RV Tire Safety: “Cold inflation pressure” and clarification of tire terms

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By Roger Marble
It appears I messed up in my previous post “More on tire cold inflation vs. ‘set pressure’.” I failed to properly define the terms I was using, so if you bear with me I will try and clear things up.

“Cold inflation pressure” – This is the pressure number you see published in tire company “Load & Inflation” charts. It is also the pressure you see on the Certification Label sticker aka tire placard that vehicle manufacturers apply to all cars, trucks and RVs. It appears that some folks have a little difficulty with the word “cold.” This does not mean the tire needs to be refrigerated or that inflation pressure needs to be “adjusted” by calculating the difference between some theoretical laboratory standard and the current air temperature. “Cold” for tires simply means at ambient air temperature and not warmed by either being driven on or being in sunlight for the previous two hours.

When I am discussing tire pressure I am always referring to the cold inflation unless we are specifically discussing the pressure increase due to sun exposure or due to being driven on and reported by the TPMS, or if the driver checked the “hot” pressure at a rest stop with their hand gauge.

Finally, the pressure number molded on the sidewall of tires is the cold inflation pressure required to support the load that is also molded into the tire sidewall. The load number is the maximum load capacity for the tire and so the cold inflation would be considered the minimum cold inflation required to support that load.

The wording on tire sidewalls does vary a bit. If you look at a variety of tire types from different manufacturers you will see some variation in the wording and IMO this contributes to some of the confusion. One fact that many do not think about is that increasing the tire cold pressure above the number on the tire sidewall WILL NOT increase the load capacity number molded on the tire sidewall.

It is also important for people to understand that tires can tolerate a significant increase in pressure due to operation under load or at speed. While I can’t provide information on the specific design limits used by different tire companies, what I can say is that in my personal experience many new tires are capable of tolerating inflation increase of upwards of 100% or more over the number molded on the tire sidewall. So the idea that an undamaged tire will suddenly explode due to an increase in inflation due to operational heat is not justified.

Tire pressure increases. In my post of March 3, 2014, I covered the science and math of pressure change due to temperature change. You can read that post HERE, or just accept the rule of thumb that pressure changes by about 2% for each change in tire temperature of 10° F.

While we are talking about pressure change you can review my post of July 8, 2011, where we pointed out that driving from Death Valley to Denver, CO, will only result in about + 2.5 PSI but the change (drop) in ambient temperature will probably decrease the pressure by more than that increase due to elevation. This is why we tend to ignore tire pressure due to changes in elevation.

Tire load is important information. You know the GAWR, or Gross Axle Weight Rating, is on your certification label. The problem is that the actual load is almost never split side to side to give a 50/50 split. While many RVs may have a 48/52%, or similar, side-to-side split on an axle, the actual scale readings have confirmed some RVs have as much as a 1,000 lb. unbalance. So without actual scale readings we could only guess which tire is loaded more.

A tire on one end of an axle has no idea about the load on the tire on the other end of the axle, so simply dividing the axle load by two is not sufficiently accurate to be confident that you “know” the actual load on your tires. The other problem is that many people simply estimate the load on their tires. The reality is that a majority of RVs (10,000+) that have actually checked the tire loads have been found to have a tire or axle in overload.

This data demonstrates the importance of learning the actual load on your tires. While learning the load on each tire position is not easy, at minimum RV owners need to confirm the load on each individual axle and this is easily done with a visit to a local Truck Stop. This needs to be done with the RV loaded with as much “stuff” as you ever carry. With the axle loading known and until you can get individual tire position weights, I suggest you assume one end had 53% of the axle load.

My final point for this post is reserve load, and this is where we get to the “set pressure.”
First, we need to remember that reserve load is the load capacity of the tire at its cold inflation pressure that is in excess of the measured or calculated load of the RV on the tire. Some use the term “safety factor,” but as an Engineer, this term is not really appropriate.

In general, it is suggested we have at least a 15% reserve load. Most new cars come with 20% to 30% reserve load, and this is a major reason why we seldom see tire failures on cars. An exception was seen in the ’90s when one vehicle manufacturer provided for less than 10% reserve load and a number of tire failures occurred and even made the TV news.

Many motorhomes may have less than 10% reserve load even if the inflation pressure and the loading shown on the Certification pressure are followed. IMO this is a major reason for the relatively high failure rate of tires in RV motorhome application. RV trailers have it worse. In addition to having 0% to 10% reserve load, the suspension design contributes to high interply shear due to being dragged rather than steered around corners.

So what should an RV motorhome owner do?

