Does Grass Pose a Danger to Racecar Drivers?

Now that most tracks have put SAFER barriers on any possible surface, it might seem like racetrack safety is a done deal.

That’s not what we’ve heard this week at Daytona, though. Some of the drivers have some strong opinions about grass.

“Grass belongs on golf courses. We need asphalt around here to slow the cars down, control the cars.’’ — Jimmie Johnson

“There’s absolutely no reason to have grass at any of these facilities. I think that needs to be one of the next biggest pushes we all have.” — Kyle Busch, last July

“If his (Johnson’s) nose would have snagged the grass wrong, the car would have flipped over and he could have ended up if not in the lake” — Ryan Newman

Anti-Grass Arguments

Argument 1: Grass has Really, Really Crappy Frictional Properties

This is the biggest and most convincing argument against grass at racetracks. Remember that a NASCAR race car going 180 mph has about the same energy as is stored in 2.2 lbs of TNT. That’s a lot of energy to dissipate.

Much of the energy is dissipated by friction. Let’s assume you get spun out going 180 mph. You get off the gas and on the brakes, locking the wheels. Sliding friction between the tires and the track provides the majority of the force needed to slow down the car.

Friction always opposes motion: if you go left, friction acts to the right. If you go right, friction acts to the left. It’s just contrary that way.

A vector drawing showing the forces that are exerted when the drivers is on the gas vs. off.The coefficient of friction tells you how much friction you get between two surfaces. The larger the number, the more friction you get.

The graph below shows some ranges of friction for a normal passenger car tire on a couple of different surfaces.

A bar graph of the coefficient of friction for rubber against a variety of surfaces.

The coefficient of friction on grass is significantly lower than the coefficient of friction on asphalt or concrete – it’s even much lower than the coefficient of friction on wet asphalt or concrete.

The numbers would be even worse for racing tires because treads make a huge difference when it gets wet. But basically, a race car on wet grass isn’t all that different than a race car driving on ice. We can all guess how well that works.

Let’s put some numbers on it. I started by figuring out the numbers that would give me a race car that would take 5.0 seconds to come to a stop from 180 mph. That gave me a number for the stopping force. Then I halved that number, then quartered it.

The dark blue solid line on the graph below shows you how the speed changes (assuming constant deceleration) for the highest stopping force. 180 mph to 0 mph in 5 seconds.

The dark orange line (the next one up) is with half the stopping force. In five seconds, the car’s still going 90 mph. It’ll take another five seconds for the car to come to rest.

The mustard yellow line shows what happens if the car has one-quarter of the stopping force. After five seconds, this car is still going 135 mph. It’ll take this car another fifteen seconds to come to a stop.

  • If the coefficient of friction decreases by half (which is a good approximation going from dry asphalt to dry grass), the time it takes for the car to slow down doubles.
  • If the coefficient of friction decreases by a factor of four (a good approximation going from dry asphalt to wet grass), the time it takes for the car to stop goes up by four times.

And, of course, that means the car travels a much greater distance before it stops, which means (in turn) that the car is much more likely to run into something.

A sub-argument is that your ability to steer is also compromised by the lack of friction. Ever tried avoiding something while you’re driving on ice? Driving on wet grass is essentially the same situation. If you’re headed for a wall, your chances of changing direction are slim.

And don’t forget: cars don’t speed up when they go from the track into the grass! It just looks that way.

Argument 2: Steps Up/Down Cause Torques, which Cause Flips

There’s a lip between the track and the grass. This creates a torque on the car. Torque causes rotation. See below and watch what happens both when Sadler goes from track to grass and from grass to track.

When Sadler comes from the grass back onto the track, the roof of the car catches on the track surface and that starts the car rolling again.
Okay, I’m cheating a little because this argument would still hold if there were something other than grass. Any disparity in surface heights will cause this problem. It’s one more argument against grass, but has to be considered with other solutions, too.

Argument 3: Sogginess/Destructiveness

I know – who calls grass destructive? It is, especially when it gets wet. The car’s wheels sink into the grass, which puts parts of the car body in contact with the grass. Watch Jimmie Johnson in the Sprint Unlimited last week – look at about 45 seconds in where you can see the car’s front fascia torn entirely off by contact with the grass.

We see plenty of times where drivers spin, don’t hit anything, and are able to continue — but not when the grass eats parts of the front body. We’ve seen splitters come back onto the track with a load of turf as well (which doesn’t help the track surface and can block the radiator inlet and cause overheating).

The changes in the ride height rules keep the cars much lower to the track than they were in the past. That’s why the autoniverous tendencies of the grass are more of a problem now than they were in the past.

Yes. I did make up the word ‘autoniverous’.

OKAY – If Grass is so Bad, Why is it There?

Some people think the only reason for grass is because it’s easier to put advertising on. Some have suggested that grass is cheaper than asphalt. I don’t think that’s correct. It may be cheaper to put in initially, but grass takes a lot more care and maintenance than asphalt. In the long run, I bet the asphalt is actually cheaper.

So why is it there? Primarily, grass is there for drainage.

Take a look at Daytona from the air. Remember that the track is banked at 31 degrees in the turns and even the front stretch has 18 degree banking. The track is like a giant bowl — as as most racetracks.

Daytona Beach gets about 50 inches of rain a year. That rain has to go somewhere and the last place we want it is on the track or in the infield. Saturation of the ground is a huge problem in urban areas. There’s so much concrete in sidewalks and parking lots, so when the little grass there has absorbed all the water it can, the rest of it just sits there. We call that a flood.

