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Friday, June 15, 2018

What should Airstream and similar type trailer owners do?

One of the Forums I follow is on Airstream trailers. There was a post from an owner who was having problems getting the actual loads on the trailer, hitch, and TV. There were a number of posts on what he needed to do.

However, I saw no mention of tire loading or tire inflation so I added the following post to the thread.


"When you are done making adjustments and moving stuff around and get new scale readings. You need to look at tire loading and inflation.

Ideally, you would get the individual axle load as they are probably not split 50/50

You are also probably not 50/50 side to side loaded either.  The good news is that you don't have slides or residential refrigerator so are probably 51/49 side to side or 48/52%

The best approach is to take the heavier loaded axle and assume a 52/48% split and compare the 52% number with the tire load capacity. You should have at least a 15% load capacity margin.  (52% of the heavier axle is no greater than 85% of tire max capacity.

Finally, with a multi-axles trailer, you should use the tire sidewall pressure number as your "Cold Tire Inflation" to try and lower the Interply Shear forces that are trying to tear the tire belts off the carcass.

You can learn more on my RV tire blog."

Note: While I wrote this in response to a specific post/question on an Airstream forum the information on tire load & inflation would apply to similar trailers that do not have slide-outs or heavy residential refrigerators or other obvious unbalanced equipment.

Friday, June 8, 2018

Setting Motorhome inflation for "Newbie"

Reading a forum post that said:
"We are moving across the United States for our FIRST trip in our 06 Mountain Aire. Don’t think that’s a great idea for a first trip, but that’s what’s gonna happen. LOL. I do not have access to a four corner scale. I do have access to a cat scale. I downloaded the Michelin tire pressure guide. From what I remember here, I take the axle weight and use the Michelin guide? I seem to remember someone saying to add 5 psi to compensate for the lack of a 4 corner scale?"
My reply:

You can read my RV Tire blog for more details for Motorhomes (but not towables)

1. If you have no scale reading follow the Vehicle Certification Label AKA (Tire Placard)

2. As soon as possible, get on a truck scale when you are at your heaviest (full of fuel, food, water, propane, clothes, people etc).

3. Using the axle load numbers: Confirm no axle exceeds GAWR (gross axle weight rating) (on placard) and the RV does not exceed GVWR (gross vehicle weight rating) (on placard). If you exceed either max weight rating you must move or eliminate some "stuff" in the RV.

4. If your weights are lower than GAWR then you MIGHT be able to lower your inflation a bit but there is some effort involved.

5. Since you will not have "4 corner weights" you will not know how much out of balance side to side you actually are. Some RVs have been found to be 1,000# out of balance on an axle.

6. I suggest at a minimum you assume one end of each axle is carrying 53% of the total axle load. You could use the 53% number to consult the Load & Inflation tables.  WARNING Michelin tables assume perfect 50/50 load balance which IMO is not realistic, so you need to calculate the individual axle end load ( divide Michelin number by 2)

7. Using the 53% load number and the calculated Michelin load number you now know the MINIMUM Cold Inflation aka MCI number. It is suggested you add 10% to that inflation number for inflation when setting tire inflation. This is your daily measured Cold Inflation Pressure aka CIP. This 10% covers you for day to day temperature and elevation changes.

8. If or when the measured cold inflation (or inflation reported by your TPMS) drops to your MCI number you need to add some air to get back to your 10% margin number.

9. I see no reason to ever bleed off pressure from your daily measured CIP unless your reading is higher than the inflation number molded on the tire sidewall. If you get that high you can bleed down to the tire sidewall pressure but never below the MCI.

Ya a bit of work but once you think about your goal - Stay above the MCI and below the inflation molded on the tire sidewall you are good to go. A TPMS makes the job of checking inflation on each day of travel much easier as you simply check each morning before you start out. TPMS will also give you a warning when or if you get a puncture od have a leaking valve and are losing air. A good TPMS will have some form of "early warning" when you start to lose a couple PSI while running down the road. This could prevent a serious tire failure.

Friday, June 1, 2018

ST type tire speed ratings. One man's opinion.

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 fundamentsl format of the formula.

The "K" shown above is an important concept as it is really a factor based on the expected service.
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 catigory" 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 
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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 DT does not recognise 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" engineering are tire companies puting 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? There are construction features that will improve the test performance of tires, maybe even improve the speed rating a step, but I have serious questions on the ability of tire company to increase load capacity by 20 to 25% which is effectively what some appear to be trying to do.


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.

Wonder why they didn't increase the speed rating of these tires using similar construction features they applied to the GYMarathon to turn it into a GY Endurance.
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