What inflation should I run on my Tow Vehicle and is my RV trailer different? draft

 I monitor a number of RV forums and when I see someone asking about tires, I will make the effort to learn about the problem and offer possible solution.

Recently there was a question on inflation for both the TV and the TT. The comments seemed to be all over the place so I decided there was a need for some basic foundation of information needed. Following is my reply and attempt to provide the basic information needed by RV owners.

TV  Tire size, type and inflation should go by the Certification Sticker on the driver door jam. This was established by a team of tire and vehicle suspension engineers over a period of two to 4 years with numerous changes and tests run on different specification and performance parameters that were fine-tuned to meet the goals of the vehicle design team. The sticker provides a recommended inflation that normally delivers 30 to 35% Reserve Load so those numbers should be followed unless you have done a lot of research and understand the trade-offs. Do not forget that few people in the tire store have technical training beyond how to sell tires, so I would be careful with the information they provide.

TT  Tires for almost all RV trailers have a single spec. That being Low Cost. I have never heard of any vehicle testing or evaluation that compares different tire constructions or ratings for the application of a tire to the TT.

 To achieve low cost , tires are only required to meet a single requirement: That they meet the minimum load capacity specification needed to support the RV. In 2017 RVIA established a Reserve Load of 10% but for RVs built before 2017 there was no Reserve Load requirement.

 

Those of you that have read the owner's manual and reviewed the Certification Label information and the Load and Inflation information molded on the tire sidewall may have noted that for most RV applications, you can only achieve the tire's 10% Reserve Load capacity when inflated to the level required for the  maximum load capacity.

Yes this basically means you must run the max inflation for the tire's Load Range if you want to have the minimum margin of Reserve Load recommended by the RV Industry association.

RELATED INFO. The words "Max Inflation" on the tire sidewall is NOT the highest inflation the tire can tolerate, but is the highest inflation that will provide the greatest load capacity of that tire. The published Load & Inflation tables show the direct relationship between inflation and load capacity. If you want to increase load capacity you MUST increase the inflation. BUT once you reach the stated "Max Cold Inflation" there is no further increase in load capacity available for that tire, even if you increase the inflation above the number molded on the tire sidewall.

In the subject post there was some confusion about the Michelin "LTX" line and one poster thought this was some special "Hybrid" type of tire so felt the need to clarify tire "TYPE" nomenclature.

The letters before the numbers in the tire size is critical and important but too often people seem to ignore that important info. The P is for Passenger, LT is Light Truck, and ST is Special Trailer.   ST should only be used on trailers and are not approved for use on passenger carrying motor vehicles. if a P type is applied to a trailer its load capacity is to be reduced by dividing the load capacity stated on the tire or in the Load & Inflation tables by 1.10 but if an LT tire is applied to a trailer no reduction in load capacity is required.

A few days later the following was added to the Forum thread

"------------------ Steer Axle ---Drive Axle --- Trailer
Weight #1 - Truck only/hitch/bars -- 3180 3520 (exceeds 6500 GVWR)
Weight #2 - Truck and Trailer with WDH bars -- 2980 4320 4880 "

Side note the owner had previously stated the TV GVWR was 6500 so his scale reading so the initial scale reading seems to exceed his GVWR !

My response

I am assuming the "Trailer" scale was for the 4 tires on RV. If so it sounds like the trailer was empty when you did the weighing.

While waiting for a response I saw a comment about Michelin tires

"- It was helpful to learn of both the Michelin "hybrid" LTX and the Cooper XL alternatives to LT tires although when I looked up the Michelin LTX I saw that it comes in A/T and A/S versions ... even more confusion ... is the A/S the "hybrid" and the A/T the real LT??."

 

 

My response

I'm not sure what you mean by "Hybrid". Michelin has a "line" of tires it calls "LTX" this includes both passenger type and LT type tires.

Think of this as with General Motors who has a "line" of cars called Chevrolet with many type vehicles from sedans to SUV and pickups.

In my post I covered the 3 basic families of consumer tires Passenger, Light Truck and Special Trailer.  Each "family" has many different sizes and many different tread patterns like the A/T that is a traction tread pattern and the A/S which is All Season and provides less off road or snow traction.

There are also within each family different levels of Load Capacity.

