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Dr. Diandra: Pit road speeding from the driver (and engineer) perspective

TrackHouse Racing President Ty Norris joins Motormouths to talk about Daniel Suarez's future with the team and what he told Ross Chastain after receiving criticism about his driving style.

Last week, I explained how NASCAR monitors pit road speed and how their measurement method allows drivers to momentarily exceed pit road speed without incurring a speeding penalty. This week, let’s look at pit road speed from the driver’s — and the engineer’s — perspective.

We can break a pit stop into three parts:

  • The driver gets onto pit road (where pit speed begins) and stops in the pit box.
  • The pit crew changes tires, adds fuel and makes adjustments as needed.
  • The driver leaves the box and exits pit road.

Although the driver plays a role in the second step (positioning the car correctly), his primary job is getting in and out of the pit box.

Speeding on pit road is the most frequent pit road penalty. Through Sonoma, NASCAR Cup Series drivers have incurred 77 pit road speeding penalties. While that might seem like a large number, those 77 penalties happened during more than 3,700 pit stops.

About a quarter of the speeding penalties occurred during green-flag pit stops. Only 16 percent of all stops happened under green, which means drivers are more likely to speed during green flag pit stops than during yellow flag pit stops.

A driver caught speeding during green flag pit stops must serve a pass-through penalty. Yellow flag speeders restart at the tail of the field. While drivers can often recover from early speeding penalties, late-race mistakes can take a driver out of contention for the win.

Given the impact of speeding penalties, why do drivers even take chances on pit road?

They have to.

The graph below shows each driver’s fastest time getting into and out of his pit box at Sonoma last weekend. This graph doesn’t include the time in the box. Variations in pit stop speeds are in addition to these numbers.

A vertical bar graph showing driver time getting on and off pit road

Alex Bowman traversed pit road the fastest: 34.347 seconds. The graph shows the 22 drivers whose fastest runs were within one second of Bowman’s time. Seven drivers were within 0.18 seconds of the fastest run. When track position is paramount, as it is at Sonoma, every tenth of a second on pit road is critical.

Speed limits without speedometers

In 2016, NASCAR moved from analog gauges (gauges with dials and needles) to a digital dashboard. Digital dashes offer more information in a format customizable to each driver. But NASCAR still specifies what sensors may be used to display information.

Engine RPMs? Check.

Oil pressure and temperature? Check.

Lap time? Check — if the driver wants it.

The gear the car is in? Not only checked, but required to be displayed at all times in the top right corner.

But no speedometer.

The pit road speed limit at Sonoma was 45 mph: 40 mph plus the five mph NASCAR allows before calling a penalty. Drivers had to obey the speed limit without benefit of a speedometer.

NASCAR drivers instead rely on tachometers, which measure engine rotation rate in revolutions per minute (RPMs). But just because the engine rotates at 7,000 rpm doesn’t mean the wheels do. Sonoma tires were 89.61 inches in circumference, which means the car travels 89.61 inches every time the tire makes one rotation. If the wheels rotated at 7,000 rpm, the car would be going 594 mph.

The transaxle, which replaces the transmission and the rear gear from the Gen-6 car, steps down the rotation rate from the engine to the wheels. Once you know the transaxle gearing and the tire circumference, you can calculate exactly how many rpms the engine should rotate in any gear. Here’s this calculation using the Sonoma gearing and tire circumference in the figure below.

A flow chart showing the route from engine to wheels to transaxle and how rpms change

The five gears change the same engine rpm to different wheel rpm and thus different car speeds. That’s why pit road speed specs must include engine rpm and gear. Before the digital dash, the crew chief would remind the driver as he entered pit road: second gear, 4460. At Sonoma, that would put you at 44.97 mph.

Even if NASCAR allowed speedometers, drivers would still choose the tach because a tachometer provides better precision. In second gear at Sonoma, one engine rpm corresponds to 0.01 miles per hour.

No time for numbers

Although the crew member programming the dashboard sets up a numerical tachometer display, most drivers also utilize a light system so that they don’t have to watch the numbers so closely. Some pit road segments are only two seconds long. In addition to maintaining their speed, drivers also have to avoid other cars and pit crew members.

Jose Blasco-Figueroa, lead race engineer for Sonoma winner Daniel Suárez, noted that a typical dash page might include six red lights and a few blue lights. The programmer would link engine rpm to the light display so all six red lights illuminate at the target engine rpm. If the first blue light blinks, the driver knows he’s okay, but better not push it any further. If the driver sees two blue lights blink, he’s gone too fast. He’ll need to back off the throttle until only five red lights illuminate to compensate.

Back when there was plenty of practice time, Blasco-Figueroa explained, teams could verify pit road speed settings during practice. Now, they have only one opportunity to check their settings when the cars come down pit road at pit road speed just before the race.

During that transit, NASCAR flags speeding cars and tells their crew chiefs in which segment their drivers sped. That’s exactly what happened to Tyler Reddick at Sonoma.

A tweet from Zack Alberts about Tyler Reddick's speeding issues at Sonoma

This is where the digital dash is especially helpful. Blasco-Figueroa said that teams plan for those eventualities by programming pages with slightly different rpm settings. Instead of the driver trying to remember a new target rpm, he just switches to a new dash page.

But even contingency plans sometimes fail. Sonoma featured only one speeding penalty: Reddick’s second stop. That’s the 34.394 second stop represented in the first graph — his fastest of the race.

If teams had needed wet weather tires at Sonoma, the driver would have switched to yet another dash page for trips down pit road. According to Goodyear, wet-weather tire circumference is 89.06 inches — smaller than the standard tires. That difference matters: 4381 rpm with the slick corresponds to 45 mph. The wet weather tire at the same engine rpm corresponds to only 44.72 mph.

This, by the way, is why you have to recalibrate your speedometer and odometer if you put bigger or smaller tires on your car.

In addition to pit road speeds, teams can program other pages of the dash with additional information: one for the driver to focus on lap time and specific pages for troubleshooting if issues should arise with, for example, the cooling system or the transaxle oil temperature.

Eventually, NASCAR will likely communicate directly with drivers though the digital dashboard. Race control will be able to hit a button and drivers will know the yellow flag is out.

Or that they were just caught speeding on pit road.