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Wednesday, November 23, 2022

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.

Thursday, November 10, 2022

Have a broken wheel stud? Here is what I would do...

 Read a post about someone discovering a broken wheel stud on their RV. The wheel had been changed after there was a tire problem and the reported labor done at the tire shop is suspect.

In my experience studs bet broken from car crashes or over tightening of the lug nuts.

Here is a picture of the wheel with the broken stud at the 

8 Oclock position

Click image for larger version

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From my 40 years Endurance Sports Car racing


and mounting hundreds of tires while doing evaluations as a tire design engineer, I STRONGLY recommend that all studs on that tire position be replaced and a HAND torque wrench be used to fully seat the studs unless a stud hydraulic press was used to install a full set of studs.

Also you should check all the studs at 50, 100 and EACH 50 miles thereafter till ALL the new studs have been confirmed to NOT need any additional turning of the torque wrench for two torque checks.

 I believe so strongly in the importance of lug nut torque I required our lug-nuts be torqued whenever we changed tires. In thousands of miles racing, including winning six consecutive 24 hour endurance races my cars NEVER lost a wheel due to broken stud or loose lug nut.