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JimGolden · 12-30-2005, 11:10 AM

I should make a revision to the little test method I mentioned earlier...

Rather than starting low and working up, start at the max pressure listed on the sidewall and work down. You certainly won't burn up the correct type tire for your trailer at max pressure. So start there, then back it down in small increments.

It'd be neat to see a chart that somebody could put together on this. I'd be curious to see the relationship between pressure and temperature. Also be good to figure out some way to tell how rough the trailer is riding at each pressure.

To do this right would take some time. You measure your pressure with the tire cold. The air expands as the tire warms up, so that the 65psi you put in when the tire was cold may be more like 70psi when it's warm. You'd then need to let them cool down again before backing it down to say 55psi, which may wind up being more like 60psi once the tires warm up.

Have to compare apples to apples.

2airishuman · 12-30-2005, 01:53 PM

hi jim and others

seems i mentioned the temp issue in post 3 of this thread and starting hi then going lower.......see "tinkering". i do have a guage and have done this exercise on my car at the track and on the truck while towing. don't recall the numbers since i was also testing brake pads and rotor temps that day. i haven't done it on the trailer since i run 65psi and think its best.....i have measured tire temps at this pressure and the increase is small from ambient temps/road surface temps. my goal on the truck was to get similar temps on front compared to rear while pulling and hauling.

and jim if one plans to do the pressure measurements you suggest.....if the tires are properly inflated the pressure rise during use is very small....actually if the tires are underinflated there will be the biggest increases when warm.....so as odd as it seems the process is to run the questionably underinflated tires for a while and if warm pressure goes up much (>3-5psi) ADD air.....drive a bit and re measure.....as you approach the ideal psi the warm tires will cool some and the pressure differential will decrease.....of course this process is done will staying below the max psi on sidewall. yes check the cold numbers later too.

this is really a better exercise for properly finding t.v. tire pressures with/with out tow or load, or if, for some reason, running different brand/model tires on a multiaxle trailer or tandem/tag set up on a moho.....i used to do this to get the dualies tuned on my moho. turns out it is very common for the inside tire on a dual side setup to be underinflated.

but really this tangent has moved far from the original post......and the best advice has already been posted several times....

run sidewall max psi OR measure axle loads and use the brand chart and be sure the tire / rim load ratings are compatable. recheck frequently. that is all....

this is starting to read like one of those god awful threads on the yahoo site.....yikes!

cheers
2air'

**azflycaster** · 12-30-2005, 10:12 PM

I read this thread and then went to a couple of tire sites and read some more. Tire pressure is a moving target. For every 10 degrees of temperture increase your tire pressure will increase around 1/2 PSI. For every 1000 feet of altitude your tire pressure will increase around 1/2 PSI. Changes in the weather will also change the tire pressure slightly. So, it's early morning at sea level and 30 degrees and you inflate your tires to 50 PSI. Later that day you are at 6000 feet elevation and it is 70 degrees. The tires will most likely be running around 90 degrees because of road friction. Your tire pressure is around 56 PSI. It is a little over a 10% increase. You could remove the heat factor from the equation if you fill your tires with nitrogen. The pressure change from altitude can not be removed. It's just like the bag of patato chips you take to the mountains and then it pops open!

**Action** · 01-01-2006, 06:25 PM

AZ Flycaster,

I don't get the 30 degrees in the above post. Please explain.

"So, it's early morning at sea level and 30 degrees and you inflate your tires to 50 PSI."

>>>>>>>>>>>Action

Rob30044 · 01-01-2006, 08:15 PM

I have never run my Marathons at max 65 psi. These tires have a load weight rating of 2540 lbs. per tire or 10,160 lbs for 4 tires. My 89 Excella has a GVWR of 8,300 lbs. I inflate to 58 psi.

JimGolden · 01-02-2006, 07:05 PM

Yup, sho 'nuff you did mention that in post #3. Sorry I missed that. Least we're thinkin' on the same lines

I did email my buddy last week about this to see what temp his bike tires run at. Figured that'll give us an idea anyway.

