- Like many aspects of getting the peak performance out of your car, tyre pressures
can be a really complex subject.
The methods you use to both record and adjust and track your tyre pressures will depend on
the type of racing you're doing and the equipment you've got access to.
Here I'm just going to go through a few basics to help you do a better job of
getting your tyre pressures right while you're on track.
The vertical load on the tyre, the internal construction and the tyre pressure are
things that are pretty dominant when it comes to determining the surface pressure
distribution on the tyre.
This is something that's quite often referred to as the contact patch,
and how that pressure is distributed throughout the contact patch has a really
big impact on how the tyre's working when you're on track.
Once the tyre pressures have been set in the pit lane, that's what you're stuck with until
the next time you return to the pits.
They're generally not adjustable while you're on track.
The tyre pressures are also continuously changing throughout your stint and
throughout the lap.
If you leave the pit lane with cold tyres, as they warm up, the pressures will all come
up and that means as you continue to run lap after lap, the pressures will be changing
and evolving as you go through your stint.
The other thing to consider is the tyre pressures are continuously changing as
you move through each part of the circuit, so depending on the energy input from one
corner to another, you will find that the pressures will vary at different parts of
the circuit.
This is one of the things that makes tyre pressures such a challenging part of
car setup.
This also means tyre pressures are a massive compromise.
Just because it's right in one part of the track, doesn't mean it's going to be right for
the rest of it so you do always have to make a compromise on getting the optimum
tyre pressure over a single lap.
The most dominant parameter that controls tyre pressure is the internal air temperature
inside the tyre.
There are a couple of different sources of temperature that changes the internal
temperature of the tyre.
One of them is the hysteresis of the tyre as it works.
That simply means, as the rubber deflects and moves as the car moves around the
track, it naturally generates heat which tends to warm the tyre and therefore
the air inside the tyre.
Another one is the brakes, the brakes give off a lot of heat and this heat naturally
results in the tyre pressures rising inside the car at the same time.
The age of the tyre, type of tyre construction and compound and the tread depth also have
a big impact on how the tyre pressures evolve.
The cold pressure is the pressure of the tyre before it's been run on the car.
That means that tyre and the rim will be at ambient temperature.
You can generally only check and set the cold pressures before the tyre has been run on
the car that day or if it's been sitting around for at least a few hours to equilibriate to
the ambient temperature.
A hot pressure tyre target is what you're generally most interested in when it comes
to maximising the performance from your tyre pressures.
Each combination of tyre and car and track will have an optimum hot pressure you'll
be going for.
The hot pressure recommendation is best to come from either the tyre supplier or
the tyre manufacturer.
Once you've been given a hot pressure target from the tyre supplier or manufacturer,
this is when you need to go to the track and test across that window to see what works
best for your car at that track.
If you're going to get advice from a tyre supplier or manufacturer, they'll have a few
questions for you, this will generally be what type of racing are you doing?
What type of car are you using?
And from there they'll be able to give you a pretty good pressure range that you should
be aiming for.
If your tyre supplier or manufacturer can't give you a recommendation for your target
hot pressure, that's probably a pretty good indication that it's time to look for a
new supplier or manufacturer.
This is pretty basic information that you really should be getting if you're investing money
in their tyres.
It's worth remembering that tyre pressure not only affects performance, it's also a
safety issue.
It's quite easy to damage a tyre by running really low pressures and damaging the
construction.
Your tyre pressures are constantly evolving as you move throughout a lap and a stint.
It's worth remembering that if you're doing many consecutive laps, particularly when
you're starting from a cold tyre, the tyre pressure will be constantly evolving as
you move through a stint.
There will be a point where the tyre pressure reaches equilibrium where it stays
relatively stable as long as it's being driven in the same way consistently.
But this can be as many as 20, 30 laps depending on the application and the
type of tyre.
Because your hot tyre pressures are constantly moving throughout your
stint, this means that you need to set your cold pressures based on whether
you're doing a short run, like a qualifying run or a long race run.
For a qualifying run you would typically start with a lot higher cold pressure.
That means you'll reach your target hot pressure in a lot fewer laps than you
would during a race run.
By the same token, you would typically set your race pressures a fair bit lower
with the cold pressures.
That's just to allow for doing many consecutive laps so that you don't blow way over your
hot target early into your stint.
The simplest way to get your cold pressures from your hot pressures is to run the car
on track.
Ideally you want to be bleeding down rather than having to boost the pressures when
the car's in the pit lane.
This means sending the car out on track and every time it comes back to the pit lane,
bleeding the hot pressures down into your hot pressure target.
For this method to work, you need to be running approximately the same number
of laps on track as you're setting the tyre pressures for.
This means if you're trying to find your qualifying cold pressures, you would go out
and run a similar number of laps that you expect to run in your qualifying session,
come in, bleed the tyres down to target and that's going to get you pretty close with
your qualifying pressures.
In a similar way, if you were doing your race pressures, you would head out and do
close to a full race run, come in, bleed the tyres down to target and that means you're
pretty close to your hot pressure target.
