[Long] Overheard Interesting Campfire Chat About Tire Load/Inflation

[Theo-FRF]

This summer, while trying to fall asleep in the bunk of our Roo, I heard the campfire chat going on at our neighbor's campsite. It sounded like a rather loud and sonorous RV "expert" was imparting his "wisdom" to several newer RVers.

The part of the chat I heard concerned how he determined the lowest tire inflation pressure that he could safely use with his rig. IIRC, his algorithm was to take the CAT Scale (WDH-connected) trailer axle weight of the TT and divide it by 4 to get the load each tire must support for the current trailer's weight. Then, given that load value, look up the psi-setting required to support it.

Well, I've always set my tire pressure to the TT's tire and loading sticker. In my case its 65 psi.

However, after remembering this overheard chat, I decided to see what the difference in psi would be using his method. I went back through my CAT Scale tickets and found one when we were loaded within a couple hundred pounds of the Roo's GVWR. The ticket below shows weights with the WDH attached.

When we ditched our old Castle Rock tires, we replaced them with GY Endurance 215/75R-14 tires. Their Tire Load/Inflation Chart is below:

At 65 psi, each of our tires can carry 2200 lbs. Multiply that by 4 and (according to the "expert") our tires can support an 8800 lb axle-weighted trailer. That's PLENTY for our hybrid's needs.

However, according to the "expert's" algorithm, at the 6000 lb CAT Scale ticket's Trailer Axle weight, each tire needs only support 1500 lbs. Referencing that load in the Endurance load/inflation chart, it appears that we could run our tires at 35 psi!

I am certainly NOT a tire expert, so I find this all quite confusing. Is this "expert" correct? The difference between 65 psi and 35 psi seems to be causing me some cognitive dissonance.

Thanks for any help that can given!

 

[TitanMike]

Your best course would be to follow what the factory put on the sticker found side of the trailer.


This and use good tires.

 

[Theo-FRF]

Your best course would be to follow what the factory put on the sticker found side of the trailer.

This and use good tires.

Yep. 65 psi (as on TT sticker) and Goodyear Endurance.

But is the overheard "expert" technically correct or is his algorithm bunk?

 

[Mike134]

Yep. 65 psi (as on TT sticker) and Goodyear Endurance.

But is the overheard "expert" technically correct or is his algorithm bunk?

The truth falls some place in-between.

Take my TT for example. OEM tires where Castlerock 205-14s trailer sticker says 50PSI.

Put on aftermarket tires Goodyear Endurance with a MAX fill of 65 PSI. I rode them about 15,000 miles at 60 psi Treadwear was only in the center of the tread which as any knowledgeable tire person will tell you is a sign of over inflation. Still have some tread left and will be running them at 55 to see how they wear.

How they wear is your best indication if your over inflated or under inflated. Either condition puts less tread on the pavement under heavy braking.

 

[TitanMike]

The truth falls some place in-between.

Take my TT for example. OEM tires where Castlerock 205-14s trailer sticker says 50PSI.

Put on aftermarket tires Goodyear Endurance with a MAX fill of 65 PSI. I rode them about 15,000 miles at 60 psi Treadwear was only in the center of the tread which as any knowledgeable tire person will tell you is a sign of over inflation. Still have some tread left and will be running them at 55 to see how they wear.

How they wear is your best indication if your over inflated or under inflated. Either condition puts less tread on the pavement under heavy braking.

My trailer came with same tires/size and recommended pressure.

I replaced with GY Endurance LRD tires. To determine what I considered the best pressure I made 1" wide chalk marks across the tread face (only the first two ribs) and down onto the sidewall an inch or two. Starting at 50 psi and trailer loaded as I travel (I'm a full wanter tank traveler) I observed how much chalk was worn off and the highway stability. I then observed where the chalk had worn off in relation to the edge of the shoulder. For my load it was dead on and chalk line end matched edge of shoulder but if line wears off more toward sidewall, add 5#, re-chalk and repeat. If chalk remains on tread, reduce and check in 5# increments.

Don't ever go below the mfr's load/inflation table's min cold inflation for tires load.

 

[Chuck_S]

The past year or so I've come to agree with the (lets call him) Campfire Expert.

When I put new Goodyear ST tires on my Roo in the summer of 2023 I thought I was going to hammer the trailer to death on the road. Just bringing the camper home from the tire shop (7 miles): Cabinet doors came open, dishes and other items all over the floor, etc. Bought auxiliary latches, etc. and after a week long trip checked the inflation information (same as posted above) and determined this doesn't make sense to have tires inflated to support 1300 pounds more than the trailer's GVWR -- and only 90% of that is on the trailer wheels.