JimGolden · 01-05-2006, 10:34 AM

Got a reply from mechanic biker buddy. He says 105-110 degrees on his truck and 115-125 degrees on his bike with gooey sportbike soft compound rubber.

startrekker2001 · 01-05-2006, 05:34 PM

Tire pressure is one of the topics addressed by the RVSEF (RV Safety Education Foundation, Inc. 4575 Annette Court Merritt Island FL 32953). They are heavy into RV Safety. While at the Florida State Rally last year I had my tow vehicle and trailer weighed. They weigh each wheel and determine based on the tire manufacturers recommendations, the amount of pressure to carry on each ... AXLE ... .

Since most TV's and trailers are unbalanced side to side, (they should be within 10%) the pressure for the heaviest side on an axle is the pressure to be used on both ends of the axle.

It is true that running at too low a pressure will cause an early failure due to excessive flexing of the sidewalls which builds up heat and weakens the tire. Too high pressure will wear the tire in the center and actually decrease the amount of rubber on the road. The whole idea is to have the tire's designed footprint on the ground for best traction and tire performance, which is what we are all looking for. Which is why we should run the correct pressure for the load.

Another factor in tire failure is the age of the tire. Every tire has the date it was cast imprinted on the tire. For tires cast before 2000 it is a 3 digit number. the first two digits are the week it was cast and the last digit is the year. For tires cast in 2000 and later, the date is 4 digits. The first 2 are the week and the last two are the year. I just purchased a 1992 Cadillac with 18000 miles on it. The tires had a date of 202. Just for grins I took the car to several tire dealers and out of 5 tries only one could tell me the correct date which was week 20 of 1992!!! The tires were 12 years old!! I have replaced them since. The point of all this is that tires have a shelf life of 5 to 7 years. Beyond that the material in the tire has broken down to the point that failure is imminent. When you purchase tires, insist on seeing the cast date. If it is more than a year old, reject it and tell them why. Unless of course they want to knock 20% off the price of the tire...

In many cases where people experience tire failure they cant tell you how old the tire was or what pressure they carried. It is usually, "oh, just what the guy at the tire shop put in, I guess" or "gee the tires all looked good and the 1996 unit only had 30000 miles on it."

I have developed an Excel spreadsheet that when the design weights, ie GVW, GVAW, for the TV and trailer and the GCVW for the TV are entered plus the results of the TV and Trailer actual weights on each tire, calculates the correct pressure for each axle on the TV and Trailer, the tongue weight, and highlights any value that is out of spec. It also tells the remaining capacity at each weight point and for the TV and trailer.

It uses the tire data for that can be found on-line at any manufacturer website. The industry is standardized to the point that for the tire size and type, the weights and pressures are nearly the same.

One last thing. There is a difference between the sidewall construction of truck/passenger tires and trailer tires. You should use only tires that are prefixed with ST as in ST225/75R15.

The upshot of all this is... use the CORRECT tire pressure. It will extend the tire life, tread life and your life.

**Action** · 01-06-2006, 08:18 AM

Thanks Vic!

And I believe there is a significant amount of people that can not and/or will not follow the above. The consequenses are shorter tire life or unexpected tire failure. And that may be an acceptable result.

And to the RVer that uses the unit once a year and gets 12 years out of the unit then has a flat due to side wall cracks and replaces all of the tires, that person would praise the tire manufacturer. And if it was a blow out at speed, the reaction might be something different.

And there are so many different maintence habits between the above and filling the tire when it looks low. That different outcomes occur.

>>>>>>>>>>>>>>>>>>>>Action

Tinsel Loaf · 01-08-2006, 05:47 PM

So what have we learned..... Basically follow the manufacturers inflation instructions for the type, style and use the tires are used for. If you should ever be the cause of an accident the investigators will eventually check if your rig was over loaded. You were using the proper style tires that are in good repair and inflated with the proper tire pressure all around.