Tire inflation

[SteveSueMac]

Originally Posted by CapriRacer

Snip...

Rule of Thumb: For every 10 degrees F change in ambient temperature, the pressure in a tires changes 3%.

So far a passenger car tire, 3% is about equal to 1 psi for every 10 degrees.

- AND -

A tire measured at 65 psi at 55 degrees would measure at 115 degrees, 3% times 6 (60 degrees) = 18% more or 12 psi. That needs to be accounted for when doing pressure build up.

So a tire operating at those conditions could run as high as 85 psi.

Snip...

Let me see if I understand this concept correctly. My p-rated 15" Michelins have a max of 50 PSI. So let's say it's 70 degrees outside at 8:00 am and the trailer has been sitting overnight. I ensure the tires are at 50 PSI and start a long road trip.

By noon, it's 100 degrees outside so the ambient temp is +30. So I should expect about 3% of 50 PSI increase for each 10 degree rise in ambient - or a total of 4.5 PSI increase just due to the rise in ambient temperature, correct?

And what then should I expect for the 4 hours of 65 mph towing? Is that where your 10% window comes in to play - and if so, is that on top of the increase for ambient? In other words - if the PSI was 59.5 in this scenario where 4.5# was due to rise in ambient temp and 5# (or 10%) was due to travel friction - is that 19% total increase in PSI acceptable? Or is it dangerous because it's more than a total of 15% increase in PSI?

I ask because I've set my TPMS alarm to go off at 57 PSI thinking if I get over that 12-15% range I'm in trouble. Wondering if it's safe to set the alarm at 60 instead to account for both factors?

I've usually seen almost exactly 10% increase in PSI and 30 degree temp above ambient no matter how cold or warm ambient actually is and no matter how many miles I drive - though maybe once or twice to 56 PSI on triple digit days....

[jcanavera]

Originally Posted by dkottum

I have heard this when parking the trailer, the sideways scrubbing. Using LTX M/S Michelin tires on our Airstream and truck, it seems to me similar sideways scrubbing happens when we steer our truck around a curve or corner at speed. Surely the LTX M/S must be designed to handle that? Maybe the ST trailer tires are not?

Tandem axle trailers are subject to much more scrubbing due to the fact that in sharp turns, the tires based on their position on the trailer aren't rolling as much as a motorized vehicle. This causes some tremendous forces as the tire sidewall attempts to pull from the wheel.
ST tires are built with stiffer sidewalls that resists collapsing under that torque. Since LT tires are truck tires, their construction also provides additional rigidity over passenger car tires.

Inflation pressures also play a big part in giving those side walls support. At a campground I once watched a SOB tandem owner attempt a back in to a site. His tires on the trailer seemed to be a little low and as he made the pivot, a tire on the inside of the turn collapsed as the tire bead separated from the wheel. He promptly put on the spare and continued the back in. That spare was also under inflated and that tire also separated from the wheel. My last view of him was leaving the trailer in the campground road as he took one wheel and tire to get it inflated it properly.

Jack

[CapriRacer]

Originally Posted by SteveSueMac

Let me see if I understand this concept correctly. My p-rated 15" Michelins have a max of 50 PSI. So let's say it's 70 degrees outside at 8:00 am and the trailer has been sitting overnight. I ensure the tires are at 50 PSI and start a long road trip.

By noon, it's 100 degrees outside so the ambient temp is +30. So I should expect about 3% of 50 PSI increase for each 10 degree rise in ambient - or a total of 4.5 PSI increase just due to the rise in ambient temperature, correct? .......

Correct.

Originally Posted by SteveSueMac

.........And what then should I expect for the 4 hours of 65 mph towing? ......

No more than a 10% buildup! And that should occur within the first hour and a half.

Originally Posted by SteveSueMac

...... Is that where your 10% window comes in to play ........

Yes

Originally Posted by SteveSueMac

......... - and if so, is that on top of the increase for ambient? ........

Yes

Originally Posted by SteveSueMac

........ In other words - if the PSI was 59.5 in this scenario where 4.5# was due to rise in ambient temp and 5# (or 10%) was due to travel friction - is that 19% total increase in PSI acceptable? ............

Yes, from the point of view that your starting psi was adequate.

Originally Posted by SteveSueMac

........... Or is it dangerous because it's more than a total of 15% increase in PSI? .........

No, it is NOT dangerous, because the increase in pressure due to running was 10%.

Ya' see, the point of the test is to measure whether you have adequate load carrying capacity and by measuring the pressure build up, you are indirectly measuring that.

Originally Posted by SteveSueMac

........ I ask because I've set my TPMS alarm to go off at 57 PSI thinking if I get over that 12-15% range I'm in trouble. Wondering if it's safe to set the alarm at 60 instead to account for both factors? ........

