What margin should be used for Low Pressure warning on TPMS?

 In response to a reader from Australia with a question on TPMS Low Pressure Warning level, I responded...

We need to separate the settings specified by US DOT for regular passenger car applications and the more specialized world of Caravans (RVs for us Americans) (both motorized and pull behind).

DOT is trying to protect the worst case which is operating a tire in overload. Most cars have their tire inflation set based on the goals of the manufacturer to meet fuel economy goals while delivering acceptable ride & handling the expectations of the consumer. With our current Federal fuel economy standards, this results in fuel economy being #1 or very near the top of the priority list. This means that almost all tires have sufficient pressure to significantly exceed the load capacity requirements. I have seen some data that shows +30% is not an extreme level. So that means a loss of 25% of air pressure still keeps the tire out of the "overload" situation.

Now when we move to the Caravan market, there are no regulations for fuel economy. So marketing pressure of low cost takes priority. This means the RV manufacturer selects the lowest cost tires possible. While in many cases this means tires that just barely meet the DOT safety requirement tests, it also means the smallest tire size sufficient to support the load as this translated to smaller (lower cost) wheel well space and smaller (lower cost) wheels. This results in most tires needing to specify higher inflation (more load capacity) than if the tire were applied to a car. The bottom line is lower levels of actual reserve load capacity with data showing that the average user ends up overloading the tires or axle or both.

I am a strong supporter of tires & inflation that provides at least a 10% reserve load for motorhomes with towables goal of 15% reserve load being needed to address the Interply Shear forces seen in multi-axle towables. These are what I would consider MINIMUM reserves.

To achieve these smaller margins we need to recommend the Low Pressure setting of TPMS is closer to 0% air loss from the cold "set pressure". If we were to allow a 25% air loss before the TPMS Low Pressure sounded, the owner could have been driving hundreds of miles in an overloaded tire condition.

 

Here is a post on how I set the TPM system on my Class-C motorhome. 

 

##RVT1056

Things to do while not traveling.

Ya it's really a pain to not be able to enjoy our RVs.

But There are a couple of "Projects" that may make life a bit better down the road.

Tire Inspection

Recording your DOT Serial

Testing your TPMS

Reading to increase your general tire knowledge.

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First is Tire Inspection. Here we are focusing on possible separations. This is a bit easier for Trailers but also it may be more important for them because of higher separation rate. First off you need to be safe. This means ensuring the trailer or Motorhome can not move. It may be best to hook up the trailer to the TV and set the brakes. Motorhomes can use the parking brake and put the unit in gear. For either type of RV you should block the tires that are on the ground so the unit does not move.  Not everyone will be able to do these steps, but if you can, here is what I suggest.
Jack up one end of one axle (motorhomes this means one end of the front) You then want to place a reference bar or pointer or stick near the tire, then rotate the tire watching the space between the reference pointer and the tire. Do this for the side of the tread and center of the tread. If you can see the relative movement of more than 1/4" you have a suspect tire. Mark the location where the tire is moving outward. you might snap a picture where you can also read the words on the sidewall at the location of interest for future reference. You might even consider a 10 second video that shows suspect movement.  I cover the general visual inspection and have a youtube VIDEO in THIS post.  I have a series of posts with more details on tire inspection HERE.
While Trailer owners can do this for each of their tires Motorhomes are probably limited to just "spinning" the front tire and even then need a HD jack.  Remember if unsure or do not have a solid surface to work on, or the proper tools, you may not want to do this rotating inspection.

If you have a suspect location you should contact your tire dealer. If you have a video you can show them that. Since they can see the tire close up if necessary they are in the best position to confirm there is a problem or to say all is OK.

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DOT serial. If you haven't recorded your full DOT serial numbers and kept that information with other important papers, this "downtime" might be a good opportunity when you have a nice weather day. Having this information might save you some time in the future if you hear there is a recall on your tire brand and size. Recalls are based on the DOT sericl and tires will be replaced for free if covered by a recall.

