A story and some questions about polymer bonding and NASCAR tires

I am not a chemical engineer, just wanted to share this and get some insight on some observations.

I’m guessing not many of you have too much knowledge on the tire situation in Bristol so here’s a little background.

NASCAR runs two races a year at a short concrete oval called Bristol. Over the decades the track has transitioned from a bottom groove bump and run style track into a top lane dominant track. NASCAR has experimented with ways to bring the track back to it’s roots. This has typically been done with resins that you may see at drag strips. Ultimately I believe the track owner will have to redo the turns to bring back their original glory.

The night race does pretty good for ticket sales, but for many reasons the spring date has suffered. It feels like it was rained out for several times in the past ten years, two dates with the 2nd date being a crown jewel event, and the invisible hand of aerodynamics slowly choking out quality racing. They actually decided to cover Bristol in dirt for the spring race for a few years to give it some help. I didn’t mind that, but the suits tend to listen to certain voices that have a deep passion for traditional Bristol so they went back for another shot on the concrete.

Here’s a brief overview of my shaky knowledge of tire physics at Bristol.

When cars race at the limit of grip they deposit rubber onto the surface of the track. You may see this as large marbles to the side of the where the cars run, as well as darkening of the racing groove. The rubber in the groove at least in NASCAR typically is slower than a green (non-rubbered) race track, but it protects tires from wearing quickly. At Bristol currently, the preferred groove starts at the bottom and as that rubbers in the cars search for speed by going up until the entire track is covered in rubber and the top is the dominant line. In theory you’d hope that faster cars would be able to then pass to the inside or that the groove would work back down, but at Bristol it just hasn’t worked that way with the new car. And to be honest it doesn’t work super well with the more old school Xfinity car (2nd tier series).

Now the interesting part. It’s the 2024 Spring Bristol race. It’s mid March so it’s chilly in Tennessee. Practice has signs of higher than normal tire wear. Qualifying comes up and all of a sudden cars are only able to make one fully committed lap. Teams are puzzled, Denny Hamlin speculates that someone peed in the tire mixture. Still no one thinks it’s going to be a major issue. The track always rubbers up in the race.

Next day comes around and the race is set to start. It’s around 60f (16c) and with cloud cover track temp is around 80f (27c). Race starts pretty normal. The drivers think it’s hammer down time for 500 laps so they’re getting it. Most are in the resin, but some are making slight progress on the outside lane. This lasts until lap 21 when Byron gets knocked into the marbles and loses the car and busts a toe link. He hits pit road and a caution is brought out for debris. The entire field dives onto pit road on lap 26 except for Tyler Reddick who had been running around 20th. Reddick was taking a chance that it was gonna be a typical Bristol track position race. Unbeknownst to anyone this would be the least track position dependent race in perhaps all of automotive racing history.

On the restart he was quickly swallowed up and his lack of grip caused a spin and a minor pileup. During that caution drivers began to talk about the massive marbles around the track. Next green flag run happens and it’s incredible what’s happening. Drivers seem to be able to drive at will to the front, then after leading a bit the car loses pace just in time for another volunteer to set the pace. This is interrupted by the first blown right front tire. We’re 70 laps in and people are putting on their 3rd set of tires. 500 lap Bristol races usually use 4-5 sets. They are given 10 in total. I’m sure you might be seeing the potential math problem we may run into. Here's an example of the cording on the tires after that 30ish lap run.

See the marbles on the high side, with no signs of rubber actually sticking to the surface.

Anyways long story short the rubber was never able to adhere to the surface and we were treated to one of the most entertaining races of all time. Drivers would pit under green and literally look like they got the star in Mario Kart (only have a twitter link for this in car vid). It was a beautiful chaotic mess. For some reason Goodyear held a press conference during the race. I know it was a CYA type of deal since they did have a legit dangerous tire issue at Indy in 2008(?), but it’s Bristol baby, let that shit ride. Tires were generally just wearing out and if they blew it was the drivers fault for not listening to the warnings.

Everyone loved what we were seeing. Literally 50+ lead changes and still no one would say the winner was gimmicky. It came down to Hamlin’s clear skill of managing his tires.

The night race comes around at the end of summer and NASCAR and all of us fans are hoping to see more of the same. There were a lot of hypotheses floating around about what exactly happened. Cold temperatures? Goodyear pee pee tires? Resin vs PJ1 track bite? Initially the night race was intended to be ran with exactly the same variables other than obviously temperature. For some reason they opted to use PJ1 last minute. I was upset about that as I was in the track temp camp and wanted no doubt about the resin.

Speaking of track temp it was around 100f (38c). Anyways we were treated to one of the worst curb stompings of all time. Kyle Larson led about 450/500 laps. Running like it was qualifying all night.

Since two variables were changed it wasn’t conclusive and I thought it wouldn’t be easy to confirm especially since the spring race was moved to a potentially warmer April for 2025. Well good news. PJ1 was applied for the spring, and Saturday practice and qualifying was cold and overcast so we were finally getting a good one variable difference from two different sides. Ambient was 50’s f (10c) track temp 70ish (21c). It looked like a carbon copy of the previous spring. Tires were cording in practice, drivers getting one usable lap in qualifying. I looked at the forecast for race day and got excited as it was showing ambient in the low 60s. The only question was how much sun was gonna beat down on the track. 

Answer ended up being a lot. Track temp was, if memory serves me right, 100f (38c) on one end and high 80s-90s (31-33c) on the other. Track took rubber like the greedy bastard it can be and we saw another Kyle Larson full throttle ass kicking.

Here's a comparison image i made of the difference between the spring races

Top cold track and awesome race. Bottom warm track and crappy race

Having observed this from a fans perspective I find this concrete track temperature barrier both interesting and frustrating. It seems to be a narrow window somewhere in the low to mid 80s (High 20s C) track temp wise. I just have a handful of questions that if you have any knowledge on I’d like to hear your perspective.

Do any of y’all have knowledge of polymer bonding as it relates to temperature? If so, what could be going on?

How might rubber bonding to concrete be different than asphalt? (haven’t seen cold pavement races have these issues but not sure what other tracks they use this compound at)

Is there anything that could be done with these tire compounds that could raise that bonding threshold closer to 100f? I know that's probably a very specific race tire chem question.

I'd really like to know what Goodyear is thinking now since (at least publicly) they had speculated 2024 spring was resin rather than temp. How would you feel as a chem engineer if a few million people wanted you to formulate something that fails (in a manageable way) like these tires? It's literally the opposite of what you do for road car tires.