Contributor: Andrew Brilliant
(Editors Note:) We were contacted by Yokohama who were happy with the technical content of the article. We would like to thank them for reaching out and we would like to clarify that all tyres have a failure point. As a personal note I have used Yokohama tyres over the years and have always had a great experience. It must be remembered that in Pro and even in Open the cars are producing far greater loads than almost any other class of motorsport.
I woke up early Thursday morning, 4am in my location and somewhat later in Sydney. I had a lump in my throat, it was as if I knew something had gone wrong. I opened up my computer and tried to get my eyes to focus in the glowing light, I could tell my inbox had exploded. A flood of messages from team mates, scattered information, “Under, Huge crash”… “left in helicopter for hospital”, “pray for Under to be ok”. I couldn’t help but think even if he was ok, is this going to be the end of his efforts? The phone was ringing from Danny Nowlan (Under’s race engineer) on an international call. All everyone seemed to know at the time it was a sudden, hard hit and mysterious. It seemed likely that Under would be OK, but the car was definitely in bad shape. Tire failure seemed possible, but oddly they were still inflated, something totally different than the overloading and sidewall failures we had seen in the past. The car was being held, surrounded by officials and the team were not allowed to touch it. That meant nobody could even work on figuring out what had happened until we got official word he was OK. That fueled the internet speculation when not even the team knew what happened. When people asked what was the cause I didn’t have any answer for them, but it was anything but a cover up … we just had no idea and were firstly concerned with Under’s health. The reality is that it takes lots of work to understand how severe the damage and the cause can sometimes never even be determined when all you know is its all come undone in an unknown order.
I have been researching the tire loading and safety issues since the days of Nemo, so more than ten years and most of the research was done in the earlier days. I have done tire rig testing, built tire simulation models, measured downforce methodically even on competitor cars, inspected take offs dozens of times and sent rubber off for analysis off my own teams and competitor teams cars. I cultivated relationships with engineers and various manufacturers just to gather information and build countermeasures that have worked. I sent the tires off for flat trac testing in 2012. Be it wrong or right, it is what I believe and work I tried to be diligent about. Of course, we have limited information about what happened to the other teams, so we can only make assumptions. I hope they will chime in with some comments of their own as well.
Why and how the tires fail from load
OK, this is actually really simple. The tires fold under as they are loaded up, take a look at this picture, this is the V-Sport 86 running in open class. It has high downforce for the class, similar to that of MCA hammerhead in pro class but runs a lowish top speed, gunning for the naturally aspirated record. You can see the tire has quite a bit of displacement, but this amount is actually fine, it only happens for a short burst down the straight and in Turn 1 so it lasts a handful of laps, more than the number of laps before it heat cycles out anyway and goes in the bin. Mission accomplished. Note that the displacement looks a lot more severe when you look at the inboard tire face.
Think of the deformation as a standing wave in the rotating tire and building up heat in the sidewall. It’s only when this heat exceeds the limits. You can reduce the occurrence of something like this by reducing camber or increasing inflation pressure (within safe maximum).
If you spend too much time at this kind of displacement you build up heat in the sidewall and it will fail. This failure mode is identical to an under-pressurization or a big truck with too much load. The sidewall fails, all pressure is lost and it looks like you cut a nice clean circle right through the sidewall with a knife. The manufacturers looking at a picture like this would say to run more pressure and less camber, it would probably look totally reasonable if you did that.
When you build up heat into the tread it has a way to exchange that heat: into the ground. The sidewall however has far less ways to remove heat and anything made of rubber and glue has a heat limit.
There is a maximum deflection after which it simply cracks open. You might have found this the hard way when running over a curb even at 2kmh you can pinch the sidewall and crack it open. Using a stiff suspension means that all the energy of bumps (for which SMP Turn 1 is infamous) wind up transmitted into the tire or the car since the suspension takes up little of the movement. A stiffer suspension is more likely to find this limit than a softer one. This is the conundrum of a high DF car. You need the support to keep it out of the ground but you also need to be nice to the tire so you don’t destroy it. Second order effects here like energy absorption via viscosity heating the sidewall also add up. The recipe for fastest Turn 1 time is actually only as much downforce as you need to saturate the tires to let the suspension be more free. Meaning for some of the cars reducing downforce could make them have a faster Turn 1 bottom speed… assuming they can get the vehicle dynamics just right as well which is no easy task.
