Originally Posted by TT_Vert
Good to know thank you. I will increase my cold tire pressure to 70/75 and keep an eye on pressures. I was under the impression that these were maximum acceptable pressures cold or hot.
FWIW, my canned thoughts on the settings of the TPMS system for a motorhome:
- WHAT DOES A TPMS TELL ME, REALLY?
- A TPMS on a motorhome 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 DRIVING
- As long as the cold pressure is set appropriately for the weight any pressure increase while traveling is pretty much irrelevant UNLESS the tire is overloaded or under-inflated.
- 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 MEAN, REALLY?
- 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 DRIVING
- 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.
- HOW DO I DETERMINE WHAT PRESSURE TO SET MY MOTORHOME TIRES TO?
- Motorhome tires should have their MINIMUM pressures set by the actual "as weighed" weight according to the tire manufacturer's manual, also called the load inflation chart.[B]
If you have never had the motorhome weighed, start with the pressures on the inside sticker. Those pressures, by law, are the MINIMUM pressure needed to fully support the design weight rating of each axle, its GAWR (gross axle weight rating) when using the tires installed by the motorhome manufacturer.
If you are using an axle weigh, such as a CAT Scale, the generally-accepted wisdom is to add 5% to the total axle weight before looking up the inflation chart. The load inflation charts assume an even weight distribution left-to-right and that's a good assumption for a car or a truck, but not an RV.
RV's can have as much as a 10% difference in weight between the driver side and the passenger side. If you use the load inflation charts with the real axle weights you can be under-inflating one tire, the heavier side. On ours that could be a 5 PSI under-inflation and if the weights were shifted a bit it could be a 10 PSI under-inflation on the heavier side tire.
NEVER try to extrapolate the load inflation chart numbers. Always use the next higher weight in the chart if you're between two.
I actually add an extra 10% over the inflation chart to accommodate for day-to-day temperature variations without having to "chase" the tire pressures. I only have to add air maybe once a year on our Class A.
For example, before we leave Florida in early April I'll have the tires at about 100 PSI even though my sticker pressure is 90 PSI. When I get to northern Ohio a week later with temps in the mid-30's the tire pressures will be in the low 90's for all tires. I will not need to adjust the tire pressures until we leave in November, maybe not even then.
NEVER let air out of a hot tire even if the pressure is higher than the molded sidewall pressure. The pressure will drop as the tire cools. DO NOT exceed the pressure on the sidewall when inflating the tire cold either.
- LOW PRESSURE ALARM
- This is the critical 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 lower 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.
For example, if your tires are supposed to be set cold to 90 PSI to handle the weight they're carrying, then the Low Pressure alarm should be set to alarm at no lower than 81 PSI and 72 PSI is considered flat.
- HIGH PRESSURE ALARM
- As long as the cold pressure is set appropriately for the actual weight of the motorhome axles (or corners), the pressure increase while traveling is pretty much irrelevant.
Why? Because the tires are designed to easily handle those pressure increases as long as the tires are not overloaded. The pressure increases are normal and they are expected. Again, NEVER let air out of a hot tire. Never.
- HIGH TEMPERATURE ALARM
- On a motorhome it's pretty much useless as long as the tires are not underinflated or overloaded. That's why the default is set so high, close to 160 degrees F. I leave mine set to the default. It's the temperature of the layers of tread that's important, not the temperature of the air in the valve stem.
That large metal wheel also acts as a heat sink. Unlike a trailer with smaller wheels and tires, the large wheels and tires can dissipate some heat from a bad wheel bearing or a dragging brake. That makes a TPMS "high temperature" alarm less useful on a motorhome for detecting mechanical failures.
- 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? Or be futzing with a phone when an alarm goes off while driving? 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.
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.
THIS IS IMPORTANT ->
A motorhome has dualies 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 dually motorhome that means the remaining tire is now carrying 100% of the weight on the rear 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. And then you go through the whole problem again except you may now have also damaged the tire you just replaced.