Initial TPMS Tire Pressure Setting?

[reubenray374]

I will be taking my maiden trip in 10 days. I won't be able to get my Vibe weighed before then, but I will be well below the max. I have ST205/75R14LDR tires. The max pressure for these is 65 psi.

What should I set the TMPS for?

[markb5900]

I set mine at 65.

[pclevel1]

I have 225's max 80psi. I air to 60 to allow for expansion and set the TMPS at 65-70.

[NXR]

My canned thoughts on using a Tire Pressure Monitoring System (TPMS) on a trailer (the advice is a bit different for a motorhome):

- WHAT DOES A TPMS TELL ME, REALLY? - A TPMS on a trailer, while towing, should be considered more of a trend monitor. In other words, if all of the tires are behaving essentially the same and all of the tires are behaving essentially as you know they have in the past, the absolute numbers AND how much they change while towing are not that important.

Note that the direct sun on one side will raise the temps and pressures of those tires, more than you might expect. You can even see this effect while parked. That's why tires must have their pressure set while "cold".

"Cold" simply means "Today's outside air temperature BEFORE the tires have been used and BEFORE the direct sun has a chance to warm the tires."

"Cold" is NEVER a specific temperature.

- WHY YOU SEE PRESSURE INCREASES WHILE TOWING - As long as the cold pressure is set appropriately for the weight, and for a trailer with more than one tire on each side that almost always should be the pressure molded into the sidewall due to interply shear stresses, any pressure increase while traveling is pretty much irrelevant UNLESS the tire is overloaded OR a mechanical failure is occurring, such as a wheel bearing failing.

A failing wheel bearing can cause the tire to heat up because the tire cannot spin as freely. That temperature increase will also cause some increase in the tire pressure. How much? More than the other tires is all anyone can say for certain.

- WHY ARE PRESSURE INCREASES NORMALLY NOT A CONCERN? - Because the tires are designed to easily handle those pressure increases as long as the tires are not overloaded OR under-inflated. The pressure increases are normal and they are expected.

- WHAT DOES THE PRESSURE ON THE TIRE SIDEWALL REALLY MEAN? - The pressure molded into the tire sidewall is a constant source of confusion. It is NOT the maximum pressure the tire should ever see.

That molded pressure is the air pressure needed in the tire to safely support the MAXIMUM weight that the tire is designed for WHEN COLD.

- WHY YOU SEE TEMPERATURE INCREASES WHILE TOWING - It's the same reason as above. When the tires are being used there is road friction and their temperature will go up, the same as how their pressure goes up. Also, the sun shining directly on one side will raise the pressure and temperatures on just those tires.

That being said, on a trailer, an abnormal TEMPERATURE increase might be indicative of a failing wheel bearing or a dragging brake. "Might" being the operative word.

THIS IS IMPORTANT -> If you do not already do so, consider buying an infrared temperature "gun" and get in the habit of "shooting" the wheel hub temperatures each time you stop, as soon as you stop because they can cool quickly. If one wheel hub is significantly hotter than the others you have a problem, perhaps a big problem.

- LOW PRESSURE ALARM - The critical alarm on a TPMS is the Low Pressure alarm. A tire that is 20% below the required pressure is considered flat and subject to hidden damage.

The Low Pressure alarm should be set to no more than 10% below the required cold pressure to give you an adequate amount of time to get pulled over, add air to get to a tire shop, or whatever is needed.

So if your tires are supposed to be set cold to 65 PSI to handle the weight they're carrying, then the Low Pressure Alarm should be set to alarm at no less than 59 PSI and 52 PSI is considered flat. No, that's not much of a difference.

- HIGH TEMPERATURE ALARM and HIGH PRESSURE ALARM - Because these are somewhat useful on a trailer, they should be set to something higher than you see while towing. I know, that's a somewhat useless answer. [emoji4] Read the TPMS manual for suggestions.

In other words, you need to do some trips to know where they should alarm. For example, if the highest pressure you ever see on your 65 PSI tires is 80 PSI, you might consider setting the HIGH PRESSURE alarm to 85 or 90 PSI.

