Run-flat tires and reduced inflation pressure 1

[NormPeterson]

As I understand it, "run-flat" tires are rated for some number of miles at such-and-such a speed at zero inflation. 50 miles @ 50 mph @ 0 psi seems to be common.

What sort of distance rating might apply to run-flat tires that are occasionally used at an inflation pressure temporarily reduced by a few psi from the mfr-recommended value for activities such as driving off road or for a little more compliance to avoid shocking the tire into spinning on the pavement at drag racing? Say, down on the order of 20 - 25 psi.

Would there be a huge reduction in rated speed from such a tire's rating at a more "proper" inflation pressure?

I'm guessing that this is a fatigue matter, and that it might even have application under "normal driving" circumstances should a RF tire be allowed to lose a little pressure but not enough to trip the TPMS lamp (thereby goading the driver into doing something about it).


Norm

 

[CapriRacer]

I don't think you will find RunFlat tires suitable for off roading. Typically, they are low profile (low aspect ratio) which is the opposite of what would be best for an off road tire.

Drag Racing? Again, a RunFlat tire is just not suitable for that activity. It has a stiff sidewall, making the tire not good for absorbing the initial shock of hard acceleration.

But I suspect your question is more about understanding how a RunFlat tire works, rather than trying to do those activities mentioned.

Yes, the problem with operating a RunFlat tire - or any tire for that matter - is fatigue. In regular tires, reducing the inflation pressure causes the operating temperature of the tire to rise - and the same applies to RunFlat tires in exactly the same way. So the reduced life of the tire is the same for both types of tires - and the failure mode would also be the same - a belt leaving belt separation, commonly called a *tread separation* - and the reduction in life is a percentage of the original fatigue life - with the percentage being a function of the amount of underinflation.

If you continue to reduce the inflation pressure - say you have a slow leak - there is a point where a regular tire bottoms out. That is, the tire collapses to the point where the upper sidewall comes in contact with the lower sidewall - and that causes 2 issues.

1) Those parts of the tire were never intended to come into contact with each other, so they were not designed to handle the load on the tire in that way. They can cause the sidewall to fatigue from too much bending.

2) There is a difference in rotation between those 2 parts so they have to slide against each other - causing the rubber to be rubbed off, eventually wearing through.

A RunFlat tire is designed with a much stiffer sidewall to address those 2 issues. However, the current technology level results in a tire that has a severely limited fatigue life. The failure mode is a separation between the stiff support rubber and the sidewall at about mid-sidewall.

When operated in that mode, the life of a regular tire is measured in feet or miles, depending on how much touching is going on. By contrast a RunFlat tire's life is measured in tens of miles to hundreds of miles, depending on how severe the underinflation is.

So where is this point where the tires transition from one failure mode to the other? An educated guess would be about 50% of the specified inflation pressure. Please note, this is a guess as I don't know of any data on the subject.

 

[JayMaechtlen]

One other aspect - when running at reduced inflation pressure - how quickly does the tire become unsafe for further use at specified inflation pressure?

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[CapriRacer]

Jay,

As I indicated in my response to Norm, there are 2 different failure modes when we talk about reduced inflation pressures. With that in mind:

If we talk about anything below above the pressure were the sidewalls are touching internally, we're talking about an SN Curve type of fatigue. If you look at an individual tire, you won't be able to tell where you are on the curve until it starts to fail - and by then it is too late. One might be able to observe early signs of failure, and be able to remove the tire before it reaches a critical point, but it is irreversible.

And, certainly, less pressure = a quicker failure. Also a tire never forgets a loss in fatigue.

So where is that point where the tire pressure was so low for so long that it shouldn't be returned to service? Rule of thumb says that if there is rub marks on the outside sidewall or rubber dust inside (or even a heat ring on the innerliner), those are signs of the sidewalls touching and the tire should be scrapped.

In other words, if we aren't aware that we are in the second mode of failure, then we assume the risk of failure is low enough.

Now when it comes to RunFlat tires that don't show the symptoms of this second failure mode - we just do not have the tools to judge. Some tire manufacturers assume the worst and say that RunFlat tires should not be repaired if they are punctured. Others say to treat the tire like a regular tire when it comes to repairs. Choose your poison.

 

[NormPeterson]

Many thanks, Capri.

However, I do have a further question. Should

If we talk about anything below the pressure were the sidewalls are touching internally

actually read

If we talk about anything above the pressure where the sidewalls are touching internally

or

If we talk about any pressure reduction below where the sidewalls are touching internally

? I am reading from first reply to mean that SN effects govern until contact occurs, when the scrubbing wear takes over.

Yes, it was all about me learning more about these tires, as I'm not personally interested in driving off-road or making passes at the local drag strip myself (with any tire type). But the question has started to come up on some of the car enthusiast forums where people don't know who, where, or even what to ask, and I'm not going to be the one who opens the floodgates to every run-of-the-mill automotive question.

I'd have replied sooner, but I didn't get any notices of reply.


Norm

 

[CapriRacer]

Norm,

Thanks for the correction - *below* should have been *above*. I have corrected the post.

If you have any more questions that I can answer, try emailing me through my web site: Barry's Tire Tech [[Moderators: The URL is an informational website and not a sales website. The *.com* sort of predates the use of the suffix to indicate what type of web site it is. Check it out if you don't believe me.]]

 

[NormPeterson]

Thanks again. I'll have to make a note of this other way to get in touch, which I won't generally share either. Or privately unless you OK it.


Norm

 

[CapriRacer]

Norm,

I set up the website to address issues that I encountered frequently. A byproduct is I get questions from time to time. I'm OK with anyone asking questions so long as it's not overwhelming.

 

[NormPeterson]

Privately it is, then. Else you really could get swamped with questions like "What's the best tire for my XXX" and "Can I put tire size X on my car's stock wheels".


Norm