RV Tire Safety: More on tire cold inflation vs. “set pressure”

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By Roger Marble

I got this question from a reader of an RV Forum:

Thank you for all of your informed comments regarding proper tire care. I need one clarification. I have always considered the cold psi on the side of my 22.5 RV tires to be the minimum to carry the maximum rated load, but have assumed that psi was also the maximum COLD psi the tire should see. From your recent post, am I to understand that unless the tire states that it is the maximum cold pressure, I can exceed it by 5-10 psi?
Thank you for your time, Doug

My answer:
The wording on a tire sidewall, IMO, was written by lawyers, not engineers or users. Info on the sidewall is the inflation needed to support the Max load. The difficulty is that few understand that the pressure changes with temperature and the only meaningful pressure measurement is when the tire is “cold.”

This still confuses some because some want to apply chemistry lab practice of adjusting to theoretical 72.5° F when what “cold” really means for tires is at “ambient temperature” and does not include any pressure buildup. In real-life terms this means “Not warmed by being driven on or in direct sunlight for the previous 2 hours.”

Now we need to address what is meant by “cold inflation” vs. the psi to set your tires to or what I like to call your “set pressure.”

I like to suggest the “set pressure” for motorhomes to be the minimum needed to support the maximum load on the tires PLUS 10% inflation.

RV trailers are different because of their interply shear problem. For RV trailers, I would like to see a minimum of +15% load capacity over the measured heaviest loaded tire, with +20% to +25% reserve load capacity being better. Sadly, most RV trailers come with tires that provide +0% to +10% load capacity vs. GAWR (gross axle weight rating).

NOTE: I am not even addressing the tendency for most RV owners to overload their tires.

So for trailers I try to simplify, as follows: To lower, but not eliminate, the interply shear problem I suggest the “set pressure” when the tires are “cold” to be the pressure on the tire sidewall. BUT I still want trailer owners to confirm they have at least 15% “reserve load” over their measured scale reading.

 

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

 ##RVT962

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28 Comments
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Steve
1 month ago

Goodyear Tire Company Recommends that trailer tires should be inflated to the maximum pressure stamped on the side of the tire. Trailer tires need stiffer sidewalls because of sway and bounce. Load the trailer properly and there shouldn’t be a problem. I think you may confuse some readers by not keeping it simple.

Roger Marble
1 month ago

OK I clearly messed up with this post. I guess I sometimes assume people understand the tire related terms that I use. I am writiing a new blog post and it will be available by the end of this week and RV Travel will have it available by this weekend (Aug 29/30). So if you can hang in there I will try and do a better job that will make things clear on “Cold Inflation Pressure”

Jeff Craig
1 month ago

Thank you for this article! I’ve has 22.5″ Michelins on my Class A for 11 years now, and recently learned about the load tables on their website (from another article you wrote) so this ‘cold’ info helps me with inflation. Turns out they’ve been running over-inflated since I bought the rig. It’s always been set to 95PSI, when the tables and weight on the scales show they should be at 88PSI ‘cold’ on all six tires, with our usual load out..

David
1 month ago

So, as a trailer owner I should pump my tires to the max pressure listed on the tire. Right? What do you mean by “BUT I still want trailer owners to confirm they have at least 15% ‘reserve load’ over their measured scale reading.”?

Roger Marble
1 month ago
Reply to  David

OK so lets assume your trailer tires say Max Load 2,000 at 80 psi. So you inflate to 80 but when you get on a scale that measures each tire (such as at Escapees or FMCA etc) you learn your actual load on the heavy end of your axle is 1850#. Now that is less than the tire MAX but it is telling you you only have 150# Reserve which is 7.5%. RVIA recommends a minimum reserve of 10% You might want to review this post to learn more about Reserve Load https://www.rvtiresafety.net/2018/12/safety-factor-or-reserve-load.html

David
1 month ago
Reply to  Roger Marble

Your comment and the linked article were very helpful. I wish there were places where one could measure the actual load on each tire other than at these RV events that are never near me. Is there a way to replicate the tire loads some other way?

Roger Marble
28 days ago
Reply to  David

You really only need individual tire position loads once then you can use a Truck Scale to watch for weight creep on each axle. You can look for scales at Gravel pits, Grain & Feed stores, Some State Police can help if you are not in a hurry.

TIM
1 month ago

I read the tire side wall as stating; The minimum cold pressure to achieve the maximum load rating. Many people will argue it is the maximum pressure but anyone should see if they read carefully it doesn’t say anything about maximum pressure.

That said, the answer given in this article is vague and doesn’t give clarity.

John
1 month ago
Reply to  TIM

Agreed… If we could be more clear and give examples it would surely help us “non engineer” types. I keep my tires at different “cold” pressures depending on where we are or where we are going. If I am in Death Valley my tires are at lower pressures than in North Dakota in the winter. I have seen tires add more than 15 PSI on the road in hot weather. When its cold I add more PSI. Still… if my tires say 80psi at maximum load is running them at 72psi cold too low? I have scaled my RV and calculated that my tires are just under maximum load.

