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You don’t hear about tyre pressures too often but when you do, it’s because teams
are pushing their luck against the limits. In fact, the FIA had to enforce mandatory
minimum tyre pressures recommended by Pirelli as teams just kept running lower and lower
pressures, destroying their tyres and then asking… why? Why would Pirelli DO this?
And even after compulsory minimum pressures were brought in, drivers and teams complained
that they sucked, that they were too high, that the cars didn’t feel right and, gosh
it’s just like they want everything to be easy don’t they?
Flippancy aside, so big a pain in their side were these enforced high tyre pressures that
there was even rumour and gossip of some teams tricking the sensors. That maybe they tyres
were at a legal pressure when they were measured before the race but… something happened
by the time they started the race. That was all hearsay but it does speak to
just how important tyre pressures are to a race so let’s explore WHY.
Let’s start by reminding ourselves of the ins and outs of gas pressure in the first
place. An F1 tyre is inflated with a gas - it’s
not actually air for reasons I’ll get to in a bit. Now the inner structure of an F1
tyre is more complicated than this but for the purposes of this I’m just going to treat
it like a thick-skinned doughnut shaped balloon. Delicious.
So, as you know, all the molecules in a gas are much more spread out than solids and liquids.
In fact, if we took all the gas molecules inside an F1 tyres and condensed them down
to something like carbon it would take up less than 1% of the space it currently does.
And yet the tyre stays firmly full and rigid and doesn’t collapse if you push on it.
And that’s because of the air pressure. Gas molecules, due to the energy given to
them by their temperature are constantly moving, flying around in their container like ping
pong balls, just absolutely relentlessly clattering off the walls of the tyre.
And, of course, every time they hit the outer rubber skin of the tyre, they push against
it. They exert a force on the rubber, pushing the tyre outwards with every ricochet.
Obviously these molecules are very very small, but there are billions of them all rattling
against the surface, holding the tyre inflated, stopping it from collapsing even if you push
against it very hard. A few things can affect the magnitude of the
pressure in the tyre. One is the density of the gas. Now this is
a trade off of two things: how big the capacity of space is – how much volume the gas has
fill up – and how much gas you’re trying to shove in there. Basically we’re asking,
how crowded is it in there? If we have heckloads of gas molecules flying
around inside the tyre, we’re going to get more frequent ricocheting against the walls
of the tyre, adding up to a bigger overall pressure pushing against the rubber.
Now, for the most part, the volume of an F1 tyre is pretty static, so we fiddle with the
tyre pressure by adding more gas or letting some gas out to increase and decrease the
pressure respectively. But, technically, if you push against the
tyre to deform it and try and shrink its volume, this will force the air pressure inside to
increase and push back against your efforts. So you’ll have some fluctuation from the
tyre deformation as it responds to different loads.
The other important factor to consider is the temperature. With higher temperatures,
the molecules are given more energy so they are pounding against their sides of the tyres
even harder than before, boosting the pressure. This is important, because as you probably
know, a big part of Formula 1 competitiveness is keeping the temperature of the tyres in
a sweet spot and temperatures of the “oven” within the rubber may rise and fall depending
on external conditions and teams need to account for that.
This is one of the main reasons why F1 tyres aren’t just filled with air but with Nitrogen.
Air varies a bunch from place to place and has different moisture levels which would
make its pressure less predictable with respect to changes in temperature. Nitrogen is predictable
and inert and stable. So, how does this all affect the tyre performance?
Well those of you who have encountered a balloon or a football know that if they are super
inflated they become much more rigid, firm, and kind of fixed in their shapes. They don’t
have much give in them, they won’t flex easily if you try and bend them or squish
them. But if you under-inflate a balloon you can
squish it and stretch it; it has so much give that it will deform around other surfaces,
etc. And so it is with tyres. A very inflated,
high pressure tyre is very firm, full and unyielding. A consequence of this is that
a very small amount of the tyre tread surface area will be in contact with the track at
any one time. This area is called the contact patch by the
way, for obvious reasons. At any instant this contact patch is doing
all of the work demanded of the tyre. Remember, the tyres are the only physical link between
the car and the track so when a driver puts their foot down and demands power, it’s
up to the tyre, through the rubber to push the car along the track.
When the driver steers, it’s up to the tyre to pull against the tarmac and yank the car
sideways, and so on. This is a lot of energy demanded of the tyre
and if there’s only a small contact patch, we’re asking just a small amount of rubber
to deal with all that energy and this will lead to that rubber overheating quickly and
from there we get degradation and blistering. Blistering is when the rubber overheats and
superheated pockets of air build and explode through the tread. And then your rubber kind
of sucks and won’t have the same grip levels you need and your performance will drop.
So, it’s better to reduce the pressure a bit, give the tyre a bit of malleability,
let it sink and hug the track a bit once it’s bearing the weight of the car.
Now, we’ve got a nice big contact patch and the energy demands in accelerating, braking
and turning are shared across a much bigger amount of rubber. This way we’re not overwhelming
just one little bit of rubber but spreading the workload - a bit like when you really
should hire more staff and not keep piling it all on Jenny who’s doing the best she
can but is about to have an absolute breakdown if this carried on MARTIN.
A bit like that. This is also good for when you want to add
a bit of camber to your wheels. I’ve a whole video on camber, but it’s essentially angling
your wheels in vertically which means that when the car rolls as it take a fast corner,
the outside tyre taking the high loads still sits fairly flat.
Keeping the tyre pressure low enough allows the tread of the tyre to flex and use as big
a contact patch as possible whether the car is driving straight or rolling onto its loaded
tyre. Well, grand, then. So as long as we don’t
lower the pressure so much that the car is hitting the floor or the tyres are so floopy
they’re barely supporting the car, we’re sorted then, right? What’s the problem?
Why are Pirelli being so fussy about it? So, even though on an out and out performance
basis dropping the pressure a bit on tyres is always the winning move … in terms of
tyre integrity… that’s where things get ropey.
So, this lower pressure tyre allows us more give in the rubber. As it interacts with the
track, forces pull on the tread it stretches and releases the rubber.
This does a couple of things. Firstly it does heat up the rubber. As all the long chain
molecules wind and unwind they rub together and put heat into the rubber. This also takes
energy, deforming the rubber like this. Energy that is supposed to go into driving the car,
lost into the rubber through what’s called hysteresis.
The heat put into the rubber through hysteresis, under some circumstances can start to superhear
the gas inside which can cause a tyre blow out as the pressure inside rapidly increases.
Secondly, and most importantly for F1, this energy pulling through the rubber puts a massive
strain on the tyre construction - meaning we’re stressing out the joins of the tyre
- specifically at the shoulder where the sidewalls meet the tread.
Too much of this will weaken the shoulder and result in tyre failure along this line
and this is something we don’t want to see, especially through a high speed corner.
Therefore, for safety reasons, Pirelli recommend certain minimum pressures based on what they
expect the tyre construction to be able to take with respect to the nature of the track
and the energies driven through the tyres. You may ask - why can’t we have it all?
Tyres that are malleable enough to give us plenty of lovely contact patch without tearing
themselves to bits in a single stint? For that you’ll have to ask Pirelli.