Tire Management

The Rules

NASCAR is very clear about what teams can and can not do to the tires.  Teams may not use any chemical or physical means of altering the tires, which means no siping (cutting grooves into the tires), no chemical softening, not even heat blankets to pre-warm the tires are allowed. Basically, you can’t do anything to the tires.

This means that when a tire blows out, it’s the tire’s fault. Right? Because there’s nothing the teams can do to change the tires.

Phoenix Spring 2016

Last week’s race at Phoenix featured a number of tire problems.

The cause of each of these problems was quickly identified by Goodyear.

So there must be something wrong with the tires if the beads are melting, right?

Not necessarily. Manufacturing defects are extremely rare in race tires. The tires made for NASCAR are custom made and go through really stringent quality control. The problem comes when teams push tires past their limits.

The limits aren’t necessarily well defined. There’s no tag on the tires that says “this tire may fail if you subject it to more than 2.35 degrees of camber”. That’s something the teams have to figure out. It’s part of setting the car up.

You’ll notice that all of the tweets note that failure of the tire was due to bead melting and that the bead melting was due to excessive tire heat. This is Goodyear’s way of telling you that it wasn’t their fault.

Tires

Let’s look at how tires work to understand the problem. Here’s the anatomy of a tire.

The bead is the part on the inside circumference of the tire. It’s a metal (often metal fibers supplemented with synthetic, ultra-strong polymers like Kevlar or carbon fiber) that is surrounded by rubber. The bead seals against the rim of the wheel to keep air in the tire.

So when the bead melts, the air comes out of the tire and it’s very hard to steer a car when there’s not enough air in one of your tires.

The question is: why do the beads melt?

To understand that, we have to understand why tires get hot. We’ve talked a lot about how the treads get hot because of friction between the track and the tire.

But the bead isn’t rubbing against anything. It stays stationary relative to the wheel. Why would it get hot?

 

Brakes

The wheel and tire are attached to the rotor. You can match up the five studs in the picture below to the five lug nuts to imagine how the wheel goes on.

Heat is generated by the brakes when the calipers press against the rotor. That friction heats the rotor — you’ll be able to see the rotors glow at places like Bristol and Martinsville in a few weeks.

The wheel itself (at left) is made of metal – and metal is an excellent conductor of heat. Since the wheel is in contact with the rotor, heat is transmitted through conduction from the rotor to the wheel.

Heat is also transmitted by radiation. Not radiation like alpha rays and beta rays. Radiation is a mechanism for transferring heat between two objects that aren’t in contact. The Sun radiates heat through the vacuum of space (and through our atmosphere) and heats the planet. If you put your hand near a fire, you feel it getting warm, even without touching the flame.

Even the parts of the wheel that aren’t touching the rotor get warmed by the heat of the rotor because of radiation. Since the wheel pretty much envelops the rotor assembly, all that heat is trapped by the wheel, which means the wheel heats up. Since the bead is in contact with the wheel, when the wheel gets hot, the bead gets hot. Rubber melts at a much lower temperature than metal. The moment the bead melts, you lose the seal between the tire and the wheel and all the gas that had been in the tire.

Brake Cooling

This is why teams must cool the brakes during a race. To do this, they use brake cooling ducts and fans.  A very simple application is show below.

Air comes in through an opening in the front of the car (the white piece represents the front fascia). The grid over the opening prevents anything (dirt, marbles, small children) from getting in. Neoprene brake hosing routes that air back to the rotor. The air blows on the rotor to try to keep it cool. (Also note in the picture above that you can see that the rotor isn’t solid. It’s got vanes in it that also help heat escape from the discs.)

NASCAR allows only one opening per brake, but you can use three 4-inch hoses per brake. There are very specific rules on how those hoses can be mounted and where the air can come from because NASCAR doesn’t want to offer teams opportunities to mess with how the air moves under the car. The back brakes are cooled via air brought in through NACA ducts in the rear quarter windows.

The diagram above shows an external blower, but NASCAR requires teams to use inline blowers like the one shown below. This fit right into the ducting, very much like a blower you might use in the heating/cooling ducts in your house.

You’ll notice that this is spec’d at 12 Volts. When you hear drivers and their crew chiefs talk about turning on or off their brake fans, this is the piece they’re talking about. If there are electrical problems in the car, sometimes drivers will have to turn off the fans to save their battery.

The more air you can bring in, the cooler you can make the brakes. The limitations on the duct size limit how much cooling power teams actually have.

Why Was There So Much Brake Heat?

Excessive brake heat means there’s not enough cooling. If you trace the path in the first diagram of the brake ducting system, you can see a lot of possible reasons why the cooling could be compromised.

There could be a hole in the ducting, the ducting could come lose, the fan could fail or… there simply could be not enough air coming in to do the job. If you let the teams decide what equipment to use, they’d all put a bunch of really big ducts to the brakes so the brakes could get too warm.

But of course, NASCAR limits that. They also limit the sizes of the openings in the front and back (the back brakes are usually cooled via air brought in by NACA ducts in the rear quarter windows).

But because the new low-downforce package has teams searching for ways to increase downforce, teams are turning to taping up the openings on the front of the car as much as possible. We all know about tape on the radiator grille and the perils of having too much tape – you risk overheating your radiator, pushing water and eventually overheating the engine.

The same thing can happen with brake intakes. If you cut off how much air is getting into the brake ducts, you reduce the car’s cooling ability. You also increase front downforce. But all the front downforce in the world does you no good if you don’t have a tire.

That’s most likely what happened with four out of five of the cars at Phoenix last weekend. Keselowski, you’ll notice, below a rear tire, while all the others were right fronts. So they may have had a different source of the problem, even though it was the same problem.