The Science of …Setting Pit Road Speed

Early in the Las Vegas race, Elliott Sadler, Kurt Busch and Greg Biffle were all caught speeding on Pit Road. NASCAR has a speed limit on pit road for safety reasons. At Las Vegas, it’s 45 mph. You can go five mph over the speed limit, but after that, you are assessed a drive-through penalty: You have to go through pit road, at pit road speed while everyone else continues around the track at race speed.

Kurt was especially incredulous about the penalty because he was in the second pit stall from the front and they were tagged for exiting too fast. Kurt wondered how he could have reached 50 mph in the short distance from his pit stall to the exit of Pit Road. Jeff Gordon suggested over the radio that the large number of speeding penalties might indicate that either NASCAR or the teams had incorrectly calibrated the cars for pit road speed.

You probably already know that NASCAR cars don’t have speedometers. They have tachometers, which measure engine rotation rate in revolutions per minute (rpms). The race cars follow the pace car during the parade laps. When the pace car reaches pit road speed, the pace car driver radios the NASCAR control tower and the tower relays this information to all the spotters. You’ll hear the spotter tell the driver something like, “Pit road speed…now.” The driver will respond with a number and a gear, like “thirty-seven hundred, second gear.” Let’s decode what that means.

A revolution per minute is a unit that tells you how fast something rotates. It’s like miles per hour, but miles per hour corresponds to a linear distance rather than a rotational distance. The minute hand on your clock, for example, makes one revolution every hour. The seconds hand makes one revolution per minute.

The engine on a NASCAR car runs as high as 9500 revolutions per minute (or rpm), which is incredibly fast. How fast do the tires rotate? Math alert: I’m showing the math at the request of a couple of high school teachers who asked if I could include some problems they could use in class. You can skip the calculations and read the rest of the article without any problem.

The tires used at Las Vegas were tire codes D-4142 (left-side tires) and D-4144 (right-side tires). The left-side and right-side tires have difference circumferences (which the crews call “rollout”). If you measured the distance around a left-side Vegas tire, you’d find a measurement of 87.4″, while the right-side tires have a circumference of 88.7″. (Bonus points if you can explain why right-side tires have a larger circumference than left-side tires.)

If the car is traveling 45 mph, how many times do the tires rotate each minute? The first thing to do is to calculate the speed in terms of inches and minutes when the car is going 45 mph. I know to use those units because I’m trying to get an answer in revolutions per minute, so I need to convert hours to minutes. I also know that every time a right-side tire makes one complete rotation, it has traveled 88.7 inches, so I’m going to convert miles to inches because I know I will need that later. Let’s convert 45 mph to inches per minute first:

2008_03_pitroadspeed

So 45 mph corresponds to 47,500 inches per minute. Looking at the right-side tires (for no particular reason), one revolution of the tire corresponds to traveling 88.7 inches. The number of times the tires rotate each minute is 536 rpm, as shown below.

2008_03_pitroadspeed2

NASCAR engines are optimized to run best in the 8500-9500 rpm range and they struggle to run at very low speed. If the tires are rotating at 536 rpm and the engine is rotating at even 2000 rpm, it is clear that you can’t connect the engine directly to the wheels because the rotation rates are so different. The match is made using two sets of gears.

The set of gears closest to the rear (drive) wheels is the rear-end gear. The gear ratio tells you how much the rotation rate is changed by that set of gears. A set of gears consists of a larger gear and a smaller gear that are meshed together. The smaller gear has to turn faster than the larger gear, so picking the right gear ratio lets you control how much the rotational rate changes by.

NASCAR gives teams a selection of two rear-end gears at each track. At Las Vegas, most teams were using a 389 rear-end gear, which means that the smaller gear makes 3.89 rotations for every 1 rotation the larger gear makes. Just ahead of the rear-end gear, the rotation rate changes from 536 rpm to about 2085 rpm (which is 536 times 3.89).

That still isn’t enough of a change in rotation rates, so a second set of gears is employed, this time in the transmission. Josh Browne, the Chief Race Engineer for Red Bull Racing (who served time as Elliott Sadler’s crew chief last year) took a break from testing in Phoenix to tell me that a good number to use for the gear ratio in second gear is 1.8. Drivers are usually in second gear coming down pit road. In second gear with a gear ratio of 1.8 increases the rotation rate by 1.8, so it changes from 2085 rpm to 3753 rpm. When the pace car hit pit road speed, the driver of this car would have called out on the radio something like, “Second gear, thirty-seven fifty”. (They don’t have fine enough markings on the tachometer to get all that precise.)

This is a theoretical rotation rate and the actual rate may be a little different; however, it gives the crew chief a general idea of what the tachometer ought to read when the spotter reports that the car is at pit road speed. I usually listen to the drivers and their crew chiefs at the start of the race to hear them go through this ritual on TrackPass; however, I was driving back from the Omaha airport as the race started and didn’t get to hear. The drive-through penalties, along with some unfortunately timed cautions put Biffle, Busch and Sadler down a lap for most of the race. All three fought their way back to the front, although Busch blew a tire and ended up with a finish that he didn’t really deserve. Biffle came back to finish third and Sadler twelfth, making them wonder, I’m sure, what might have been if they hadn’t gotten pit road penalties.

Addition: I http://onelugnutshort.blogspot.com/2008/03/three-races-in-and-what-do-we-know.html“>read that Elliott originally gave his team a reading of 3950 rpms in 2nd gear, while Rodney Childers, Elliott’s crew chief, said that the number should be more like 3700 rpm. They ended up going with 3850 rpm (2nd gear), which apparently put him about a half mph over the speed limit

A couple other comments on Sunday’s race:

  • Having to listen to the first half of the race on the radio instead of watching it on television made me appreciate what a great job PRN and MRN do. They are able to paint a great picture of the race without the benefit of video. I don’t know how they slide seamlessly from one reporter to another without talking over each other. It must be the result of a lot of practice.
  • Thanks to Chad Knaus for proving that he is actually human. The 48 team, in a very uncharacteristic showing, missed the setup entirely. Chad finally had to tell his driver “We can’t fix the car, you’re just going to have to do the best you can with it”, which ended up being 29th, two laps down. I’m sure that finish gave some of the other crew chiefs a little glimmer of hopeā€“for this week, at least.

Also published on Medium.

4 thoughts on “The Science of …Setting Pit Road Speed”

  1. There is one slight in your statement that the 19 car was a half mph over the limit. He was actually 5.5 over, as NASCAR allows a 5 mph grace zone.

    Otherwise, good blog!

  2. Thanks for the comment, Snafam. I was imprecise. Elliott was apparently clocked at 50.5 mph. The thing is that 50.5 mph should corrsespond to a tach reading of 4210 rpm, which is pretty far away from their target of 3850 rpm. Anyone have any insight into how he could have been almost 360 rpm off?

  3. I am not sure how Elliott’s team was off on the calculations, but I don’t think that he was the only team.

    I was listening to David Gilliland’s scanner at the race, and I heard some discussion of discrepancy between what was calculated, and what was used due to the pit road clocking during the pace lap.

    As far as ‘being clocked’ on pit road, the timing markers are set up on pit road, and if one listens to the scanner, they commentary of where a car can speed up and slow down is quite common, as long as they know where the timing lights are located.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: