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Friday, February 25, 2022

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!

  • 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.
    Why do tire valves leak?


Friday, February 18, 2022

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.


Friday, February 11, 2022

Age old question tire pressure?

  Please forgive me if I sound like a broken record.        I found another post on an RV forum on inflation. It's almost as if no one knows that the answer to most questions on RV life has already been asked and answered. That is certainly the case when it comes to tire inflation. But I just can't ignore these questions as I hope that if some learn how to properly inflate their tires they may avoid a costly failure.

"I have a 2003 Fleetwood Pace Arrow 37A about 20,000 lb loaded. Tires are Hercules 255/70R22.5. Sidewall states 120PSI. Camping World who I do not trust and will not go back to ( whole other story) set tire pressure at 80psi. I think they are way low. Am I right or wrong? Have not had each wheel weighed. Recent CAT weigh in: Front axle 6300 and Rear axle 14640. Any recommendations? "
My reply: 
Well the RV owner gets points for getting some of the information needed and he provided that data in his first post. I started off doing an Internet search to learn if this RV has slides and it does have a large slide on the driver side and a small one at the rear of the passenger size. This suggests the potential for some significant weight unbalance side to side.
Since we do not have the "4 corner weights" I am going to suggest we assume a 4% side to side unbalance because of the weight of the large slide.
With a Front axle scale reading of 6,300# that would suggest that on end could be supporting 3,402# on the heavy end.  The heavy end of the rear could be supporting 7,906#.

There was an answer posted that the owner could use a Firestone load/inflation chart since he could not find one for Hercules brand. This information is correct in that almost all brands other than Michelin have the same numbers in their tables. I had a Bridgestone chart that shows a 255/70R22.5 LR-G tire can support 4,190# @ 80 psi in single (front) position. his size in a dual position (rear) is rated to support 3,970# per tire at 80 psi. or 7,940# per dual pair.

So what would I recommend for this specific application?
1. Since we do not have the actual "4 corner weights" and are estimating a 4% side to side unbalance, we need to be careful as we could easily be wrong with that estimate. I have heard of some large Class-A being out of balance side to side by over 1,000#. We should probably estimate a greater margin jsut to be safe.

2. A 6% unbalance could mean 3,528# on the heavy end of the front which would suggest that our suggested 80 psi is probably enough for the front position. The rear position calculates to 8.198# on the heavy end or 4,100#.   The table suggests that inflation of 85 psi would be sufficient to support 4,110# per tire in dual application.  This gives us a goal minimum inflation of 80/85 F/R.
3. If you have been reading my posts you know I like to have a bit extra margin on the inflation number so the RV owner doesn't have to chase inflation as the Ambient temperature changes so an extra 10% on inflation is good.
4 We end with 88F/ 94R  but lets round to a nice 90/95 psi.
I trust that if people follow the concepts I covered in this post they should be able to arrive at a reasonable inflation guide till they have the opportunity to get their actual 4 corner weights.

Friday, February 4, 2022

Tire Loads & Inflation - A refresher

 Just read another post on a FB page where more incorrect info was published....Soooo lets go over this topic again

 Today’s key points: Know the minimum tire inflation based on manufacturer estimates. At a MINIMUM Check your inflation with a good gauge at least monthly and every morning before travel. Better yet get, program and use a Tire Pressure Monitor system  TPMS to the knowledgeable.

Tire inflation seems to be a topic that confuses some and has others believing in misleading or just plain incorrect information. Tire inflation is one item that directly affects the safety of your RV, truck or car as you travel down the highway. Many of my posts have been background information that you really don’t have to know or fully understand. The intent of these topics is to give you a better foundation of understanding more about tires but, if you only pay attention to one series of posts this is it.

Tires do not carry the load. They are just a container of air. It is the inflation air that does the work. Think for a moment of an impact wrench. It can’t do the work of loosening or tightening nuts on your wheels without the air, and a tire can’t do the work of carrying the load or provide the traction needed to turn, start or stop if it doesn’t have air. The load a tire can carry is basically based on the air volume of the tire and pressure of that air inside the tire.

If the load was carried by the tire construction we would have Tire Load and Construction tables. But we don't. We have Load and Inflation tables because, for a given type and size tire the load is just a function of air pressure. More pressure gives more load carrying capacity.

If you want to get a feel for how little load your tire can carry without air in it, you can test this yourself. Simply take an unmounted tire and stand on the beads of the tire. I think you will find that the tire can support less than 5% of the rated load before it deflects more than an inch or two. Some people believe that tires with higher Load Range can carry more load at the same inflation. This is just not correct. You would be hard pressed to measure the uninflated load capacity difference between a Load Range D and E tire or between a G or H Load Range tire because almost any significant load would compress the tire to be flat.

The term Load Range replaced to old "Ply Rating back in the early 70's when tire construction materials became stronger and with the introduction of Radial construction.

If you are not sure, the "Load Range" is marked on the sidewall of your tires. If you don't see the words "Load Range" look for "LR" followed by a letter usually between C and G. "Load Range" may not be in large letters but you should be able to find them on your tires.

TireRack has a good information page on "Load Range" HERE.

The simple answer to how much air you need is on your Certification Label AKA Tire Placard that I wrote about last week. Depending on the year your RV was made, this information is on a sticker on the side of your trailer or near the driver’s seat in your Class-A or on older RVs it might be glued to the wall inside a cabinet or on the inside of your entry door. Yoyr car or Pick=up has the sticker on the driver door jam. Where ever it is, you should also have the information in the stack of owners manuals you received when you bought the new RV. Find this data sheet and write down the information for loads, minimum inflation, tire size and load range and place this info where you can easily find it. This inflation is the minimum based on what the RV manufacturer thought you would load into your vehicle. They may have guessed correctly or you may have gone way over that estimate depending on how much “stuff” you carry.