I think the alarm set at 57 would be OK, as what what would happen if the outside temperature dropped? If the alarm is going off because of the outside temperature increase, and you know that, then you can ignore the alarm. Personally, I'd rather have an alarm going off prematurely ratgher than too late!

Originally Posted by SteveSueMac

......... I've usually seen almost exactly 10% increase in PSI and 30 degree temp above ambient no matter how cold or warm ambient actually is and no matter how many miles I drive - though maybe once or twice to 56 PSI on triple digit days....

It sounds like you've kind of answered your own question.

[SteveSueMac]

Thanks CapriRacer! Very helpful. And that last bit was really just an observation on my part - not so much an answer to the question. Your response helped me understand more about the tolerances and test for load capacity. That 10% increase in PSI that I see usually happens within the first 60-90 minutes on the highway and remains there for the duration of the trip - though as noted, if triple digit weather is happening they can rise another pound.

Also liked your reason for keeping the alarm at 57. Makes sense to me and should I experience higher PSI in very hot weather (or, in temps that are 40-50 degrees higher than when I hit the road) I can relate that to the ideal gas law and not lose too much sleep over that. (Roger Goodell would do well to read this forum ... But I digress 😃🏈 ).

Thanks again - very helpful insights!

[slowmover]

Originally Posted by mike28

What is the proper inflation for the 16" Michelin tires on a dual axle trailer with a gross max weight of 9100 lbs? I get all kind of suggestions.

It's simple enough to avoid problems by using max sidewall pressure. It's not just in tight turns, but also in braking. The forward axle of the tandem winds up with the main load

It's the TV that takes more time to figure out, and that by using scale readings when hitched correctly. Correctly hitched is itself a topic. But it's also the only way to find INDIVIDUAL tire loads. The worst load on the axle would be the minimum (after road testing). The TV is what takes work, not the trailer.

[switz]

If one has the individual weights of each tire, then one can look at the Michelin weight capacity versus tire pressure.

80psi 2,680 (E)
75psi 2,560
70psi 2,440
65psi 2,335 (D)
60psi 2,190
55psi 2,060
50psi 1,940 (C)
45psi 1,790

We figured 72.5 psi could be optimum with a extrapolated weight capacity of 2,500 pounds. We tend to make life easier and put in 75 psi.

[BlackAces]

In the industry where all of your RV trailers were manufactured there are regulations the trailer manufacturer MUST follow. One of those regulations is unique to the trailer manufacturer, they are directed to set recommended cold tire inflation pressures for every trailer they build and certify. Those recommendations are going to be displayed on the federal certification label, tire placard and in the vehicle owner’s manual.

Tire industry standards - the rules used for consumers - are supported 100%.

Tire industry inflation charts are used by manufacturers to set recommended cold tire pressures. They are used by retailers/tire shops when installing replacement tires that use different charts than the OE tires use. They are not intended to be used to jack around tire inflation pressures where established recommendations have already been set. Unless, there is reserve inflation pressures above the recommended pressure values. Any pressures used below the established recommendations is considered under-inflation. Any pressures used above the maximum pressure molded into the tire’s sidewall is considered over-inflation. Any reserves in-between is there for the using.

Vehicle owner manuals may have optional pressures for vehicles that tow things. Automotive industry tire fitments must provide a percentage of load capacity reserves. Trailer tire fitments do not have that provision. For RV trailers there is seldom any wiggle room with inflation pressures and those needing extra load capacities are probably already over-weight. If you’re looking for a smoother ride I suggest you upgrade your suspensions. Fooling around with tire pressures on RV trailers is like asking for some time on the side of the road to change tires or waiting for someone that will.

Industry standard: Replacement tires need to have a load capacity equal to or greater than the OE tires, by inflation. The installer should inflate them to a pressure value equal to the load capacity of the OE tires. To take advantage of a stronger tire an inflation pressure allowing more load capacity than the OE tires will provide a relaxing amount of reserve load capacity. Auxiliary tire placards are allowed to be placed adjacent to the current placard. It should display the new tire size and recommended inflation pressure.

When you’re operating outside of the established norm you’re on your own - so to speak - and solely accountable for your actions. If retailer’s/tire shops offer different options make sure they are in writing and on the bill of sale. If they wont do that eye brows should go up.

Note: Inflating your tires to the load carried is a product of other regulations in the trucking industry (FMCSA) and has no place in the safety regulations (FMVSS) for RV trailers. FMCSA has no provision for recommended cold tire inflation pressures. Their results are strewn all across our country, on every major highway.

[tjdonahoe]

I check all my tires before we go on a trip, and do a look see every time we stop,never had any problems....my big truck has 120 lbs cold , in the steer tires, after 500 miles on a hot day, when I checked them out of curiosity they will read having 132 lbs, never had any problems,why worry about it?...