The key is to have the "HIGH" alarms set high enough that they will NEVER go off when nothing is wrong. I made that mistake once and trust me, it is not something you want to try to deal with at 65 MPH just to find out it was the hot sun that did it.

- SEPARATE TPMS DISPLAY OR A PHONE APP FOR THE DISPLAY? - Do you really want a critical safety feature dependent on a phone being charged or on some phone update breaking the TPMS functionality? Not me but perhaps you are OK with it.

- IS IT IMPORTANT TO HAVE A TPMS THAT IS FULLY CONFIGURABLE? - Note that at least one TPMS manufacturer does NOT let the owner set their own alarm thresholds. They try to do your thinking for you by using a "percentage of baseline" for all alarms. When selecting a TPMS system be aware of that limitation. Personally, I would never buy that system but others like it.

- BUT A TPMS IS SO EXPENSIVE! - A TPMS is not cheap but it is cheap insurance and only has a one-time premium payment. [emoji4]

In most cases the price of a TPMS is less than the cost of one tire and much less than repairing the damage from a tire that disassembled itself at highway speeds because it was driven on flat for many miles and you never noticed.

- WHY ALL-METAL VALVE STEMS ARE IMPORTANT - When installing a TPMS, strongly consider having the tire valve stems replaced with all-metal valve stems. The small, extra weight of a TPMS sensor screwed on the end of a valve stem WILL cause the valve stem to flex more because the valve stem is a lever, and now it's a lever with a weight on the end.

THIS IS IMPORTANT -> An RV can have two tires (or more) on one side because that number of tires is required to safely support the weight on that side. When one tire fails, the other tire(s) must assume the weight that the flat tire was supporting.

On a two-axle RV that means the remaining tire is now carrying 100% of the weight instead of 50% and undoubtedly is massively overloaded. The longer that tire is overloaded the more likely that it will fail sooner.

If you drove on one tire and have no idea when the other tire failed, you REALLY should replace BOTH tires on that side. The remaining tire will fail sooner but no one knows it if will be in weeks or months, but it will fail. And then you go through the whole problem again except you may now have also damaged the tire you just replaced.

And NEVER let air out of a hot tire. Never.

Make sense?

Ray

[bob213]

When I bought my TST monitor it recommended 20% above and 10% below your normal inflation psi.

[jimmarako]

The TPMS has nothing to do here. The correct pressure to set the tires to is on a label on the side of the trailer. The odds are it says 65PSI for you tires. Set it to that and measure however you want: gauge, tpms,...

Jim M.

[PhilFromMaine]

Set it to what is "stickered on the front left side of your trailer at a "cold" temp . Cold temp means a tire that hasn't been rotating for some time.

[GolfingDave]

Originally Posted by reubenray374

What should I set the TMPS for?

Specifically asking about TMPS not cold PSI. NXR and Bob213 are explaining that correctly. And the manufacturer of the TMPS system might have it in their install manual.

Now if he is asking about cold tire temp in his post but saying TMPS.............


Edit: Original Poster clarifying would help.

[5picker]

Originally Posted by reubenray374

I will be taking my maiden trip in 10 days. I won't be able to get my Vibe weighed before then, but I will be well below the max. I have ST205/75R14LDR tires. The max pressure for these is 65 psi.

What should I set the TMPS for?

No clarification needed... the OP asked what to set the TPMS pressure to.

For MAX pressure in the TPMS, most manufacturers recommend +20% of MAX tire cold inflation value.

In the OP's case for high pressure alert... 65 PSI +20% (13 PSI) = 78 PSI.

Low pressure alert is often several PSI different between manufacturers but I like 10%. So again for the OP... whatever they decide on for cold load pressure, (according to THEIR load and tire manufacturer recommendations) deduct 10%.

[pclevel1]

With the TMPS 77, I set my baseline alittle higher. Just my preference.

[SailorSam20500]

Originally Posted by PhilFromMaine

Set it to what is "stickered on the front left side of your trailer at a "cold" temp . Cold temp means a tire that hasn't been rotating for some time.

Seems most RV manufacturers of towables are now specifying cold PSI as the sidewall max of the tires they install when built.

[reubenray374]

Now that I have my answer for the tire pressure I have another question. The paperwork that I have shows the lugs to be torqued to 90 to 120 ft. lbs. This is a wide range for this.

