Low Pressure as a Failure Cause in Steel-Belted Tires

[BrucePerens]

I was reading today about Zipper Rupture in tires, which injures and kills its share of tire technicians. There is a video of mannequins being destroyed in tests

here

and of a tire being destroyed in a shop

here

.

The failure mode is interesting. The tire is run at low pressure for some reason, for example it's run flat. Or it's simply overloaded. The sidewalls over-flex. The steel cords in the sidewalls undergo work hardening because they are flexed many thousands of times. The hardened cord breaks instead of flexing. The tire later bursts in the sidewall at the point of broken cords. It's called "zipper" because there are steel wires to the left and right of the rupture, which look like an open zipper. The burst is so sudden that it sends the tire flying, potentially killing people in the way.

It's not inconceivable that some of our blowouts start with broken cords due to over flexing, and the rubber over the broken cords bursts, then the tire disintegrates. Lots of folks have commented on the suddenness of the blowout, well here's a reason for that to happen.

We've been thinking about the cause for tire failure in TrailManors as rubber degeneration due to age. Steel belt failure doesn't really have anything to do with the rubber. One of the safety features of the nylon overlay in premium trailer tires is that it is not subject to work hardening, and thus will retain the pieces of the tire in place through a steel belt failure.

This is a pretty strong argument for running your tire at the cold inflation pressure on the sidewall, and not below that pressure. Cord flex increases with lower pressure.

[RottieMom]

Bruce, I'm in a bit of a quandary on my tires... I just had some Maxxis load range E tires put on my TM a few months ago and assumed the recommended psi was 65. I noticed the other day that the tires actually say 80 psi max. I still have the original rims on which I believe are rated for 65 psi. So, I'm wondering if I should just inflate tires to 65 psi? Or, do I need to look into new rims?

[BrucePerens]

In this table, Maxxis derates your load rating based on pressure. According to Maxxis, your tire becomes load rating "D" instead of "E" at 65 PSI and is rated for 2540 lbs instead of 2830. It could take 1450 pounds at 25 psi and would bear the "B" rating.

So you don't actually have an "E" rating with that wheel! Yikes! You still have the strongest tire you could mount on that wheel, and the important nylon overlay.

This is a good point for people who would buy the 10-ply tire from Maxxis. You may need to buy wheels that can sustain higher pressure than your current set, to get the full value of the tire.

I guess the derating is related to overbending the sidewall and the work-hardening issue.

I am more concerned with folks who keep their tires "soft" to reduce shock to the trailer.

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So how do you account for when the tire heats up while driving. If you inflate your tire to 80 psi cold and start driving and your rims are rated at 80 psi, after a while your tires heat up and the air inside expands and then the pressure expands to 90 psi well above the pressure rate for your rims! Then....KA_BLOOOOEY!! whap whap whap whap....oh, was that my plumbing going down the road? I knew I should have built that cage...
Tom ... who is wondering what to do

[Bailey'sMom]

Every tire I have seen gives you the maximum PSI rating on the side and specifically states that it is Maximum Cold rating. This means that they have allowed for the fact that tires will heat up.

[RottieMom]

I believe Bailey's Mom is correct. PSI on tire is for cold.

Thx for your response Bruce. It sounds to me like I'm ok for now with inflating tires to 65 psi. And, if I want to get max benefit with these tires, I'll have to get new rims at some point to inflate more.

[Padgett]

Thank you Bruce for the chart but it demonstrates what I have been saying: that chart is not how Maxxis rates their tires, it is a tire industry chart that all manufacturers build their ST (special trailer) tires to. Some just meet those requirements, others (I suspect Maxxis is like this) exceed them.

Key is to buy tires (and wheels) that are more than what you need on paper. My 2720SL is always lightly loaded when I travel, under 4,000lbs. Less 460 (measured) tongue weight and 60lbs for the tires is 3480/2=1740 so TM was probably reasonable specifying 1870 lb @ 50 psi LRC tires. Mine are rated 2150 at 65psi LRD which gives me about 20% margin which I like. On a 90+F day at 65 mph my tires are warm but not hot to the touch.

Think I have finally found a TPMS that has enough range. Suspect they use a metal valve stem as part of the antenna so may have much shorter range with a rubber valve stem.

Have seen the zipper effect but did not know the name but is why you need to consider a tire that has been run flat for any distance as ruined. That said, flex is why the sidewall is tupically a lesser number of polyester plies. Radial tires are designed to have some flex.

However the bottom line is that the majority of non-impact tire failures are caused by overloading and/or underinflation and the temperature skyrockets.

ps the load rating is based on tire pressure times tire volume. The Load Rating is based on the tire construction with industry defined steps. The ratings are pure math. Tire construction is how the manufacturers achieve those ratings. I look for a nylon cap or overlay in a trailer tire because it is a good thing and if it has that then I feel I can trust the other ratings as well.

pps the stronger the tire, the more it weighs so the faster you go the faster the tire spins and the more force is acting on the tire. Part of the reason I like 15s better than 14s & shudder when I see a pickup pulling a trailer with 8" tires at 70.