THANKS TO OUR SPONSOR!

THANKS TO OUR SPONSOR!
Your Ad here
Be sure to sign up for the weekly RV Travel Newsletter, published continuously every Saturday since 2001. NOTE By subscribing to RVTravel you will get info on the newest post on RV Tire Safety too
. Click here.
Huge RV parts & accessories store!
You have never seen so many RV parts and accessories in one place! And, Wow! Check out those low prices! Click to shop or browse!

Friday, June 18, 2021

Are tire pressure recommendations changing?

A question from an Airstream owner.

Hello All,
I have a 2020 Ford F250 and an Airstream 33' Travel Trailer. I've read a lot of discussion about tire pressure, but nothing real current. What I mean is, the newer F250 door column states for these Michelin tires now, 60psi front and 65psi rear. The tires max at 80psi on the sidewall which was my experience with prior year F250's on the door column too, but the tech at the dealership said they've changed and that with normal driving you can run 80psi, but harsher ride and you'll run the "middles out of them."

On to the Airstream. All four tires are Michelin LT225/75R16, Load Range E. The Airstream sticker says 80psi for these tires. I went up to my local Dobbs Tire Dealer and he pulled up the Michelin chart for these tires, and said with a trailer weight on dual axles of between 7980lbs and 8600lbs, you could run the pressure between 60 -- 65psi. I asked if I would have a problem with tire heat or wear and he said no. I did put a 3" lift kit on the Airstream, so as to have better clearance at gas stations and various road conditions. The dealer puts in 80psi, but from time to time I get popped rivets, (if ride is the cause).

My question is this: With an approximately 8200lb trailer, 1300lb hitch weight, and just me and some luggage, (300lbs of luggage), what would you recommend on tire pressure on both vehicles for mostly interstate driving, despite the door stickers, (if recommended)?

My reply:
Not sure why you think the information on appropriate tire pressure is different today than it has been for the last 10 years.

TV Car companies have teams of engineers that work closely with the team of tire engineers to select tires and inflation numbers that will deliver the best overall tire and vehicle performance. The consider and test for ride, handling, steering response, fuel economy and safety in emergency situations. The inflation on the sticker along with tire inflation information in the owner's manual should be followed. Mechanics and service people at the car dealership are not involved with the selection or testing and evaluation of the new tires that were selected for the new vehicle. While they have some knowledge I see no way they can know the performance  features of the tread rubber or the construction features of the body of the tire that was designed, selected and manufactured for that specific vehicle.

TT While I am not aware of any similar testing and in vehicle evaluation on tires for trailers in the RV world, there are still some requirements the RV Certification Label aka Tire Placard sticker has to meet BY LAW. Tires selection is the responsibility of the RV mfg. The inflation specified on the sticker along with the tire type and size must be capable of supporting at least 50% of the stated GAWR per DOT regulations for each tire. A few years ago RVIA decided that having at least a 10% load capacity margin would be a good thing and that having such a margin should improve tire reliability so the sticker inflation for the specified tires must provide AT LEAST 110% of the GAWR and assumes a perfect 50/50 end to end of each axle oad split. So if your RV is RVIA certified the sticker as applied by the manufacturer must meet these standards.

I think a point of confusion is the wording on some tires concerning the Max Load capacity of a tire and the appropriate inflation needed to provide that load capacity. Each tire has an absolute Maximum Load capacity number and that is the number stated on the tire sidewall. The confusion comes in when people think that they can obtain more load capacity if they increase the inflation but that idea is incorrect once the maximum load capacity of the tire has been reached. While it is possible to increase the inflation doing so will not increase the laod capacity and that is why some tires say "Max" inflation but everyone should know or realize that tires warm up when in the Sun or when driven and that pressure increases with temperature (about 2% per each 10F) BUT when discussing inflation numbers or when setting tire pressure we are ALWAYS talking about the pressure when a tire is at the prevailing Ambient temperature i.e. temperature in the shade and the tire has not been warmed by either being in the Sun or being driven on for the previous 2 hours. i.e. "Cold Pressure".

As an experienced tire design engineer I can only advise that people follow the car company and RV company recommendations for tire inflation unless they follow the specific guidelines provided by actual tire engineers if they want to fine tune tire pressure based on actual scale measurement and include the suggested tolerances and margins on load and inflation.

##RVT1005

Friday, June 11, 2021

Are ST trailer tires made with "Magic" pixie dust?

 Lets see if I can bring some Science and Engineering facts and history to this issue of speed limits on ST tires.