1. Learn your actual loads on your tires by getting on a scale for each tire position.

2. If you can’t get individual axle end loads assume the heavy end has 53% of the axle load.

3. Use the tire Load & Inflation tables to learn the minimum cold inflation needed to support your actual (or 53%) load.

4. Consider applying a +15% to the load figure to give yourself a reasonable reserve load and consider that your minimum cold inflation. You could also consider adding 10% to the load table pressure if that is easier for you to calculate.

5. Consider adding 5% to the inflation in #4 and use that as your “set pressure.” This gives you a cushion for day-to-day temperature variation which can change inflation pressure 2% to 5%.

So what should an RV trailer owner do?

Do 1, 2 and 3 above.

4. If you want to try and lower the interply shear, I recommend you increase the inflation to the number on the tire sidewall and use that for your set pressure. If you have increased the tire load range from, say, a LR-D to LR-E, you can use as an inflation number the 65 PSI for LR-D and the 80 PSI for LR-E tires, and use that as your set pressure.

5. Try and learn the wheel max pressure rating and do not exceed that number.

 

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT963

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19 Comments
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Wayne
23 days ago

Hello Roger. You stated “One fact that many do not think about is that increasing the tire cold pressure above the number on the tire sidewall WILL NOT increase the load capacity number molded on the tire sidewall.” Would you explain what limits the load carrying capacity. In my pea brain it stands to reason that if reduced pressure reduces load capacity, increasing the pressure would increase load capacity up to the burst point. Thanks for the guidance you provide to us RV Travel readers.

Roger Marble
23 days ago
Reply to  Wayne

I understand your thought process but it is tire industry practice to establish a Maximum Load Capacity for a tire and increasing inflation beyond the stated inflation for that load capacity will not give extra load capacity. I do not think you will find any tire company suggesting otherwise. FYI a few years ago Goodyear issued a Technical bulletin addressing the 65 mph max speed limit on their ST type tires. They said you could increase the max speed if you increased the inflation but they said specifically that the increased inflation did not increase load capacity.

Wayne
22 days ago
Reply to  Roger Marble

Thanks for your reply. I have never inflated tires above the maximum pressure molded on the sidewall but have had friends that did as they knowingly overloaded their tires and entire rig. It made me cringe but they “got away with it” if you disregard any damage that may show itself in the future. I prefer to get tires that are over rated for my application. It probably costs a little extra but I hate trouble on the road.

Rich
24 days ago

Four corner weighing showed a need for 125psi in the front tires that have maximum load carrying at 130psi. I have seen pressures as high as 147psi on a hot day. I typically set my CIP at 130 and I do carry a 12v compressor. Would you recommend a higher CIP to get closer to your 10% reserve?

Roger Marble
23 days ago
Reply to  Rich

I would stay at the 130 and try and decrease the weight of the RV. The closer you operate to the Max Load capacity the more you are shortening your tire life.

Joseph
24 days ago

Roger, thank you for all the information you have posted in the past. It has been very helpful to me. Based on this well written article, will you see if I understand correctly?
Steer axle weight = 6860 lbs X 53% = 3636 estimated load per side
Drive axle weight = 11900 lbs X 53% = 6307 estimated load per side
Michelin Tire Pressure Chart:
75 PSI 3645 lbs Single
70 PSI 6320 lbs Dual
75 X 115% = 86 X 105% = 91 PSI
70 X 115% = 81 X 105% = 85 PSI
Max pressure on sidewall is 110 PSI

Thanks, Joe

Joseph
24 days ago
Reply to  Joseph

I applied your percentages to the PSI instead of the load!
Try again (2011 Tiffin 30GA on Ford F53 chassis)

Steer axle weight = 6860 lbs X 53% = 3636 estimated load per side
Drive axle weight = 11900 lbs X 53% = 6307 estimated load per side
3636 X 115% = 4181
6307 X 115% = 7253
Michelin Tire Pressure Chart:
90 PSI = 4140 lbs Single (95 PSI 4300 lbs)
85 PSI = 7230 lbs Dual (90 PSI 7530 lbs)
90 PSI X 105% = 95 PSI
85 PSI X 105% = 90 PSI
Max pressure on sidewall is 110 PSI

Thanks, Joe

Roger Marble
23 days ago
Reply to  Joseph

I think your 95F and 90 Rear would be good and give you a good reserve Load until sometime in the future when you can confirm with actual “4 corner” weights. When you get the 4 corner numbers you still inflate all tires on an axle to the inflation needed for the heavier loaded tires. On my Class-C I have 4 corner weights. I took the heaviest loaded axle end and consulting charts rounded up on load capacity to the chart number to get my “minimum CIP” for each axle. I still added another 5 psi so I don’t have to worry about day to day changes in ambient. Following this approach with the extra 5 psi ( which amounts to about 8% ‘cushion’) It took a 40F drop in temperature before I needed to add air to keep my reserve where I wanted it at when I made a cross country trip a couple years ago. While I am pretty “a**l” about tire pressure, that doesn’t mean I enjoy inflating my tires more often than necessary. I check pressure every AM when I turn on my TPMS monitor and as long as I am above my minimum CIP I am good to go.

