Trailer Tire pressure ? - max on side wall or ?

My dad had a new set of tires put on his flatbed trailer. They are a 7.5 x 16 with a max pressure of 70 psi (7K axles). The tire dealer only wanted to put 60 psi in the tires but Dad thought they should be at full pressure 70 psi. The dealer tried to explain that once the tires heated up they would be a full pressure. Dad and I both think that the dealer is wrong since we have been running the orginal tires at 70 psi since the trailer was new and the sidewall reads maximum cold pressure 70 psi. So are we runnning to much pressure in the tires? Normal loads of 9K - 12K on the trailer.

The tires on our 7K+ travel trailer are rated at 50-psi. I run them at 50.

Tires on my truck are rated at 80-psi. I run all four of them at 65-psi when towing. A little less in the rear when not.

MP

All tire pressures are cold pressure and no manufacturer that I know of ever suggests that you let air out as the tire heats up because the tire material softens as the tire heats up so it needs the higher pressure when hot. You are right especially trailer tires.

Which is purple? The farmer or the sheep? Hmmmm...


70 PSI cold is just that. Ignore "hot" tire pressures, as they can vary widely, even from tire to tire on the same vehicle. Tire compounds soften and air expands as they heat up. It can change the pressure a great deal, depending on the conditions. Tires heat up for several reasons. Surface friction, ambient temp, surface temp, braking, etc. Tread and carcass design also effect the operating temp of a tire.

Your "tire man" is an idiot, for lack of a better term. If he was a tire man, there should have been no discussion about hot/cold pressures. That's a pretty tall order for him to guess what the pressures would be when you are working the tires.

Here's the forumula. Scientific, absolute, and the intentions of the tire manufacturers, as well as the DOT. The tire is rated to support X amount of weight at Y PSI. Period. Ideally, you should adjust your tire pressure every time the load on them is changed (as well as normal periodic checks). What is the actual weight the tires are bearing? Is the front/rear the same (most aren't)? Left/right? Never mind all that. You'll never keep up with that stuff.

What is the GCVW of the trailer, and what is the weight distribution on the axles? You'll need to scale it to know for sure. If you haul the same load every time, it's simple.

Determine the required PSI by the actual weight on the tires. What percentage of the tire's capacity is the actual weight it is bearing? Use your answer to figure the percentage of the 70 PSI. For example, if a tire is rated for 3000 pounds @ 70 PSI, and the tire's actual load bearing weight is 2000 pounds, the ideal PSI would be about 47 PSI, plus a ~10% safety margin. The tire is bearing 66% of its capacity, therefore, the PSI should be adjusted accordingly. In the example, 52 PSI would be the ideal pressure.

In your case, I suggest you set your (Dad's) PSI according to the trailer's GVWR. That way, the tires won't be underinflated. As long as you keep them between the calculated PSI (above example) and max. pressure stated on the sidewall, you won't have problems with underinflated tires. Overinflation (up to the max. pressure) won't be a safety issue, but causes tires to wear less than optimally, and more pressure means a more harsh ride.

If the trailer was at the same weight all the time, it would be simple. Most cases aren't that simple. Even my 5'er can vary as much as 2000 pounds. I run the tires at max pressure all the time. I've never used up the tread on a trailer tire. Time and elements is usually what kills them. If you haul a trailer for a business, or are a full time RV'er, you might get full tread use.

The reason for dealers and tire shops (maybe not in your case, but most) airing up tires to rated pressure is simple. Liability. They have no way of knowing how you will load the tires once you leave their shop.

put 'em at max (cold) and forget about it. I work for a marina - we always set the tires at max for our boat trailers. Like DMax said, you could set them a little lower if weight allows, just so the ride isn't as hard. Besides, putting them at max helps with fuel mileage (not a lot, but every little bit helps!)

Tires pressure is always @psi cold. get a digital pressure gauge, set the psi early in the morning prior to sun up. Then without driving, check them on a sunny warm day about 2:00PM and see what you get? :rolleyes: Now see the differance between left and right tires? :rolleyes: The sunny side will be higher, and hotter.
On tandem trailers, if when hitched it is not sitting level on level ground, you will heat up the tires on the axle that has the most weight on it. So always make sure you are level when towing. [img]smile.gif[/img]

I'll second what DmaxMaverick said with one exception - your tire man might actually be an 'imbicile' rather than an 'idiot'. ;)

Most tire mfg's websites have tire load inflation tables that you can download for the reasons he stated.

