Does your TPMS "control" you? or can you control it?

I saw a post on an RV Forum about a tire "Blowout" on an RV that had a TPMS.

The RV owner said they had TPMS but still had a sidewall failure. He also said, he "set his  "baseline" PSI which is the threshold for the low end pressure reading and then the upper limit alarm will tell you if you''re exceeding normal pressures and headed for a blowout. Same thing for the temperatures as well....set the baseline and then it alerts you if the tire temperature is above normal and too high. I used my unit last year on 2 long haul trips to MI and it performed well. Gave me the piece of mind knowing I could keep an eye on the tire pressures and temps. I do recommend the unit. I purchased the unit which can support up to 22 sensors, but got the TPMS system with 6 sensors on Amazon, then bought 2 additional sensors for a total of 8 - 4 for my TV and another 4 for my TT. My trailer has dual axles. I mounted the wifi extender on the front of my TT just behind the battery box and use the clip on leads from the battery to power it. The wifi unit is in my truck and I tuck it in the center pocket in my cab. Works well."

I pointed out that not all TPMS let you set that actual low pressure warning level. Some use a % or number of PSI below the "set pressure" AKA Baseline. Others let you set the low pressure warning level.

I recommend that the low pressure warning level be no lower than 5 psi below the minimum pressure needed to support the measured load on the inflation for the tires on that axle. With some TPMS, this will require some extra calculation and work as you will need to set the Baseline higher than your actual "cold" inflation.

Example

If Tire minimum inflation needed to support the measured load was 70  your low warning level should be no lower than 65.

If your system has the warning at -25% from "baseline" then you need to set your Baseline to 87 psi  (75% of 87 = 65). Some systems use the pressure in the tire when you first connect the TPM sensor as the "baseline" so you would need to pump up your tires to 87 before first connecting the TPM sensor. Once the TPM system is fully connected you could then lower your 87 to  80 (70x 1.15).

The 1.15 is a suggested margin of inflation so you are not pumping up your tires if the Ambient temperature drops. Remember that a drop of 10°F will result in about a 2% drop in the "cold" tire pressure.

 

Yes this is complex but that is a problem with the TPMS MFG not giving the user full control of your warning and baseline inflation levels. Please learn how your TPMS is programed. Your instructions should tell you how to set the "Low Pressure Warning Level" but if it doesn't you will need to do the above math.

 

##RVT1002

Another post on ST type tires, Speed and tire life

 Post on an RV Advice forum:

"We have ST rated tires. Do we have to run less than 65 mph? We see a lot of RVs passing us."

 

"Speed Symbol" is actually a relative measure of heat resistance. 

The speed number is based on a brand new tire being able to run for 30 minutes on a perfectly smooth steel drum,  at a constant speed without coming apart. Any tire running the test is considered scrap after the 30 minute run. 

 

 65 mph is the assumed Max operational speed for ST type tires. 

This is part of the calculations used to establish a tire load capacity. While it is unlikely that your tires will come apart as soon as you run 66 mph, it is true that the faster you run the "faster" you are using up the life of your tires. People running faster than 65 is one of the contributing reasons for ST tires to have shorter life than tires on your car or Pick-up.

 

I often compare tire speed rating like your engine "Red Line" speed. If your car indicated 4,500 as the Red Line you can probably run 4,495 RPM for some time without the engine blowing up. BUT I would think you know this will significantly shorten the life of the engine. Same with tires.

##RVT1001 

Can you change tire size or type on your RV?

 As an actual Tire Design Engineer I can assure you that there is more misinformation or partially correct information out there on the internet, than technically accurate information.

It is true that the original tire selection is the responsibility of the RV Mfg. The issue is that once the RV is sold it seems that most RV Manufacturers have little or no interest in standing behind their choices with any actual warranty service when it comes to tires.

It seems that OE tire selection for most RVs is based on one goal. Find the smallest, lowest cost tire that will meet the requirements.

The only Federal (DOT) requirement is that the tire load capacity, times the number of tires on the axle, be AT LEAST equal to the maximum load rating of the axle. While RVIA now requires 10% Reserve Load capacity, DOT does not. As a point of reference most cars come with a 20% to 30% or higher Reserve Load capacity

A smaller tire can mean the RV Manufacturer can get away with less costly (smaller) wheel and maybe a smaller wheel well so this is extra pressure on purchasing Dept to get the minimum possible tire that can meet the requirements.

Given the above, it is up to you, the owner, to decide if you want any, some or more "Reserve Load capacity" for your RV. You may have the option of larger tires or you may be restricted to trying to find tires of the same dimensions but with higher load capacity.

You need to educate yourself about the requirements and limitations of the four "types" of tires that are in the market.  'P" is Passenger type. If used on an RV (trailer or motorhome) the load capacity Must be reduced by dividing by 1.1 but not everyone will know or do that. LT type can be used in RV service but you will soon discover that LT tires with the same dimensions and Load Range (ply rating) have a lower load capacity than the same dimension ST type.

ST type have the highest load capacity rating for a given set of dimensions, but you need to remember that the ST tire Load formula that is used to calculate the tire load capacity is based on an assumption of a 65 MPH Max speed. We all know that there is "No Free Lunch" and the trade-off for increased load capacity is lower speed capability. The "Speed Rating" symbol on many ST type tires is based on a 30 minute test so you need to decide if you want to depend on such a short term test when making a tire selection.

Finally there are actual "Truck / Bus" tires. These have no leading letter and are usually on 17.5" or larger wheels. These tires have higher Load Range, usually F or higher These tires are almost all rated for 75 MPH in RV use on the highway.

Do your homework. Ask questions, but remember there are very few really knowledgeable people out there who have the training or experience in tire engineering. Just having driven on tires for 40 years is not the same as having been held responsible for designing tires for Truck, Passenger, Trailer, or Indianapolis racing application. Also, being able to read Federal Regulations is not the same as having to work within those regulations while meeting the goals and demands from GM, Mazda, Toyota, Honda, Freightliner, MB, Nissan, Ford, or Chrysler.

I am only aware of two actual Tire Design Engineers who regularly post on various RV Forums.

##RVT1000

What is the "BEST Tire"?

This question is usually framed as "What tires should I buy for my RV Trailer?"

I usually stay away from brand recommendations. As a certified Forensic Tire Engineer I will say that what ever you get, you should ABSOLUTELY be sure the new tires have a load capacity that is at least equal to the OE tire capacity. Better yet get tires that can support 120% of the load of the OE tires.

If the RV was made before Dec 2017 each tire may have only been rated to support 50% of the GAWR (see your certification label). If mfg after Dec 2017 each tire might have been rated to support 55% of the GAWR.

I strongly recommend you be sure your tires are of sufficient size and Load Range to be able to support 120% of the measured load (scale weight, not printed limits or someone's estimate) of the heaviest loaded tire on the axle. All tires on an axle should be inflated to the same "cold" inflation.

RE TPMS  I recommend the Low Pressure warning be no lower than 95% of the goal Cold pressure. Your High pressure can be set to 125% of your Cold inflation.  Your high temperature is probably set by the TMPS mfg to 157°F (70°C) which is a reasonable level for aftermarket external TPMS..

Finally, I will go so far as saying you will probably be better off if you ensure that what ever tire you select shows that it has Nylon aka polyamide listed as one of the materials under the tread in addition to the steel ply. This will improve heat resistance and high speed durability.

##RVT999