P type "family" tires come in "Standard Load" that has no special markings and also XL for Extra Load.

LT and ST "families" have greater load capacities so they have letters assigned to the "Load Range" or LR of short with the different LR having letters starting at C, D, E and on up in some brands. Each letter has a specific maximum load capacity at a specific inflation and the load capacity is published in tables available in some owner's manuals or tire data books or even on the Internet as seen HERE.

All the above applies to tires made to US Tire & Rim Association standards. Europe has some similar but different standards with different codes. So do tire made to the standards in Asia. I will not go into those standards as I probably have overloaded you with the above.

Clearly the owner of the truck and trailer needs to learn more facts about tires and proper loading of his TV truck.

I am wondering how many readers would be able to offer advice to this fellow RV owner.

Can you change tire size or type on your RV?

 As an actual Tire Design Engineer I can assure you that there is more misinformation or partially correct information out there on the internet, than technically accurate information.

It is true that the original tire selection is the responsibility of the RV Mfg. The issue is that once the RV is sold it seems that most RV Manufacturers have little or no interest in standing behind their choices with any actual warranty service when it comes to tires.

It seems that OE tire selection for most RVs is based on one goal. Find the smallest, lowest cost tire that will meet the requirements.

The only Federal (DOT) requirement is that the tire load capacity, times the number of tires on the axle, be AT LEAST equal to the maximum load rating of the axle. While RVIA now requires 10% Reserve Load capacity, DOT does not. As a point of reference most cars come with a 20% to 30% or higher Reserve Load capacity

A smaller tire can mean the RV Manufacturer can get away with less costly (smaller) wheel and maybe a smaller wheel well so this is extra pressure on purchasing Dept to get the minimum possible tire that can meet the requirements.

Given the above, it is up to you, the owner, to decide if you want any, some or more "Reserve Load capacity" for your RV. You may have the option of larger tires or you may be restricted to trying to find tires of the same dimensions but with higher load capacity.

You need to educate yourself about the requirements and limitations of the four "types" of tires that are in the market.  'P" is Passenger type. If used on an RV (trailer or motorhome) the load capacity Must be reduced by dividing by 1.1 but not everyone will know or do that. LT type can be used in RV service but you will soon discover that LT tires with the same dimensions and Load Range (ply rating) have a lower load capacity than the same dimension ST type.

ST type have the highest load capacity rating for a given set of dimensions, but you need to remember that the ST tire Load formula that is used to calculate the tire load capacity is based on an assumption of a 65 MPH Max speed. We all know that there is "No Free Lunch" and the trade-off for increased load capacity is lower speed capability. The "Speed Rating" symbol on many ST type tires is based on a 30 minute test so you need to decide if you want to depend on such a short term test when making a tire selection.

Finally there are actual "Truck / Bus" tires. These have no leading letter and are usually on 17.5" or larger wheels. These tires have higher Load Range, usually F or higher These tires are almost all rated for 75 MPH in RV use on the highway.

Do your homework. Ask questions, but remember there are very few really knowledgeable people out there who have the training or experience in tire engineering. Just having driven on tires for 40 years is not the same as having been held responsible for designing tires for Truck, Passenger, Trailer, or Indianapolis racing application. Also, being able to read Federal Regulations is not the same as having to work within those regulations while meeting the goals and demands from GM, Mazda, Toyota, Honda, Freightliner, MB, Nissan, Ford, or Chrysler.

I am only aware of two actual Tire Design Engineers who regularly post on various RV Forums.

##RVT1000

Another post & question of "Trailer tires vs Truck tires"

Original post and question on a 5th wheel RV forum I follow.
"Searched this forum (no, didn't search travel trailer forum) for any info on using LT (truck) tires instead of "trailer" tires on a 5th wheel.
Nope, didn't find anything - So,
Any problems with using a 6 or 10 ply LT tires on my '99' trailer instead of the replacement tires for trailers?"

He gave the dry weight and stated GVWR but no actual scale readings. He also did not provide information on his current size or type or Load Range of his tires. He did provide info heard around various campfires. In the forum, there were a number of replies. A few OK and sadly a number just plain incorrect.

