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Old 03-09-2012, 03:46 PM   #1
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Adding more air to tires then spec'd

I was told years ago from a tire rep that it was o.k. and even advisable to add 5-10 psi more then what the side wall said when towing at max weight.

What is your opinion and do you know or have known anyone doing this.

I have at times on various vehicles and trailers. I guess this is more of a what's your take on this.
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Old 03-09-2012, 03:48 PM   #2
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When cold, never go over the tires rated pressure.
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Old 03-09-2012, 03:59 PM   #3
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Remember as tire temperature goes up from rolling resistance the air pressure will expand in the tire do to heat build-up. This could cause the wheel and or tire to fail do to over pressure of the air introduced into the tire above rating on tire.
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Old 03-09-2012, 04:08 PM   #4
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Quote:
Originally Posted by pwr2tow View Post
I was told years ago from a tire rep that it was o.k. and even advisable to add 5-10 psi more then what the side wall said when towing at max weight.
ABSOLUTELY NOT. The sidewall psi is the MAX (cold) inflation for that tire and should NOT be exceeded (when cold). Cold means UN-DRIVEN at the current outside air temperature you are at. Adding air above the sidewall psi WILL NOT increase its load carrying capacity - which is already maxed out at the sidewall psi.

Now, given there might be a misunderstanding from that tire rep you talked with, it is common for folks to add 5-10 psi to the inflation psi listed by the manufacturer of the tire in their inflation tables for that specific tire at the measured weight resting on that tire - SO LONG AS THAT DOES NOT EXCEED THE SIDEWALL. This is typically to offset potential errors in weight (like side-to-side differences) and psi measurement.

And yes, as the tire is driven it will heat up and yes the psi will increase. This is all safe and is built into the inflation tables and this INCREASE IN psi should NEVER EVER BE RELEASED.
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Old 03-09-2012, 04:12 PM   #5
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Yah I know psi go up as tire temp goes up. Was just wondering if over inflateing was a myth or just something the tire manufacturers could not recommend due to uncontrolled circumstances and liability due to that.
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Old 03-09-2012, 04:21 PM   #6
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I've never heard of adding that kind on air pressure over and above what the tire calls for. Remember adding pressure is done when the tire is cold. Never try to add pressure after a tire is warmed up. I don't know how tires inflated that high could even be controllable.
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Old 03-09-2012, 06:07 PM   #7
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I have heard the same from people I trust HOWEVER..... And I stress this. it is brand of tire specific. IN the case they were relaying info which was supposed to come from the tire manufacturer (Who I will not name here)

Today's tires are far too often made very cheaply and overseas, I would not want to risk it with those.. The tires I was advised to over-inflate are high quality ones that can and do take it.

That said: IN any case. inflate per the load... NOT per the sidewall, it is all but guaranteed the sidewall pressure is too high. (NOTE: There are exceptions, I'm one of them) .
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Old 03-09-2012, 06:12 PM   #8
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Quote:
Originally Posted by ottffss View Post
ABSOLUTELY NOT. The sidewall psi is the MAX (cold) inflation for that tire and should NOT be exceeded (when cold). Cold means UN-DRIVEN at the current outside air temperature you are at. Adding air above the sidewall psi WILL NOT increase its load carrying capacity - which is already maxed out at the sidewall psi.

Now, given there might be a misunderstanding from that tire rep you talked with, it is common for folks to add 5-10 psi to the inflation psi listed by the manufacturer of the tire in their inflation tables for that specific tire at the measured weight resting on that tire - SO LONG AS THAT DOES NOT EXCEED THE SIDEWALL. This is typically to offset potential errors in weight (like side-to-side differences) and psi measurement.

