Why did I have three Blowouts? A question on an RV forum

 So I came across another post on an RV forum. I am sorry to report that many of the posters did not understand the concept of Cold Inflation.  The OP asked:

Why did I have three Blowouts?

I’m pulling my hair out. I’ve had three blowouts in 18 months of ownership of our 2020 Keystone Outback. We have ST225 75 R 15 Load E.

The blowouts have been on 3 of 4 points on our dual axle TT.

First blowout was road hazard / excessive speed (I was routinely doing a few mph over on the interstate), and second and third appeared to be heat related in that both tires were running hotter than others and PSI had climbed to 95 on our max 80 PSI.

I feel like I’m inflating my tires every few days when leaving a camp site. This can’t be normal. When starting this trip from SC to FL, I found them all down 3-6 PSI in GA after I had inflated all to 80 (cold) two nights before in the SC storage yard.

Before this last blowout they were all sitting around 74/75 PSI in the morning and I topped them up to 80 PSI according to TPMS. I had barely left the campground when each had climbed to 82. One went to 95 within an hour and temp was running mid to high 70s and the other three were in mid 60s. I pulled over and let air out of them all. But the eventual blown tire just kept running hotter and then popped within 2-3 hours later. Center tread separated completely from tires.

No I haven’t weighed my trailer but I’m just a guy with a wife and three kids 5 and under, no modifications to my trailer, etc, and I don’t pack every inch of floor space with gear that then spills out at the campsite upon arrival, so I can’t imagine I’m overloading.

I don’t understand why I’m having to top up my tires all the time, and it’s not unclear why the last two blowouts have occurred when I’ve been trying to stay on top of the cold PSI and staying under the speed limits.

We are “on the road” so I’m contemplating buying whatever replacement tire I can get, and then asking my local RV service center to check bearings and alignment upon our return. Could either of these be the underlying issue?

Then if that isn’t the case, I’ll consider investing in a complete set of tires with a better reputation, in case the current tires are truly bad.

I will welcome any thoughts or comments!

My reply:

• There were 3 tire failures. A puncture is not the fault of the tire as any tire can be punctured or cut. An increase of 20 to 25% in pressure is an indication of a combination of excess load and excessive speed. A dragging brake or wheel bearing problem can also generate excess heat which can affect both TPM temperature reading as well as increase tire pressure. We do not know what the Ambient was or the TPM pressure readings were which might provide additional clues as to the reasons for their failures. Others have posted that bleeding air out of a hot tire is definitely the wrong thing to do. There is the potential that this action resulted in two tire failures depending on the actual loads and how much air was actually let out. We do not know the actual truck scale load for each axle so this is important information that is missing. 80 psi on the tire is the minimum inflation needed to support the load number on the tire sidewall. It is NOT the max operating pressure. The ONLY pressure we need to be concerned with is the "cold" inflation which is the inflation measured before the tires are driven on or exposed to direct sunlight for the previous 2 hours. 

   

  Inflation pressure when the tires are stationary and out of direct sunlight will change about 2% for each change in the ambient temperature of 10 F. A 6 psi drop with no other cause would indicate a drop in Ambient of about 37°F for a tire inflated to 80 psi. We do not know the Ambient at the time of tire measurements but that is a considerable drop in temperature so I think there is probably some other reason for the reported pressure loss. We do not have to fill every space with "stuff" to end up overweight. The RV Certification sticker indicates the maximum load for each axle when the RV is fully loaded. This load GAWR should not be exceeded. If a scale indicates the RV has an axle at GAWR then the tires MUST be inflated to the pressure stated on the sticker BUT we can still have one tire overloaded as most RV have a side to side imbalance of their axles. They also have an imbalance between the two axles so that is why we need to confirm the actual load on each axle. This can be learned on truck scales as long as we get readings for each axle which requires careful parking on the platform scales. I have seen air loss due to small tread punctures, leaks around rubber valve stem, Leaks between TPMS and the valve stem. leaks through the aluminum due to casting errors. and leaks between the tire and the wheel due to improper mounting. I have posted example of leak through the valve core. See the link below. I and a couple other actual tire engineers follow some of the RV posts. Plus there are many self-appointed "experts" so you do need to always consider the source of the information you find on the Internet as not everything you read here is the truth believe it or not. https://www.rvtiresafety.net/2012/11/why-do-valves-leak.html

  

##RVT1041

4 questions on inflation, a long back story and RVs are not the same as HD Trucks

 [QUOTE=yeloduster;6070640]This is a question for Tireman.

