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[1:12]You don't hear about tire 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 tire pressures recommended by Pirelli as teams just kept running lower and lower pressures, destroying their tires 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, 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 tire pressures that there was even rumor and gossip of some teams tricking the sensors. That maybe the tires were at a legal pressure when they were measured before the race, but something happened by the time they started the race. Now that was all hearsay, but it does speak to how important tire pressures are to a race, so let's explore why? And let's start by reminding ourselves of the ins and outs of gas pressure in the first place. An F1 tire 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 tire is more complicated than this, but for the purposes of this, I'm going to treat it like a thick-skinned, donut-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 tire and condensed them down to something like carbon, it would take up less than 1% of the space it currently does. And yet the tire stays firmly full and rigid and doesn't collapse if you push on it. And that's because of 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 tire. And of course, every time they hit the outer rubber skin of the tire, they push against it. They exert a force on the rubber, pushing the tire outward with every ricochet. Obviously, these molecules are very, very small, but there are billions of them all rattling against the surface, holding the tire 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 tire. 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 to 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 heck loads of gas molecules flying around inside a tire, we're going to get more frequent ricocheting against the walls of the tire, adding up to a bigger overall pressure pushing against the rubber.
[3:45]Now, for the most part, the volume of an F1 tire is pretty static, so we fiddle with the tire pressure by adding more gas or letting some gas out to increase and decrease the pressure, respectively. But technically, if you push against the tire 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 pressure fluctuation from the tire 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're pounding against the sides of the tires 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 tires in a sweet spot, and the 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 tires 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 all of this affect the tire 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 underinflate a balloon, you can squish it and stretch it, it has so much give that it will even deform around other surfaces and such. And so it is with tires, a very inflated, high-pressure tire is very firm, full, and unyielding. A consequence of this is that a very small amount of the tire tread surface area will be in contact with the track at any one time.
[5:42]This area is called the contact patch, by the way, for obvious reasons. At any instant, this contact patch is doing all the work demanded of the tire. Now remember, the tires are the only physical link between the car and the track, so that when a driver puts their foot down and demands power, it's up to the tire, through the rubber, to push the car along the track. When the driver steers, it's up to the tire to pull against the tarmac and yank the car sideways, and so on. This is a lot of energy demanded of the tire, 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 it 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 you won't have the same grip levels you need and performance will drop. So it's better to reduce the pressure a bit, give the tire 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 bigger amount of rubber. This way, we're not overwhelming just one bit of rubber, but spreading the workload. A bit like how you should really hire more staff and not just keep piling it all on Jenny, who's doing the best she can, but is about to have an absolute breakdown if this carries on, Martin. A bit like that. This is also good for when you want to add a bit of camber to your wheels, I have a whole video on camber, but it's essentially angling your wheels in vertically, which means that when a car rolls, as it takes a fast corner, the outside tire taking the loads still sits fairly flat. Keeping the tire pressure low enough allows the tread of the tire to flex and use as big a contact patch as possible, whether the car is driving straight or rolling onto its loaded tire. Well, ground then. So, as long as we don't lower the pressure so much that a car is hitting the floor, or the tires are so floopy, they're barely supporting the car, we're sorted then, right? We know what's the problem, why Pirelli being so fussy about it? So, even though on an out-and-out performance basis dropping the pressure a bit on tires is always the winning move, in terms of tire integrity, that's when things get ropey. So this lower pressure tire allows us more give in the rubber. As it interacts with the track, forces pull on the tread and it stretches and releases the rubber. This does a couple of things. First, it heats 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's 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 superheat the gas inside, which can cause a tire blowout as the pressure inside rapidly increases. Secondly, and most importantly for F1, this energy pulling through the rubber puts a massive strain on the tire construction, meaning we're stressing out the joins of the tire specifically at the shoulder where the sidewalls meet the tread. Too much of this will weaken the shoulder and result in tire 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 recommends certain minimum pressures based on what they expect tire construction to be able to take with respect to the nature of the track and the energies driven through the tires. You may ask, why can't we have it all? Tires that are malleable enough to give us plenty of lovely contact patch without tearing themselves to bits in a single stint. And for that, you'll have to ask Pirelli.