And yes, as the tire is driven it will heat up and yes the psi will increase. This is all safe and is built into the inflation tables and this INCREASE IN psi should NEVER EVER BE RELEASED.
ON TRUCK size tires the cold pressure on the sidewall is the MINIMUM required to support the maximum weight rating of the tires. Same with the tire charts, it's the MINIMUM cold pressure to support the weight. So YES you can exceed the pressure on the sidewall by a few psi.
Quote:
From page 2 of the 06/07 Michelin RV Tire Guide: "If you look at the tire's sidewall, you'll see the maximum load capacity allowed for the size tire and load rating, and the minimum cold air inflation needed to carry the maximum load."
From the Firestone/Bridgestone RV tire guide:
Quote:
Bear in mind that these are maximum ratings. The sidewall of the tire
shows maximum load and minimum inflation pressure for that load
From th eGoodYear RV Tire Guide:
Quote:
How much air is enough?
The proper air inflation for your tires depends on how much your fully loaded RV or trailer weighs. Look at the sidewall of your RV tire and you’ll see the maximum load capacity for the tire size and load rating, as well as the minimum cold air inflation, needed to carry that maximum load.
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Old 03-09-2012, 06:28 PM   #9
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Quote:
Originally Posted by pwr2tow View Post
I was told years ago from a tire rep that it was o.k. and even advisable to add 5-10 psi more then what the side wall said when towing at max weight.

What is your opinion and do you know or have known anyone doing this.

I have at times on various vehicles and trailers. I guess this is more of a what's your take on this.
If you don't know your actual four "corner weights" or you don't check pressures very often and are using just the axle weights then yes that is advisable.
On car tires never exceed the pressure on the side of the tire, on RV tires see my post above.
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Old 03-09-2012, 06:44 PM   #10
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Before we left for Florida Feb 14th I had two new tires put on our MH, the tire store also talked me into nitrogen air in all six, [ 80psi ] max inflation. The results was this, as we got into the warmer climate from MN. the MH ride got harder, the passenger seat began to shake, I forgot it and enjoyed Florida. On our way home just south of Atlanta the MH was shaking so bad we pulled into a truck / RV tire store, I told them my problem, they spun the front tires and found they had both balloned and couldn't be balanced, keep in mind these were brand new coopers. We had to buy two new tires and the tire presure was set to the chassis label. When we parked at the RV park that night north of Atlanta, I checked the tires and they were all hot and 17psi higher than where the tire store had put them. Our label on chassis 65 front / 60 rears. Good luck with your decision.
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Old 03-09-2012, 09:41 PM   #11
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All three of my trucks with E tires say : max 3042 lb or 3195 lb load ..... max 80 psi" stamped on the sidewall.

The wifes tires on her Impala reads "max load 1600 lbs .... max 44 psi stamped on the sidewalls.

I sure wouldn't overpressure a tire on a truck or heavy trailer on purpose.

Now if your looking for higher mpgs on your car with no load then the old trick that the car manufacturers drive across America back in the 50s/'60s of pumping those old 35 max rated car tires to 40-50 psi to see who got the best mpgs may work for you. But I wouldn't add a load or go over 50 mph.
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Old 03-09-2012, 10:21 PM   #12
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Mr_d - I am NOT going to play the max/min psi for max load semantic game. Nor should anyone else. The sidewall is the MAX inflation, period.

If you don't understand what the semantic game is, this exceptionally well written and well researched post will clarify

Tire pressure?
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Old 03-10-2012, 02:53 AM   #13
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If I was about to leave on a trip on a 110° day, what should I set my tire pressure to when my laser temp gun shows over 170° on the sunny side tire sidewall and 110° in on the shady side tire sidewall? If I set the sunny side tires to sidewall pressure, wouldn't they be under inflated after I get moving and circulate some air around them?

I just check my GY truck tires and it says Maximum Load at 80 PSI. Nothing about Maximum pressure. Same with my trailer tires.
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Old 03-10-2012, 11:36 AM   #14
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I've been watching this thread for a while and frankly, most of the info in it regarding overinflation and tire safety, is malarky.

I've raced cars and been around people at the highest levels of motorsport since the 1980s, talked frequently with tire engineers, and after reading this last night, wrote a friend who's a tire engineer and discussed this thread, as well as compared notes on our experiences with trailer tires over the years.

First of all, tire durability is overwhelmingly related to running temperature. Running temperature goes up because of flexing in the tire carcass. The higher the pressure the less flexing and lower overall tire temperatures (especially in radials).

