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Old 08-17-2013, 05:58 PM   #21
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B47:

I was in the landing gear and brakes group at Boeing for several years. I did a lot of research into tire bursts.

The accident with the magnesium wheels was a Swissair Caravelle. There were 69 fatalities. Magnesium alloy wheels were banned on passenger airplanes after that, as was the Mil-H-5606 hydraulic fluid.

Boeing had an in-flight tire burst on a 727 that punched a 15" or so hole in the wheel-well aft bulkhead. The tire bead had failed in tension, which we calculated would have taken an instantaneous pressure in the tire of about 12,000 psi. We figured that a dragging brake during taxi and take-off had caused gases in the tire material to mix with the inflation air and a spontaneous explosion caused the burst after the gear was retracted. There have been several others, including a USAF Galaxy, but none since nitrogen was made a requirement.

Conclusion - if your RV can accelerate to 150 mph in two miles with a dragging brake, and then you put the wheels in a closed box, inflate your tires with nitrogen. If not, save your money!
OK, let's start a brand war...since I have a Ford, I can accelerate to 150 MPH in two miles with a dragging brake!
Just kidding! LOL! Seriously, that's interesting reading. One of my side jobs is FOD coordinator and I saw the test and RCA analysis on the Concord accident. Very interesting. That accident was from a tire hitting a piece of another airplanes engine shroud and the rubber tore open a fuel cell.
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Old 08-18-2013, 12:43 AM   #22
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That's interesting, B47. My research was done in 1974, shortly after I came back to Boeing after being laid off. I'm amazed it took the FAA that long to get their act together. As I remember, Boeing issued an all-airlines service bulletin recommending nitrogen as an inflation medium in about 1976, based on the research I and my colleagues had done. It wasn't mandatory, since Boeing couldn't do that, but it had a "strongly recommended" header.
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Old 08-18-2013, 12:48 PM   #23
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Originally Posted by F and E Damp View Post
That's interesting, B47. My research was done in 1974, shortly after I came back to Boeing after being laid off. I'm amazed it took the FAA that long to get their act together. As I remember, Boeing issued an all-airlines service bulletin recommending nitrogen as an inflation medium in about 1976, based on the research I and my colleagues had done. It wasn't mandatory, since Boeing couldn't do that, but it had a "strongly recommended" header.
A couple of things:

1. From my FAA experience, it's not unusual for a manufacturer or vendor to issue service bulletins before the FAA issues an Airworthiness Directive (AD). As you know, many AD's adopt the manufacturers or vendors service bulletin as they are written and then the FAA makes compliance mandatory in an AD. ( ref 14 CFR 39)

Manufacturers and vendors are required to monitor the performance of their products and report unsafe conditions to the FAA. Thats how the FAA learns of many unsafe conditions.

2. The FAA is a Goverment agency and like most Goverment agencies, doesn't always act and move as fast as they sometimes should.
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Old 08-20-2013, 12:46 AM   #24
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Nitrogen was always the fill in my airline career. Aside from the safety advantages, it only made sense to use the same bottles for tire inflation as those used for struts and accumulators. Different regulators of course

For my TV and trailer it is air. Available anywhere (most importantly from my own compressor), they're not race cars so maintaining exact pressures aren't critical, no need to worry about a blown tire feeding a fire and I am continually adjusting the rear tires on my truck between 80 PSI and 50 PSI whether I am towing or not. If nothing else, nitrogen is just too much trouble for any advantages it may have in vehicle use.
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Old 08-20-2013, 10:15 AM   #25
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N2 from the view of a tire design engineer

As a tire engineer I understand the theoretical advantages to inflating tires with N2. While sometimes measurable they are not really meaningful for normal road vehicle usage, and even when I had a "bottle" of N2 available when I was racing, I seldom used the N2 to inflate the truck or trailer tires.

I have two posts on my blog on the topic.

It is MUCH more important to check your inflation every travel day. Even BETTER to constantly monitor the inflation with TPMS just as you monitor your engine oil pressure and temperature.

If you have a trailer, especially multi-axle unit you really should be ALWAYS setting the tires to the inflation on the tire sidewall as this will help lower the internal shear forces that are unique to multi-axle towables.
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Old 08-20-2013, 10:45 AM   #26
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Interesting thing I saw an a "How it's made" show. They blow N2 into potato chip bags. I believe the reason is to flush out most of the O2, and the source of oxidation. It prolongs freshness.
I guess we can expect the tires and wheels to have fewer issues with rust and interior dry rot.
Yes, I realize the tires will fail from sun-rot long before they fail from internal oxidation.