Tommy Molnar
24 days ago

IMHO, about the only way you can get individual wheel weights (on trailers) is if you come upon a state truck scale that is closed but keeps the scale operating and has a display you can see. Then you can spend the time to move around and position each wheel where you can get a reading, with no pressure on you to get out of the way of the next person or truck. Truck stop scales usually only allow two weights before they start charging for additional weighs (not cheap). And, there are working drivers there that need to weigh and get going. They will most likely have little patience for an RV’er trying to figure out what they are doing.

Roger Marble
24 days ago
Reply to  Tommy Molnar

Agreed. That is why I offer the 53% approximation as an alternative to individual tire weights. You can get individual weights at some large RV conventions such as Escapees and FMCA

Tim Burke
24 days ago
Reply to  Tommy Molnar

Truck Stop scales have 3 platforms Steer wheels, drive wheels, Trailer wheels, so you really cannot get accurate individual weights of each wheel. BTW, Cat Scales are under $15 for 2 weigh ins.
There aren’t many, but Escapees has their Smartweigh program at a few of their parks.
Trailer height measurement is included in the price also.

Al C
24 days ago

Actually, this clarification confused me. Specifically #4 for a trailer. If you change tires and go up a load level as noted LR-D to LR-E are you suggesting that we should use the CIP of the LR-E assuming the wheels are rated for the higher air pressure? A clearer recommendation would be to always use the CIP for the max load when installed on a multi-axle trailer if this is the case.

Will the “ride” be rougher at the higher LR-E pressure than it was at the lower LR-D CIP?

Roger Marble
24 days ago
Reply to  Al C

Yes use the higher CIP if wheel is rated for it. It’s not a good idea to make hard, absolute suggestions on changing parts and ratings as we do not know all the related facts. If something goes wrong there is always the potential for some liability.

Phil Atterbery
24 days ago

Roger, thank you for your continuing efforts to educate the RV tire consumer. I have recommended folks with tire questions go to your web site for answers. Tire care doesn’t have to be rocket science.
Are you a fan of servicing tires to a pressure within a plus or minus range? After weighing my coach, consulting the tire manufacturers inflation chart I determined that my tires should be at 95psi. Can I add a range to that number, say, a +/- 2 psi (93-97) and still satisfy the intent of the chart? Of the charts I’ve looked at, there has been no definitive tolerance called out. Reply when you can.

Roger Marble
24 days ago
Reply to  Phil Atterbery

Well the chart numbers are actually minimums. So if you want a tolerance It would be -0 to +5. But I see no problems with running +5 as I run +10 on my Class-C motorhome tires.

Roger Marble
19 days ago
Reply to  Phil Atterbery

Additional details on the “tolerance” WHEN SETTING PRESSURE.

I do not expect people to try and set pressure much closer than +/- 2 psi. When I was racing that was a different situation and knowing the gauges my crew used I established a +/- 1 psi range for the tire pressure. But for regular use +/- 2 psi from your goal is just fine. If you follow my guidelines you will have already built in some margin so +/- 2 psi should be good and is easier to set than +/- 1 psi. Also don’t forget I am using my TPMS for my morning check and I know that is “off” by +/- a couple psi. Since we always “rounded up” when using Load/Inflation charts and when confirming our gauge accuracy that allows us to have day to day variation on the TPMS of a few psi. TPMS are NOT a substitute for a good digital hand gauge. You set your pressure with the hand gauge and the TPMS is just a rough quick check for air loss. After a while you will learn that one TPMS always reads a couple psi low and another may read a couple psi high

Steve C
24 days ago

You still haven’t defined “cold”. Morning temp = 15 degrees somewhere where there is snow on the ground or afternoon temp in a warm state in the shade at 90 degrees. Neather being in the sun or having been driven on. So, what do you consider cold?

Glenn
24 days ago
Reply to  Steve C

In that case either scenario would be considered the “cold” temp. If in a warm climate I would check them in the morning when it is slightly cooler. He described “cold” as not in sun or been driven on. Not really any set temperature.

Roger Marble
24 days ago
Reply to  Steve C

Steve, It may be easier for you to ignore the word “cold” . The tire is at “Ambient” air temperature. Sometimes given as “Temperature in the shade”. If you are in Phoenix, in the shade and the temperature is 90 then set the tire pressure with the tires in the shade. If in Montana in Dec and it is 15F with snow on the ground you can set the tire pressure then.