By Roger Marble
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 original poster 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). 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.
Inflating my tires every few days
“I feel like I’m inflating my tires every few days when leaving a campsite. 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.
I haven’t weighed my trailer
“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. 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. It’s not clear 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 three 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. That can affect both TPM temperature reading as well as increase tire pressure. We do not know what the ambient temperature was or the TPM pressure readings were. That information 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. That 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 temperature 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.
Gross Axle Weight Rating
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 (Gross Axle Weight Rating) 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 RVs 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. A couple other actual tire engineers and I 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. Here is my previous article on why tire valves leak.
Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.