The two words we heard most frequently last week in Pocono (after “still raining”) were “green racetrack”.

Rain doesn’t just delay racing. When the race is finally run after a rain delay, it’s run on a different racetrack. To get the details of how a racetrack changes from “rubbered up” to “green”, I talked to Greg Stucker (Goodyear’s Director of Racing).

The Process

We often talk about a racetrack rubbering up as though it’s a one-way deal and the only time rubber is removed is when there’s a rain or the track is cleaned. Greg Stucker explained that the racetrack is constantly changing because rubber is being laid down AND taken up throughout practice and the race. Here’s the overview, as a picture, then I’ll go through the steps.

Rubbering Up

The right-hand side of the graphic (the red circles) shows the process by which the track gains rubber.

The faster cars go, the hotter their tires get. While rubber is a solid at normal temperatures, when tires reach 200 F and higher, the surfaces of the tires are so hot that the bonds between rubber molecules weaken to the point where the abrasion between the asphalt and the tire can pull rubber molecules off the tire and stick them to the road.

Once that first layer of rubber has been laid down (which can take awhile with, for example, a brand new racetrack), subsequent layers go down on top. The more hot tires in contact with the track, the more rubber you lay down.

Rubber adheres to aggregate and to the asphalt matrix that holds the aggregate together. The adhesion of the second layer of rubber to the first layer of rubber is weaker than the adhesion of the first layer of rubber to the pavement — and this becomes important.

More Is Not Always Better

If you’ve ever painted something that’s been painted many times (and this could be anything from drywall to your fingernails), you know that there is a point at which your coating becomes too thick. It starts to crack and peel and come off in nasty little pieces. If you’ve ever painted something that’s been painted many times (and this could be anything from drywall to your fingernails), you know that there is a point at which your coating becomes too thick. It starts to crack and peel and come off in nasty little pieces.

Despite behaving differently, rubber and paint are both polymers and they share a lot of similar properties. The same type of peeling and cracking happens with rubber as you find in paint. As you build up more and more rubber, the layers become stressed. One layer can crack, which pulls on the layers it’s attached to.

Because the bonds between layers aren’t equal strength, eventually you get cracks that go through many layers . While that can lead to the rubber delaminating from the track, that’s not the main process by which rubber is removed.

Below, the ‘BEFORE’ picture shows the successive deposition of many layers of rubber. I’ve idealized it a bit. They aren’t necessarily continuous and you can’t really tell the difference between the layers — but otherwise it would’ve been one giant black blob and what would you learn from that, huh?

‘AFTER’ shows what happens when the layer is thick enough that the layers begin to crack and peel. It looks very much like the paint above, but we’re talking smaller cracks and much thinner layers, so you don’t necessarily see this happening, especially at early stages.

Caution: Rubber Coming Up

Here’s something I hadn’t thought of. The deposition of rubber depends on high speeds because those produce hot tires.

So what happens when we slow down? Let’s say we’ve had a nice green-flag run and built up a nice layer of tire rubber on the track.

The yellow flag comes out and the cars slow down — which means the tires cool down.

Now, instead of depositing rubber, the cooler tires actually start picking it up from the track. It either sticks to the tires or gets flung out onto the track. Greg pointed out that this process is much more noticeable in F1. (This isn’t a knock against Pirelli, who supply tires for F1. Any tire company would have to provide the same types of tires to satisfy the needs of a F1 car. But the type of tires F1 requires just produce a whole lot more deposition and take-up of rubber.)

The pieces that stay on the track are good-sized chunks of rubber called “marbles” because drivers say driving over them is like driving over marbles. On the left, you can see the huge amount of marbles on the racetrack that accumulate during the course of the race. They clean the racetrack before the race starts. Everything you see is created during the race. The cars push the marbles out of the racing line, but heaven forbid you get a little out of sorts and get off the clean track. There’s an example of where rubber on the racetrack does NOT improve grip.

On the right is a tire that just came in from the track after a caution. The pattern you see is not due to tire wear: The snake-like surface texture is all rubber picked up from the track. Greg tells me you can see the difference between tires that come in from a green-flag stop and those that come in after the car’s run two or three laps of caution. Something to look for the next time you’re on pit road.

You’ve probably seen the tire specialist using a propane torch to clean off tires that have just come in. That’s to remove the picked-up rubber that is covering up the tread indicators. If you tried to measure how much tread you used without cleaning the tires, you’d get garbage results.

When drivers don’t get new tires, they have an additional challenge because they’ve got all the rubber crap on their tires. Yes, swerving does heat your tires up, but more importantly, the back and forth motion scrubs the chunks of rubber sticking to their tires off. It’s like scraping your boot on a step to get the mud off.

A chunk of rubber picked up on a tire is like a piece of chewing gum on your shoe. It sure ain’t gonna make you go faster and it’ll probably slow you down.

Just for comparison, here’s a NASCAR tire from Martinsville. It’s not nearly as bad as the F1 tire, but I wanted to show you how big the chunks of rubber are. Soft rubber is like chewing gum: you just keep gathering it up. Your take-away?