What do I do them at? Also I was surprised that my 4 way lug wrench would not work on my tires.

[ependydad]

Originally Posted by 5picker

No clarification needed... the OP asked what to set the TPMS pressure to.

For MAX pressure in the TPMS, most manufacturers recommend +20% of MAX tire cold inflation value.

In the OP's case for high pressure alert... 65 PSI +20% (13 PSI) = 78 PSI.

Low pressure alert is often several PSI different between manufacturers but I like 10%. So again for the OP... whatever they decide on for cold load pressure, (according to THEIR load and tire manufacturer recommendations) deduct 10%.

Agreed, and for others coming along- here's a handy little calculator that helps with this math:

TPMS Configuration

[5picker]

Originally Posted by reubenray374

Now that I have my answer for the tire pressure I have another question. The paperwork that I have shows the lugs to be torqued to 90 to 120 ft. lbs. This is a wide range for this.

What do I do them at? Also I was surprised that my 4 way lug wrench would not work on my tires.

What size are the lugs?

These lug nuts/aluminum wheels often take a thin wall socket. 4 ways mostly don't fit. Thin wall, deep well socket a short extension, a breaker bar and a torque wrench.

Don't be surprised you don't have a jack either.

[darrellr]

Originally Posted by SailorSam20500

Seems most RV manufacturers of towables are now specifying cold PSI as the sidewall max of the tires they install when built.

As they should. They cannot specify for tires installed after they build it. PSI specs are for tires, not trailers.

[reubenray374]

Originally Posted by 5picker

What size are the lugs?

These lug nuts/aluminum wheels often take a thin wall socket. 4 ways mostly don't fit. Thin wall, deep well socket a short extension, a breaker bar and a torque wrench.

Don't be surprised you don't have a jack either.

I was able to find a 3/4" socket that fits the lug and my torque wrench. My torque wrench is 16" long and I have a 2" and a 6" extension for it.

I don't plan on changing many flat tires if possible. I have three road hazard plans to do this for me. I guess now I don't have to pack the 4-way lug wrench.

[jimmarako]

Originally Posted by reubenray374

I was able to find a 3/4" socket that fits the lug and my torque wrench. My torque wrench is 16" long and I have a 2" and a 6" extension for it.

I don't plan on changing many flat tires if possible. I have three road hazard plans to do this for me. I guess now I don't have to pack the 4-way lug wrench.

Double check your spare. On my Flagstaff (and the Rockwoods) the spare uses a different size lug nut so you need two wrenches to change a tire. Possible 13/16" for the spare.

Jim M.

[jim1]

Do yourself a big favour and replace the crappy tin covered factory lug nuts with the solid no tin covered ones of the same size. The tin covers on the factory nuts will spin sooner or later making it a P in the A#@ to change. And buy at least one set of extra lug nuts that will work with the spare tire rim. Its usually a different lug nut required for the thiner rim, or you might be stuck waiting for a tow.

[PhilFromMaine]

Originally Posted by 5picker

No clarification needed... the OP asked what to set the TPMS pressure to.

For MAX pressure in the TPMS, most manufacturers recommend +20% of MAX tire cold inflation value.

In the OP's case for high pressure alert... 65 PSI +20% (13 PSI) = 78 PSI.

Low pressure alert is often several PSI different between manufacturers but I like 10%. So again for the OP... whatever they decide on for cold load pressure, (according to THEIR load and tire manufacturer recommendations) deduct 10%.

I read the OPs question too quickly. I should have said fill the tires to the value on the side of the TT. I believe my tire pressure sensors automatically add the extra PSI percentage to account for increased pressure- temp variation. I will have to check the sensor specs again.

[reubenray374]

Originally Posted by jim1

Do yourself a big favour and replace the crappy tin covered factory lug nuts with the solid no tin covered ones of the same size. The tin covers on the factory nuts will spin sooner or later making it a P in the A#@ to change. And buy at least one set of extra lug nuts that will work with the spare tire rim. Its usually a different lug nut required for the thiner rim, or you might be stuck waiting for a tow.

I have a full size socket set in my trailer toolbox.