Welcome to part #5 of this racing concepts series. Remember: the most dangerous words in the English language are “I already know that.”
Ok, here we go.
Knowledge of tire pressures and their effects on performance is a key element to racing success.
Unfortunately, this aspect of racing is vastly overlooked and ignored by amateur racers.
I have never attended an autocross race where entrants were using a reliable method to find their optimum tire pressures.
Typically, amateur drivers will add a few extra pounds at the start of the day and let a bit out as the tires become hotter.
I hardly have to explain that this method is entirely guesswork, with no scientific evidence to justify any kind of change to the tire pressure.
However, I saw this as an opportunity to use their shoddy methods to my advantage and create a program that would give me an edge.
But we’ll get to that later in the series.
The basis of a winning tire pressure program is an intricate understanding of how pressures affect the performance of your car.
Regardless of how much money, horsepower, or time you have invested into your vehicle, the tires are the only thing connecting your car to the ground (unless you have drag bars, in which case I hope you remove them immediately because wheelies are cool).
Even with the most expensive tires, inadequate pressures (either too high or too low) can have a drastic effect on performance.
Tire engineering and production is an extremely complicated topic that I would never dare claim to fully understand.
Lucky for us, we have Dunlop, Hankook, and Kumo who are experts in the tire knowledge paradigm. If I were to hazard a wager, I’d say that we need to know less than 1% of their knowledge to become great drivers.
For now, let’s just focus on what is most relevant to our purpose.
- Tires are a complicated combination of rubber, oil, steel or synthetic cord, and a whole mess of engineering.
- Tires can be made to be soft or hard (referred to as compounds).
A soft compound tire produces huge amounts of grip but wears out quickly.
A hard compound doesn’t grip the driving surface as well, but can last for several thousand miles.
Furthering your knowledge of tire design is an excellent outlet for your free time, but I feel that it falls outside the scope of this series.
Road VS. Racing
Road tires and racing tires are very different. A street tire is designed to provide adequate traction under normal commuting parameters.
A high quality road tire is typically constructed from a hard rubber compound, allowing it wear slowly and cover tens of thousands of miles before it requires replacing.
In addition, a street tire is designed with strategic recesses to shed water away from the tread and reduce the possibility of hydroplaning.
Racing tires, on the other hand, are only concerned with one thing: providing huge amounts of grip at various vertical loads.
They are typically constructed from a soft rubber compound which provides much more traction than a hard one and often only lasts several miles when used to their fullest potential.
Racing tires (the ones designed for use on a dry surface) are usually slick, with no recesses at all.
This allows for a larger contact patch surface area (we’ll talk about it later), and provides more potential traction between the tire and the ground.
Slick racing tires are only designed for use on driving surfaces that are dry. Even a slightly wet track can cause a slick tire to hydroplane.
Because there are no grooved recesses to channel water away from the contact patch, the entire vehicle is can be separated from the ground by only a thin film of water, leaving the driver with zero control.
In these conditions, a rain tire is the perfect solution. Recesses cut into the tread of the tire allow water to funnel into the grooves and out from between the tire and driving surface, providing enough traction to continue racing (although at lower speeds).
Because a rain tire is constantly being doused with water and therefore experiences continual heat removal, rain tires are designed have a much lower optimal operating temperature.
For this reason you cannot use a rain tire on a dry surface–it will overheat and deteriorate very rapidly.
Depending on how much a vehicle weighs and current weight being transferred, a tire can experience various vertical loads and therefore various amounts of potential traction. But we will discuss this later.
Tire Pressure Conditions
Typically, tires are filled with dry compressed atmospheric air.
It’s easy and the equipment needed to check and alter your pressures is very inexpensive–easily under $50 dollars (we’ll get to that).
However, as tires filled with atmospheric air become hot during use the air inside will expand, causing an increase in pressure.
An alternative to air is nitrogen, which is quite inert and doesn’t react to changes in temperature.
Unless you’re racing professionally (and have deep pockets) I don’t recommend nitrogen–atmospheric air will suffice perfectly with a little planning ahead.
Optimal tire pressures have many variables such as
- ambient air temperature
- tire temperature
- track surface temperature
- tread compound
- the tires optimal operating temperature
- the drivers skill
- the car suspension setup,
Just to name a few.
There are three main conditions that pressure (from either compressed air or nitrogen) can create inside a tire.
- Perfect tire pressure (Figure 12).This is the optimal tire pressure – the one every racer should strive to find.With only a small patch of contact to the ground (four of them, actually – one on each tire), it is critical that every bit be used.This ideal pressure produces an even contact patch across the entirety of the tires width, maximizing its potential traction. We’ll discuss contact patch in more detail later.
- Low tire pressure (Figure 13).This tire does not have enough air, and is therefore soft and unsupportive.A low tire pressure tends to bulge out at the sidewalls and lift the center of the tread away from the road surface.As you can see, only 2/3 of the tire is contacting the ground, greatly reducing its traction potential.A low tire pressure allows the sidewall to cave under hard cornering, allowing greater lateral weight transfer.If driven in this state, the inner and outer treads will wear out before the center.In addition, the added flexing of the tire creates extra heat as it constantly conforms to the bulging shape. This condition can cause a tire to overheat and exceed its optimal temperature range.
- High tire pressure (Figure 14).With too much air inside it, this tire tends to balloon around the circumference, lifting the inner and outer treads away from the driving surface.Note that only 1/3 of the tire is available for traction. If driven with a high pressure, the center of a tire will wear out far before the sides.
So what is the magic tire pressure number for your tires?
Unfortunately there is no uniform answer to quell your curiosity…but later in the series I will show you exactly how to find your perfect pressures.
NEXT WEEK: Contact Patch and Vertical Load
The next part in this series is only a week away. Go back and read through this again. Make sure you truly understand what you just saw, because the series progressively builds as it continues.