Popeye427 - your help please

[CapriRacer]

Popeye427,

I see you've just joined, and you have expertise in an area I need to confirm some things on.

Tires on trailers tend not to have enough load carrying capacity. As a result, it is common for people to go to higher load range tires and use more pressure to increase the load carrying capacity. But what about the wheels?

I don't think there are any regulations requiring maximum inflation pressure on wheels (unlike tires) - and I suspect this is so because inflation pressure doesn't have a great affect on the stress levels on wheels (unlike tires).

Can you confirm both of those assertions?

 

[popeye427]

There are significant stresses on the wheel rim from tire inflation and loading and significant differences in stresses with bias or radial tires. Other old timers may remember major rim split issues with 16.5 truck wheels running the early radials in the '70s.

That said, its unlikely that a reasonable increase in tire sizes will produce serious concerns in field usage, and if it did, under extreme conditions and many miles, an inflation related fatigue failure would likely appear as a rim split with a resultant flat tire. SAE J1204 covers trailer wheels, but I don't have a copy of that spec in my files. I do remember that trailer wheels are NOT designed to the same severity levels as car and truck wheels.

I'd make sure that the tires were load capable, inflate them to max limits, carry a spare, and inspect them frequently. The normal white paint or trailer wheels is useful for identifying any small cracks that might appear. Max inflation or even slight overinflation on the tires is always safer that underinflation.

Hope this helps

 

[CapriRacer]

Popeye,

Thanks for the reply. Here's the situation.

I'm a retired tire engineer. I surf the net for questions and problems with tires and offer advice.

I frequent an RV web site, where the discussion of tire failures is a major item of discussion. The folks there attribute failures in trailer tires to the tire manufacturer - and they have a point. But I have seen trailers where the tire is marginal for load carrying capacity - and considering that trailers are usually loaded up all the time, that's an unusually large amount of stress (compared to a car or a pickup truck).

Because many trailers use 15" tires, there aren't a lot of options for fixing the problem - but one of them is going up in load range and using a higher inflation pressure. That usually results in people being concerned about the wheel and its limitations. Sometimes the wheel isn't stamped with inflation pressure limitations. Sometimes it is.

I have come to the conclusion after many discussions that compared to the load on the wheel, inflation pressure is not a major factor in stress. Agree?

 

[Tmoose]

Among Studebaker owners there are tales that running radial tires on original wheels has >> caused << wheel failures.

Your radial tire comment seems to indicate tires with greater cornering power, when used hard, can overstress even more modern wheels, at least in the lug nut area, which is not specifically mentioned in the quote below.
Statements by experienced, thoughtful folks like this are hard to dismiss - " I've seen the OEM Stude wheels fail at the lug mount interface. I've also seen them split at the tire bead area."

Would you speculate the rim seat area failures are in any way the result of "differences" in radial tires?

 

[GregLocock]

The main rig test that causes spider failures is the SAE cornering fatigue test, where the wheel is mounted at a very large camber (30-45 degrees from memory) and then run hard against a turntable.

So, since there is in reality no such thing as an endurance limit for steels, old rims subjected to greater Mx (vehicle coordinates) due to wider radials with squarer profiles might generate shorter fatigue lives than expected.

It would be interesting to know which axle tends to break the wheel first, but I expect cars like that get their tires rotated.

Cheers

Greg Locock


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[popeye427]

Capriracer,

I definitely agree that many concerns in trailer wheels and tires arise from a proliferation of new offshore suppliers rushing into the US market with poorly designed and often untested product. New brands are readily apparent in tires, but unfortunately new brands of trailer wheels have also appeared and may be untested as well. Trailer wheels never really been a mainstream product, and really aren't tested to the same severity levels as normal roadwheels. And trailer wheel ends are generally built to the lowest price with marginal sizes.

That said, I agree that increased inflation pressure is the best alternative. Increased inflation pressure generally isn't a huge effect on the wheels, as long as the tire type, i.e. bias vs radial remains the same. Radial tires greatly alter the stresses in wheel rims and its as apparent as the bulge in the sidewall. The greater lateral deflection of the radial tire increases lateral wheel rim loading. The worst offenders are the radial tires with the least structure. Stiffer belts and maximum flippers-fillers in the tire sidewall delocalize stresses in the wheels and improve fatigue life, if the overall loading remains the same.

Wheels survive by delocalizing stresses, and this is especially true of steel wheels. Many metal fatigue specialists use a rule of thumb for heavily loaded structures that a 10% stress increase reduces fatigue life by 50%. If the trailer is heavily loaded, inflate to the maximum, inspect often and carry a healthy spare.

 

[CapriRacer]

Popey427,

Thanks so much for your reply. You are now the 4th person who agrees that increasing inflation pressure, but maintaining the same load has little impact on the overall fatigue of a wheel. The other 3 had various levels of expertise and while I have stated this to those who are interested, I felt somewhat uncomfortable without further corroboration.

The wrinkle here appears to be that some wheels state a maximum pressure - and I have argued that there is a choice between 2 options: Using the current inflation pressure (and therefore load carrying capacity), which has shown to produce tire failures - OR - using an inflation pressure that exceeds the stated max of the wheels, but with the knowledge that there are no known wheel failures as a result.

Not the best of choices, but it is what it is.

Thanks again.

 

[Xwheelguy]

FWIW,

The radial fatigue (drum) test over inflates radial tires to 60 psi throughout the test - I believe for both car, truck and trailer aluminum wheels. The variance in that particular test is in the drum rpm and total number of cycles (revolutions). My design and lab experience with trailer wheels was limited, however if I recall correctly, the safety factor in determining rotary (cornering) fatigue loads for a trailer wheel was a bit higher than a car but lower than a truck. If my memory isn't fading yet, radial fatigue loads were determined by the highest load rated tire that would fit the rim profile - drum revolution and cycles depended upon the application - cars were the lesser of the three.

That I saw, rims never had any sort of air pressure spec or test that was a requirement - a supplier, however, could perform such a test, but I would only assume that was for informational purposes only and nothing that was adherent to a particular standard (SAE, T&RA) common to the wheel industry.

Tim Flater
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