Thanks for the comments about the tire pressure and the tire man. Dad was in a bind in that he had two of the old tires come apart on sunday on the way home and did not have a lot of options of where to purchase a new set. Does not want to leave the antique tractors just sitting around to long at the hotel parking lot.
We will keep the pressure's as we have in the past.

This is certainlly a hot topic, having towed for the last 5 years, tire pressure is critical and has to be monitored the entire towing trip.
For example my cold pressures for towing are 76psi front 80psi rear to maintain the same tire footprint, however that same tire pressure after a 2 hour towing at 95F had increased to a pressure exceeding 90psi causing the truck to wonder a lot.
Point is you have to select your own tire pressure and constantly monitor it at every stop, and I might add do a walk arround to ensure every thing is secure.

drive safe and good luck

As said before, stated pressures are always cold.

As far as the correct pressure for the load, it is best to use the tire manufacturer's load/inflation tables as a guide. It should be ok to go over the recommended pressure for a given load as long as you don't go over the max for the tire. Don't run less than the recommended pressure for a given load.

Here are a couple of links...
http://www.trucktires.com/us_eng/technical/index.asp

http://www.goodyear.com/rv/tirecare/...iontables.html

Dave

Quote:

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Originally posted by tanker:
On tandem trailers, if when hitched it is not sitting level on level ground, you will heat up the tires on the axle that has the most weight on it. So always make sure you are level when towing. [img]smile.gif[/img]

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Yep, I learned this the HARD way!

We always service our a/c tires with NO2. The pressures from flt to full braking on landing will shoot up almost immediatly. Using NO2 keeps the variation in pressure more under control. NO2 is more rubber friendly as well.
A larger a/c can have cold pressures as high as 250psi and will heat up to 280psi. They are protected from extreme heat with fuse plugs. We generally will service the tires cold. The brakes are multiple disc and are on the axle inside the wheel. They will radiate heat into the wheel and into the tire for several hours. Tire pressures are criticle for tire life on airplanes and to the a/c. A low pressure tire will explode and do alot of damage to the a/c. So keeping the pressres to there subscribed pressures is economic decision as well. I keep the pressures in my tires on the trailer and the max cold pressure on the sidewall. On the truck 60 front and 65 rear when towing.
Denny

One or two years ago a member here on the Diesel Page gave an exact way of determining the correct air pressure to run in the truck as well as the trailer tires.
If my memory serves me right he was a retired engineer from Michelin.
A search would probably turn the article up.
In essence, if my memory serves me right, you would fill your tires up to say 50lbs, drive a mile or two, get out and check the pressure. Ideally it would increase only 5bs. nor more or less. So therefore you would adjust accordingly.
If anyone finds this article please let me know as I lost my copy of it when I traded trilers.

I attended a tire seminar at the RV Rally in Redmond and also had my TT & TV weighed ... each wheel. The advice I took away from this was this:

Only use tires rated for trailers (ST) as the sidewalls are manufactured differently and ALWAYS inflate to sidewall pressure (trailers) regardless of the load (tandem axles) because of lateral pressures on the tires in cornering, especially the rear axle tires. From my own experience of trying to be exact on TT tire pressure I wore out the outer 1" of one of the rear tires down to the cord, making it worthless. Cornering to the right is generally sharper than cornering to the left (since we drive on the right side of the road)and has more negative effect. A second problem of "tire dragging" around sharp corners is additional stress on the trailer chassis frame aft of the front axle. I also learned from experience. My 30' trailer 10" box frame chassis suffered a 3" crack in both sides at the cross member and had to be delivered to the chassis manufacture for repair. Which they did nicely even though it was out of warranty. My trailer has a GVWR of 11,211 lbs and weighs fully loaded at 8,865 lbs ... so I'm no where near overloaded. My 2004 Duramax fully loaded weighs 8,650 lbs .. so that's under by 550 lbs also. It all tows wonderfully .... but I'll keep my trailer tires up from now on!!

I agree, I've kept my trailer tires at the maximum inflation (on the side wall) on all my travel trailers. Our new Holiday Rambler has an 11,500lb GVWR and is equiped with LT 225/75R16 E rated tires which are rated at 2680lbs single @80 PSI. Note that these are light truck tires and not a trailer tires. :rolleyes: Thats what Monaco is putting on their trailers. We are going to Elkhart IN and taking a factory tour of that trailer plant this week, so I'll ask about that. It's possible that they don't make a ST tire with that rating, so rather than make it a tri-axle they went tandem with LT tires. When I find out I'll post it. [img]smile.gif[/img]