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.