In the'60's RV trailers were mostly smaller single axle and towed behind a car or 1/2 ton P/U. RV companies wanted less expensive tires. Speed limits and actual travel speeds were lower so Goodyear came up with "Special Trailer" tires with a 65 mph max speed as part of the load formula specification. This speed reduction and decreased tread depth theoretically offset the increased load capacity when compared to the same dimension LT type tires.

In 2002 after the Ford Explorer fiasco, new tougher standards for tires were implemented by DOT. RV companies fought to keep the old test requirements for ST type, the same as they were in 1968, while P-type and LT-type had to meet new, tougher standards for the 21st Century.

In 2017 China was accused of "dumping" cheap tires of all types into the US market. Trade restrictions by FTC (not tire safety standards) identified speed-rated tires as not having to pay the import duty. Almost overnight almost all ST type tires became speed rated. The SAE test for speed rating is stated as a "passenger car tire test" but was applied to ST type. It only requires tires to run for 30 minutes at the stated speed and to not come apart.

Back to the OP. "Dry weight" and GVWR are of no value when selecting tires. GAWR is almost useless as it is well documented that a majority of RVs have tires and /or axle in overload when actual loading is measured on scales.. The only weight number that really means anything is the actual scale reading for each tire, as RV weight is almost never exactly split 50/50 axle to axle or side to side. Some big RVs have been found to be 1,000# or more out of balance.

Reserve load is the extra load capacity above the actual load. Cars and P/U have reserve load in the 20% to 40% range while RVs have 0% to maybe 10%. That is a MAJOR reason for the difference in durability. There is also the shear forces seen in trailers that can be 24% higher than an identically loaded tire on a motor vehicle. This is a function of suspension dynamics. Tire Interply Shear was a complete unknown back in the 60's and not as well known or understood even in the 80's. You see the large tread distortion due to lateral loading when backing into a parking spot when the tire bends sideways. You never see this bending on a motor vehicle. The shear is always there in every curve or turns not just when backing into a parking spot. Even the normal sway observed when a trailer is towed down the highway is generating high Interply Shear. You can lower, but not eliminate the Interply Shear by increasing your Reserve Load.

So how do you improve your Reserve Load? An increase in load capacity would be a good approach. For most trailers, there are a number of options. Increased Load Range is one as long as you confirm wheel limits on load & inflation. Changing to larger tires. Even changing to larger diameter wheels might increase load capacity. I have seen some that increase all three specs, with reports of eliminating all tire failures other than road hazard or valve-related issues that can occur on any size, brand, or type tire.

If or when you replace tires the new tires should ALWAYS at a minimum have equa load capacity.

I have covered the above in numerous posts on my blog if you care to learn the facts from an actual tire design engineer.

Or you can listen to the guy in the camping space next door or the salesman at "Billy Jo Bob's Cheap Tire and Bait Emporium".

##RVT954

Remember: A tire is just a "tool" you use to get a job done

This in response to a report of tread cracking of some traction design tires placed on a custom heavy car hauler trailer. The owner was blaming the 4-year-old tires.

I do note that when there are tire problems, including actual "failure," it's common for some to say "My xxx brand tires failed, I will never buy xxx tires again".

Well, sorry to tell you but there is no such thing as "Fail-Proof" tire. This was even part of the testimony mentioned by DOT spokesperson during the Ford Explorer rollover fiasco of 2000.

Today's tires are amazingly robust. Even when they are made in Japan or Timbuktu, and we all remember how bad "Made In Japan" was when were growing up. As I pointed out in the thread, the subject tire an All season traction design was not at all appropriate for heavy trailer application. Both the tire type/size was wrong and the tread pattern was wrong.

Why is this the tire's fault?

If you put a truck tire with a heavy off-road mud traction tread design on the front of your 40' DP and had loud noise and vibration and harsh ride would that be the tire's fault? Would simply changing tire brands from say Bridgestone to Michelin solve the problems if you selected just another heavy off-road mud traction tread design? No of course not.
From my experiences as a tire engineer, I can tell you that I can probably "fail" any tire in under an hour and under 50 miles if you let me set the conditions.

