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Dr. Diandra: Darlington tire fall-off — the good kind

Ricky Stenhouse Jr. dodges a tornado to join the show and says his second-place finish at Dover was "very-much needed" but that his team still has work ahead to get a handle on the short tracks.

Engineers call the effects of tire wear ‘tire fall-off’. This can lead to confusion, especially when NASCAR is dealing with tires literally falling off cars.

Tire fall-off -- the good kind -- played an important role in last week’s Dover race. Given Darlington’s aggressive track surface, expect it to have a major impact on this week’s race, too.

What is tire fall-off?

Friction with the track wears down tires. Rubber sticks to the track, balls up to form marbles, or gloms back onto the tires during cautions. Tire wear requires teams to use multiple sets of tires during a race.

The ideal tire would grip like crazy and never wear out. Unfortunately -- like perpetual motion machines and universal solvents -- the laws of physics preclude the perfect tire.

That means Goodyear faces the classic Goldilocks problem at each track. Tires that are too soft wear out too quickly and become a safety issue. Tires that are too hard don’t wear out, but they also don’t offer much grip.

Visualizing tire wear

The best way to “see” tire fall-off is by examining lap times. Let’s use Chase Elliott’s race from Dover last week as an example. Below, I plot his lap times and running position as functions of lap number. I’ve also shown where cautions occurred by shading those laps yellow.

Two scatter plots showing the lap times (upper) and running position (lower) for Chase Elliott in the Spring 2022 Dover race

Lap times trend upward for each green-flag run of more than a handful of laps. There are variations from lap to lap, of course. That’s why I include running position. A driver’s lap times are highly dependent on whether he’s driving in traffic or not.

Elliott changed tires six times, during pit stops on laps 42, 123, 160, 191, 244 and 326. You can tell tire changes because there’s a sharp, one-lap blip upward in his running position.

Elliott led the last 52 green-flag laps of the race, mostly unimpeded by traffic. Let’s isolate these laps and, since he ran P1 the entire time, we can ditch the track position graph.

I’m also inverting the y-axis so that down means slower.

A scatter plot showing Chase Elliott's lap times for the last 50 laps of the spring 2022 Dover race

Flipping the axis makes the graph a little more intuitive because now you really see the times falling off.

If you’re wondering why I plot time instead of average lap speed, it’s because NASCAR measures time. They infer speed by dividing track length by time. Because drivers don’t drive the same distance around the track each lap, lap time is a more accurate quantity.

You’ll hear people talk about ‘two seconds of tire fall-off.’ Be careful. That’s doesn’t tell you anything. Fall-off is how much lap time increases over some number of laps. It’s a rate, just like miles per hour or revolutions per minute.

Elliott’s last run at Dover started with a lap time of about 23.4 seconds at Lap 350 and increased to about 24.4 seconds at Lap 400. That’s one second of drop-off over 50 laps. Elliott had a big lead at this point and didn’t have to push his tires very hard. A driver using up his tires trying to pass or defend a position can experience even higher fall-off.

In this year’s Martinsville race, where abnormally cold temperatures hurt tire performance, Elliott lost only one second of lap time in the course of leading 90 laps.

What determines how much tire fall-off there is?

An asphalt track is composed of an aggregate mix (rocks) and a binder (the black tarry stuff holding the rocks together.) As a track ages, the binder erodes and the aggregate edges round. Aging changes the grip and how the surface wears tires.

Many variables contribute to tire fall-off, but Goodyear director of race tire sales Greg Stucker explains that “the biggest factor in tire fall-off is the track surface.” For example: Concrete is less porous, so it wears rubber from tires differently than an asphalt track.

But every asphalt track is different as well.

NASCAR ran the same tire set-ups in 2021 at Darlington, Charlotte and Homestead. The 1.5-mile Charlotte track produced about one second of fall-off in 40 laps. Homestead is also a 1.5-mile track, but drivers there experienced about 3 seconds of fall-off in 60 laps. Darlington, with its rough, 2008-era surface, resulted in 3 seconds of fall-off in 30 laps.

The upside

High tire fall-off introduces lots of strategic possibilities.

If new tires aren’t much of an advantage (i.e. not a lot of fall-off), then there’s no point pitting unless you need fuel. But if new tires can give you a tenth-of-a-second advantage per lap, it might be worth short pitting. Short pitting is coming in for tires in the middle of a fuel run hoping to gain positions by virtue of having new rubber.

The crew chief must balance the risk and reward of pitting. How many positions will the team likely lose by coming in? How do other teams pitting (or not pitting) affect the plan? That decision will also depend on how many laps are left in a stage or the race.

The decision is complicated by the fact that tire fall-off isn’t uniform. That, Stucker says, is because there are so many different variables involved.

“In some cases, tire wear is linear,” Stucker explains. “In others, we see a rapid decline, then a slower decline.”

But even two different drivers on the same track with the same tires can experience different fall-off rates depending on their cars’ set up and how aggressively they’re driving.

Although Chase Elliott’s tire fall-off may be one second over 50 laps, that doesn’t mean he loses the same amount every lap. At Dover, tires wear faster in the first stages of a run, then the fall-off decreases a little.

I’ve tried to illustrate this by drawing two straight lines in Elliott’s Dover data. The left line was fit to the first 20 laps and the right line to the last 20 laps.

A scatter plot of Chase Elliott's lap times, with a linear fit to the start and the end of the green-flag Dover run.

The first line is steeper than the second, indicating that the tire fall-off is faster in the early part of the green-flag run than in the later part.

But again, if another driver had closed the gap in the last 20 laps of the race, Elliott’s tire fall-off might have gone up because he would have had to race harder to preserve his lead.

This non-linearity makes the crew chief’s decision even more complicated. If the tires wear really quickly in the first 10 laps, the advantage of new tires lasts for a shorter time. Can the driver get to the front before he’s worn out the tires enough that someone else can pass him?

Tire fall-off may make crew chiefs pull out their hair, but it’s a good thing for those of us watching the race because it adds one more complication to the competition.