Can We Race Stockcars in the Rain?

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.

TiresContactAs 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.



  1. A lot of that already exists – they ran the Nationwide race Montreal in the rain a few years back. The race started in dry conditions, and at lap eight the rain started, so everyone came in, installed brake lights and windshield wipers, put on rain tires, and went back out.

    Now, the racing wasn’t very good, but they did mostly get it done.

  2. NASCAR drivers are “the best in the world”? Really? If that is the case, then why is Formula One the only series to declare it’s champion as “World Champion?” The answer to that is simple. NASCAR drivers are NOT the best drivers in the world. The only people who believe otherwise are those who are too narrow-minded to see beyond their myopic views. Best stock car drivers? Maybe. Best drivers in the world? NOT EVEN CLOSE!!!!!!!.

    • Sorry, Edward – I should have used the tag around “best drivers in the world”. Long-time readers of this blog know that I am disproportionately amused by the overblown hype surrounding NASCAR. It’s typically American – we have a World Series that only covers North America. Anyway, it was meant to be ironic, not literal.

  3. While I see your point, the NNS cars do have a single brake light in the rear windshield when they race on road courses. It is turned on only when it rains. Also, the NNS teams have developed a downforce device to put on the windshield wiper to make sure they do not fly off. This was done after the first time they raced in the rain. I believe racing in the rain is possible. The first NNS race in the rain was terrible because it rained too hard to race. I think even F1 would have stopped the race due to the conditions. The second time was much much better. They can race when the track is a wet after a rain or when it’s raining lightly. No one can race in a monsoon, like they had in the first race in the rain. I am unsure of the vehicle dynamics of it all, but I will know in the coming months.

    Just thoughts from a Motorsports Engineering Student

    • Welcome, Lucas – thanks for the comments and great to have a motorsports engineering student reading! I think the question almost becomes “Is it worth racing in the rain?” They get so little opportunity to do it — Each race is likely to feature some problem until they’ve raced enough races to encounter them all. Personally, I like watching the sportscars in the rain – you can see the aerodynamics patterns in the water coming off the rears of the cars! Thanks again!

  4. I road race motorcycles with enough success to have won a few national championships. When it’s raining, one of our favorite sayings is “This isn’t NASCAR”. If we can race in the rain on 2 half dollar sized contact patches, with power to weight ratios and top speeds comparable to NASCAR on 1/1000th the development budget, there is no technical reason why NASCAR cannot do the same.

    Rain tends to separate the men from the boys regardless of that last 5% of mechanical advantage.

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