A tire is just a tool you use to get a job done. If you don't select the correct tool that is appropriate for the job you want to be done why is it the fault of the tool manufacturer? Think of the absolute best tool company. Now select one of their flat blade screwdrivers.
 OK now start using it as a chisel and pound on it as you try and cut through some rusty bolts. After cutting through a few bolts would you blame SK or MAC or  Snap-On or ???? if the point of the screwdriver is dented and chipped?

Different tires are just tools with different intended usage.

Basically I would suggest "Rib" or "All position" designs for the front position and if you intend on occasional off-highway travel (a gravel road at a campground doesn't count) then you could select something with a bit more traction capability but I would consider any "Drive" position as not a great choice for Motorhomes or trailers.

Selecting alternate or replacement tires for large / heavy trailers

Found a thread on a forum for folks who own large heavy 5th wheel trailers. This info would apply to non-5'ers too.

While I understand the concern for the tire dimensions, that is NOT the most important specification.
Number one is to ensure any replacement tire is capable of supporting the load you are placing on your tires plus a margin.

The best thing to do is to first confirm your actual tire loading.
Ideally, you would get on a scale, with the RV loaded to the heaviest you ever expect to travel with, and learn the actual load on each tire as there are very few RVs with the load split evenly axle to axle or side to side.
HERE is a worksheet you can use.  You will have to do some hunting around as you can't get individual loading on most truck stop or CAT scales. You will need to find a local building supply or feed or grain dealer or gravel pit or possibly cement delivery company.

Lacking that you could use a truck scale but to be safe you need to apply some math to estimate the load unbalances.
First, assume a split of 52/48 between axles or with a three axle trailer assume one axle is supporting 35% of the total. Then assume a 53/47% split on the heavier axle for side to side loading.  Yes, some RVs have been measured with individual position scales and found 1,000# un-balance.

So with the measured or calculated heaviest loaded tire, and the dimensions checked, you are ready to shop for tires.

You need to realize that ST tires have a higher load capacity than LT type tires. This is because the load formula for ST type tires is based on a max speed of 65 mph even if the "Handling rating" speed symbol suggests differently. So you can't just use the numbers when comparing tire sizes as an ST235/75R16 carries significantly different load than an LT235/75R16  even with the same Load Range ( D or E or F etc)

You can then consult the Load & Inflation tables for the tires under consideration. The good news is that with the exception of Michelin 99+% of the tires out there follow the same table info so you can use Bridgestone or Goodyear etc for LT and Maxxis or Goodyear for ST type tires.. You can look at different tables HERE if you want.

When selecting a tire you need to get the tire capacity at least 15% greater than for your measured or calculated tire load. This allows for sway, load shift due to road crown and wind side load to the tires you are buying.

After you do the above THEN you can confirm tire dimensions knowing the load capacity needed.

On my blog, I cover why you should run the inflation number molded on the tire sidewall (lower the Interply Shear) and why you should always run a TPMS along with other info on Interply Shear and the effect of temperature on tire pressure.  You might even subscribe.

Hope this helps.

What inflation to run when changing Ply Rating or size?

Following a few different RV Forums, I see a number of people, that clearly do not follow this blog,  have questions on what inflation to run. While many of these folks are discovering that the new Goodyear Endurance line seems to come with higher Load Range or only in larger sizes they end up not knowing what to do about inflation.
In one case the person didn't even know they were buying a larger size so I do wonder sometimes how people managed to ever buy tires for their car if all they seem to know is that tires are round and black and seem to cost too much.

When you replace tires on any vehicle, you should always be sure the load capacity of the new tires is equal to or greater than the capacity of the original tires on the vehicle.

Regular readers of this blog know that load capacity depends on BOTH tire size and INFLATION level. You also know that there is a "Tire Placard" or "vehicle certification label" on or in all your vehicles that identifies the original size tire and the inflation recommended by the manufacturer. In the case of RVs the label also should tell you the GAWR which is the maximum load you should ever have on each axle confirmed by scale readings.

So back to the original question of what inflation to run ?

 .

Example 1:  There was a change in tire size but the Load Range stayed the same.
Original tires were ST205/75R15 LR-C and you mistakenly bought ST225/75R15 LR-C
 A quick check of a Load & Inflation table for ST type tires we see that the ST205  is rated to support 1,820# @ 50 psi and the ST225 can support 2,150#@50 psi. This is a nice 18% increase in load capacity.  I would suggest that in this case the owner continue to run 50 psi and enjoy better tire life.