Tires have about a 3x safety factor in inflation - you can put more than 90 psi in a 32 psi rated tire when new and it won't catastrophically fail. Therefore a reasonable amount of cold overinflation combined with an expected amount of hot pressure increase is not going to cause failure. This is especially true when running at high speed in hot (desert sunny) conditions. I have an old college roommate who used to work the oil fields in Saudi. They used to run standard passenger car tires (32 to 36 psi) at 45-50 or else a blowout was a serious risk (they frequently ran over 100 mph out in the boonies).

I don't know what police departments do, but when I autocrossed cars we used to put 45 psi or more in higher profile tires (this was before 50 series were widespread) to keep the tire on the rim at high cornering loads - you have to stiffen the sidewall on a radial with more pressure when running at extremes or bad things will happen. Bias tires are not as much of a problem.

That's not to say there aren't tradeoffs. Inflation pressures for a given tire manufacturer and size are designed to produce a footprint that maximizes traction and durability under normal use. Raise the load and you need more pressure to maintain that footprint and lower running temperature. If you overinflate the tire it "beachballs" and reduces the load on the outer ribs, increasing the wear on the center of the tire. Because the footprint reduces in both length and width, traction capacity goes down.

If you really want to know how happy your tires are you have to get a contact pyrometer and push the needle into the tread blocks and look at temperature across the surface of the tire IMMEDIATELY after a half hour or so of intended use. For something like trailer tires and rear tires of a MH or tow vehicle, the temps should be within 10 degrees or so across the tread and consistent across all four tires - or else you have a loading issue or alignment issue. On front axles, they should be 20 deg or so hotter on the inside edge than the outside - this is due to camber. IR non-contact pyrometers are useless in this application. I'd guess they ought to be under 175 deg, but I haven't found a reference. Race tires run over 200 deg. I've commonly seen 235!

What is not BS is the warning on rated speed. I frequently pull my trailers at 75-80 with the truck, but no more than 65 with the MH. OUT here in the Mojave, it's HOT. On my old toyhauler I went through a set of Goodyear Marathons and a set of Denman STs with 7 belt failures. On my current trailer, I had almost two years of use with no noticeable tire wear using the standard cheap chinese junk OEM tires - all towed behind my MH. Then I took a a fast trip to Tucson to pick up another race car, towed it with the truck. Got home and the front two tires on the trailer are now neither round nor flat across the tread, with heavy wear on the inside. NONE of these tires were run underinflated or overloaded - in fact, they were about 20% under max load.

When I compared notes with my tire engineer friend, he indicated after 20 years of no problems with Firestone 440s, he switched to carlisles and had a blowout. Failure analysis showed no signs of rubber adhesion in the cords - a sign of poor manufacturing quality. He's now using made in USA LT tires on his trailer, rated at 44 psi but inflated to 50 to add lateral stiffness and make them run cooler - because while the LTs don't offer the sidewall protection of an ST, they are safer at highway speeds. I'm now considering going that route or the one below.

A previous post included links to the GY RV guide - but that's not the only GY reference out there - see:

http://www.tirerack.com/images/tires...plications.pdf

An interesting tidbit in there:
"Special Trailer (“ST”) Tires
Goodyear Marathon trailer tires are widely used in a variety of towable trailer applications and are designed and branded as “ST” (Special Trailer) tires.
•
Industry standards dictate that tires with the ST designation are speed rated at 65 MPH (104 km/h) under normal inflation and load conditions.
•
Based on these industry standards, if tires with the ST designation are used at speeds between 66 and 75 mph (106 km/h and 121 km/h), it is necessary to increase the cold inflation pressure by 10 psi (69 kPa) above the recommended pressure for the rated maximum load.
o
Increasing the inflation pressure by 10 psi (69 kPa) does not provide any additional load carrying capacity.
o
Do not exceed the maximum pressure for the wheel.
o
If the maximum pressure for the wheel prohibits the increase of air pressure, then the maximum speed must be restricted to 65 mph (104 km/h).
o
The cold inflation pressure must not exceed 10 psi (69 kPa) beyond the inflation specified for the maximum load of the tire."
To the original poster, your mileage may vary, but it appears a ~10-20% overinflation will increase speed margin but not load margin, and have no other adverse effects on safety.
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