Excessive pressure increase?

 Another question from a reader with a "heavy 5th wheel trailer.

 

Hopefully, this isn’t a duplicate.  I tried to post a comment before I saw the email.  Thanks for all the excellent information on your site.  It is incredibly difficult to determine who is sharing good knowledge on other forums.  

I have read through many of your posts about hot tire pressures in particular, but I’m hoping to get your experienced input on a more specific example.  How much pressure increase is reasonable in load range H (4805 max load) tires inflated to 122-125 psi CIP?  I have tandem axles on a fairly heavy fifth wheel.  All weights are within limits (trailer, axle, tires, etc.), but with the wheel-position weights, one of the four tires is carrying c. 4,700 lbs, which is 53% of the total for that axle.  I know this violates your ‘85% of max load’ recommendation.  However, I’m seeing up to 32-35 psi increases (to about 157-160 psi) after 3-4 hours on the road when in the sun.  I have always pulled over for a break at that point, so I don’t know if it would have continued to climb.  According to the TPMS, the heavily loaded tire reaches these ranges first, but the other three aren’t far behind; accounting for shade, wind, etc. all four increase in psi and temps together fairly steadily throughout.  Generally, the temperatures aren’t much higher than 30 degrees above ambient, but I’m really wondering if there’s a point I should be concerned about the pressure increases.  I know it's been said the tires are engineered to handle pressures up to 100% over max CIP, but just trying to find the threshold of when one should begin to be concerned.  Once elevated to the point where I pull over to cool things off, I can only get about 30 minutes of drive time before they start touching that 157-160 psi level again. The tires are within specs, but close to the maximum.  I can add to the CIP as you recommend, but I’m limited to only a couple of PSI before I hit the max CIP of 125 psi.  My rims are rated to 130, but not sure I have the clearance for larger tires, and I’m not sure if I need to consider that.  The current tires are Goodyear G114 215/75R17.5.  Unfortunately, Goodyear documentation or customer support doesn’t provide much guidance on this.

Any thoughts would be greatly appreciated.

 

 

My reply:

Let's get a few things settled. The pressure on the tire sidewall is actually the MINIMUM required if you are loaded to the MAXIMUM load. Ya I know it's confusing but the reason is that increasing above that pressure will not give you any additional load capacity.

You gave ranges for your pressure. Why aren't you inflating all your tires to 125 psi? I am also wondering if you have had your pressure gauge checked at the 125 psi level where you should be running your tires.  Is the gauge a "stick" type or dial or digital? Digital is best as they are easier to read and I have found them to be more accurate too.

You didn't mention your travel speed but those tires are rated to a MAX of 75 and you should consider that max just as you do the "Red Line" for your TV engine speed. I would be limiting travel speed to 70 with 65 being better since you are loading your tires to their almost their max. Something to remember is that your tires are considered "regional" service not long haul so it would normally not driven at full highway speeds for many hours on end. Think of how a commercial trailer would be used. Most of the time I bet less than an hour loaded then an hour back to the warehouse empty. You are running at Max load for hours on end.

While I have seen reports from some people seeing a +25% inflation increase to me that suggests that the combination of load and speed is pushing your tires to their limit.

While a NEW tire might be capable of handling 100% over-inflation, static in a special test chamber, that would be way too high for a used tire running down the highway.

You didn't ask about tire life but with your conditions, I would suggest you figure on a 5-year life At that point you should be able to sell your tires for a good price. Maybe in the $100 range but the main consideration is to sell them before they fail under your operating conditions.

 

When you are operating at the limit, confirming all the details such as load, inflation and speed, need to be checked. 

##RVT989

 

What information is on your tire sidewall? Could it be worth $Hundreds?

 I bet that few of you have ever bothered to read and copy down the important information provided on your tire's sidewall. There are a number of reasons for you to spend the few minutes it will take to collect and write down the information. Having the facts,  might even be worth a free set of tires!

1. DOT serial number for each tire. They might all be the same or each tire could be unique. The complete 10 to 13 character code would be used if you have a failure and want to file a complaint with NHTSA, the division of the US DOT that is responsible for initiating recalls of vehicle components that have been judged to not meet the safety standards that DOT establishes. For tires, the DOT Serial is used to identify which tires are being recalled, and replaced for FREE. Without the serial number you have no way of knowing if your tires are subject to a recall or not. It is definitely a lot easier to plan and to collect the serial number on a nice sunny afternoon, than on a rainy morning in the mud. You need to include all the letters and numbers, especially the last 4 numbers which are the "Serial Date Code" that identifies the week and year the tire was manufactured. Here is an example of an older 11 character serial number.

  In this example the Date code is 3908  which would correspond to the 39th week (October) of 2008. If you have a tire this old, I would strongly recommend you replace it ASAP as it is well over the 10 year Max age limit for any tire in RV application.

When you start looking at your tires you may find something that looks similar but the last four characters and not numbers. This means you need to inspect the other side of the tire as not all tires have the date code on both sides. I have suggested a flashlight and a 12 year old and a $5 or $10 bill might be called for as you hire some young helper to collect the Date Code from the other side of the tire. I know, that I no longer find that crawling around under my RV is easy on the knees. Some of you will discover that you managed to park your RV so the date code is hidden by the frame or exhaust so you may need to move the RV a couple feet. Don't move the RV with the 12 year old under the RV. Not Safe! Suggestion. When you buy new tires get the dealer to give you the registration sheet that has the full DOT of every tire recorded. Same when you buy a new RV. It's easier to get the dealer to crawl around to get the numbers.