Reduced pressure from 65psi to 60. ... and eventually down to 50 ("fitty"). Several hundred miles keeping track of tire temperature and pressure and while I've determined 45psi is sufficient I'm keeping them at 50. Trailer rides like it should. Dishes and other items remain in their cupboards. I don't hear it banging back there.

BIG mistake some make is fitting oversize or the next "load range up) when they replace tires. The OEM tires on current trailers are grossly over capable -- avoids warranty claims -- and so what? if you don't check tire pressure. Recall I noted 45psi was fine for my trailer.

I apologize to those in the past I've inferred were idiots for not running maximum sidewall or placard pressure.

-- Chuck

 

[NXR]

The Campfire Expert is lacking in expert knowledge.

Load inflation charts assume a perfect balance between the left side and the right side. For a car that's not a bad assumption. For an RV it can be a horrible assumption.

It's not unusual for a motorhome to have up to a 10% difference from one side of the axle to the other. When you use a CAT Scale axle weight and divide by the number of tires on each axle the result is the average weight on each tire.

So if there is a 10% imbalance and you use the load inflation chart number, which is the MINIMUM pressure for that weight you end up with one tire over-inflated and one tire under-inflated.

And that puts that tire below the minimum cold inflation pressure.


Example:

6,000 lb scale weight with two tires.

3,000 lbs per tire.

But with a 10% imbalance one tire actually is supporting 3,300 lbs and the other is supporting 2,700 lbs, a 600 lb difference.


A friend sent me his Grand Design fifth wheel weights where he got a "four corner" weigh at a rally, so to speak.

I about fell off my couch.

From memory, he had like a 28% weight difference from one side of his front axle to the other side of his front axle. The rear axle was somewhat better but still ugly. His pin weight was 23%. Yes, two side by side axles had different left-right balances.

When I pointed that out he went to the MORryde booth for their take. They shrugged and said it's not uncommon, especially with a Grand Design. He's done no mods and has no heavy options. The thing is just lopsided by design.


The "run the sidewall number" recommendation is for a trailer with more than one axle. The reason is called interply shear. When making a turn the tires do not steer and are kind of dragged through the turn. That's also different from a car. That causes a lot of sidewall stress. The purpose of running the sidewall pressure is to reduce the stress on the tire.

Does it fit every situation? Of course not. But often we blame tire pressure for ride problems when it's really the (lack of a good) suspension system.


While his theory may sound fine there is an old saying that fits here:

In theory, theory and practice are the same. In practice they are not.

Ray

 

[NXR]

I found that "four corner" 2019 model year Grand Design weight ticket and it's attached. Let's try the Campfire Expert's technique and see how well it works.

Front axle driver: 2,470 lbs
Front axle pass: 1,790 lbs
That's a 27.5% imbalance from one side of the front axle to the other.


Rear axle driver:2,440 lbs
Rear axle pass: 2,030 lbs
That's a 16.2% imbalance from one side of the rear axle to the other.


Total driver side: 4,910 lbs
Total pass side: 3,820 lbs
That's a total side-to-side imbalance of 22.2%, a whopping half-ton plus difference.


Total of all tires: 8,730 lbs.

8,730 divided by four: 2,183 lbs average weight on each tire.

Using the average puts both driver side tires way under-inflated, 12% on the front driver's side and 10% on the rear driver's side.

A tire that is under-inflated for the weight it's actually supporting at the current cold inflation pressure is considered flat when 20% low. Not under-inflated, flat, and susceptible to internal damage.


This one is a particularly bad example, consequence-wise. If one driver side tire goes low or actually flat from hitting some debris AND the owner does not use a TPMS they may not even know, especially if the tire did not go BANG.

There are two tires on each side because two tires (and two axles) are required to support the maximum design weight. When one tire goes flat the weight it was carrying is transferred to the remaining tire on that side.

That means the remaining tire on that side (or corner on a motorhome dual rear wheels) is massively overloaded, even more overloaded than if it had the correct pressure. Arrive somewhere and you get to say "Hey, look we got a low tire" but you don't know if you drove that way for one mile or for fifty miles.

Now you're replacing both tires on that side if you understand that problem. If you don't replace both tires on that side you just set yourself up for a future failure as the damaged tire fails sooner, weeks to months, and then overloads your new tire.

