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Sunday, October 30, 2011

Why should I weigh my Trailer tires individually?

Many times when I give a seminar or class to people from the RV community on proper application of tires, I get asked the above question by trailer owners.


Motorhome users seem to understand that knowing the actual load on each corner of their RV will allow them to select the tire pressure they want to run as they balance Ride, Tire Durability, Fuel Economy and Steering Response.
Owners of trailers say they have been told that all they need to do is to run the max inflation on the sidewall of their tires. Almost all the time, that is the information on their placard per the trailer manufacturer’s guideline so that recommendation seems reasonable.

Recently I have been having a discussion via email with an owner of a large 5th wheel trailer.
He even believes that because the load changes on every trip and almost every day it could be “dangerous” to get the trailer weighed as that could mislead the trailer owner to think it OK to lower the pressure to just match the minimum needed to carry the measured load. I would like to offer a different view on why it is important to get the trailer weights on each tire at least once.

More than half of the trailers that have been weighed over the years have been found to have a tire or other suspension component exceeding its maximum load capacity. We all know what happens to people who “Assume” something. When you get the trailer weighed, it does need to be with all the stuff you normally carry on your travels. Knowing this load you will have the facts to see how critical it is to re-distribute the load side to side or front to rear. You will also know if your unbalance has overloaded one or more tires even when the total load seems OK. You will also know if you have proper loading on your pin. I have heard that some axle manufacturers consider having one end of an axle loaded to more than 50% of the total axle rating as overloading the axle. This means that even if you have changed wheels & tires to not overload those components you might still be overloading the axle if you are not careful.

Another thing to consider is the “Reserve Load”. That is the “safety factor” between the actual load and the calculated maximum for a component. Most cars have a 12 to 18% reserve load. One of the other benefits of having a good sized Reserve Load is that it will allow you to occasionally bring home that big load of stuff you just had to buy at the flea market and not overload a component. One way to identify a minimum cushion would be to look at the load capacity of your tires when inflated to a pressure of at least 15 psi below the max inflation and to be sure your load does not exceed that level. A 20 psi cushion would be even better. Reserve load helps compensate for the unbalance that occurs due to side loading because of road crown and wind side loading.

You are probably asking yourself why trailers can’t lower their tire pressure the way motorized units can. The reason has to do with the mechanics of axle spacing and tire side loading whenever you turn a corner. Trailers with two or three axles put enormous side loading on the tires, wheels, and axles whenever they turn corners. A quick look at the tire distortion when you see a big twin axle trailer park will confirm that the tire needs to be as stiff as possible when side loaded. Since we all know it is the air that carries the load we also now realize that it is the air that stiffens the tire and as a result will lower the side deflection and loading.

BOTTOM LINE: Next time you are fully loaded please go to the effort of weighing your trailer and go the extra mile to do the multiple weighing and the math needed to calculate the actual load on the individual tires. Remember a blown trailer tire can lead to substantial damage and financial loss.

Thursday, October 20, 2011

Quick post on rubber valves

I continue to see posts from RV owners saying they have "Rubber" valves. This even though most rubber valves are only rated for 65 psi cold. There are some rubber valves rated for 80 and even some rated for 100 psi but these are not interchangeable.

How can you tell if you have the correct rubber valve for your application without dismounting the tires?

A quick examination should give you an idea of what you have.


What to look for

First unscrew the cap. See if the the only metal you see is the threaded part right under the cap. This is just over 3/8" long.


This valve is usually about 1-1/4 long outside the wheel. Is designed for a hole in the wheel of 0.453 and is rated at 65 PSI Max Cold inflation.


If you see metal even with the cap on like this

It is either a 600HP series LIKE THIS which is rated for 80 psi Max Cold inflation or a 801HP series rated for 100psi Maximum Cold inflation.


NOTE these are NOT interchangeable as the 600 is designed for a rim hole of 0.453" while the 801 needs a hole diameter of 0.625".

Now even if your cold tire pressure does not exceed the rated capacity for the valve you have I strongly suggest you consider switching to a bolt in metal valve with the appropriate rim hole size as if you run valve extenders or hoses or external TPMS you are probably applying more side load to the valve that it was initially designed to accept.

Also you should be running metal valve caps not the cheap plastic ones that probably were OE for if the valve core leaks due to a bit of dirt the metal cap can hold much more pressure than the plastic one.


NOTE if still in doubt you should have the valves inspected by a tire dealer. Call and ask if they have a Technician ASE Certified on tire inspection. If not, select another dealer.

Wednesday, October 12, 2011

Careful when you do research or before you buy

Recently I was asked to clarify some information an RV owner discovered as he did some research. What we see here is an error that is easy to make. Even for the more experienced RV owner.

The RV owner asked about a Michelin XTA
215/75R17.5 LR-J tire that came on a 5th wheel trailer. He had confirmed the load capacity of the tires at their max rated inflation and said they could handle the load on his 8,000# axles when inflated to the placard inflation of 120 psi. His question was about the max speed rating of the tires.



He consulted the Michelin Truck Tire Data Book and found that the subject tires were intended for Low Platform Trailer application.
In fact the brochure for his tires even identified the maximum speed rating for his tires as 62 mph.


He wanted to know if the maximum speed rating had been waved for RV application, and if it had what was the new speed limit. He found a statement from the Tire & Rim Association that specifically stating that when a tire had a speed rating below 65 mph it could not be manipulated (increased) with adjustments in inflation and load. He also found a Michelin truck tire service manual specifically saying that low bed trailer tires like the XTA, cannot have their speed increased.
Despite all this information he wanted to know how to work out the adjustments needed to allow a higher speed limit for the RV and could I please help him understand what he felt was confusing information given that the RV manufacturer had selected these tires for his trailer.

I had to tell him that in my opinion there was no way the speed limit could be adjusted. I pointed out that his error was in looking for information in data books and manuals of different type tires such as Truck Tires, RV tires and Low Platform trailer tires and assuming that since his tires were sort of like truck trailer tires and looked like tires he has seen on other large 5th wheel trailers, he could pick and choose which information applied to his specific tires.

The lessons to learn here are:
1. Not all RV manufacturers pay a lot of attention to proper tire selection for the application.
2. When doing research on tire type and specific operation limitations you need to look only at the literature for your specific tire if such literature is available from your tire manufacturer.
3. You cannot depend on the RV dealership to warn you about the real quality or limits of some components of your RV. You need to take responsibility for what you purchase. i.e. Let the buyer be aware.

Important Note:
Speed ratings are like the “Red Line” for your car, truck or motorhome engine. While it is possible to exceed the max speed rating of a tire just as it is possible to exceed the rev limit and run your engine in the red zone, I think we all understand that if we do run our engine that fast we are pushing the odds. In the same manner if your tires are rated for 65 mph max you might be able to run faster for a short time but you are consuming the finite limit of the tire structure and simply slowing down does not repair the microscopic damage you have done to the internal structure of the tire. You should not be surprised if you blow your tire just as you would not be surprised if you blow your engine after running in the red zone for engine speed.