Once you're happy with the hot pressures that you've got with the car in the pit lane
after running hard on track, in order to find the cold pressures, you need to take them off
the car and let them cool.
You need to let them cool until the wheels and tyres have reached ambient temperature.
Obviously each application's going to be a bit different but you really want to leave the wheels
and tyres out of direct sunlight for at least two to three hours so they reach ambient
temperature.
The reason we're letting the wheels and tyres get to an ambient temperature is because
what we're actually interested in is the temperature of the air inside the tyre.
Without specialist equipment, it's pretty difficult to actually measure the temperature
of the air inside the tyre so the idea is if you let them sit and equilibriate with
the ambient temperature, if you measure the rim or tyre temperature, it's a pretty
good approximation for the temperature of the air inside the tyre.
This is also why you want to make sure you leave them out of direct sunlight.
If you leave them in the shade, it allows everything to cool down faster and it means
that you can be more sure that the air inside the tyre is going to be the same
as the temperature you measure on the outside.
Now that the tyre's at equilibrium, you can go around and measure the
pressure of each of your four tyres from that set and record the temperatures
that they were when you took those pressures.
This is now your cold pressure reference.
And next time you come back to this track or the next time you run,
when it comes time to set the cold pressures, these are your references that you can use.
The reason this is a reference is because you know the hot pressures were right on track
and now that they've cooled, this is a reference you can use next time you
set your cold pressures.
It's important to realise that each different circuit you go to will require a different
cold pressure for a different hot target.
That's because each circuit has a different energy input into the tyre.
Some tracks will require a relatively small pressure split from one side of the car
to the other, others will need a lot larger cold pressure split in order to achieve
square hot targets all the way around.
One of the things you've got to remember is the air temperature inside the tyre is
the most dominant thing that controls the tyre pressure.
That means if the temperature of the tyre is different from the temperature the tyres
were at when you got your reference pressures, you need to account for that.
That's because as the temperature of the tyre rises, the tyre pressure rises with it.
A good rule of thumb is for every one degree celsius of tyre temperature,
the pressure will change approximately 0.1 psi.
As an example, if you've got a front left tyre that had a cold pressure of 20 psi
where the ambient temperature was 10°C, if you come back next time to the track and you
want to achieve the same hot pressure that you had with that reference set,
if the ambient temperature is now 12°, you would set the hot pressure to 20.2°,
rather than 20°.
This is just a rule of thumb but it will get you pretty close.
You can apply this rule of thumb to all of your cold pressures.
When it comes to getting the most out of your reference cold pressures, record keeping becomes
really important.
I would strongly encourage everyone to keep organised notes of all of their tyre pressures
when they go to the track.
That means recording the cold pressures and the temperature that the tyres were
when you set those cold pressures, the hot pressures when the car was on track,
as well as any bleeds or boosts that are done when the car is in the pit lane and then
once you've done those bleeds and boosts and you let the car settle at the end of the day,
record in the reference pressure and the temperature when you take the reference
pressure.
If you can keep well organised good quality notes on your cold and your hot pressures,
for each track that you go to with your car, you will find this speeds up your process a lot
when you want to hit your target tyre pressure each time you go back to that
same circuit.
In motorsport, it's quite common for people to use nitrogen in their tyres, rather than
the ambient air.
There are a couple of reasons for this, one reason is the pressure of nitrogen tends to
change a little bit less with temperature so it makes it a little bit easier to set the pressures.
Another one is humidity, when you have compressed nitrogen in a bottle,
it is dried and has almost zero humidity.
One of the problems with using ambient air is that if it isn't dry or if it hasn't been
through a drier when it's been compressed, it can have a lot of humidity in it.
The problem with humidity is that it can give you quite unstable pressure growth
and response to temperature.
For a tyre that has air with a high humidity in it relative to a tyre that has low or zero
humidity inside it, the tyre with high humidity will have a higher pressure
increase for a given temperature increase as opposed to the tyre with the dry air.
This really becomes a problem if you've got different levels of humidity inside each tyre
inside your tyre set or different tyre sets.
That makes it much more difficult to get accurate hot pressures based on your
cold pressures when the car's on track.
Using compressed nitrogen gets around this problem because generally when it's supplied
to you in the bottle, it's in a very low or zero humidity, this allows you to ignore all
affects of humidity.
One of the things you need to take care of when getting tyre fitted is there's not any
excessive humidity already inside the tyres.
Often this can be from condensation on the inside surface of the rim or the inside surface
of the tyre.
It's really important to get rid of all that before the tyres get fitted otherwise you're
automatically giving yourself a hard time to get accurate hot pressures because if
that humidity isn't equal amongst each one of your tyres, you're going to get much
more pressure increase for a given temperature increase when you run that
tyre on track.
Tyre pressure is a really complex subject, in professional racing you typically have
at least one person per car just looking after nothing but the tyres and the reason
is that because it is a really involved job.
However, hopefully I've given you guys a couple of insights into how to get your
hot pressures right when you're running on track.
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