RV

Special Trailer (ST) tires and load ratings

Greetings RV tire and safety enthusiasts!     In this post I will clarify at least some of the confusion around “Special Trailer” (ST) tires, supplied  on most recreational trailers today — in particular their load ratings and how important they are.

 

First, the “ST debate”

Lets first establish that trailer tires are different than Truck tires — their duty service is different because they are pulled, not driven.  No power is applied to these tires, although they do have to experience some deceleration due to braking.  Fundamentally, ST tires are optimized for tall, heavy loads, which means the sidewall design has to be different than those on a passenger vehicle.  During slow-speed, sharp trailer turns on pavement, for example,  their sidewalls must sustain very high side-to-side twisting forces as the trailer rotates about its axels.  Light Truck tires don’t have to do that.

For the above reasons, it is no wonder that one respected tire chain out West strongly recommends against putting a light truck (LT) tire on a trailer — even if you could find one with the right load and speed ratings (For example, I am not aware of any 14-ply LT tire such as the one shown below).

The Tire Ratings wars

My observation is that the RV industry is wising up to the fact that consumers are wising up.  For example, it wasn’t too long ago that “ST” (“special trailer”) tires arrived on U.S. shores without a speed rating —  the lower OE cost was apparently attractive, at least to US-based RV manufacturer’s anyway.  With very weak warranties and no speed ratings, consumers had to take the brunt of tire failures, leading many to simply ditch the OE tires in favor of more reliable rubber.  Meanwhile,  RV manufacturer’s and offshore “ST” tire manufacturer’s continued to advance their business plans.

I’m seeing signs that this is changing.  With the knowledge that all new ST rated tires are now required to have a speed rating, its time to take notice.  But hold the phone:  At what point did ST tires suddenly  start appearing with speed ratings?  I’m told that this happened in 2015, although I can’t independently verify this.  But assuming this true — lets pause on that point just for a moment:   If your ST tires do NOT  have a speed rating, then they are way too old!   Most experts will opine that the useful life of a tire is 5 years (unless of course the tread wears out sooner), so….  this is 2021:  It’s time to replace your older ST tires if they have no speed rating — they are at least six years old.

With that out of the way, lets take a look at an ST tire that is newer than 2015.  Here’s the sidewall from the Original Equipment tires supplied on my Alliance RV model 310RL fifth wheel:

Lets unpack this, line by line

  • “load range G 14 PR” means this is a 14-ply radial tire.
  • “max load single” tells me the tire is rated at 4,410 lbs when inflated to 110psi (cold)
  • “max load dual” describes the load ratings when this tire is used in a dual configuration
  • “132/127 L” is the load and speed designation, respectively
    • load rating 132 corresponds to 4,410 lbs
    • speed rating “127 L” means 75 mph

By contrast, the popular Westlake “OE” tire (found on some Grand Design fifthwheels) has the same load range (G) but not the same load rating:   Their sidewall code is “129/125L” — which means 4080 pounds — a difference of 330 pounds “per tire”.    Here’s some rough approximations to show how important this difference could be:  Consider  two 15,000 lb trailers with similar layouts, such as the Alliance 310RL and the Grand Design 310GK.   To make the arithmetic easy, lets assume a fully loaded trailer, and 3,000 pounds on the pin, leaving 12,000 lbs to distribute between the four tires.  Lets further assume that these trailers are loaded “left side heavy”  with approximately 6,600 pounds on the left side and 5,400 pounds on the right (see below for why I picked those numbers).

Consider the Grand Design:  Each of the left-side tires have to support 3,300 pounds, which means the tires a loaded at 80 percent of their capacity.  Not particularly amazing from an engineering margin point of view, but an acceptable manufacturer’s design point.  Now then look at the same calculation with the Alliance Paradigm 310RL — with the same weight distribution.  Alliance supplies a 4,410-lb tire on the 310RL, which means they are loaded to 75% of their capacity — a full 25 % of margin!

Why did I assume “left side heavy”?

A very popular fifth wheel floor plan is the the “Rear Living” with the Kitchen slide on the driver’s side.  Many such floorplans (including mine) have the bedroom slide also on the same side.  With all of the kitchen storage, the stove, water heater, and a heavy absorption-type refrigerator all on the same side, the trailer is already “left side heavy” — even dry from the factory.    Alliance RV has the transparency to provide this information to the customer so I know, for example, that my specific trailer is 700 lbs heavier on the left side than on the right — and that’s with nothing in the Kitchen cupboards, the fridge, or the pantry.    The numbers I used above (6,600 pounds on the left side, 5,400 on the right) is a worst-case estimate, reflecting the reality that the trailer is going to be substantially heavier on the left side, in spite of any efforts to compensate by loading the other side.   I have yet to measure my actual left vs right side weights.

 

What about Warranties?

When I scanned my local tire market, I was surprised to discover that there are no tread warranties, like those found on “LT” tires.  You can’t even sell your used tires back to the store for “tread credit”!   The standard answer does make sense:  There is no way to verify miles.  There is also the problem of inflation pressure — generally speaking, recreational trailers do not enjoy the same amount of inflation care as do cars and trucks, although the advent of Tire Pressure Monitoring Systems for trailer tires is starting to change that.    In any case, to differentiate themselves from the competition, the better tire stores might offer flat repair and even road hazard and rotations as part of a purchase, but the tire itself essentially has no warranty.

This is what makes my Lionshead “Sterling Steel” tires unique:  They arrived with a 5-year replacement guarantee in the event of a manufacturer’s defect.  This is strong statement, but its also one that could be difficult to implement, as there are many ways for the Lionshead to claim that a failure was the customer’s fault:  For example, without some sort of alarm-based tire pressure monitoring. system I might very well be oblivious to a slow leak in one of the four tires on my tandem-axle trailer, and it would be very easy for one tire to fail catastrophically  (blow-out) in that situation, causing hundreds (if not thousands) of dollars of collateral damage to my trailer.  And Lionshead wouldn’t be obligated to replace my tire!

So I’ll be taking extra good care of my tires, for the safety of my family but also to improve my chances of detecting a manufacture’s defect while still preserving the evidence.  As a practical matter, whether that 5-year warranty is there or not makes not difference:  My tires will get the same amount of care and feeding.

happy travels,

-doug

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