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Testing your TPMS.  I bet almost none of you have tested your TPMS. This should be done at least once a year. Some nice afternoon when you have nice weather with your traveling companion in the driver seat. AND when you have access to air of high enough pressure to top-off your tires. Go to each tire and unscrew the external sensor and have the person in the driver seat let you know that they can see and hear the warning for each tire position.  you don't even have to completely remove the sensor. Just unscrew enough to hear air leaking out. As soon as the monitor in the cab sounds you can screw the sensor back in to stop the air leak. Once you know that every sensor works you then need to go around and add back in the 5 psi or so that leaked out.
Doing this will also confirm you have your low pressure warning level set properly as the warning should go off as soon as you lose about 5 psi. I cover how I suggest you set your warning levels in THIS post.

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Finally, with all the time on your hands, you might consider simply reading through all the posts on my RVTireSafety blog. I don't expect you to remember everything but I believe that if you have reviewed the entire blog you will then know where you can go to get an answer to just about any question you might have on tires, tire pressure, valves, TPMS, weight and inflation.


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.

I run TPMS but the tire still came apart. Why?

I found a post from a fellow RV owner who has a TPMS but still had the tire come apart as seen below.

The tire in the picture shows clear signs of a "Run Low Sidewall Flex Failure" i.e. that sharp line around the sidewall with the melted body cord showing.

Yes, the driver got a warning of air loss but I wonder what the Low-Pressure warning level is set at, or if the system has a "rapid air loss" setting.

The "melt line" takes a few miles to develop with significant loss of air (maybe 25 to 40%) while still running at highway speeds. Just driving a mile or so with a 10% loss as you slow down to pull over is usually not enough time to get the body cord to the 350 to 400F required to melt the Polyester cords.

As I have covered how I program my TPMS in this blog back in August 2017. I suggested that the Low-Pressure Warning level be set such that the driver gets a warning as soon as the pressure has dropped to the pressure needed to support the tire load based on scale load and tire Load / Inflation table numbers.  Hopefully, those who subscribe to the blog or those who have gone back and read the posts followed my procedure and have adjusted their TPMS settings according to their RV vehicle needs.

This may take a little thought and effort depending on the brand TPMS. Some may allow you to directly set the low-pressure level while others may require you to calculate the required "set" level as they have a fixed percentage of air loss before they start the warning "beep".

Some systems provide a warning as soon as just a few psi is lost (from the higher hot pressure level) from a puncture.

Example  Your cold set pressure is 80 based on a minimum required inflation to support the load of 70 psi.  When running down the road at 60 mph your tire is probably at 88 to 90 psi. If your system only warns with a 25% pressure drop from the "set" pressure that means you can travel many miles before you get a warning at 60 psi. At that point, you have overloaded your time for many miles.

"Early Warning" or "Rapid Loss" feature might warn you as soon as your pressure drops from 90 to 87. This allows you to monitor the pressure as you look for a location to pull off the road.

TPMS is a powerful safety device but to get the maximum benefits it provides you need to do a little more than just screw the sensors onto your metal valve stems.

##RVT928

Will confusion on how much inflation be resolved?

Hi Roger. Do you think the tire pressure issue will ever be resolved to people accepting the values recommended by the tire manufacturers based on actual loading, or will some always be confused between the tables, the placard and quotes of Federal FMVSS Regulations from some on the forum? It seems so simple and logical, but some just don't get it. I still think a sticky written by you could settle the issue, but the moderators disagree. I suspect a concern for liability for the forum to advise something that contradicts the MH placard. I have a question for you about my RV. I carry 5 psi above the minimum for the load on my steer tires on a 60* morning @ 1100' altitude. When traveling, I had a morning @ 5000' and temp in the upper 30's. This produced a pressure of 1-2 psi below the minimum. I was going to be traveling into a warmer climate (95*), so did not adjust pressures. This type of condition can happen when we are traveling. I don't want to chase the pressures, so if traveling into warmer weather, I just go and watch the TPMS. I don't feel this is a problem but wanted your thoughts. Thanks for your thoughts and time.

