We’ve gone over in almost painful detail the rules regarding how tyres can be used
but haven’t yet given more than a passing glance at the mechanics of the tyres themselves.
In this video we’ll look at how tyres wear and degrade and understand the meanings of
words we hear a lot when people talk about tyres.
Words like: Graining
Heat cycles Blistering
Marbles (which are not the same as…)
Laying down rubber Flat spots
To get our head around a lot of these tyre phenomena we need to get a quick understand
of what the tyre is made of and how it behaves.
The inside of the tyre is a bunch of old gumpf that keep it all in shape and we’re not
too worried about that.
On the outside, though, is the rubber and we’re particularly interested in the part
of the rubber that touches the track, called the tread
Racing tyres are made are made of a nice soft synthetic rubber.
This rubber, much like a pencil eraser will heat up under FRICTION.
Tyres heat up through corners as there will always be some amount of sliding and lateral
forces acting on the tyre.
A lateral force is a force that acts across the direction the car is travelling, as shown
here, basically at 90 degrees.
Bringing the tyres up to working temperate it referred to as putting them through a heat
cycle.
Now, as the rubber in the tread heats up and cools down the chemical makeup of the rubber
– specifically, the polymers (long chains of interlaced molecules) – will start to
degrade.
Putting heat energy through the rubber deforms it.
Most of the rubber will spring back to its original shape, but some of the chemical bonds
break or deform permanently under this heat energy.
A really crude analogy for this would be like baking a cookie.
The dough starts off soft, but as you continue to cook the dough if will get tougher and
tougher until the cookie is hard.
F1 tyres aren’t exactly like this, but over time the heat will change the very nature
of the rubber itself.
The internal structure of the rubber will permanently change from the fresh-from-the-packet
rubber – soft and sticky and useful – into a less grippy, less useable tyre.
This process is the tyre “giving up” over its life span.
Heat cycles aren’t all bad though.
Tyres fresh out of the box often aren’t ready to deliver flat out speed straight away.
Putting them through a heat cycle by doing a warm up lap or two on them treats the rubber,
giving it a final bake and making it just right for baking.
Putting the tyres through a warm up heat cycle is known as scrubbing.
You’ll notice drivers often using scrubbed tyres during a race – these tyres aren’t
worn, they’ve just been race-prepared by putting them through a heat cycle.
When the rubber is heated is becomes more viscous.
This essentially means it’s a little bit more ‘melty’ and malleable.
When the tyre rolls over the rough surface of the track, the soft, melty rubber squeezes
into the surface texture and grips nicely.
It takes a good driver to get a racing tyre to a consistent temperature.
If the tyre overheats in such a way that the inside part of the rubber becomes much hotter
than the outer surface, a pocket can form and expand beneath the treat, ultimately bursting
and breaking a large chunk of the tyre tread away.
This is known as blistering.
Blistering is a lot rarer these days as racing tyre manufacturers have made better, more
temperature consistent compounds.
If it does happen, you can normally spot it as an anomalous blemish on the tyre surface.
Conversely, if the surface temperature isn’t quite right, graining can occur.
The tyres can be a little temperamental.
They need to be a fairly consistent temperature.
If the outer surface of the tyre is a lot hotter than the inner rubber, it can become
a lot easier for the hot melty surface of the tread to distort and wear under the high
cornering loads.
As the car turns through a corner, particularly fast corners, there are strong lateral forces
pulling on the rubber as it grips into the track.
Under the right (or ‘wrong’) circumstances, the rubber can deform in waves under this
abrasion.
Eventually these waves will deform so badly that the peaks will start to tear off.
These little chunks of rubber being ripped off with be hot and sticky and stick straight
back onto the surface tread.
Over time, the worn parts of the tread will take on a grainy texture, hence why this is
known as graining.
The problem with graining is that the main tread of the tyre isn’t making proper contact
with the surface of the track anymore.
The rubber in the grains is not the good, well heated rubber of the main tyre tread
and is much more slippery as the main point of contact.
This leads to grained tyres being much less grippy than a fresh tyre.
Not all of the little rubber chunks are thrown back onto the tyre though.
A lot are thrown completely clear of the car and deposited on the track.
These discarded rubber deposits are known as marbles and you’ll often see them littering
the edges of the racing line.
From the TV cameras, they’ll look like long dark patches along the edges of the track.
As with graining, running over these marbles is a lot less grippy than running the tyre
directly onto the track.
The problem comes when there are a lot of marbles and they severely reduce the raceable
part of the track.
You don’t really want to move off the racing line and onto the marbles, which means backmarkers
won’t want to jump out of the way and overtaking becomes…
more of a risk.
Now having rubber on the track isn’t always a bad thing.
Let’s not confuse marbles with the laying down rubber.
As cars drive across the track, particularly through corners, they will start to lay down
a small amount of rubber worn from the tyre.
This layer of rubber starts to build up as more and more cars take the same line over
and over again lay down more and more rubber.
Driving over this rubber is grippier than driving directly onto the track AND is less
wearing on the tyres.
Think about wearing a pair of trainers or sneakers and how much more grippy it feels
on a rubber mat to the pavement.
This is similar.
The coefficient of friction between the tyre and the rubber layer is much higher and the
surface is smoother so less wearing.
Drivers will often purposely lay down rubber on their grid spot and in the pit box by doing
a burn out.
A burn out spins the tyres much faster than the car actually travels so a nice layer of
rubber is scraped off and laid onto the track for a grippy getaway.
On the opposite end of requirements to speeding off the line – braking.
Specifically, how does braking sometimes result in a flatspotted tyre?
Flatspotting occurs when a car locks up under braking.
Ideally, the wheel should always be rotating under braking.
It means the car can still be steered, so a certain degree and the car can be brought
to a stop more quickly than if the tyre is locked – that is, non -rotating.
The maximum frictional force at the brakes is much stronger than the maximum frictional
force between the tyre and the track.
We want the brake disc to keep moving through the pads as the brakes are applied.
If the brake pads grip too strongly, the wheel will lock in place and the only braking force
happening will be between the tyre rubber and the track as it slides along the ground.
This is a weaker force than at the brakes.
Not only that, but this superheats one area of the tyre and wears it away in one flat
slice.
This is a flatspot.
A heavy flatspot will make the car judder and vibrate as it has wrecked the nice smooth
roundness of the car.
This can damage suspension (as Kimi found out in 2005) but also is a self-exaggerating
injury.
See, it’s more difficult to rotate a tyre from its flat section than it is along its
roundness.
The next time you brake, the tyre will likely slip onto its flatspot and stubbornly refuse
to rotate, thus flattening it even more.
Driving with a bad flatspot will make itself worse and worse every time you brake and makes
it very hard to continue to for long periods of time.
You can spot a flatspot a) from the vibrations in the car, and b) from the large discoloured
patch on the treat that flashes by as the tyre rotates.
So, these are some of the main elements of tyre wear and literal tear that you’ll notice
during a race or session.
Later I’ll explain some elements of the track itself – the other side of the equation.
Hopefully you’ll understand why managing your tyres is a tricky thing for a driver
to as they need to keep the rubber in the correct temperature range (the ‘working
range’) consistently to make sure the rubber is heating correctly throughout, not just
at the surface or core.
A nice, consistent temperate through the depth of the tyre – not too hot, not too cold,
but just right.
Thanks again for watching this video!
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I hope this video on the elements of tyre wear helps clear up some of the more technical
details of often heard phrases.
Next week I'll be going over the current engine proposals for 2021.
After that...
I'll probably lay off the tyres and engines for a bit and might do a video on crashes!
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For this video I've posted all the notes and sketches I might while putting this video
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