Tire failure on dyno

[Dick]

Dick one possibility could be the brakes on the dyno. I am familar with the Mustang dyno and it can slow the car down pretty quickly. If the slicks were tied down good, that unusual braking event could have caused an internal belt failure. The tires still had a lot of inertia spinning at high speeds and were set in a forward acceleration stance. Possibly if this is tested again the car should be allowed to slow down on its own or use some vehicle braking. Just not too much to overheat the vehicles brakes.
AL

Al, your idea on breaking gave me something to think about. I followed up on that and talked to the folks at Mustang to see if they had tire failure and what their thoughts might be. Seems that they have had a few of these "events" themselves, most recently on their Viper which is their "dyno mule". In that case the problem was simply caused by tires which had passed end-of-life and probably should have been replaced. One of the things mentioned was the possibility of mounting the car off-axis, which could cause heating and rapid tire wear due to the knurling on the drum. That was not the problem in my case, but had been seen in the lab at Mustang. The gentleman that I talked to did not have any personal experience with slicks, but felt in general that higher pressures might be desirable.

Your thoughts re not using the dyno break has some appeal to me. The dyno break kicks in at 5 MPH less then the "end of test speed" and is not related to the highest speed seen. This is from Mustang, as I specifically asked for clarification on that. In the case of the vehicle we were testing, the "end of test" speed was 114 MPH, however the driver continued under power to 122 MPH. By definition, the dyno break would not be applied until the roll speed decreased to 109 MPH. I have not checked the data yet, but will, to see if the tire failure occurred at that speed. More to come....

Regards,
Dick

[Dick]

Yes I have experienced similar failure.

A racing slick expands in circumfurance as it speeds up so a top fueler is riding on a very small contact patch at the top end , also 9 psi is very low in pressure for on the dyno - at the track where you are trying to maximise contact patch it is ok but on the dyno you dont want or need the sidewall distortion as in effect you only need to get the power down not move the weight of the car.

Exciting ain't it!

The issue of pressure seems to be dominant in most of the conversations I have had. Nobody "really knows", but the universal feeling seems to be that more is better. As I said in a previous reply, we have run a number of cars with slicks and some with lower pressure then this one and have had no problems. Perhaps if you do enough, it will catch up with you (us). I think I mentioned that we employ hold-down straps that pull straight down on the axel. What we do is set tension until sidewall distortion is just visible and then back of slightly. I mention this so that you would not get the false impression that we pull them down until they look low on air.

Need more air.......

Regards,
Dick

[Dick]

It seems that Goodyear has 6 Racing Tire Distribution Centers in the U.S. and one of them is located 14 miles from us at the Michigan International Speedway. We made a trip there to see if they could provide any help or insights with an eye to preventing tire failure during tests at our facility.

We were given a tour of the facility and shown a their huge selection of Racing Tires. There we also scores of Simi-trailers being loaded with tires destined for various races. The tires we saw varied from Jr. Dragster tires, all the way up to the 2024 used by Top Fuel dragsters. By the way, we were awed by how lightweight the sidewalls of the 2024 tires were. They are 2 ply woven and flimsy even compared to the wrinkle wall tires used by Pro-Modified cars. A real eyebrow raiser! Another thing we saw that was very interesting, was fellow mounting slicks on a special test fixture. He would inflate them to 20 PSI, reduce the pressure to 6 PSI and then measure the circumference (rollout) at working pressure. They would then match tires into pairs for final delivery to customers. Now that's very cool!

We were then introduced to a gentleman that travels with the trucks and Represents Goodyear at major races. In this case, he was the person who doing the follow-up on the tire that blew on Brandon Bernstein's car. At that point, we commenced discussion on dyno testing cars with slicks. They did say that they personally did not have experience with testing on a dyno, but were happy to help based on the experience they have with tires in normal applications. In discussing strapping to the dyno and in particular pull-down straps, they were NOT at all concerned. They were quick to point out that even a pro-stock car will have 2500-3000 pounds of down force at speed in addition to its own weight. When we rephrased the question and asked again, they repeated that it was not a problem. Going on, what did concern them was the possibility of a tread wave building on the front of the tire. They felt strongly that tests should be conducted with higher levels of air pressure. While they were reluctant to offer a specific number, what seemed to be implied was something in the 15ish area. The recommended pressure for bead setting is 20-25 PSI and they felt something slightly lower would be appropriate. Curse the lawyers, keeps folks from saying what they really think! The last suggestion they made, and it was strongly emphasized, check both sides of each tire for proper bead seating.

Dyno testing with Slicks, summation:
1. Don't fret too much about down force.
2. Use lots of tire pressure (i.e. 15 PSI).
3. Check for proper bead seating.
4. If used, check for proper bead screw installation.

It seems that they were in possession of the tire and wheel that failed on our dyno. While they do not "know" what caused the failure, they are of the "opinion" that it was caused by improper tire installation. First, it is critical that the beads be properly seated prior to deflation to operating pressure. Second, it is critical that if used, bead screws must be installed properly using the procedures recommended by Goodyear. In this case, bead screws were only used on the outside bead of the tires. In addition, the screws were of the wrong type and did not actually penetrate the bead. Lastly, in not penetrating the bead, the screws actually pushed the bead away from the rim. In their "opinion", during the run, the bead seal was compromised resulting in instant tire deflation. There was no evidence of tire failure, the damage to the tire and wheel appeared to be the result of the run down. The tire and wheel are being forwarded to the Racing Division engineering department, and if there is a definitive answer, it will come from there.

Regards,
Dick

P.S. Even though the Distribution Center felt the tires were not defective, they did do a courtesy replacement. That's 500 bucks worth of tires. Pretty decent of them, I think.

[Brains]

I would start looking for issues with tire GROWTH and contact with the body of the car. That is typically the problem with tire failure when it affects both, and happens at high speeds. Seen it happen several times. Speed the car up, tire contacts the fender, and starts cutting. Couple seconds later --BOOM--

[Dick]

I would start looking for issues with tire GROWTH and contact with the body of the car. That is typically the problem with tire failure when it affects both, and happens at high speeds. Seen it happen several times. Speed the car up, tire contacts the fender, and starts cutting. Couple seconds later --BOOM--

Good point, but that was not the problem in this particular case. This car had been run for several years without any problems. We are putting together a check list and will add this for new set-ups.

Regards,
Dick