Thursday, September 21, 2023

# RV Tire Safety: Tire air pressure, not the tire structure, supports the load

By Roger Marble
As part of a discussion on tire inflation and cold weather I spotted one special post. I’m publishing it here, with the permission of the author, Cushing Hamlen, who has a Ph.D. in engineering. He said his education translates into many, many classes in thermodynamics, including statistical thermodynamics, which is a real mind bender (conceptually and mathematically).

Here is what Dr. Hamlen said about tire inflation supporting the load:

A plane wing flies because of two things: the curved top of the wing which produces lower pressure on the top of the wing than on the bottom (the Bernoulli effect – which is a pressure thing and has nothing to do with density), and the angle of attack of the wing (where, when the front of the wing is tilted upward and the wing pushed forward, air hitting the bottom of the wing is deflected downward, which exerts an upward force on the wing (newtons third law – when an object exerts a force on another object, the second object exerts an equal and opposite force on the first – strictly a density/mass thing, and has nothing to do with pressure).

Your density altitude thing is mostly a result of the angle of attack of a wing allowing it to “push” downward on the air – the denser (colder) the air, the stronger the upward force (because the air molecules are closer together, and the wing pushes more molecules downward for a given amount of forward motion … kind of like throwing downward two baseballs versus one … it takes more force to throw down two of them.

Inside a tire, there is no such “pushing” of air, and so its density becomes a non-issue. The ONLY thing acting inside a tire is the pressure the air exerts on the tread, walls, and rim of the tire. This works because a given pressure pushing on the tire “stiffens” the tire, and limits how much the sidewalls of the tire will deflect for a given load. If the pressure is lower, the tire sidewalls are not held stiffly in place, and can deflect more (very much like a very underinflated balloon is easy to squeeze and deform, but a highly inflated balloon is very stiff, and difficult to deform – it can support more weight without deforming.

To understand pressure – you really need to understand statistical thermodynamics … but the simple explanation is that pressure is the result of lots and lots of gas molecules hitting the inside of the tire … it is nothing more than that. It is the summation over time of many, many small “balls” (molecules) each with very very small mass and momentum hitting a wall. So … the fewer the number of molecules inside the tire (like letting air out of the tire), the fewer will be hitting the wall in a given time, and the pressure is lower (the opposite is true when you add air to the tire).

As for temperature – it turns out that the speed a gas molecule flies through space is directly dependent on the temperature (the Maxwell-Boltzmann distribution). So for a tire with a certain amount of air in it, if the temperature goes down, the speed that the gas molecules are moving at goes down, and they each hit the inside of the tire with less momentum – and the pressure (and thus stiffness of the tire) goes down – for a given amount of weight on the tire, the tire deforms more. The tire may technically be supporting the weight, but upon each revolution it deforms more than if it were supported by a higher pressure – and it is this ongoing increased amount of deformation that causes increased stress and damage to the tire.

So, I hope this clarifies why I have been saying for years: It is not the tire construction that supports the load, but the air pressure or tire inflation.

If you doubt this, then please explain where the “Construction/Load Capacity” tables are, as all I can find are “Inflation/Load Capacity” tables.

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

Â ##RVT1024

Roger Marblehttp://www.RVTireSafety.net
Retired Tire Design and Forensic Engineer w/50+ years of experience. Currently has Class-C RV. Previous Truck Camper, Winny Brave, Class-C & 23'TT. Also towed race car w/ 23' open trailer and in 26' Closed trailer. While racing he set lap records at 6 different tracks racing from Lime Rock CT to Riverside CA and Daytona to Mosport Canada. Gives RV and Genealogy Seminars for FMCA across the USA. Taught vehicle handling to local Police Depts

1. Good article, right on point!

I would add that it is the air Pressure (force/area) that is doing the work supporting the load, while the tire construction is containing the air (providing a somewhat ‘constant’ Volume).

Recall from your science class PV=nRT (yeah, showing off now…) so good tire construction keeps the Volume while the Temperature effects the Pressure, and the Pressure in turn supports the wheel. If it were all only about tire construction supporting the wheel, why would you bother putting air in?

I think the confusion for most folks comes from the interplay of factors.

2. I would argue that the tire does support the load. For every bit of pressure on the part of the tire on the ground the same load of pressure is spread out along the entire inside tire surface. It is the weakness of a small spot in that entire surface that fails causing a blow out. In addition, the tire rim weight is not being carried by the small portion of the tire at the bottom, but is spread out along both the inside and outside tire walls. This causes even more stress on any weak spot in the wall. Although you may get a flat from a nail in the tread, it is the sidewall that is the most important.

• When you say “blow out” are you talking about belt separations or the melting of the sidewall polyester due to excessive flexing? How does high pressure generate the heat to raise the sidewall temperature to in excess of 380Â°F?

3. This relates back to a comment I made a couple of weeks ago. I was once told by a tire engineer to inflate to the number on the sidewall and not to the door jam or owners manual. I have seen so many times an owner changes his tires from say an LT to a 10 ply and service garages are inflating to the door jam. The difference being the factory LT may be 51 psi or less versus the 10 ply of 80 psi or more. Setting to the lower pressure will cause more sidewall flexing. Monitor your tire wear pattern, if wearing only in the middle tread you are over inflated. Just the opposite if wearing only on both outside edges,under inflated.

• After 40 years as an actual tire design engineer, I do wonder about the people that are claiming to be “tire engineers”. You might ask that person if they have any design patents. What tire company did they work for, and where> In Akron or at a tire production factory? How many complete tire specifications have they been responsible to sign off on? How many different tire plants have they visited to resolve production questions? Have they ever built a tire? How many failed tire autopsies have they performed? The information you received is recognized by people in the tire industry to be incorrect if that is the only advice given out. It is the responsibility of the vehicle manufacturer to establish an appropriate inflation level and for that number to be on the Certification Label.

4. Yes it is the pressure. Tire failure is the inability of the tire to withstand that pressure. Poor design and/or poor quality control of the tire (China bombs) combined with over and under inflated tires resulting in catastrophic failure. I installed inexpensive foam rubber inserts in my bike tires, eliminating the need to pressurize them. We need the same inexpensive solution for RV tires. I use a TPMS for my 5th wheel tires and built in pressure sensors in my truck – which gave no indication when one of my right rear dually tires blew. Time to apply material science and develop a cost effective solution to eliminate the high pressure tires and blow outs?

• People have thought it was “time” to do that for decades, with “breakthroughs” announced periodically and so far zero use in the real world. I wouldn’t hold my breath.

• What about the tires that suffer a belt separation yet still retain an appropriate level of inflation? How and why does this happen? How does “underinflation” in a “poor design” tire result in a failure of that tire to “withstand that pressure”? It seems you are claiming that both low pressure and high pressure cause tires to fail from excessive pressure. This is a confusing concept to me.

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