B arry's T ire T ech

This is a series of articles on the technical aspects of tires, their care and usage.

My primary purpose in these articles is to help people understand tires and thereby reduce the risks we all face every day.

..........and since tires is just about the only thing I know about..........

Please drop me a note if you have a topic you want to see:

Barry@BarrysTireTech.com

ST Tires

This article is about travel trailers (RV's) that have ST type tires. Other kinds of trailers or other types of tires on RV trailers are not specifically addressed here. HOWEVER, if you have a trailer of any sort you may find the information of value

Tips for ST tires:

  • BEFORE every major tow, check the inflation pressure of BOTH the trailer and your tow vehicle.
    • Also inspect the tires by rubbing a GLOVED hand over the tread circumference. You are looking for bulges. If you find any, replace the tire IMMEDIATELY!
  • The current recomendation is to replace ST tires every 5 years. This maybe dated as the recent upgrades in ST tires' construction may have changed this to the same recommendations as regular tires.

When I originally published this webpage, ST tires were failing all too frequently and no solution was in sight. That has changed!

Not only have the tire manufacturers stepped up (maybe not all?), but trailer manufacturers have also addressed many of the issues on their end - tires too small, overload, etc. (also, maybe not all!)

As of the date of this webpage (May, 2022) NEW trailers are generally pretty good, tire size wise. There is an issue about overdoing things, and I address that below.

- BUT -

Many old trailers still exist. That presents the problem of how to sort this out. Here's my best guess:

  • If your travel trailer was manufactured BEFORE 2017, there may be a problem - and you should follow the weighing procedure outlined below.

  • But if you have a travel trailer built AFTER 2017, then the placard pressure is adequate

    - BUT -

    There appears to be several travel trailer manufacturers who have gone overboard and specified pressures that are too high and those tend to shake everything inside them. Again, I suggest following the weighing procedure below.

And some housekeeping items:

  • Travel trailers ought to have metal valve stems, not rubber ones. Yes, there are rubber stems that can hold the pressure, but a metal one is less likely to bend due to centrifugal forces.
  • Travel trailers ought to have TPMS's. Since TPMS's are constantly changing, I am not going to recommend specific brands or models - EXCEPT that I recommend AGAINST those that screw onto the end of valve stem (in place of the valve cap). Too much centrifgual force!

What's the difference between P type tires (Passenger Car) and ST tires? What about LT tires?

I am going to start with the difference with LT tires as that is the easiest to explain.

ST tires are very similar to LT tires. They have less tread depth than LT tires. They have a narrower tread width. They have a fairly boring tread pattern: Generally simple ribs. They both come in Load Ranges (C, D, etc.). They use inflation pressures identical to LT tires, but they carry about 10% more load than LT tires.

All of that is because of the difference in intended service. Trailers follow along behind a tow vehicle, so the tires don't wear as fast. Trailers don't have an engine, so the only torque applied to the tires is during braking. The tires also don't have to steer, so there isn't a lot of sideways force applied to trailer tires.

EXCEPT

On severe backup manuevers, the trailer will jackknife and the tires will be subjected to an extreme amount of sideways stress. The good news is that this doesn't happen often, and it happens at slow speeds. Nevertheless, LT tires are generally subjected to more stress and deformation - ergo, ST tires can carry more load.

So as a general rule, the casing (plies and belts) of ST tires is similar to LT tires. It's generally the tread that is different.

ST tires are quite different than P type tires. They are inflated to higher pressures, so ST tires carry much more load - and that means a more sustantial casing.

So it is possible to use an LT tire as a trailer tire with a bit of adjustment in inflation pressure, but P type tires require much more careful consideration, due to the load carrying capacity.


Selecting the proper inflation pressure:

Every light vehicle sold in the US for the street has to have a vehicle tire placard that lists the original tire size and the pressure specified by the vehicle manufacturer. If you are looking for a good place to start, that's it.

For trailers, as of 2008, that placard is supposed to be on the left front of the vehicle. (This is more or less the same as cars and trucks where it's the driver's doorframe.)

On older trailers, it was common to put the placard on the inside of the entry door or on the inside of a cabinet.

If you don't have a placard or there's something different - like a different tire size - then read EVERYTHING below and chose a course of action approriate to your situation.

If you are using the placard pressure or are confident you have the right value, do the pressure buildup test:

Pressure Build Up Test:

  • Before starting out on an extended tow (at least an hour!), measure and record both the inflation pressures and the ambient temperature.
    • It's also a good idea to inspect the tires by rubbing a GLOVED hand over the entire tread surface. You are looking for bulges. If you find any, replace the tire immediately!
  • After an hour or so into the trip, stop and measure the inflation pressure and note how much pressure has built up.
    • You don't want more than a 10% buildup - excluding ambient temperature effects:
      • Ambent temperature effect = 2% per every 10°F change in ambient temperature.
    • If you get more than 10% pressure buildup (excluding temperature effects), you need more load carrying capacity in your tires - either more inflation pressure or a bigger tire size.

    Needless to say, you want to be aware of how heavily you are loading your trailer. More Load = More Pressure build up!

What if you have no confidence in your pressure value?

  • Determine the max tire load. See procedure below.
  • Add 15%. That's just good engineering and it's what car manufacturers do.
  • Look up the inflation pressure for that load and the tire size involved. Link below. Round UP to the nearest 5 psi for simplicity.

    TirePressure.org - Special Trailer Tire Load and Inflation Chart

    Please note that the load table in the link contains ALL possible metric ST tire sizes - including some that aren't manufactured.

  • Check to be sure that answer is reasonable by doing the Pressure Build Up Test outlined above.
  • Mark that pressure on the fenders near the tires. I've used a felt tip marker which isn't as permanent as I would have hoped. Maybe a paint stick? Or a label?

Determine the max tire load:

I am going to outline a number of ways to do this, starting with the most accurate (and unfortuantely, the most complex), then getting easier (but less accurate).

  • The US Tire Manufacturers Association (USTMA) publishes a book on tires and in Chapter 4 (Recreational Vehicle Applications), it addresses a number of issues peculiar to travel traliers and motorhomes - including the procedure to get the individual tire loads. Here’s how to get an online copy:

    USTMA's Chapter 4 - RV's

    That procedure will get you individual tire loads. Choose the max. Then add that 15%

If you only know the axles loads or only know the weight side to side:

  • Pick the worst case and add 7% to the value. Don't forget there are 2 (sometimes 4) tires on an axle. Then add that 15%

If you only know the total weight on the tires (but minus the hitch weight):

  • Divide by the number of tires and add 10% to the value. Then add that 15%.

If you don't have any actual weights:

  • Use the GAWR's (Gross Axle Weight Ratings) - see image to the left. This should be located on the front driver's side of the trailer.
    • On older trailers (pre 2008), this might be on the inside of a cabinet door.
    • And on even older trailers (Pre 1970) the trailer might not have one at all! In this case, you have no choice but to weigh the vehicle.
  • If you are going to use the GAWR's, only add 10%, instead of 15%, to the value to determine the value to look up!

Remember to round UP to the nearest 5 psi for simplicity.



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