pistons – no more
than six – within the callipers; these pistons push the brake pads into the brake discs.
As the brake discs are attached to and spin with the wheels, when the pads clamp on the
wheels, the frictional force between them will slow the spinning of the wheel and ultimately
the speed of the car. The callipers themselves are often mounted
low on the discs to keep centre of mass low, but tend to be placed closer to the 5 or 7
o’ clock position rather than lowest 6 o’ clock position. This is partly because the
bleed nipple needs to be fairly high. ‘A bleed nipple?’ you asked with horror
in your eyes. Well, remember when I said fluid was incompressible and that was what allowed
pedal force to instantly translate to the brakes? Well sometimes air bubbles can get
into the hydraulics and gas is compressible. So when the brake pedal is pushed, the gas
in the hydraulic system can deform, reducing the braking force at the other end.
To flush this gas out, you can open the nipple and – as it’s placed high up, the gas
will rise more readily to the top and be flushed out when you force fluid into the system.
You’ll often bleed the system between sessions to be on the safe side.
Onto the actual brake pad and discs, then. The brake discs cannot be larger than 278
mm (11 inches) in diameter [USE SCHOOL RULER] or 32 mm thick. A larger diameter means greater
stopping power as its easier to stop a spinning disc by grabbing it further from the pivot
point than closer to the centre. The restriction of the rules in this area is to limit the
braking power of the car so braking zones can remain somewhat competitive.
Unlike the steel-type brakes on modern road cars, F1 brakes are made of a special carbon
composite called, hilariously, carbon-carbon. It’s called this because it’s two types
of carbon composited together – a carbon lattice like graphite reinforced with carbon
fibres. Carbon-carbon is strong, can withstand very
high temperatures and has a very high coefficient of friction. The coefficient of friction of
a material just tells you how well a material grips when rubbing against another material
– ice, being slidey has a low coefficient of friction; rubber, being not slidey as all,has
a high coefficient of friction. Carbon-carbon also has a very low thermal
expansion and low thermal shock – meaning it won’t deform or crack suddenly under
high temperatures. This ability to stay robust under high temperatures is incredibly important.
The way brakes slow tyres down is by converting energy. The kinetic – or moving energy – of
the spinning wheels is converted by the braked into heat energy.
As the brake pads grip the discs, the high frictional forces turns the energy of the
wheel into tremendous amounts of heat. A cold brake can heat up by as much as 100°C
every tenth of a second in the initial phase of braking.
Carbon brakes work optimally between 400°C and 800°C, though heavy braking can often
push brakes to 1000°C or 1200°C. Brakes being overly hot causes two real problems:
One – if the brake is already hot it has less ability to absorb heat and therefore
take energy from the wheels. If, under braking, the brake disc rises from 300 to 1000°C it’s
acting as much more of an energy pump than if it could only move from 800 to 1000°C.
Two – the main driver of brake wear is thermal degradation – wear due to temperature. At
high temperature, the carbon will readily oxidise, which is essentially burning at its
surface layers. In excessive wear or prolonged overheating,
carbon deeper within the brakes can oxidise and weaken the structural integrity of the
brakes which is why worn out brakes start to disintegrate to dust. In worst cases, the
brakes can simply explode. So, the temperatures of brakes need to be
carefully managed if they are going to late a race distance and as fluid cooling is banned,
the engineers use good old air cooling to solve this problem.
The premise of air cooling is simple and exactly the same as using a fan to cool yourself off
on a hot day: By using a stream of fast flowing air – heat will transfer from a hot surface
to the air molecules passing by, which will carry this heat away from the hot body.
As a car moves quickly through the air, brake ducts channel some of the cooler air stream
into the brakes to do this job. To further improve air cooling, the brake
discs themselves are ventilated. Narrow channels run through the brake disc from the centre
to its circumference. As the brake disc spins, cool air is force
from the centre out through the brakes and away from the system, carrying brake heat
away downstream. Over the years these channels have reduced
in size but increased in number, providing greater overall volume for channelling air.
Now larger drake ducts can be more of an aerodynamic drag but the difference in top speeds between
using larger brake ducts and smaller version are only a couple of km/h.
