[CapriRacer]

I apologize for being silent for the last 3 days, but I’ve been looking for information concerning tire inflation vs various tire properties. I spent a lot of time going through a 700 page publication called “The Pneumatic Tire”. The book is published by NHTSA (National Highway and Traffic Safety Administration) . It is a series of summaries written by various authorities on the subject of tires and tire testing.

So far I’ve only skimmed the articles, looking for graphs and charts that might be helpful, and while I found a lot of stuff that is interesting, I haven’t found anything that deals directly with the subject at hand. I suspect this is because inflation pressure is usually a given in most studies.

I will have to go over the articles again in more depth to see if there are some equations that might include inflation pressure as a term – but wasn’t one of the items studied.

But I suspect that the effect inflation pressure has on various properties was studied long ago and I may be forced into doing searches the old fashioned way – in a library.

Originally Posted by Greenblazer

Hey Ya'll,

I refuse to believe that a tire manufacturer would put a max inflation number on the side of a tire that was higher than it was safe to inflate it to. If it says Max 35 PSI, then that tire better be able to handle that pressure even with high heat conditions or inaccurate guages causing the tire to exceed that number. If it could not handle that then they would be facing lawsuits when the tires failed.

They would obviously design the tire to handle even more pressure than listed on the side to have a margin of safety to take into consideration heat and tire guage variables. This is why people get away with exceeding those max numbers, but exceeding those numbers brings the tire closer to the edge of it's safety margin. The more hyperinflated tires are closer to the point of failure wheather it is a safety or actual tire failure, but only the manufacturers would know how close.

If tire X fails at a certain PSI then they would want to make sure that no one inflates their tires beyond that pressure. They would do this by putting a number on the side of the tire that has a margin of safety big enough for them to feel like tire failure lawsuits would be unlikely. The tire companies would know how big this margin of safety is, but they would not necessarily want the consumer to know because they would be more interested in maximizing profits than maximizing tire wear or fuel economy.

Capriracer, would you agree to these statements? and when can we talk about truck tires?

Later,

Allan Greenblazer

Funny you should ask a question this way. I did find something in my searching I didn’t expect:




This an S-N curve for rubber. (Stress - Number of cycles) S-N curves describe the way in which materials behave when subjected to cyclical loading. What these curves say is that the more stress a material experiences, the less cycles it can endure before failure.

S-N curves are very similar for pretty much all structural materials – in that number of cycles before failure is very highly dependent on the stress that the material experiences.

I think this curve is of interest because some discussion has centered around the phenomenon that tires will accept a lot of inflation pressure before exploding. This would be a one-time loading. Just for reference, a passenger car tire would experience 30 million revolutions in 50,000 miles (on the S-N curve that would be 3 x 10^7) If I read the curve correct, at 30M cycles, the stress at failure is only 20 to 25% of that for a single cycle.

Put another way, when a tire engineer is designing a tire, he would choose materials that appear to be grossly over designed. He’s doing that to account for this S-N curve.

That probably also means that hyperinflating tires would result in a fatigue failure many cycles down the road - due to the overall stress in the tire caused by the inflation pressure.


So to answer Greenblazer's question - ah ..... well ...... ah ....... I can't answer it in the form it is in.

The tire is designed around this fatique problem and the bursting pressure of a tire is kind of an incidental property. I haven't seem many tires that have been subject to a "pressure to burst" test, but there are a few videos on Youtube - and there it looks like either the rim or the bead is failing - and most tires fail in the are of the belt package - a completely different area.

But what is going on is that the tire standards organizations have delineated a set of operating conditions (pressure, load, etc.) - and for passenger car tires the operating range is between 1000 and 2500 # (or so), 26 to 36 psi with a provision to allow for higher pressures for special situations, such as high speed operation, and speeds up to the speed rating of the tire (with appropriate adjustments in load and pressure), etc.

So when the tire manufacturers put a maximum pressure on the sidewall of the tire, they are really telling you that this is the limitation they've designed around. So if you exceed those limitations, expect problems since this is an area that the tire manufacturers have not tested in and it may or may not be risky (OK, I think we all know that it is risky, the question is how large is the risk.)

Put another way - the tire manufacturers have warned everyone to stay away from those pressures, so the user assumes liability.

But I can tell you that when tire engineers design tires for higher pressures, they use stronger materials - larger ply cords, more ends per inch, more plies, different materials, etc.) Confounded in this is that uusually the load carrying capacity also goes up, and that's the way a tire is actually used.

Originally Posted by Greenblazer

........

and when can we talk about truck tires?.....

Anytime! I just wanted to make it clear that this discussion is about passenger car tires - and other types of tires (truck, aircraft, racing) are different and I didn't want folks to get confused.

So if you want to talk about truck tires, just make it clear. BTW, there are several many different types of trucks - and the tires for each group are indeed different (and delineated so in some way), so please be aware of that when you post.