At least in a pinch (say stranded in the desert) we could keep our chips fresh longer by emptying the tires....

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Old 08-20-2013, 07:44 PM   #27
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Originally Posted by Tireman9 View Post
It is MUCH more important to check your inflation every travel day. Even BETTER to constantly monitor the inflation with TPMS just as you monitor your engine oil pressure and temperature.

If you have a trailer, especially multi-axle unit you really should be ALWAYS setting the tires to the inflation on the tire sidewall as this will help lower the internal shear forces that are unique to multi-axle towables.
Obviously your recommendation to check/monitor tire pressure is spot on.

However, I question your statement regarding inflating tires to the inflation pressure indicated on the sidewall. So
I could be wrong on this but I always understood the inflation pressure indicated on the sidewall was the maximum cold inflation number. Since it is a given that your load should never exceed the max tire weight rating at the max pressure, one would logically surmise that you should inflate your tires to the recommended pressure for the load thereby obtaining the proper ride and flex caused by those shearing forces. Another critical piece of the shearing force issue is use of the proper tread design for this application. I do not know of any tire manufacturer that recommends over inflation. So please do point me at some authoritative documentation to support your position because, as I said, I could be wrong and really would like to know that so I can make appropriate adjustments.

Meanwhile, I offer this document from Goodyear to justify my position:

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf
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Old 08-20-2013, 08:13 PM   #28
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Obviously your recommendation to check/monitor tire pressure is spot on.

However, I question your statement regarding inflating tires to the inflation pressure indicated on the sidewall. So
I could be wrong on this but I always understood the inflation pressure indicated on the sidewall was the maximum cold inflation number. Since it is a given that your load should never exceed the max tire weight rating at the max pressure, one would logically surmise that you should inflate your tires to the recommended pressure for the load thereby obtaining the proper ride and flex caused by those shearing forces. Another critical piece of the shearing force issue is use of the proper tread design for this application. I do not know of any tire manufacturer that recommends over inflation. So please do point me at some authoritative documentation to support your position because, as I said, I could be wrong and really would like to know that so I can make appropriate adjustments.

Meanwhile, I offer this document from Goodyear to justify my position:

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

OK last item first. Yes those are the Load & Inflation tables from Goodyear but I see no statement concerning what the inflation should be based on actual load or what the "safety factor" should be etc.

Now to the real point.
Normal self powered vehicles such as cars, pickups & motorhomes are usually told to 1. learn the actual load on each position. 2. Based on the heavier end of each axle find the minimum inflation that exceeds the actual load in the tables and that is your minimum "cold" inflation. Many suggest you add 5 to 10 psi as a cushion to allow for day to day temperature changes and to eliminate the need to constantly set the pressure.
A key point is when you go around a corner or even make a small direction change such as changing lanes the wheel axles basically point to the center of the circle.

However multi-axle trailers are unique in that the axles do not point to the center of the same circle so the tires are forced to slide sideways. This side loading results in increased interply shear (force at the belt edges trying to tear them apart). Increased inflation will lower the slip angle which results in lower interply shear.

One of the main reasons multi axle trailers have a higher tire failure rate even if loads and inflations are within design limits is this higher than normal side loading and higher interply shear.

This can get pretty technical as it takes Finite Element analysis to learn about these structural forces. While the big name tire companies have the computer capability to run these programs I doubt that any manufacturer of low cost tires bothers and certainly the RV manufacturers do not do this level of design and structural analysis.

I do plan on laying out the math in a future post on my blog but I try to stay away from complex engineering analysis as those posts do not get a lot of readers.
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Old 08-20-2013, 08:13 PM   #29
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The Goodyear chart clearly states these are "tire load limits at various cold inflation pressures", not recommended pressures. I don't know why one would want to run at the maximum rated load for his tires when they could increase their safety margin by just increasing pressures a few pounds. I run the E's on my rig at 80 PSI and as long as wear is even, that's where they stay.
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Old 08-20-2013, 08:53 PM   #30
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The Goodyear chart clearly states these are "tire load limits at various cold inflation pressures", not recommended pressures. I don't know why one would want to run at the maximum rated load for his tires when they could increase their safety margin by just increasing pressures a few pounds. I run the E's on my rig at 80 PSI and as long as wear is even, that's where they stay.
Here's more from Michelin. Most dealing with Class A's but some RV multi axle application as well.

https://www.michelinb2b.com/wps/b2bc...s_Brochure.pdf

I'll work on the updated Goodyear specs as they are the ones that are on most of the units in this section.

BTW, Michelin speaks directly to why you should not just run your tires at max cold pressure.
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