In '60's & '70 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' long and I doubt there were many if any 5th wheel tri axle trailers on the road. 
Today we see speeds across the country of 70+ and there are many locations where you could set the cruse at 70 and never slow down for an 8 hour drive here in the US. Trailers over 30' are normal with some pushing 40 feet and most have tandem axles with more tripples showing up every day.

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 and while RV are loaded almost all the time, when ST type tires were "invented" we didn't have slide-outs or 35' 5vers or pickups capable of running 80 mph for hours on end.
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 "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 lets 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 SUV's came along they were places in the "Multi-purpose category" and 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.

Lets 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

Moving on to Pick-up 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 the higher percentage time spent carrying more load.
Before we move on lets 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.

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 lower the speed would reduce the higher heat and that was part of the original ST tire standard.

Now lets 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.
The problem is that Speed Symbol does not have any standard DOT test or requirements as in the US 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 and 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.
What "magic" pixie dust are tire companies putting in their ST 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 Goodyear Tire Care Guide (https://www.goodyearrvtires.com/pdfs/tire-care-guide.pdf) 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? 

##RVT1004

Friday, June 4, 2021

Are "Blowouts" the result of running a tire inflated to more than the tire sidewall number?

I think people are over-thinking tire over-inflation.

1. Any mention of tire inflation is about "cold" inflation unless there is a specific mention of "hot inflation"

2. "Cold" inflation does not mean you need to refrigerate your tires or to get them to some artificial chemical laboratory "standard" of 68 or 70F. Cold simply means at the prevailing Ambient temperature. i.e the air temperature in the shade. Tires generally get to "cold" after being parked for 2 hours or more, in the shade.

3. All tires warm up when running. Sometimes when driving the Interstate, we even see the tires on one side are 10-20F hotter than the other when constantly in direct sunlight. Think about traveling due East or West in Kansas or Iowa. I know I have seen this happen in my Class-C more than once.

4. If you have your actual tire loads (scale weights when fully loaded) and use that number to learn the minimum inflation from the tire Load & Inflation tables you can add a margin and then be in good shape and not worry about tire temperature.

5. What "Margin" of inflation? If using the tables and you identified the inflation that can support no less than the "heavy end" load. (Always go up to next 5 psi step in the table.) Do not round or try and calculate an inflation that exactly matches your scale number. Just use the table numbers. 

6 Now that you know the MINIMUM inflation required, it is desirable to have a margin so you do not have to get out the compressor every day because the Ambient temperature has changed. Given that tire temperature will change about 2% for each change of 10F in Ambient and that a 20F to 40F change in morning Ambient is in the "normal" range, that would suggest that having a margin of +  8% to 10% will make life easier. If you are running 10% more inflation than the minimum required for the load you will find that the "hot inflation" will not increase as much than if you are starting at the minimum + 2psi.   EXAMPLE Suppose you need to run 70 psi MINIMUM based on the measured load and the information in the tables. I think that if you start out inflating to 77 psi you will see less of an increase in pressure  than if you start out at 72 psi.

7 Maximum Inflation? This causes some unwarranted concern for many. They read the tire sidewall and see something like "Max inflation 80 psi, Max Load 2780 Lbs" IMO it would be better if the tire said "Max Load 2780 at 80 psi" This is because there is an absolute maximum load a tire is rated for and the inflation pressure needed to support, when cold, is 80 psi. This means in reality that 80 psi is the MINIMUM inflation required to support the MAXIMUM load. While I can not speak for all tire companies I do know, from my experience as a tire design engineer where testing for the maximum inflation a tire can tolerate was part of the standard process I saw that for normal street tires. P, LT or Truck type the new, undamaged tires could tolerate anywhere from 100% to 250% over-inflation and not fail. In a few cases some tires were able to tolerate as much as 400% for a short time before failure. Now I need to make it clear that running tires at 100% over the number molded on the tire sidewall is not safe. The tests were conducted in an explosion chamber and not while tires were running. Tire engineers test new designs to confirm they can tolerate significant over-inflation. We run tires in overload on test wheels continuously longer than possible for anyone to run their tires on the highway so we know thy can tolerate significant increase in both temperature and pressure.

8 "Blowouts" do not happen because a properly loaded, undamaged tire was run at highway speeds. "Blowout" simply is a word that is used to explain that there was a sudden loss of air, that made a loud noise, and the driver was surprised. Running a tire at hot inflation of 110 to 130% of the tire sidewall pressure is not, in itself going to cause a "blowout".

##RVT1003