Whenever I read of someone complaining about multiple tire failures in a short period of time i wonder if that was the real cause, a flat on one side only that caused the other tire to overload leater.

And if you follow the Campfire Expert's advice you just set yourself up for future failures even if you replaced both tires on the one side.


But it's your credit card and your inconvenience so you get to do what you want.

And yes, please double-check my math.

Ray


PS: So how can you effectively use a CAT Scale axle weight? Me, I add 5% to the CAT Scale axle weight and only then look up cold inflation pressure in the load inflation chart. That extra 5% helps compensate for the left-right imbalance. It's a WAG, of course.

Then I add 10% to the load inflation chart pressure to help accommodate for area-to-area and season-to-season temperature variations and use that result, as long as it does not exceed the pressure molded into the tire sidewall.

It works for me and that's all that matters to me.

 

[Theo-FRF]

WOW!

Thank you all for greatly clarifying why there is danger in what I overheard that night! It's kinda' been in the back of my mind and I hadn't pursued the issue until I was reminded while taking the Roo to winter storage this week.

There is some very valuable info in this thread's posts! I hope that newbies and veterans alike stumble across its contents. Please feel free to add more to the discussion as info and experiences arise.

The one thing I DO know after only 5-years of RVing is how inexperienced I am. It is so valuable to have a resource like the FR Forums and its seasoned and knowledgeable community members.

I've followed many of you for years and you have significantly added to my RV knowledge and enjoyment!

Thank you!

 

[NXR]

BIG mistake some make is fitting oversize or the next "load range up) when they replace tires.

Maybe, maybe not. If you stay with the same tire size but go up a load range, say from C to D, you have precisely zero more weight-carrying capacity unless you also increase the tire pressure. The air supports the weight, not the rubber. The rubber just hold the air in.

The OEM tires on current trailers are grossly over capable ...

With emphasis on the word "current". Otherwise that is definitely a "maybe". Even for a current trailer it's still a "maybe" and almost certainly not "grossly over-capable".


The RVIA changed their standard for towables built on or after September 1, 2017 to require tires that can carry at least 10% more weight than the axle GAWR, its Gross Axle Weight Rating a.k.a. its maximum design weight.

So if an axle can support 5,000 lbs then the two tires installed must be capable of carrying 5,500 lbs together.

In contrast, a passenger car's tires often have a 20% or more margin. 10% is pretty darn minimum, especially with the left-right imbalance being a known problem with RVs.


For towable RV's built prior to September 1, 2017 the RVIA had no standard. Legally and practically if they had a 5,000 lb-rated axle they could install two tires that together could only hold 5,000 lbs when inflated to the sidewall pressure.

So if you have a towable built prior to September 1, 2017 you may want to dig into just how adequate the OEM tires are. They may be barely safe, and only if you are 100% certain you never overload a tire on that axle.

Per my previous example, it is kinda easy to be under the axle GAWR as reported by a CAT Scale but still be overloaded on one of the marginal tires.

Ray

 

[ErnieSimons]

Typically the sticker pressure is way high for the actual load. With the max pressure when cold the pressure when hot is going to be extreme. On our Super C I use Michelin's air pressure load recommendation with minor tweaks. I'm running 20+ psi lower than the sticker and psi on the tires. I monitor pressure and temps with TPMS and the ride is way smoother than at max..

 

[TitanMike]

Typically the sticker pressure is way high for the actual load. With the max pressure when cold the pressure when hot is going to be extreme. On our Super C I use Michelin's air pressure load recommendation with minor tweaks. I'm running 20+ psi lower than the sticker and psi on the tires. I monitor pressure and temps with TPMS and the ride is way smoother than at max..

Just remember that with TPMS you aren't measuring the actual temps of the tire body itself but rather the temp of the valve stem and whatever air that is residing in the stem.

Tire damage from heat begins in the tread/shoulder area and only a needle type pyrometer can read those temps accurately. Second best method is using an infrared "gun" and measuring temps immediately upon stopping.

FWIW sometimes a nice ride can be detrimental to tire life.

 

[ErnieSimons]

Ok. But here's the facts. The Michelin recommendation is 75 PSI with the load we are running. The pressure on the tire and the sticker is 110. I've settled on running 87/83 after many adjustments. My point is running the max pressure when your load is way under the weight value for that PSI makes zero sense. We have to spend a little time understanding load and tire manufacturer PSI/load recommendations vs just hitting the easy button and maxing the tire pressure out. While talking tires I'll add one more concern. Tire speed ratings. Might want to understand those as well as the recommended air pressures.