Back in the days of Nemo I intended to make sure the car would be safe, I had flat trac testing carried out. Flat trac testing is when the tire is placed on a moving belt and hydraulic ram presses it down and spits out measurements on the tires. https://www.youtube.com/watch?v=96ME_I5s-gQ
You really must do the measurement this way rather than with any sort of static loading test, because without a rolling road you will get fairly inaccurate results. As you can see the tire behavior is totally non-linear. The initial spike in vertical stiffness is because the tire is cambered and after about 13mm of displacement both sidewalls are engaged the tire spring rate spikes up quite a bit. After 13mm is when you would start to see vertical deflection of the tire when you look from the outboard face of the tire. This was very important data for us in wind tunnel and CFD so we could model the correct shape of the tire in cornering attitudes we created that CFD shape for the tire both in high load and high cornering load situations.
At any rate say you reach about 23mm in displacement which will look like about 10mm looking at the outboard face of the tire (the other 13mm you can see mostly when looking at the inboard face of the tire), this can be about 4 tonnes total normal load on the tires or 3 tonnes of downforce and one tonne of car weight.
The anatomy of an overload failure and why this wasn’t one.
All of our load related failures in the past had been the same. The tire was completely cut through the sidewall, it would look as if you had taken a knife to the tire. Zero pressure left, 100%. That failure mode is identical to under inflation or overload you would see on a large truck or any road vehicle.
The rated loads for a tire do not account for any sort of dynamic conditions they leave a lot of margin on the table because it can for example endure much higher loads if the vehicle suspension is softer, the manufacturers have to assume a worst-case scenario. Leaving the question open to teams like us of just how far we can push the envelope. We have been pushing this past the recommended number since Nemo. I feel the responsibility for this is squarely in the hands of the teams not with Yokohama tire. They fairly warned that they hadn’t carried out this testing and that the rated loading was far exceeded, we all carried on.
There is a picture floating around of RP968 with a tire flattened but my guess would be the tire was just flat already from failure. There are other pictures in a similar place on track with a perfectly round tire. I think we can probably rule that out based on the evidence and the flat track tire spring rate testing above.
RP968 with a probably JUST FLAT tire.
back to round again in these two
The real cause of Under’s tire failure! (maybe)
OK this is a bit shameful to admit but there was only one set of tires with the red painted sidewalls. These had been provided by Yokohama as display tires for use at Tokyo Auto Salon. They were the tires that under crashed on, but they were also the ones the car was loaded onto The boat with you can see them in the loading day pics WTAC posted. So, the ‘roller’ tires wound up put on the shelf with all the tires and thought of as any other sticker set. For those that don’t know, that’s a really bad thing to do.
I will let Under and Danny comment on what happened.
“Really I had gotten so confident that the new suspension had done its job and given us tire life we had dreamed of. On the first day of testing we did twelve laps on a set of old tires from Tsukuba 2017, that’s probably a time attack record and we were running in the low 1:22s which would have been fast enough for second place, so it wasn’t like they were babied, even though that wasn’t much for what the car to do it. Felt like I was running lightly and I wasn’t leaning into it at all in the corners it was still that fast.”
“Obviously had I known those were loading tires I would have never ever put them on the car, I just asked for a sticker set from the team and that’s what I got. There was some kind of communication breakdown internally. Sometimes those tires actually even fail during transport because the strapping of them can be quite severe. We can’t say for sure that was the cause but we cannot rule it out either. It is definitely odd that we went 12 laps the day before and then 1.2 laps the next day on a similar set in terms of age but with far less laps run on them and a similar lap time.”
“Initially we thought the suspension had failed because there were photos just before the car went into the wall of the right rear at various crazy amounts of toe. Later upon inspection the team believes that the delamination of the tread caused the tread to come loose and destroy the tie rod and that’s what put the car totally out of control.”