Not sure if there ever will be a resolution to the Inflate to the placard vs Inflate to the actual tire load. I would liken this to the change oil every 3,000 miles vs change when the car's computer advises.

DOT has a goal of trying to make things simple and keep people safe. DOT also knows about the data that indicates that over half of the RVs on the road today have one or more tire overloaded (either too much load or too little air for the actual load) I really can't fault their approach as IMO many RV owners aren't willing to make the effort needed to learn the proper inflation and to then maintain it.

We have had tire inflation stickers in cars for many decades and there have been massive vehicle recalls because their tires were underinflated, I have read that many as much as 30% low. We don't hear anyone making a case that the accidents and fatalities were the driver's fault. That would be blaming the victim which isn't acceptable, even if true.

OK to your question. I am guessing your CIP is in the 90 - 110 psi range so IMO +5psi isn't enough to avoid the pressure fluctuation that results in your overload. This is why I suggest a +10% value for inflation margin over the table minimum number..
In my Class-C RV, with LT225/75R16 LR-E tires, I am lucky as my RV is rather light. I really only need 60 psi F & R based on my "4 corner scale readings. My certification sticker says 65 / 80 and my dealer delivered it at 64 psi all around. 

For my application +10% would indicate 66 psi and a  +15% margin > 69 psi. I run 70-75 
Even though I can be a bit "anal" about inflation I also wrote THIS post and I really do not mess around with my tire inflation.

My TPMS is set to warn before I would be in overload and I run across a scale once a year just to confirm I am not too far from my original "4 corner weights".

In 2014 I drove Ohio > OR > Calgary > Yellowstone >OH over a seven-week period with temperatures ranging from 90's to snow and elevation of 20' to 8,000' and never had to adjust inflation.

How I program my TPMS

As you know I am a strong supporter of having all RVs being equipped with Tire Pressure Monitor Systems. Since 2004 cars and many pickups with ratings less than 10,000# GVWR have come with TPMS installed by the factory.

RV companies have not been providing TPMS even as optional equipment from the factory, so we owners have had to make the purchase and install the warning systems ourselves. This means we need to ensure the warning levels are appropriate for our RV.

While I can't address how to program every different type or brand system, I can tell you how I have the system I purchased set up. I hope that after reading this post you can sit down with the information that is specific to your RV,  i.e., load on each tire position, tire size, Load Range and inflation info, and ensure your settings are appropriate for your personal situation. So I suggest you dig out the instructions that came with your TPMS and review your settings.

First off, some terms:

MOP - Minimum Operating Pressure - This is the minimum pressure that I think you should ever operate your tires. This pressure is found in the Load/Pressure tables and is the minimum pressure needed to support the actual measured tire load.

CIP - Cold Inflation Pressure. This is the pressure you would set your tires to before driving on them. The tire should not be warmed by either driving on them or having the tire in direct sunlight for the previous two hours. Some folks call this their "Set Pressure" or "Baseline Pressure". I suggest the CIP be set to MOP + 10%, or +10 psi, but not to exceed the pressure molded on the tire sidewall associated with its max load, as this tire pressure may be the intended max CIP for the wheel.

Tire Load - This is the actual measured load on the tires when the RV is at the heaviest you would ever expect to be. Ideally this has been learned by having individual scales under each tire position on your RV. If you can't get this number then at least get the RV on a truck scale such as a CAT scale at a truck stop and learn the actual load on each axle. We want to know each tire position or each axle, as loads are almost never completely evenly distributed. It is not unusual to have tire on an axle or on one end of an axle to have 500# to 1,000# more than another position. There are a number of posts in this blog on load and how to get the loads on each tire position. Note duals are considered one position. If you can't get individual axle ends measured then I suggest you use the measured axle load and assume one end has 53% of the axle load. This would provide at least a partial margin for error.