 

[TitanMike]

Ok. But here's the facts. The Michelin recommendation is 75 PSI with the load we are running. The pressure on the tire and the sticker is 110. I've settled on running 87/83 after many adjustments. My point is running the max pressure when your load is way under the weight value for that PSI makes zero sense. We have to spend a little time understanding load and tire manufacturer PSI/load recommendations vs just hitting the easy button and maxing the tire pressure out. While talking tires I'll add one more concern. Tire speed ratings. Might want to understand those as well as the recommended air pressures.

Now you just need to know why the vehicle manufacturer put 110 psi on the CERTIFICATION sticker. If you're running well under vehicle GVW you might get away with lower pressures but that would be a bit of a rarity in the RV world. Another factor would be handling/stability and it's not unusual to see max pressures stated due to stability issues revealed during testing.

As for speed ratings, they're simple. It's the maximum speed the tire can be driven and still maintain the performance it was built for. Higher speeds jeopardize the integrity of the tire. The speed rating is independent of road and weather conditions which on their own can dictate a lower speed for safety.

 

[NXR]

To make a blanket statement that the sticker pressure is "typically" too high is too much of a generalization and flat out wrong for vehicles in excess of 10,000 lbs GVWR.

The pressure on the sticker is set by law, FMVSS 120 for vehicles in excess in 10,000 lbs GVWR. That's a federal recall when the vehicle manufacturer gets it wrong, as Tiffin and others have done on occasion.

The correct tire pressure also depends on the actual weight on the axles and the actual weight distribution.

There are several examples where an axle is almost overloaded as delivered from the factory and some DPs had to get axle upgrades because they were so underrated.

It's good that yours has a bunch of margin but that usually is not the case. You seem to understand the concepts except for one thing.

There is no Michelin "recommendation" for tire pressures. The load inflation chart pressure for a given weight is not a recommendation. It is the bare minimum pressure needed to support that weight at the current ambient temperature.

Ray

 

[ErnieSimons]

The vehicle manufacturer puts the max psi on the sticker because they have no idea what load the vehicle will be carrying. So they say pump em' up to max. We have 11,000 pounds of CCC and are 8,000 under that number. Yes I understand speed ratings but clearly a lot of drivers don't and are running over the rated speed. No wonder so many are on the side of the road with a blown tire. No manufacturer is testing these rigs at various tire pressures for handling, lol.

 

[RamManMN]

Yep. 65 psi (as on TT sticker) and Goodyear Endurance.

But is the overheard "expert" technically correct or is his algorithm bunk?

He is probably technically correct i.e. each tire needs to be inflated to at least the pressure to match the weight actually on it. But it's not practical. Is he going to weigh it every time he sets out? Does he believe he can measure the pressure with enough precision to actually make it work? Also, there are too many other variables. Finally, even if it could be made to work, its way too much ofna PITA for me. The sticker on my trailer says 65 psi. The goodyear endurance tires on it says the max load is at 65 psi. So that's what I set them for. As long as I'm not over loading the trailer I'm good to go.

 

[ErnieSimons]

Actually Michelin does make a recommendation. From the Michelin document. "For RV use only, Michelin displays the loads per axle end in the load and inflation tables, as we recommend weighing each axle end separately and using the heaviest end weight to determine the axle's cold inflation tire pressure." My interpretation is Michelin is recommending a cold tire pressure. I could be wrong. Yes this can be a PITA. The range the psi/load covers is not super granular. Our load does not change significantly. 25K pounds +/- 500 lbs is nothing. For us traveling 6+ months, 20K miles per year, tire pressure is really important. Good discussion regardless of conclusions

 

[ErnieSimons]

Also you don't do a per tire pressure. As Michelin states "For control of your RV, it is critical the tire pressures be the same across an axle, while NEVER exceeding the maximum pressure limit stamped on the wheels." Michelin is recommending set both tires on an axle to the highest psi dictated by load on that axle end. With our particular tires we have a 35 psi range so it really isn't difficult. Like I said I've settled on 87/83 which is almost in the middle, gives us a good ride, steer tires handle best, and tire wear looks even.

 

[Oaklevel]

Many opinions..............

The only thing I want to add is take a picture of the sticker on your trailer as in a few years it WILL probably fade out...