Roger Marble
1 month ago
Reply to  John

Once you learn the actual load on your tires in your motorhome and consult the Load & Inflation Tables to learn the MINIMUM inflation needed to support that load you should never run with a lower inflation. If you measure 76 but the table says 80 then you need to add inflation to get up to 80. You should NEVER start out with a lower pressure and plan on running heat to increase the pressure to the number you want.

Trailers are not the same as motorhomes, and normally should inflate to the pressure molded on the tire sidewall. You should be checking inflation not when the tires are hot or in direct sun but when they are at Ambient temperature which means not driven on for previous 2 hours.

Marcia
1 month ago

So you are talking about trailer tires only right?

Roger Marble
1 month ago
Reply to  Marcia

No. People with Motorhomes or Trailers should not run lower inflation and depend on the traveling heat to increase the tire pressure.

dkjen
1 month ago

Purchased 4 GOODYEAR® ENDURANCE SPECIAL TRAILER (ST) TIRES from Discount Tire in May for my 1975 Avion TT. I was looking for load range C or D but it appears they only come in load range E.
Goodyear’s site has a tire finding app and only shows the E when you use it.
I had used Marathons the prior 6 years with good results and was happy to buy Goodyears again even though I did not need E carrying capacity. I was very surprised to find Discount Tire have inflated my tires to 80 PSI (max pressure) when my wheels are stamped rated for 55 max psi. I had no luck finding the Load/Inflation tables for these tires on the Goodyear site. I finally called their help line. I first talked to a guy who had no idea what I was talking about (this is GY’s customer support team). He finally asks other support folks and eventually was able to get me a pdf showing load/psi info. There is plenty of carry capacity for my Avion at 55 psi.

dkjen
1 month ago
Reply to  dkjen

part 2
I am under the impression from the Discount guy that GY is just making the load range E now. If your wheels are not rated for 80 psi and you have the Endurance tires you need to check the load table to see if you have adequate carry capacity. 

Ed Fogle
1 month ago

I’m confused. You seem to be bouncing from tire pressure to load rating in the article. Are you saying a trailer tire needs to be rated for a weight 10-25% greater than the maximum actual weight at the wheel? Or were you talking about pressure? I couldn’t follow.

As to adjusting cold pressure for departure from standard, for tires anyway, I believe it is correct to adjust for this. I have read many times to adjust 1 psi per 10°F for car tires that are normally in the mid 30 psi range and 2°F for higher pressure tires such as those on motorhomes which run closer to 100 psi.

Thanks

Tony d
1 month ago

If possible, can you provide a simple ‘laymans’ explanation? Thank you

Tommy Molnar
1 month ago

So, should I fill my travel trailer tires to the “max tire pressure” for the max load in weight? Or underfill a bit to allow for the increase in pressure once I hit the road and run at 60 mph in 90 degree heat?

Dan
1 month ago
Reply to  Tommy Molnar

Fill to the max cold psi on the sidewall. The tires are engineered for the expansion due to heat generated and ambient temperature.

Roger Marble
1 month ago
Reply to  Tommy Molnar

No you NEVER should plan on the higher “HOT” pressure to get to the tire pressure want. All tire design engineers know that tires heat up when running and we take the associated pressure increase into account. We only talk about “cold” pressure which really means Not Warmed Up from being driven on or being in direct sunlight.

Tommy Molnar
1 month ago
Reply to  Roger Marble

Thanks Roger!

TIM
1 month ago
Reply to  Tommy Molnar

The maximum pressure is not given on the sidewall, read carefully.

Tommy Molnar
1 month ago
Reply to  TIM

I guess I didn’t explain what I meant. There is a pressure listed that gives the max load at a given pressure for both single tires and dual tires.

Last edited 1 month ago by Tommy Molnar
peterb
1 month ago

I need a little help understanding also

Roger Marble
1 month ago
Reply to  peterb

OK what is a specific question? “Set Pressure” is the pressure you want to inflate your tires to when they are not warmed up. Some folkes call this their “Cold Pressure”. I think the problem is some people think “cold” means in the refrigerator or when on ice. “Cold” for tires means not warmed up. “Cold” for tires means at Ambient Air temperature which is the temperature in the shade after a tire has “cooled down for at least a couple hours after being driven on or in the shade for at least a couple hours.

Does this help?

Melvin Bowden
1 month ago

I think you need to explain your explanation. Maybe you could define some of the terms you are using?

Roger Marble
1 month ago
Reply to  Melvin Bowden

Maybe the term “Ambient Temperature” is defined as the air temperature of any object or environment where equipment is. A tire is considered to be “cold” when it is at Ambient air temperature. “Interply Shear” and “reserve Load” are defined if you follow the links. If there are more questions, let me know.

BrianR
1 month ago

Ok, I’m confused. It’s still early so I’ll read it again after another cup of coffee 🙂