By Roger Marble
Let’s see if I can bring some Science and Engineering facts and history to this issue of speed limits on ST trailer tires.
In the ’60s and ’70s, when ST-type tires were “invented” and started to be applied to travel trailers, the national speed limit was 55 mph and tires were bias. Trailers were considered “big” if they were 24 feet long. I doubt there were many, if any, 5th wheel tri-axle trailers on the road.
Today we see speed limits across the country of 70+ mph. There are many locations where you could set the cruise at 70 and never slow down for an 8-hour drive here in the U.S. Trailers more than 30 feet are normal – with some pushing 40 feet. Most have tandem axles with more triples showing up every day.
Formula for determining load capacity
The formula for determining the load capacity for all tires follows the basic format
Load = K* x (air pressure) x (air volume)
Now the calculation for air volume is the complex part, as aspect ratio and a theoretical rim width and other factors such as tread depth come into it. But these details do not change the fundamental format of the formula.
*The “K” shown above is an important concept as it is really a factor based on the expected service. Trucks are expected to carry heavy loads but not all the time. Passenger cars are not expected to be heavily loaded much of the time. While RV tires are loaded almost all the time, when ST-type tires were “invented” we didn’t have slide-outs or 35-foot 5ers or pickups capable of running 80 mph for hours on end.
Standard passenger cars seldom carry max load
Standard passenger cars seldom, if ever, carry their max load. The GVWR and GAWR are not even in every owner’s manual or on the Vehicle Certification label aka the “Tire Placard.” They are expected to be run at posted speeds but on paved roads for hours on end and driven 10,000 to 20,000 miles a year, i.e., used fairly frequently with many being parked in a garage.
For the sake of this discussion, let’s assume the “K” is set to 1.0 for passenger cars.
Now, what do you do with station wagons and other “multi-purpose” vehicles? These vehicles were expected to carry more load more often, so the service is obviously more severe. When SUVs came along they were placed in the “multi-purpose category.” If a passenger-type tire was applied to a trailer, that was also considered more severe service. So the load capacity was reduced. Many are aware of the “de-rating” of P-type tires when used on trailers or SUVs, etc. So K (multi-purpose) = K (passenger) divided by 1.10, and we end up with lower load capacity – about 90% of passenger.
Let’s look at the actual numbers:
P235/75R15 105S 35 psi
2,028# @ 35 psi 112 mph on a passenger vehicle
1,844# @ 35 psi 112 mph on an SUV or P/U or trailer
Formula for LT-type tires
Moving on to pickup service, we have LT-type tires. The formula is still K x pressure x air volume. But with trucks expected to carry even more load most of the time, their K factor is different.
Their numbers give us:
LT235/75R15 101/104Q LR-C
1,985# single 50 psi 99 mph
This lower load capacity on truck service is clearly because of the higher percentage time spent carrying more load.
Before we move on, let’s look at the ST numbers:
ST 235/75R15 LR-C
2340# @ 50 psi 65 mph
To me the obvious question should be: How does the addition of the letters “ST” on the sidewall allow a 26% increase in load capacity over a P-type tire (adjusted for trailer service)
or a 29% increase over the heavily loaded but occasionally empty truck? The only reason I can see is the significant reduction in speed.
More load on the tire generates more heat
We all know, or should know, that more load (more deflection or bending) generates more heat. So what could you do to counteract the increase in heat due to the increase in load? Obviously lowering the speed would reduce the higher heat. That was part of the original ST trailer tire standard.
Now let’s look at the tire type that is of real interest: ST type as used on many RV trailers.
In 2014, new duties were imposed on imported tires, but ST type were exempt, sort of. There were various requirements, some of which were requested to be changed or eliminated. The speed symbol was one of these requirements.
Starting in 2017 (possibly earlier in small quantities), many ST-type tires started showing up with a Speed Symbol selected from the table as published by US Tire & Rim Association in the LT section.
U.S. Speed Rating is a marketing tool
The problem is that the Speed Symbol does not have any standard DOT test or requirements as the U.S. Speed Rating is really a marketing tool and not a strict performance requirement. A review of various ST tires shows a range of speed symbols from L (75 mph) to R (106 mph) and possibly higher.
Further compounding the confusion is that the speed symbols are from the SAE – Society of Automotive Engineers. According to SAE, their test criteria J1561 apply to “‘standard load,’ ‘extra load,’ and ‘T-type high-pressure temporary-use spare’ passenger tires.” This raises the obvious question of what test procedure, if any, are various tire companies following when they assign the Speed Symbol? While we are talking about SAE symbols, we need to remember that DOT does not recognize or test for these ratings.
Let me close with a question I have asked a number of times but as of now have never received an answer for.
Are tire companies using “magic” pixie dust?
What “magic” pixie dust are tire companies putting in their ST trailer tires that allows them to run 75 or 81 or even 106 mph without making any adjustments in load or inflation? And if they have this “magic” engineering available, why aren’t they using it in their LT tires?
NOTE: The Goodyear Tire Care Guide clearly shows a blanket 75 mph max speed for 17.5 rim diameter and larger tires.
Some may want to argue that tire technology has improved since 1970, and that is certainly true. But I would ask why haven’t load capacities for passenger or LT or heavy truck tires been increased over the past 50 years?
Have a tire question? Sign up for Roger Marble’s new Facebook Group: RV tire news, information and discussion, hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.