In sports car racing, the only discernible change the viewer sees when it rains is that the normal “slicks” (which have no treads) are changed out for rain tires. Thus the calls for NASCAR to develop a rain tire good enough to allow us to continue races, even when it rains.
The surface of slicks (shown at left; from the Goodyear site) are flat sheets of rubber that maximize how much rubber is gripping the road at any given time. The contact patch (the part of the tire in contact with the road) for a NASCAR tire on a Sprint Cup car is about the size of a man’s size 11 average width shoe — which isn’t all that much considering that those four tiny patches of rubber are responsible for producing 200 mph speeds.
The number that tells you how grippy your tires are is called the coefficient of friction: The higher the number, the grippier the tire. For regular car tires, the coefficient of friction between dry asphalt and rubber ranges from 0.7-0.9, depending on the specific surface. When you make the asphalt wet, that can decrease the coefficient of friction down to 0.25-0.50. (These numbers are all ranges because the specifics – the type of asphalt, how old it is, the oil content, the specific type of rubber, etc. all affect the coefficient of friction.) For race tires on dry asphalt, we’re looking at coefficients of friction between 1.1 and 1.3 — or so I’ve been told, because tire companies keep that type of information under wraps.
As I’ve tried to show at right, water gets between the tire and the road, which decreases friction and thus decreases grip. All you need is a very, very thin layer of water and you lose your grip. You’ve probably heard the term hydroplaning – there is a plane of water between your tires and the road. The faster you drive, the smaller the contact patch area and thus the faster you drive, the more likely you are to start hydroplaning.
Rain tires are made with grooves (as shown below) – just like tires made for passenger cars. The grooves point away from the center of the tire in the middle. The pressure of the tire presses the water away from the flat spots and into the grooves. The grooves give the water a way out from under the tire.
The Nationwide series at Road America will have rain tires on hand, but the Sprint Cup Race at Sonoma will not. People have always wondered why you would do this for one series and not for the other.
Reasons to Race in the Rain
1. The fans. How many people spend all year anticipating a race, only to have the race postponed and they can’t stay for the following Monday because they have to work or have other obligations? There’s a lot to be said for making sure that races are run the day they are scheduled.
Reasons Not to Race in the Rain
1. Windshield Wipers. When it starts raining, I’m guessing the first thing you do is to turn on the windshield wipers. On a race car, you have to have a motor and a post available on the car so that a wiper could be mounted. That’s not so difficult, but the teams can’t really head out to Auto Zone and buy windshield wipers. The wipers would have to stand up to high speeds. Ever gone through the dryers at a car wash and watched your windshield wipers get pulled off the windshield?
Another issue is that the windshield wipers would be much trickier to use effectively at high speed. Remember that you go about a football field a second at 200 mph. The rear tires of the race cars throw up a huge amount of spray – the windshield wipers would have to run pretty fast. If you’ve ever reached for your wiper speed control and found that it was maxed out, you know that there’s a limit as to how fast the wipers can go and still be effective. Sometimes, there’s just too much water coming down to remove.
Finally, if you’re going to use tear offs, you’ve got to have a wiper material that works with tear offs. I don’t see how you can eliminate tear-offs unless you were positive it would rain the whole time — and even then, the rain isn’t going to wash all the track debris off your car.
2. The Swiffer Effect. Remember Carl Edwards at Montreal in 2008 with a Swiffer mop trying to clear the fog from the inside of his windshield when they tried to race in the rain there? When the air is moist, it will condense on any cold surface. Glass tends to be much colder than other parts of the car because glass is a good thermal insulator. When it’s moist, you get a thin layer of fog on the inside of your windshield.
A defogger uses heated air to increase the temperature of the glass so that the water doesn’t condense on the glass. You need a heater, ductwork and a blower. Most passenger car systems also pass the air through a dehumidifier so that you’re not making the problem worse by increasing the moisture content.
3. Brake Lights. Coming over the Cumberland Pass this morning, I encountered a lot of fog in the mountains. It’s beautiful, but really hard to see through. In Maryland, people turn on their hazard blinkers when the weather gets like that so that you can see them more clearly. Race cars don’t even have brake lights. In the afore-mentioned Montreal race, Joey Logano wrecked under caution when someone stopped in front of him with no warning. Not only do race cars not have hazard lights, they don’t even have brake lights. If we give the drivers brake lights, they are likely to start playing mind games with each other using the brake lights, so perhaps that’s an area we don’t even want to go.
4. The Fans. I was at Petit Le Mans in 2009 and I can tell you first hand that being out there trying to cover a race in a monsoon was not even close to fun. I spent ten hours in a damp firesuit with wet feet. My notes got so wet I couldn’t make half of them out. Do you really want to sit for three hours in the rain — even a light rain — trying to watch a race that you may or may not even be able to see because of the moisture?
5. Safety. Rain often is accompanied by thunder and lightening. Lightening is dangerous, as I wrote about after a fan was killed and several more were injured at Pocono in 2012. You’ve got tens to hundreds of thousands of fans, often seated at high points and surrounded by metal. That’s a disaster waiting to happen. Pit crews face additional hazards, including slipping (them and/or the car).
6. The Drivers. There is something to be said for experience. Novice drivers make a lot more mistakes, even if they are being as careful as they can be. I know… NASCAR drivers are the best in the world… but very few NASCAR drivers have experience racing in the rain. There are going to be accidents. Since the tires are responsible for making any directional changes, wet tires are going to make it harder for the driver to make sudden changes in speed and/or direction, which means they will have a harder time avoiding accidents.
7. The Racing. In passenger cars, hydroplaning can happen at speeds as low as 35 mph. The higher the speed, the more likely you are to have hydroplaning, so racing in the rain means lower speeds. In the Montreal race, the average speed went from 90 mph to 75 mph when the rain tires went on. Racing on an oval in the rain would probably look like a funeral procession when they cars aren’t busy crashing into each other. Remember that the series that race in the rain race a lot more road courses and have a lot more downforce. The increased downforce allows them to keep the speeds a little higher than you could with low-downforce stock cars.
Stock cars weren’t designed for racing in the rain. Could they be? Sure – the technical challenges can be overcome given enough time and money. Goodyear would probably have to invest some money in research and testing to come up with a tire optimized for wet stockcars. Motors, blowers, wipers, etc. could be designed and tested. But you’re still left with the question of whether a race in the rain can be just as good as a race without rain. I think not. Add in the safety concerns, and it seems like the smartest thing to do when it rains at a NASCAR race is still to stay indoors.