Example 2:  There was a change in tire size AND in Load Range as can happen with the new Goodyear Endurance.
 Original tires were ST205/75R15 LR-C and you mistakenly bought ST225/75R15 LR-E
Back to the Load & Inflation table for ST type tires we see that the ST205  is rated to support 1,820# @ 50 psi and the ST225 can now support 2,830# @ 80 psi. This is a 55% increase in load capacity.  If the owner were to run the 80 psi as indicated by the tire sidewall the "ride" would probably be hard on the TT. In this case I would not follow the inflation on the tire sidewall. I would suggest that in this case the owner continue to run 50 to 55 psi and enjoy better tire life. In this example the owner expressed concern about running lower inflation than marked on the tire. Some people even incorrectly said that running less than 80 psi would somehow overheat the tire. I pointed out that I saw no problem with running less than the sidewall inflation as long as the actual load had been confirmed with actual scale readings and that there was a good margin of capacity over the actual loading.

Example 3. A Class-A owner wanted to change from a Michelin 275/80r/22.5 to a Firestone FS 591 295/75r/22.5. His reasoning was two-fold. First he was concerned about the historical reports of  delays in getting the sometimes hard to find the Michelin size and second was the significant for him, cost difference.
Both tires were LR-H . The owner did a good job of consulting the tire manufacturer's data sheets to learn the actual tire dimensions. One poster had offered a generic tire size comparison web page but the numbers the page generated were clearly different and could lead to problems.
The load capacity for the Michelin and Firestone tires is 7,160 @ 120 The owner had also confirmed the dimensions were acceptable for the application.

Tow vehicle tire question

Dear Roger,

"Last week I purchased new E rated tires for my F150. Went with the BF Goodrich KO2 (275/65/18) and so far I'm very happy with them. These are larger than the tires that came from the factory (265/60/18). I have a quick question.

"The tire shop originally put the PSI at 35 based on the door sticker, but after reaching out to BF Goodrich they suggested 45 PSI for everyday driving. They didn't give any recommendations for tire pressure while pulling a trailer since there are so many variables like trailer size, weight and cargo. The tire shop suggested going to 50 pounds and then deflating back down after the trip. I'm just curious if others actually bump up their PSI while towing, and have you noticed a difference in doing this? Just trying to figure out if there is a benefit to doing this.

Some important information is missing, so let's identify our assumptions:

1. We are talking about the tow vehicle tires - the F150.

2. The original poster didn't identify the OE (original equipment) tires as being P type or LT type. This makes a major difference and providing that information up front will eliminate guesswork for those trying to help with the answer. With a 35 psi door jam sticker number, I suspect the OE tires are Passenger type and the replacement tires are LT type.

3. Since it is the air pressure and not the tire construction that supports the load on a tire, buying LR-E (80 psi) tires when you never run higher than 65 (LR-D) would be a waste of money. Also if you are changing from P type tires with a max inflation of 36 to 41 you need to confirm the wheels are rated for higher load and inflation of LT tires.

4. The door jam pressures are based on the car company making some estimates on how much and how often you have the vehicle empty or fully loaded. You might check the owners manual and see if they give an empty and loaded tire inflation suggestion. The inflation in the OE tires is what is needed to support the GAWR but few people run that heavy all the time with their pickup.

5. I suggest you get the F150 on some truck scales when empty and again when fully loaded with the trailer also fully loaded, and learn the real facts of the various axle loads under both conditions.

6. Knowing the real loads, you can use the tire Load & Inflation tables to learn the MINIMUM cold inflation pressure for the F150 for the two situations. I suggest you use a margin of +10% of the table inflation to establish your CIP (cold inflation pressure). See other posts on weight and inflation if you need more details.

7. Trailer tires are a completely different situation and should always be inflated to the tire sidewall inflation. You should also run no more than 85% of the trailer tire max load rating when on a scale. Cornering, sway and side wind loads have been shown to shift loads side to side by 10% or more. Also the Interply Shear on tires in trailer application needs to be considered as this is a major contributor to trailer tire belt separations.

Hope this info helps clarify.

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##RVT781