A little background first. My older brother was an independent OTR for years. The last 15 years or so of his career he had a contract that took him from his home in Roosevelt, UT to Houston, TX. His gross weight were ~80000 lbs. He typically left early Monday morning and was home by 4 PM on Friday.

In the winter he might leave Roosevelt at 5000 ft elevation and maybe -20°. He would proceed through Colorado and top out on Berthoud Pass at 10,300 feet and proceed to Houston where it might be 85° and elevation 100 ft.

His tire cold inflation pressures would experience a difference of ~5 PSI due to elevation change and ~10 PSI due to temperature change. In addition because he was hauling oil field equipment on a flat bed his load could be heavy on one side because of the odd shape of his load. I know he couldn't measure side to side weights. He relied on axle weight to set his air pressure.

He and I talked about tires a few times. I don't recall talking about specific pressures but I know he checked his pressures each morning before leaving wherever he was but didn't change the pressure unless he found a low tire.

He drove millions of miles and wore out several trucks and a truckload of tires. He says he had very few blow outs because he used good tires and monitored his pressures. The few blowouts he experienced were road hazard type events.

His experience would suggest that tire manufacturers load charts take into account a certain amount of side to side imbalance of the load and account for large changes in elevation for over the road trucks.

Actually I guess I have 4 questions:

• If the engineers who developed the load charts took into account reasonable side to side load imbalance and large elevation changes shouldn't we just follow the chart?
• If the engineers who developed the load charts took into account changes in ambient temperature when developing the chart would it be better to set your cold inflation pressures according to the chart and add or subtract about 1.5 PSI for each 10° the ambient temperature varies from 72°?
• Most RV owners do not have a calibrated tire pressure gauge. I've read that moderately priced tire pressure gauges are accurate ±3%. Knowing all these uncertainties about inflation would a RV owner be wrong if he set the pressures with his gauge according to the weight on his axles and checked them regularly. As long as the pressures were his original set point ±2-3 PSI everything is OK?
• Some of us are OCD and getting 4 corner weights, studying the charts and getting the exact pressure according to the chart and then setting pressures to ±1 PSI according to our gauge satisfies our OCD but does being that precise really matter for safety, driving comfort and tire wear?

I'm thinking if we inflate to the chart value for our axle weights taking into account the actual cold inflation temperature we should be good![/QUOTE]

 

First off it's important to remember that HD Trucks are weight limited not tire limited so many times they do not have to run as high a pressure as some RVs do. HD trucks also may have tandem duals which means 18 tires while Large RVs may only have 8 tires total so don't get hung up with that comparison.

1. The charts have the MINIMUM psi required to support the stated load. There is no stated or implied "safety factor" that would allow overloading.

2. Tires are to be inflated to the appropriate inflation when the tires are at ambient temperature before you start driving. 72°F is not some special temperature. The chart is based on AMBIENT air temperature wherever you are.

3. My hand gauge (Accutire brand) cost $12 from Amazon and is accurate to +/- 0.5 psi at 80.0 psi ( 0.6% accuracy) when checked against an ISO Certified Laboratory gauge. Paying more for a gauge is no guarantee of better accuracy. I prefer digital gauges over dial gauges as they are easier to read. I do not like stick gauges as they can get way off if they get worn from use or dirty. Dial gauges can sometimes be hard to read.

4 I would suggest if possible to get 4 corner weights when the RV is at its heaviest. Consult the tables for the inflation for the heavy end of each axle. That is the MINIMUM pressure to set the low pressure warning on your TPMS. I would add 10% to that MINIMUM for your "cold" tire (ambient) pressure.

5 If you are traveling back and forth between Phoenix and Pikes Peak just be sure that in the morning at Pikes Peak in the snow you are not below the MINIMUM psi shown for your scale weight and the tables advise.

6 It is the under inflation we tire engineers are concerned about. In my Class-C I tend to run +15% to +20% higher inflation than the tables suggested minimum because I have a light weight RV based on my 4 corner weights.

7. Run a TPMS that you have set properly and test at least once a year.

##RVT1040