Pit Road Speeding Explained

The black helicopters were out over Indy on Sunday, or so suggest some Juan Pablo (a.k.a. Juablo) fans. This happens every time someone leading a race (or contending for the lead) gets a pit road penalty.

On This Week in NASCAR, Micheal Waltrip opined that if NASCAR caught you speeding, you were speeding. Juablo maintains that he wasn’t. It’s possible for both statements to be true.

I’ve already covered in detail how teams determine the tachometer reading that puts them at pit road speed plus five mph minus just a little. In brief, the engine rotates around 9500 times each minute and the wheels somewhere around 2000 times per minute. Between the engine and the wheels are two sets of gears: the one closest to the engine is the transmission and the one closest to the rear wheels is called the rear end gear. The diagram shows the gear ratios for a Borg Warner MM6 manual transmission and a GU6 3.42 rear-end gear, as might be found in a Corvette. When you turn a larger gear with a smaller one, you decrease the rotation rate. If the gear ratio is 2:1, the smaller gear turns twice every time the larger gear turns once. Get a set of K’Nex and you can prove this to yourself. The gear ratio is the ratio of the number of teeth on one gear to the number of teeth on the other. (I have a two-speed K’Nex transmission on my desk. I have had to literally take the transmission out of the hands of more than one Ph.D. who was just amazed that simple machines actually work.)

Anyway, the teams know the gear ratios in the transmission and the rear end gear – you can see the calculations in the previous blog. Even before getting to the track, they know what the tachometer should read when the car is at pit road speed during the warm-up laps. NASCAR allows the cars 5 mph over the pit road speed, so 60 mph was the maximum speed you could go on Pit Road at Indy without getting a penalty. The engineer and driver will agree on a speed during the parade laps. If you’re listening in, you can hear the process.

tachThis is what a tachometer looks like. The new ones are a little fancier – they have lights that can be set by the driver so that a yellow light comes on when they are getting close to the target rpm and a red light comes on when they darn well better get off the gas if they don’t want a penalty. Everyone always asks why they don’t just put a speedometer in the car. You can actually control your speed very precisely with a tach.

The divisions on the gauge are 100 rpm. If the driver can read the gauge to 100 rpm, for a typical gear ratio (i.e. let’s say a 1.45:1 second gear and a 4.22 rear end gear), each 100 rpm step on the tach corresponds (for 82.1 inch circumference tires) to 1.37 mph. If you assume that the driver can read the tach to 50 rpm, that’s 0.64 mph. So for a driver, there’s really no point in having a speedometer and a tachometer. They need the tach to help with shifting, and knowing how hard they are pushing the engine. If driver’s had speedometers, I assure you there would still be speeding penalties.

Ever looked at the speedometer from the passenger seat? What you see is different from what the driver sees due to something called parallax error. Look at something with just your left eye (closing your right eye), then with just your right eye. The object looks like it’s in different places because your two eyes are not located in the same place – they see things from different angles. When both eyes are open, your brain automatically interpolates between the two. (This is one reason that people who have lost the sight in one eye have problems with depth perception.)

The tach in a racecar is usually about 5 inches in diameter. Put yourself in Juablo’s place, coming out of your pit box, trying not to hit any other cars, trying to maintain your lead, and watching the tach. All you have to do is be off by 50 rpm.

I was looking back on a previous blog I started and didn’t ever publish and found something from Red Bull Racing engineer (and former crew chief) Josh Browne. Teams sometimes see discrepancies between their calculations and what they read on the track during the parade laps. If the pace car speedometer is off, that raises a bit of confusion, as you have to decide if the discrepancy is due to a problem on your end or on NASCAR’s. The other problem, he mentioned, is that tachometers are analog devices (in contrast to digital). And they aren’t always exactly accurate. Each team’s engineer gets a report each week that tells them what the offset is on the particular tach in the car. If the team is part of a multi-car company, they know the parameters for the other cars and can get the rpm reports from the other drivers and compare. Pit road is divided into segments and the car has to have an average speed less than pit road speed (+ the 5 mph buffer) in each segment. After the first pit stop, teams can ask NASCAR for their speeds on pit road and double check their calculations. They can get their numbers for each segment from NASCAR.

I know of at least one case in which people incorrectly set up the spreadsheet most teams use to find the pit road tach reading.

NASCAR measures the speed of each car in a series of segments, which are defined by wires embedded in the track. The car has to be below pit road speed + 5 mph in each segment. If you’re over in anysegment, you get penalized. Juablo was going 60.06 mph in Zone 2 and 60.11 mph in Zone 4. Do the math: 0.11 mph corresponds to 8 rpm and 0.06 mph corresponds to 4.4 rpm. Look at the tach and tell me you can tell the difference between 3850 and 3858 rpm.