Hot Running Inflation - This is the pressure we see on a tire after it has been running down the road. I have previously covered the relation between Temperature and pressure in THIS post. Normally for properly inflated tires this will be 10% to 20% above your CIP.


When I bought my Class-C unit in 2008 the first "add-on" was a TPMS. At the time I was not really aware of the selection of TPMS available for the RV aftermarket but I found a system made by German electronics company Hella. It was an internal system but it was designed for passenger cars so I had some problems as my hot tire pressures occasionally exceeded the high pressure warning level programed by Hella. I learned to ignore the occasional high pressure warning levels. In 2009 I started to go to FMCA RV conventions and rallies where I discovered the aftermarket systems designed for the RV use. Based on features I felt important, I purchased a TireTraker TPMS.
When we bought our new 2016 Class-C coach, I needed a new TPMS and selected the TireTraker TT500 as I felt the features plus the lifetime guarantee made it the best option available for me.

Programing the TPMS:
This is usually a two-step process. First, you need to have the monitor/display "learn" which sensor is on which tire. For the TT500 it involves stepping through the monitor settings, identifying a position and installing a sensor at the appropriate position. Other brand TPMSs may have different setup steps or even may come pre-programed with each sensor marked by position. It's the next step that I think is also important to get right for your specific needs.

Setting the warning levels:
Before we start the actual process of setting the levels, it is important to know what the various levels should be. You also need to know which pressure warning levels you can set and which may need extra work if you can't set the levels yourself. Again I can't address every brand TPMS so you may need to re-read your manual or even contact the support people at your brand TPMS.

1. Low Pressure warning level. Some systems have a fixed % below your baseline pressure; others may allow you to set this level independent of the CIP. You need to know how to set this level or what the % below the baseline your system works on. My TT500 is pre-programed to warn at -15% from the "Baseline" or CIP.

2. High Pressure warning level. The TireTraker is pre-programed at "Baseline" + 25%.

3. High Temperature level. The TireTraker, as are many TPMS, is set to warn of high temperature at the 70 C to 75 C (158 F to 167 F) range. IMO it should be remembered that it is low pressure that generated high tire temperature. If you get a High Temp warning but the pressure is OK, then your sensor has detected high temp from metal components that transfer heat faster than rubber. This might indicate a bearing or brake problem which should be checked as soon as possible.

Here things get a bit complex:
We do not want to operate tires when under-inflated but if we set the CIP or Baseline pressure to just what is needed to support the load the TPMS will not warn till we are 15% underinflated. If our CIP is MOP + 10% (See definition of MOP above) we still could end up with an underinflated tire. So to meet my goal of never operating in an underinflated condition, I will need to set my TPMS "Baseline" pressure to MOP + 15%.

My situation and solution:
As measured by RVSEF (RV Safety & Education Foundation):
My front tires are loaded LF 1,900#   RF 2,100#   and
Rear Duals  LR 3,550#   RR  3,850#
Tires are LT225/75R16 LR-E

Based on Load & Inflation tables for my size tires, my MOP for my front tires on my Class-C is 60 psi based on the 2,100 load and also happens to be 60 psi based on my read dual load of 3,850.

I add 10 psi to 60psi and get 70 psi for my CIP.

If I want my TPMS to warn me before I get to my MOP, I need to set the "Baseline" pressure on my TPMS to MOP + 15%, which for 60 psi is 9 psi. So in my case my "Baseline" for the TPMS and my CIP are within 1 psi of each other. I don't worry about trying to measure or set my tire pressures closer than +/- 1 psi. I would say that if you are within +/- 2 psi when setting your CIP you are in good shape.

We need to be careful when discussing safety margins or pressure changes as people with Load Range C, D and E tires will have different calculations than the folks with LR-G & H tires where 15% might be 18 psi.

The High Pressure warning level for TireTraker is Baseline +25%, so with a Baseline of 69 to 70 psi, that equates to 87 psi. On a recent trip to Indianapolis in the heat of the day I noted my tire pressure had gone up about 15%, so my settings worked out for me.

##RVT805