How Do We Fix It?

Idea 1: Remove the Grass

Removing all the grass isn’t a feasible option in many places because of drainage issues. Another issue you’ll hear batted around on motorsports forums is that the grass plays a role in the race because grass constrains the drivers’ option. The opposite is Phoenix, where drivers have the option of taking a giant shortcut. An all-asphalt track presents no penalty for overdriving.

Another issue we often forget is that Daytona hosts much more than just NASCAR. If you give a motorcycle driver the option of falling off their bike and landing on grass or landing on asphalt, I’m betting they’d like the grass there. If you thought the differences in slowing down between grass and asphalt were big, think about the impacts on bones and flesh.

Idea 2: Gravel Traps

Gravel traps do a very good job of stopping a car; however, the chances the driver will get the car out unassisted again are pretty darn small. The gravel traps are even more quicksand-like than wet grass. Once the car gets in, there is nothing rigid for it to push against. That means a yellow flag and the need for emergency personnel to come onto the track to remove the car.

And if the car does make it out of the gravel trap, there’s a possibility that it carries some of the gravel out onto the track -which poses a hazard for the other cars. When only a few square inches of rubber hold a car onto the track, a piece of gravel can make the difference between navigating a turn and crashing.

There’s also a likelihood of the car being damaged by the gravel pit, but probably not as high as grass. Finally, you’ve got an issue in splashing gravel everywhere. If you’ve ever gotten hit by a piece of flying gravel, you know it can be darned dangerous.

Other Ideas?

There is actually something called “high friction asphalt“, which is pretty much what it seems. It would stop a car faster than the track asphalt. You still have drainage problems to contend with, however, and there would have to be a lot of studies to ensure that the asphalt didn’t damage the tires. The last thing you need when you’re spinning out of control is to have a tire blow out.

A number of F1 tracks use an Astro-turf-type material, but it’s not clear to me whether it’s  there for looks or safety. I did learn that some tracks put artificial turf down after replacing gravel traps with asphalt because the motorcycle riders were using the extra width, which made the track easier to run. In 2014, two high-profile Moto-GP crashes were attributed to motorcycles loosing traction on the wet artificial grass surface and riders lobbied to get rid of it.

There’s a product called Flexamat (suggested in a previous blog by reader RAEckart) that is designed to control erosion on slopes. It’s a bunch of concrete pieces woven together. There are enough gaps that grass can grow between the concrete. From the pictures, it looks like a good amount of the concrete sticks through.

You’d still have issues with it being dangerous for motorcycle riders, but some type of hybrid, engineered system like this might be the solution. (So you civil engineers, get thinking – you’ve got a small, but anxious market base!)

Charlotte Motor Speedway tried something unique for last Fall’s race: a 6-foot-wide transition border they put between the pavement and the grass on the front stretch. The barrier (shown below from an article on is made up of 140 tons of sand and oil mixed with rye grass. (Yes, I did check that twice. It is rye grass. I don’t know whether they intend for it to grow grass in that base or…?)

The idea is to make a more gradual transition from track to grass. The track PR mentioned that the transition surface would give the drivers a little extra room to run on, so it’s got to be pretty well tamped down and solid. It’s only been up there for the one race, so it’s hard to evaluate at this point. Charlotte has no plans to remove the rest of the grass.

As usual, there’s no simple answer — otherwise the problem would have been solved already.

1 Comment

  1. Your theory that the grass is there for drainage is spot-on. Housing developments, warehouses, retail projects – almost all carve out an area for drainage since most new construction already has development around it. (We can’t just sheet flow to an open field like 30 years ago.)

    One thing we use along highways, in warehouse truck-courts and in inner-cities is “drop inlets”. Instead of depressed curb inlets that you see in housing developments, a drop inlet is a grate over a hole in the ground that achieves the same function – allowing rainwater or other surface waters a place to go.

    Of course, in warehouse truck-courts and inner-cities, we put them directly in the driving lanes and don’t worry about it since the speeds are low (<30 mph). But you do see them on highway shoulders in busy parts of town, and occasionally when forced, we have to put them in higher-speed areas. (That's usually a last resort and often temporary, since we tend to "crown" road surfaces to get water to flow away from the center of the road.)

    But you "could" switch from grass to asphalt, put in drop inlets and then install storm drain to take the water somewhere else outside the track. The only thing is – this is going to be expensive. You're probably going to have to bore under grandstands, then hope you can find an area below the racing surface outside of the track that can take the water. After all, storm drain is gravity-fed.

    The construction crew that builds this setup will be the miracle workers, since they're going to have to level the grate to the surface around it. And bolt down the grate hard! Indy Car racing through inner-cities has demonstrated that manholes not bolted down hard tend to get sucked up by an aerodynamic car traveling over it.

    I believe Texas Motor Speedway did this in the late 1990's after having severe drainage issues. There are drop inlets in the infield, but they were surrounded by grass. (Since Dale Junior's trip through the Texas infield grass a couple years back, they paved more infield, so I'm not exactly sure what the setup is there now.) And Texas avoided some of the storm drain expense by making these "French drains", which means the bores were made into looser subsurface and the water simply goes deeper into the ground instead of outside the track.

    The bottom line is expense, and like the projects I work on day-to-day, the geotech report of the ground underneath of you is going to tell you how much you have to spend to get rid of the water. It'll vary widely across the United States.

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