OK, the DOT serial is the most important number to record and keep in an easy to find location along with the manuals for the furnace, stove, AC and other features of your RV.

2. A picture of your Certification Label AKA Tire Placard, would be a good idea to have on your computer or phone. The placard has a statement of complete tire size, Load range and the GAWR PLUS the recommended inflation. Here is an example from an Airstream.

All of this information is important and should be easily available. It is very helpful when buying a new set of tires. I have read more than one post of an RV owner getting the wrong Load Range for their RV which would be a serious safety concern.

3. All tires also have information on their construction like this:

 This is really just FYI and is more like truth in advertising to let you know the materials used in the sidewall and center of the tread of your tires. In this case there are two ply of Polyester in the sidewall and in the tread there are two ply of Polyester + 2 ply of Steel + 1 ply of Nylon. Most 19.5 & 22.5 tires will only have 1 ply of steel in the sidewall. This information might help you understand why "Ply" and Ply rating" is no longer used since most tires only have 1 or maybe 2 layers in the body. This is why we use Load Range terminology.

Don't worry as we tire engineers have a large selection of materials and different strength of those materials so can choose the appropriate materials for the tire we are designing.

Just as I did, you can capture a picture (Bright Sunlight is best) and just keep that picture so you can refer to it in the future.

OK so now you are asking "When do I get my free tires?" 

You can check to see if you vehicle or any of your tires or other parts are covered under a recall HERE.  You will need the vehicle VIN and or the tire DOT serial to check the list.

So where does having this information handy come into play?  Well, lets assume someday you have a tire failure or simply wear a tire out because an axle is out of alignment. So you replace it and give the worn or damaged tire to the dealer. BUT if that tire is on the DOT Recall list you could get it and any other tire covered by the recall replaced. FOR FREE. But here is the catch. You have to turn in the old tire to get the free replacement. Just saying "I had a tire" will not work. Even having a picture is not enough. So just think of how you would feel if after replacing one tire or even your entire set, you were to discover the tires were subject to a recall but you no longer have the tires as proof.

NOTE: Tires are not the only parts that get recalled. I have heard of shocks or door locks or brake parts or wheels and many other parts being recalled and REPLACED for FREE.

##RVT988

What is a "C" tire? 185/R14C

 There are a number of Class-B motorhomes that are being built on Van chassis. Some of these vehicles come with European "Commercial" tires. I had a discussion with one owner that wanted to find the Load & Inflation tables for these tires.

Well for US tires there are books published annually by the US Tire & Rim Association that have the tables used for almost all tires in the US. In Europe they have a similar organization the European Tire and Rim Technical Association. Both manuals have dimensions and load limits but the ETRTO  manual does not have tables as they expect people to inflate to the tire sidewall pressure or the pressure specified by the car or truck company. BUT they do provide a formula. It is a bit involved but I worked up a spreadsheet that you can use if interested. You do need to know the Max Load for a tire and the inflation level required to support that load but you can then work out the results for your application.

What size spare tire?

 There have been a few people ask about what size spare they can use. The easy answer is simply use the same type, size and Load Range as the tire you are replacing. BUT for some this may not be easy to do. Lets look at a few scenarios.

1. You have a trailer with ST type tires. Your only option is an LT type tire or wait for a few days. Now I don't have any personal experience with needing an ST tire in a rush. The one time I needed to replace an ST tire on my race car hauler, I already had a spare. I decided I could wait till I got home to buy a replacement. So my advice to trailer owners is to carry a spare. When you do I advise you either keep it in a storage bay or under the trailer or under a white vinyl cover to prevent the Sun's heat from damaging it. The spare doesn't even have to be a new tire. Just be sure it is inspected inside and out. I would also suggest you keep the inflation low (10 to 20 psi) and be sure you have a compressor available. However, if I simply had to continue on my trip and had no spare, I probably would have bought an LT that had a load capacity that was greater than the original ST. This would have been either larger in size and or higher in Load Range. While the axles would have looked strange with one tire being larger in OD, I felt that I could continue if I simply dropped my travel speed to the 50 - 55 mph range.

2. You have a Motorhome with same size front and rear tires that have significantly different levels of wear. This requires a bit more work. With duals on the rear it is critically important that you have each tire in the pair of duals, matched in size and load capacity. I covered how to match tire OC in dual position HERE and HERE. If you can't get a tire that allows you to have a "matched" pair, it might be possible to take the two fronts and use them as a pair and have different size tires on the front till you can get to a tire dealer. 

3. Your motorhome has different size fronts and rears. This presents other problems, especially if the fronts are much larger. There is a critical dimension called "Dual Spacing" that needs to be considered. Learn more HERE. I suggest you spend a few minutes now, while you are calm, and review your tire sizes. You can learn the Dual Spacing for your tires by consulting the dimensional information on the web site for your brand tires. THIS post shows how you can measure and confirm your current spacing. Knowing what you have and what you need can answer the question of what you might be able to do in an emergency. You might discover that you need to review your situation with your tire dealer. 

4. Another fly in the ointment involves your wheels. Many Class-C motorhomes have identical steel wheels in all positions but some Class-A motorhomes may have both steel and aluminum in the rear dual position and you may have to have tires dismounted and switched around as not all wheels can be used as both inner and outer in dual position.

Depending on your situation you may have no choice but to be towed to a truck stop where you can wait to have the correct tire delivered.

You may find that you need to review your options with your tire dealer as it would be better to know your options now rather than make an expensive and incorrect decision on the side of the road while under pressure.