We may argue for centuries about who killed JFK, but if there was someone suspicious on the grassy knoll, I  can guess he didn’t make his getaway in a car with lots of torque. Given my online racing experience with iRacing, he likely would have been another easy target for Jack Ruby since he’d be spinning his wheels on the grass while the car was stationary.

I remember cursing the slippery grass at various tracks and praying for the wizards at iRacing to “fix” it. After all, going off track at high speed at a place like VIR with wide fields next to the track means you could be in for a long excursion before you can collect your bearings (in every sense of the word), and turn on the GPS to figure out how to get back to the track. Surely it couldn’t be. In the end, I just used extra caution in those parts of the track to ensure I kept the rubber on the road.

Let's not distort the facts . . .

Let's not distort the facts . . .

But on occasions where I would misjudge the exit of a turn and touch the grass, I was in for a nasty spin with all the foul language that accompanies it. So after about 500 such spins, and taking into account Einstein’s definition of insanity, I decided to alter my driving technique: when I would see the edge of the road getting unavoidably close, I eased-off the throttle.

Yes! With this freshly patented move (I’m sure no-one has ever attempted this) of just easing-off the throttle, one of two fortuitous things happen. Either the car makes the corner and avoids going off the edge of the track due to the induced oversteer, or when the car could not be saved from “going agricultural,” it no longer spun out of control; and with some creative use of the steering wheel I quickly regained my path to the theater.

But in spite of my evolving driving genius I still believed the folks at iRacing were out in left field when it comes to grass physics. So I decided to put it to the test. I took the Daytona Prototype to – where else? – Daytona International Speedway and went for a wee spin in the grass. Actually, I went as fast as I could over the Daytona logo field in a makeshift grass skidpad to see how fast I could go without losing grip in a tight radius (~25m). Telemetry measured the lateral acceleration at about 0.70 Gs. Then I did similar radius circles on the adjacent asphalt and got upwards of about 1.5 Gs or so. Hmmm. The grass felt like I was driving on ice, but it wasn’t a conspiracy, because 0.7 Gs is actually not bad grip. Just under half of what I was getting on the track. I found the same results with accelerating and braking in a straight line.

Daytona International Speedway, aka, my personal grass skidpad.

Daytona International Speedway, aka, Ray's personal grass skidpad.

Then I Googled ‘tire friction grass’ and was shocked to find that, sure enough, street tires have a friction coefficient on dry grass of about 0.35 compared to asphalt at about 0.72. Racing tires have special compounds to bring the friction coefficient on the track up to 1.8, but there is no data on racing tires in grass. Assuming the ratio of grip on asphalt and grass is similar for racing tires, then iRacing have gotten it almost exactly right with the ratio of grip on tarmac compared to grass. If, however, the racing tires maintain the same grip as street tires on grass at about 0.35, then actually we have three times as much grip as we should on grass! I think the former assumption is closer to the truth, and actually now have to admit I was in error about my former grass = ice assertions. In fact, tires on ice have a friction coefficient between 0 and 0.1, so the grass probably has at least 4 times the grip of ice.

So no need to get upset about the sim accuracy. In fact, my donuts in the Daytona grass made me think I may have a future in Oval racing after all. I just have to remember how the steering wheel goes: “Back, and to the left…. Back, and to the left…. Back, and to the left….”

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So you were in the middle of the field smoking grass and you had the epiphany that the friction was right after all? 🙂

A very interesting read! Thanks for writing it up and sharing.

February 6th, 2010 at 5:49 pm

Nice reading! I’ll surely direct someone to it if he’s asking about tyre grip on grass. 😀

Fabrizio Cuttin
February 7th, 2010 at 2:58 am

Does iRacing figure in if say 1/3 of the tire is in the grass (or above) and 2/3 is still on the pavement? A lot of racetracks, the car could be over the edge of the track, and still be above the grass (stiff tires).

Dennis K
February 12th, 2010 at 12:31 am

Yes, the coefficient of friction (“grip”) is probably not far off. However the perceived grip is far too low. Why the difference? Real grass slows you a lot more than iR grass, which helps keep you from sliding 500 yards at full speed into walls and allows you to regain control faster as the car slows to a controllable speed. Real grass also gives, resulting in a tendency for the car to sink in and travel in little ruts (this also causes extra drag and more loss of speed – especially for low cars), this helps stabilize the car (thus little tendency to do donuts at 1/10 throttle and 2 mph trying to get back on track.).

Bruce Funderburg
February 23rd, 2010 at 7:47 pm

wonderful diary you’ve enjoy

Kathey Lacount
November 8th, 2010 at 6:31 pm

Although, my Mother used to say, No guts, no glory.

Lawanna Fiore
November 13th, 2010 at 10:56 am

Thanks a lot for posting, it was unbelieveably informative and showed me tons

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November 15th, 2010 at 8:27 am

Nice post! I always love when people experiment.

Peter Lai
December 17th, 2010 at 10:43 pm

This is an old post but while the author is accurate about the “coefficient of friction of grass”, they are incorrect in the conclusion of how this plays into car control iRL. As one of the other commenters pointed out, iRacing will allow a low power car to just pivot in place with throttle input while on grass. This is not real life and reflects the absolutely flawed tire/car dynamics in iRacing. Clearly the on-track physics of the tire is better but being an experienced track driver I can tell you that in almost every case (not all), dropping two wheels off track results in dropping two wheels off track without the car spinning. You just drive back on track. This is normal. Most other examples are exceptions due to weather, mechanical failure, etc. It is important to note that I am not mentioning excessive speed for the turn here. That is because when the speed for the turn is excessive you are dropping four tires off the track and not generally in parallel with the track. Furthermore, iRacing seems to have issues with transitions from one set of tire physics to another so even if you do manage to spin, you would have to slow down to a crawl to be able to rejoin the track or you will spin while GAINING traction as well. Anyone with any mediocre amount of track experience should be able to recognize this issue.

March 28th, 2020 at 10:05 pm

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