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Friday, February 14, 2020

Heat, High Speed and the "Magic" in ST tires.



Heat generation is primarily the result of a combination of High Speed, High Load, and Low Inflation. Factors that can counter some of the negative effects of heat on tires can include the use of more expensive rubber compounds, reduced thickness tread and other construction features that are out of our control as tire owners. Without access to secret formulas and other proprietary information, I know of no way to learn which of these features is in which tires if any. There is one exception, of the inclusion of full Nylon Cap Ply. This would be identified in the material list molded on the tire sidewall along with the number and material in both the sidewall and under the center of the tire tread.

However, the driver] has complete control over the Speed/Load/Inflation factors.

IMO it would be better if all tires had at least a 15% margin on load capacity, with 20 to 25% being better margin. We need to remember that road slant & crown and curves along with significant sideload from wind can easily result in overloading tires that were measured on a level scale. Multi-axle trailers should shoot for a 25% load margin. (read about Interply Shear to understand why )

Ever wonder why RV trailers seem to have a lot of tire problems but your daily driver (car or SUV) doesn't?
Let's look at Reserve Load Capacity of your car:
Many / most have 30% to 40% Reserve Capacity on the tires.












  


Now when we look at an RV trailer we see only 6% with many having 0% margin.
 




 
 If we assume we want to have 25% Reserve load capacity on the above RV, then 13,290/(Tire max load X 4)= 0.75
and each tire max load shall be 4,430 lb.    So the right choice would be an all-steel
ST235/85R16 132/127 (14 with a max load of 4,410 lb.)


Previously I indicated that Inter Ply Shear can be the equivalent of adding 24% more load on the tire while in operation. Using the above tire the IPS effect is reduced considerably.

NOTE in 2017 RVIA and the National Fire Protection Association, guidelines for tire selection for RVs was updated to require a MINIMUM of a 10% margin in tire load capacity over the GAWR. Some RV companies achieved this margin by upgrading their tire load capacity while others simply lowered the GAWR number on the vehicle certification label. Do you know what your RV company did?

   

Speed rating. We should think of this like the engine red-line. Everyone seems to understand that running faster than red line will shorten engine life but that doesn't mean that 5,900 on a 6,000 redline is good to run for hours on end and may contribute to shorter engine life just as running 62 mph on tires rated, based on their original load calculations, for a max of 65 mph.
The high-speed test is a 30-minute step speed test of a brand new tire on a smooth test wheel. No potholes or curb damage. All that is needed to "pass" the test is to meet the target speed and not come apart. The tire is considered scrap after the test.  Would you consider your tires to be "scrap" after you run 70 or 80 mph for an hour or so?
Remember damage (internal cracks) done, even at the molecular level, in tires never repairs itself but only increases in size.

While this post is primarily about Trailer application the limit on high speed also applies to 16 through 22.5" tires that have a max rating of 75 in RV application which would be their "Red Line" but I hear many Class-A owners talking about running over 70 for miles on end but being surprised when they suffer a tire failure.

LT tires would be my first choice in any trailer application where heavy loading was required. I do not understand why so many think that there is some magic engineering used on ST tires that allow them to support 10% to 20% more load than an LT tire of identically dimensions. The load formulas used today for ST type are the same ones used in 1969 when they were limited to 65 MAX (red-line).

I would be very interested to hear an answer from the engineers at the companies that make ST type tires why they were able to suddenly, almost overnight, increase the speed rating from 65 to 75, 87 or even 99 on their ST type tires. Do they have some new materials? Why don't they put those same super materials in LT tires and increase the load capacity of LT tires to be the same as or near to the loads seen claimed for ST type tires?

Sorry, I just don't buy the new claimed high-speed capabilities of ST type tires that exceed the load and speed capabilities of equal size LT tires.

##RVT935

Friday, February 7, 2020

Air Compressor - How Big do you need?

While air volume output might be a consideration, IMO if you are properly inflating your tires and properly (TPMS) monitoring inflation I don't understand how anyone can get in a position of needing more than about 5 psi unless you have an active leak.

If you are only adding 5 psi then tou do not need to worry about how long it would take to inflate a 275/80R22.5   All you need is a compressor rated for more psi than what your tire needs. I would think that a 150 psi rating for the compressor fro tires needing 125 psi   or 100 psi rating for 80 psi tires would be enough.

As I have covered a number of times in this blog, I recommend your cold inflation pressure to be at least 10% above the minimum needed to support your actual measured tire loading (Minimum inflation would be based on the heaviest loaded tire on any axle or lacking individual tire loading numbers,  then using an assumed 53/47% side to side split for motorhomes and trailers with big slides or residential refrigerators and at least 51/49% load split for smaller trailers)

So assuming you have LR-C or LR-D tires you would be inflating to 50 or 65 psi with your TPMS warning set to no lower than 49 or 64 with your minimum inflation in the load tables being 45  and 58. So how would you ever need to add more than 5 or 6 psi assuming you let your tires get that low? Why not do your "top-off" as soon as you need 3 psi? Now you do need to consider the 1 or 2 psi difference between your calibrated hand gauge and the TPMS reading. I set the warning based on the TPMS reading AFTER setting the tire using my certified hand gauge.

Yes pressure changes with temperature (about 2% for change of 10°F Temperature)  A change in morning temperature of 40F from day to day is unusual and that would only result in a pressure drop of 5 psi on your LR-D tires.

Motorhomes should be running a +10% margin on air pressure based on the measured tire loading which means there would need to be a 50°F drop in temperature for them to need to add 10 psi (assuming a 100 psi minimum).

If you need to add more than 20% (20psi) of the needed pressure in your tires with steel body ply, that means you have technically been operating on a "flat" tire according to tire industry standards and you should have a professional inspection and have them re-inflate your tires AFTER the reason for the sir loss was identified and repaired. Large 19.5 and 22.5 tires should only be re-inflated in a cage just in case there was damage to the steel body cords which can lead to an explosion due to zipper rupture.

LR-E (80 psi polyester body tires) as found on most Class-C and some larger trailers need to consider the above information and adjust for their higher cold inflation numbers. I would consider a 20% drop to put you in the safety cage re-inflation level if you drove on the tires when that low. While they are not likely to suffer a true "zipper" failure from fatigued steel body cords, there can still be internal structural damage to your tires.

Bottom Line:  Monitor your tire pressure and don't let the pressure drop more than 10% before you re-inflate your tires. Know why the pressure dropped and if not due to a drastic change in temperature overnight, inspect for leaks. I find that spray cleaner like Windex or other cleaners tend to foam at the location of the leak.


##RVT934