A cold tire running around the track during a caution is like a Swiffer for rubber

Why Some Tracks Don’t Rubber Up

How well a track rubbers up depends on the track as well as the temperatures. I know you’ve seen this graphic a couple times, but it’s important to remember how tracks wear. The asphalt between the aggregate (i.e. the rocks) wears away. The rocks themselves wear a little, but smaller rocks may actually be pulled out of the track as they have less and less matrix surrounding them.

When a track is brand now, the aggregate is nice and level and the asphalt comes up to the top of the aggregate. It’s smooth and grippy. But look what happens after the track wears: the aggregate pokes through a little more. The track is even grippier. Everyone loves a track like this. When the track wears down, you start losing aggregate and the aggregate that is there rounds off.

NASCAR laser maps their tracks. The pictures below show the maps for Kansas and Atlanta. Zero (think of it as sea level) is in that muddy place right between the red and the green. As you move from red to orange to yellow, the track is getting higher — you’re looking at hills. As you move down to blue and violet, the track is getting lower and you’re looking at valleys. The topmost yellow corresponds to 2 millimeter (2 mm) and the bottommost violet corresponds to a negative 2 millimeters. The area we’re looking at in each one is about 20 mm x 20 mm.

Kansas, which was recently paved, is pretty darn smooth. It’s mostly green and red — there’s no yellow and only a very little blue.

Atlanta, however, has big peaks and low valleys. What you’re seeing here is the real-life equivalents to “Too New” and the “Just Right” in the graphics above.

Imagine we multiply the scale so now 2 millimeters is two miles and I ask you to walk from one side to another. If you walk Kansas, you’ll go a distance that would be equivalent on our scale to about 20 miles; however, if you do Atlanta at its peakiest part, you’ll travel further than 20 miles because you’ve got to go up the hills and down it.

ASIDE: Did you know that physicists have scientifically proven that Kansas is indeed flatter than a pancake?

Back to work. In case you don’t see it, Compare the two shapes below. One is a flat surface and the other is the same sized flat surface with a mountain in the middle.

I traced the surface and then extended the lines out. See how much further you have to go with the bump in the middle?

Now let’s apply that to our track.

I do not have the patience or skill to do the same thing with the complicated lines in that picture that I did above, but hopefully you can see that there is more red on the “Just Right” track surface than either of the others. When a track is ‘just right’, it gives the rubber more places to stick to. Those tracks rubber up better than the other types of tracks.

This is the same reason why you need a lot more pain to paint a textured wall than a plain flat wall. Ever tried painting brick? You need a ton of paint because of all the little dips and ridges and holes. Surface area is bad for painting, but really good for racetracks.

Concrete

Greg Stucker pointed out that it is much harder for rubber to stick to concrete than on asphalt. Goodyear developed compounds specifically designed to lay down rubber on the concrete tracks so those tracks do get grippier. One more thing (in addition to speed and safety) Goodyear has to consider when designing a tire.

Rain, Rain, Wash Away…

One of the questions I hear a lot is some variant on this:

If rubber on a racetrack can stand up to a 3500-lb racecar going 200 mph, how come the same rubber comes off with just water?

Some people think that rain dissolves the bonds between the rubber and the track and the rubber is slowly eroded away. It’s actually a physical interaction, the same as if you went out on the track and scrubbed it.

A raindrop in a drizzle might be going 5 mph when it hits the ground, but in a good storm, a raindrop can splat with a speed of 15–20 mph.

It has a force of about one Newton (which is about the same as the weight of an apple). But remember that the force is spread out over a very tiny area, since the average raindrop is on the order of 2.5 millimeter (which is about a tenth of an inch). Pressure is force divided by area. A woman in a high-heeled shoe can exert as much pressure as a small elephant because all the force is focused into a point.

A good strong rain (like the one they had in Pocono last weekend) is similar to sandblasting or pressure washing. If there was already a good buildup of rubber, that rubber layer is already stressed and just waiting to crack and come up, so the rain gives it the extra impetus and physically washes the track clean.

Air Titan Plays a Role, Too

The old way of drying the track used heat from jet engines to evaporate the water. In addition to being slow, the sudden temperature changes and intensity of the air removed rubber from the track as well.

The Air Titan works by first pushing water off the track using air. Greg told me that the Air Titan purposely uses as low an air pressure as they can to remove water while preserving as much rubber on the track as they can.

So… Is Rubber on the Track Always Good?

Rubber is always being laid down and taken up during the course of a race, with the amounts depending on the track conditions and the temperature.

Relatively thin layers of rubber on the track are the best. They give the tires more to stick to and improve grip, but…

When rubber layers get too thick, they start to crack and peel like old paint, which means they can get picked up the the cars’ tires and either reduce the tires’ grip or end up on the track as marbles, which are also reduce grip.

So, like anything else, rubber on the track is good for increasing grip — as long as it’s the right size and in the right place!

*** Many thanks to Mike Siberini of Goodyear for coordinating the interview — especially on short notice.

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