Previously we looked at the tyre rules, at how tyres wear and the different types of
corners we’ll see at tracks around the world.
One of the most-requested topics I’ve been asked to cover is How Pirelli choose which
three dry compounds to bring to each race.
This is naturally quite a complicated process but I’ll try and lay out the basics for
you here.
As a reminder, there are now seven (previously five) dry compounds that Pirelli can bring
to a race.
The softest of these compounds are grippier, have a shorter lifespan and have a lower ideal
operating temperature.
And naturally, the harder the tyre, the less grippy the rubber, the longer the lifespan
and the higher the ideal operating temperature.
Pirelli will bring three different compounds to each race and we want to look at how they
decide which ones to bring.
Let’s remember that Pirelli make their choices based on a core philosophy behind what part
the tyre should play in races.
At the moment, this philosophy is to encourage two-stop races with a consistent tyre degradation.
This is the full range of tyre compounds Pirelli selected through 2017.
We’ll come back to this diagram later in the video once we’ve gained a bit more understanding.
So there are two main factors that govern Pirelli’s choice of tyre compound: track
qualities and what I’m loosely going to label “environmental factors”.
Overwhelmingly the choice comes down to the qualities of the track, so let’s look at
those.
We start with the track layout.
How many slow corners does a track have?
How many fast corners?
Does it have a lot of straights?
Slow corners put a lot less energy into the tyres than fast corners.
The main reason for this is that when the car is at higher speeds it’s able to demand
a lot more lateral friction from the tyres to pull it into the corner.
This is because a car at higher speeds has a lot higher downforce and this maximum non-sliding
frictional force is directly proportional to the amount of force pushing down on the
tyres.
That was a little aside into frictional forces there, but the takeaway is that slow corners
put less energy into the tyres.
This means that the tyres will be slower to heat up if the corners of a track tend to
be slower.
In this case we may want to consider a tyre compound with a lower operating temperature.
Of course, the reverse is true for high speed corners, which energise the tyres a lot.
Tracks with a lot of high speed corners will result in high tyre temperatures, which are
best handled by harder tyres with a higher operating range.
Similarly, straights give the tyres a lot of respite and time to cool down.
If there are a lot of straights or a couple of very long straights, this might encourage
Pirelli to head in a softer direction as the tyres as naturally allowed a breather after
taking a pounding through the corners.
A track with very few straights might encourage a harder compound as the tyres as allowed
little recovery time between corners.
Now let’s consider the track surface.
There are two qualities of the track surface that affect its relationship with the tyres
– its macro roughness and its micro roughness.
To understand these we need to first consider what the track surface is.
Asphalt or tarmac is a mixture of bitumen and ground up stones.
Bitumen is a tar like substance made from a mixture of oily hydrocarbons.
Hydrocarbons are what make up oils, petrols, diesels, plastics, tars, etc.
Mixed as bitumen, it’s a sticky, syrupy substance that sets under normal temperatures.
It’s mixed with ground up stones and gravel (known as ‘aggregate’) and laid out onto
the track surface.
If you look at the surface of a road or track you can see all the aggregate – the tiny
mashed up stones – set in the sea of black bitumen.
It’s this texture of stones that make up the macro-roughness of the track surface.
The size, shape, spacing and sharpness of these stones can make for a very rough, or
much smoother surface.
Freshly laid asphalt can often be much rougher in its first few races or years of service.
The aggregate will have a lot of pointy edges and won’t necessarily lay evenly across
the surface of the track.
This macro roughness makes the track more grippy but much more aggressive in how it
wears the tyres.
A track resurfacing requires special attention from Pirelli as it may call for a harder wearing
tyre compound.
Over time – possibly many years, depending on how often the track is used - repeated
wear will eventually start to smooth out the raw aggregate over time.
In fact, eventually track wear will start to wear the surface level bitumen clean from
the aggregate, reducing the grip significantly.
Fresh asphalt is also loaded with oils when first laid and is much more sensitive to temperate
change.
Rising temperatures will cause some of the oils in the bitumen to leak out from the mixture
during its early use.
More oils present in the asphalt make it less grippy and obviously, oils sweating to the
surface will be much more slippery during its bedding in phase.
The evolution of a new track surface requires a lot of monitoring.
Pirelli are putting a lot of work into studying the new track surface at Barcelona, where
all the pre-season testing is taking place.
You’ll also see compound choices shift with new tracks as a Pirelli anticipate changes
in the qualities of the asphalt in early years.
The other element of track grip quality is micro roughness.
This actually involves looking at the surface of the asphalt itself and testing the graininess
of its texture.
The quality and texture of the track surface relates to its direct relationship with the
surface and texture of the rubber of the tyre.
This is pure chemistry.
The friction between two materials is borne out from bonding between the molecules as
they push up to each other.
High fidelity scanning with give Pirelli a good sense of how the tyre and track will
rub against each other at different temperatures.
Pulling away from the track surface now, let’s have a quick look at some of the environmental
qualities Pirelli will have to consider.
The track temperature will have a slight impact on the sustained temperatures of the tyres.
In a lot of cases, a cooler track is actually more helpful as it helps to balance the energy
put into the tyres through the corners.
Hotter temperatures can also cause the bitumen in the asphalt to deform and make the track
a little less predictable.
Of course, Pirelli can’t know what temperature the track will be on a race weekend but the
track climate will offer some insight.
Speaking of climate – if it’s likely to rain between dry periods this will both keep
the track cool and start to wash away rubber that get laid down onto the tarmac through
the sessions.
A track without a rubber line would do better for a hard compound, but only if it doesn’t
get too cool!
Track locations also play a big part.
Bahrain is often dusted with sand whipped up by winds, which reduces the grip level
and increases sliding through the corners.
Street circuits are temporary tracks and will always feature a lot of track evolution as
the cars clean up the surface of the track and stress the bitumen in ways the track rarely
sees at other times of the year.
So with that all in mind, let’s have a quick look back at some of the selections Pirelli
made in 2017.
Let’s start with Monaco.
It’s got the slowest corners of anywhere, the slowest average speed and it’s a temporary
track.
All these factors contribute to very low tyre energy so the softest possible compounds were
chosen.
Azerbaijan has a street track, but unlike Monaco it has a mixture of slow corners with
some medium and high speed corners and heavy braking.
In this case, Pirelli brought compounds one step harder than Monaco.
The Catalunya circuit in Spain is a permanent, heavily used race circuit with lots of high
and medium speed corners, an abrasive asphalt and reliably high track temperatures.
These all contribute to high tyre temperatures which suit the harder end of the spectrum
of compounds.
Silverstone also has a lot of medium and high speed corners but with a higher chance of
rain and more unpredictable temperatures so Pirelli opted to go one softer than they did
in Spain.
In Brazil, Pirelli also went for the middle road with compounds as it has a rough, abrasive
track surface but also a lot of slow corners so there’s a balance in how much the tyres
are put to work.
What’s actually quite frustrating in looking at this is that with only five compounds,
the Hards all but out of action, and Pirelli only choosing three adjacent compounds, there
are actually only two sets of compound choices.
UltraSoft, SuperSoft, Soft or SuperSoft, Soft, Medium.
This year, Pirelli have a wider range of tyres and will not necessarily use adjacent compounds
so we should see more variation in compound choices.
We’re seeing this already in the first selections for 2018.