“I could see Under making all the right moves to control a car with a burst tire but once tire is toed out 30+ degrees there’s nothing any driver on Earth could hope to do. In that lap it was also his first time actually pushing into some of the corners at all, he just gave it a little go in T7 but did the fastest time through there of any car potentially ever, this car is going to be able to go flat through T7.”
I would argue a few of these internet rumors are totally wrong:
- The tires this year were different
To be sure and rule this out, I had samples analyzed and at the very minimum the compounds are identical. Also, Under’s failed tires were stamped manufacture in 2017 and were used at Tsukuba two years ago but others with a similar failure mode were 2019 mill. Under has flat out refused to use any different tires than others, he is just that kind of sportsman. I don’t think those tires exist or if they did that he would use then knowingly. I’ll go ahead and call that highly unlikely.
- The failures were caused by too much downforce
The failures on both Under’s car and Tilton were a delamination, not an overload failure. The tread simply separated from the carcass. This is indicative of an overheat situation, not an over load situation. It is the first time Under has seen such a failure. It was like we had solved the sidewall failures and got into the next problem, the raw heat generated running a lap with those kind of cornering Gs, or it could be due to them being the loading tires. We have no way of knowing about that…
The precursor to a tread delamination like this is blistering when chunks of the tread delaminate. Basically, the glue holding the tread to the carcass fails. Tilton has been having that issue for a long time. Nemo did as well, I had always assumed it was the front heavy evo basic layout that caused the fronts to take on too much heat in T1. Nemo was also equipped with a lot of sensors and I had a personal belief that changes in the ackerman could have improved a bit of the T1 heating but on a mega high power 4WD car like that the fronts can spin and that creates tremendous heat in the tread. Based on Garth’s comments in the live stream it sounded like they knew they shouldn’t go for a second lap on one set of tires but got blocked on their flier and did anyway knowing the risks.
I would like to address those mentioning the idea that reducing downforce will solve the problem
Here is a video from 2017 RP968 , top speed 252kmh https://www.youtube.com/watch?v=02TpGiNDlm8
Here is a video from 2019 RP968, top speed looks much better with the Thor engine in action, let’s call it 278kmh. Assuming they had the exact same aero forces (at a normalized speed) they would have increased the peak tire loading by about 25%. This combined with the headwind this year, they could have seen as much as 40 or 45% more peak aero loading, with the same aero. So not just the aero design is causing increases in downforce. Increases in engine power are.
What I’m trying to say is the weather and the engine power caused the largest increase in tire peak loading than any aero development had. If you reduce the downforce and therefore drag, you’ll increase the top speeds and the aero peak loads come back up even on lower downforce cars. You have to control engines as well, if you want to control peak tire loads. Chopping the aero down won’t even solve that problem unless you do it in such a severe way as to essentially make it a mechanical grip class. You lose the soul of what we have done. If the failures are related to heat more wheelspin will make that problem worse and reducing downforce will surely increase wheelspin on a 1400+hp monster of a car. I just don’t see that as an answer that will get the intended result even though it would probably mean a lot of extra business for us! I would be bummed out because the cars being wild is what made this class. I think we need to find a better solution to the problem.
Why Aero controls will fail I thought through each possible control for aero to its completion and its not a pretty picture.
Rear wing: Wing design becomes a focus of major attention, off the shelf vs one off becomes a big deal. Wings will run higher angles focusing on downforce and increasing drag. Cars with higher engine output will gain a huge advantage and total downforce will change little. Teams with budget to take on a complete re-tooling of the rear wing will dominate. The current winning car will improve the most as they have very little to do this year except focus on reg compliance.
Front wing width: Again those with the developmental budget for a smaller wing will spend the time and money to rebuild the whole unit and excel over those that are simply cutting down.
Tougher tires are out there, Under thinks he can find a way to cope with the tires moving forward as well. The decision unfortunately will be up Yokohama and the collective of teams. Hoping for the best and a solution that gives the sport a bright future.