Races shouldn’t be won or lost on being 0.11 mph over. After all, the whole point of the Pit Road speed limit is for safety and you aren’t going to hurt someone any less if you hit them going 60 mph or 60.11 mph; however, you have to draw a line. If you give them 6 mph, everyone will be going 61 mph and then people will complain about 61.06 mph being penalized. As Juablo said, “It is what it is”. Them’s the rules and each team chooses how close they want to get to the line. If you’re leading the race or have a chance to win, you have to balance loosing positions on Pit Road because you’re slower than other cars with being sent to the tail end of the longest line for speeding.


Also published on Medium.

16 thoughts on “Pit Road Speeding Explained”

  1. For whatever it’s worth, I was “watching” the race using raceview at nascar.com, and after it happened I backed up and watched the saved tach for Montoya. Mostly I watch Tony Stewart’s info, and his crew chief will be saying ‘4200 in second’, and Stewart will be at 4300-4400, within the fudge. One other driver I listened to that race was being told 4100 in second.

    Montoya’s tach showed 4900 on entry, and 4800 on exit. So there was no question in my mind that he was definitely over. I know the resolution on the web tach isn’t great, but I doubt it’s off by more than 100-200RPM.

  2. Smart racing is smart racing. This was not. If you have the field covered like Juan did there is little sense in speeding. Back off 100 RPMs (which we all can read on a tach), don’t get a penalty and go on to win the race.

  3. Juan got caught coming in. Here’s the deal, as Kyle Petty asked what were the speeds of all the other drivers on pit road, selective enforcement, and why wait to show the infraction until after the race? It’s amazing that Juan went through the first 2 pit stops with no problems but this one with a 5 second lead he gets caught. Oh, by the way, I have a good source who said Juans green light was set to 75 RPM below the max speed, and it never went yellow.

  4. “Races shouldn’t be won or lost on being 0.11 mph over.”

    He was 5.06 and 5.11 miles per hour over the speed limit, not .06 and .11 over.

  5. Another example in a continuing series of excellent posts, congrats Diandra.

    It all bring to a point my contention NASCAR should institute some type of pitroad electronic rev limiter.

    Formula One has used a limiter for several seasons with little to no problems.

    As an F1 driver nears pit entry all he has to do is hit a button/switch and all the work is done automatically.

    If possible could you bounce the idea off one of the engineers you are familiar with.

    I don’t see a problem with the possible exception of NASCAR’s lack of engine electronics as compared to F1 and reliance on outdated dinosaur-like carbs.

  6. I just wanted to make a comment on all of the figures that go into this. Understandably your point of trying to see 8 rpms on a tach is rediculous but drivers and teams need to use judgement on when to push the speed limit vs. being more conservative. Assuming pit road is the full length of the front-stretch (3300 feet) the difference between trying to maximize a full 60mph vs. being more conservative and say maintaining 58mph would cost the driver 1.3 seconds. Considering JPM was 5 seconds up on the field the team/driver could have easily been conservative and still maintained a healthy lead.

  7. What about any variables? Seems to me no one has mentioned heat build up in a tyre over a long green flag run. Dont tyres expand? Are tyres inflated with air or nitrogen?

  8. Diandra,

    I may not have understood all the ratio numbers but, I sure understand that it means Juablo was speeding.

    Thanks for the explanation.

    BTW, we call him Juablo, too.

  9. From someone that has been carring a tire tape for 52 years, they are not all the same size. It also makes a difference where you measure them. I know they are fighting for every split second, but you have to balance risk vs. reward. I think that I might figure my calculations based upon 4.5mph over the speed limit.

  10. Hey Bob – good to hear from you! You are right. If the tires get larger when they heat up, then you go further for each rotation of the wheel, which would mean you are going faster. Juablo got tagged, I believe in a segment before and a segment after the pit stop, so two different tire circumferences (hot and cold).
    What a great calculation to do…

  11. Just wondering how much the tires actually grow, and also how much rubber is worn off the tires between tire changes as this would also change the size of the tire. Would the amount of rubber worn off the tire offset the amount the tire would grow? I think my head is hurting.

  12. Steve says: F1 can do that because they have computers on board, something NASCAR very much wants to avoid.”

    First of all you’re incorrect, NASCAR has telemetry and computers installed in their race cars. They are used by crew chiefs and some computer data is used to gather crash data.

    That aside, it doesn’t take a computer to limit revs, rev limiters have been around long before computers.

    Would any potential NASCAR system use one, most assuredly, but it can be done without one.

  13. What went wrong on Pit Road. The gear ratio was changed in Jaun car last week but Engineering didn’t go back and reset the Tach with new Pit Road speeds so the Tach was set too high. As a result his Tack was seening 3 green lights instead of 7 green lights for Max Pit Road speed. If he reset the Tach behind the Pac car he would have been at the corrected Pit Road speed. Be hind the pace car his Tach saw Yellow lights that told the driver he was set to high. Behind the pace car he should have seen 7 green lights.

  14. I don’t understand why Nascar doesn’t allow speedometers in the race cars. Knowing your speed from the tachometer is pure, unadulterated, horse crap. You don’t know exactly how fast you’re going with a speedometer. Whoever decided this rule in Nascar is a complete moron!

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