Engineers call the effects of tire wear “tire dropping”. This can be confusing, especially when NASCAR is dealing with tires literally falling off cars.

Tire drops – the good ones – played a big role in the Dover race last week. Given Darlington’s aggressive track surface, expect it to have a major impact on this week’s race as well.

What is tire drop?

The friction with the track wears the tires. The rubber sticks to the track, inflates into balls, or rolls back onto the tires during warnings. Tire wear forces teams to use multiple sets of tires during a race.

The ideal tire would grip like crazy and never wear out. Unfortunately – as perpetual motion machines and universal solvents — the laws of physics rule out the perfect tire.

That means Goodyear faces the classic Goldilocks problem on every 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 don’t offer much grip either.

Visualization of tire wear

The best way to “see” tire drop is to look at lap times. use Chase Elliotthe Dover race last week as an example. Below I plot his lap times and race position as a function of lap count. I also showed where the caveats occurred by shading those towers yellow.

Lap times tend to increase for each set of green flags over a handful of laps. There are variations from round to round, of course. That’s why I include the race position. A driver’s lap times are highly dependent on whether or not they are driving in traffic.

Elliott has changed tires six times, during pit stops at laps 42, 123, 160, 191, 244 and 326. You can tell the tire changes because there’s a one-lap upward jerk in his race stance.

Elliott led the final 52 green-flag laps of the race, mostly unhindered by traffic. Let’s isolate these laps and since P1 has been running the whole time, we can ditch the track position graph.

I also reverse the y-axis so that down means slower.

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

If you’re wondering why I’m plotting time instead of average lap speed, it’s because NASCAR measure time. They deduce the speed by dividing the length of the track by the time. Since drivers do not cover the same distance around the track each lap, lap time is a more accurate quantity.

You will hear people talk about “two seconds of tire drop”. Be careful. This means nothing to you. The drop is the increase in lap time over a certain number of laps. It is a rate, just like miles per hour or revolutions per minute.

Elliott’s last race at Dover started with a lap time of around 23.4 seconds on lap 350 and increased to around 24.4 seconds on lap 400. That’s a one second drop over 50 laps. Elliott had a big lead at this point and didn’t have to push his tires very hard. A rider who uses his tires trying to overtake or defend a position may suffer an even greater fall.

In this year’s Martinsville race, where abnormally cold temperatures hampered tire performance, Elliott lost just a second of lap time while leading 90 laps.

What determines the amount of tire drop?

An asphalt track is made up of a mixture of aggregate (rocks) and a binder (the black tarry substance that holds the rocks together). As a track ages, the binder erodes and the aggregate edges become rounded. Aging changes the grip and the way the surface wears the tires.

Many variables contribute to tire drop, but Goodyear’s racing tire sales manager Greg Stucker explains that “the number one factor in tire drop is the track surface.” For example: concrete is less porous, so it wears tire rubber differently than an asphalt track.

But every asphalt track is also different.

NASCAR used the same tire configurations in 2021 at Darlington, Charlotte and Homestead. The 1.5 mile Charlotte track produced about a second of drop over 40 laps. Homestead is also a 1.5 mile track, but riders there experienced around 3 seconds of crash in 60 laps. Darlington, with its rough surface dating back to 2008, resulted in 3 seconds of downfall in 30 laps.

The advantage

The high tire drop introduces many strategic possibilities.

If the new tires aren’t much of an advantage (i.e. there isn’t much drop), then there’s no point in swooping down unless you need fuel. But if new tires can give you a tenth of a second per lap advantage, it might be worth a short pit stop. Short pitting comes for the tires in the middle of a fuel run hoping to gain positions with the new compound.

The team leader must balance the risk and reward of pits. How many positions is the team likely to lose by entering? How do other opposing (or not opposing) teams affect the plan? This decision will also depend on the number of laps remaining in a stage or race.

The decision is complicated by the fact that the tire drop is not uniform. This, says Stucker, is because there are so many different variables involved.

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

But even two different riders on the same track with the same tires can experience different crash rates depending on their car setup and how aggressively they drive.

While Chase Elliott’s tire drop may be one second over 50 laps, that doesn’t mean he loses the same amount every lap. At Dover, the tires wear out faster in the early stages of a run, then the drop decreases a bit.

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

The first line is steeper than the second, which indicates that the tire drop is faster at the start of the green flag race than in the last part.

But again, if another driver had closed the gap in the final 20 laps of the race, Elliott’s tire drop could have increased as he would have had to run harder to maintain his lead.

This non-linearity makes the team leader’s decision even more complicated. If the tires wear very quickly in the first 10 laps, the advantage of the new tires lasts less. Can the driver get to the front before the tires have worn down enough for someone else to pass him?

Tire drop can make team leaders tear their hair out, but it’s a good thing for those of us who watch the race because it adds an extra complication to the competition.