A greater reason from adjusting the size of the ducts is more to do with the braking nature
of the circuit. If you’re having to brake a frequently and/or heavily, the brakes will
need more intensive cooling as you aren’t coming off the brakes as often and giving
them enough time to lose their temperature. You don’t want to keep heading into braking
zones will the brakes at 800°C. So larger brake ducts will more intensively
cool the brakes in the periods between braking zones.
On the other hand, the brakes don’t actually work very well when they are cold. You ideally
want them at at least 400°C when you hit the brakes. If you’re not braking very often
on a circuit, so there are long periods of time between braking zones for the brake temps
to come back down, you’ll probably opt for smaller brake ducts so they don’t lose too
much temperature. When you hit the brakes at cold temperature,
the brakes can take a few hundredths or even tenths of a second to kick in properly, which
isn’t ideal. The other interesting problem to manage is
that of feeding the thermal degradation problem. As I said, at high temperatures, the carbon
oxidises. This means the carbon atoms bond with oxygen atoms in the air, forming carbon
monoxide or carbon dioxide. Now, the brakes take a while to cool down
and all the time they are at a high temperature, they are still ripe of oxidation. And all
this while the brake ducts are feeding the carbon more and more air, including oxygen,
which can accelerate the process. A tricky problem.
You’ll often see engineers blanking off brake ducts with – aptly – duct tape if
the ducts seem to be feeding too much air into the brakes either temperature or degradation-wise.
So that’s the simple end of the braking system – the front brakes are powered by
a straightforward hydraulic system. The rear end – that’s more complicated.
Since the hybrid power unit was introduced, the MGUK is a significant part of the system
that slows down the rear wheels. This duty is now shared between the brakes and the MGUK.
To manage this effectively, the rear brakes are not operated by a simple hydraulic system
but by brake-by-wire. A brake-by-wire system (sometimes obliviously
referred to as BBW) means the physical action of the brake pedal is not directly attached
to the physical action of the brake callipers. Instead, there’s a computer in between telling
the brakes what to do. The MGUK can take up to 2 mega joules of energy
from the rear wheels per lap. How much energy the MGUK harvests under braking at any given
time is decided by things like brake pedal pressure, harvesting settings and battery
level. The rest of the deceleration is performed by the actual brakes.
The Electronic Control Unit (or ECU) is fed live info constantly, calculating and delivering
exactly how much work the physical brakes and MGUK perform in decelerating the car when
the brake pedal is pushed. Any excess hydraulic pressure not used to
brake the car is automatically fed back into the system via a release value.
This all happens on the fly and is incredibly sophisticated and, while all this is going
on, it has to feel like real braking to the driver.
Now, because the rear brakes don’t have to do as much work as they are sharing the
load with the MGUK, the brake discs themselves are a lot smaller than they previously were.
But if there’s a failure of the MGUK and brake by wire system, the rear brakes will
have to do all the work and this is suddenly a massive problem. Larger discs can manage
and dissipate heat much more efficiently than small discs which overheat very quickly.
This happened to Ricciardo in Monaco after his MGUK failure so he had to move the brake
bias forwards to take the load off the rear brakes.
Brake bias (or brake balance) sets how the braking force is shared between the front
and rear of brakes when the pedal is pushed. Ideally you want each brake doing the exact
amount of work necessary for the weight load it’s managing. At rest an F1 car’s weight
is distributed roughly 45:55 – i.e. 55% of the weight is supported by the rear tyres.
But under heavy braking, the weight shifts forward to as much as 55:45, so you’ll tend
to end up setting a brake bias to about 55% frontwards.
Too much front brake bias and the fronts will grip too tightly and lock the wheels, causing
heavy understeer. Too much rear bias and the back wheels can
lock and cause the car to become unstable and spin.
Ideally, you want all of you brakes to each deliver their maximum force and, if you pushed
slightly too hard, all wheels should lock in unison. But erring on the side of front
bias is wise as a lock up of the front at least keeps the car stable, not throwing it
into a spin. Drivers can adjust brake bias between corners
from within their cockpit but this is only allowed while the car is off the brakes.
F1 brakes are a complicated technology with the potential for phenomenal stopping power.
With such state-of-the-art materials and design, half the battle continues to be managing brake
temperature and bias throughout each session to keep degradation at bay and try to ensure
the brakes are in the perfect temperature region into every braking zone.