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Old 10-18-2020, 07:02 PM   #1
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High Tire Pressure?

Not sure if I am posting this in the right forum but here is my question.


I am a new owner, Thor Windsport 31S, and I have just added a VESAFE TPMS. I have set the cold pressure at 82 on all tires as recommended on the safety sticker in the coach. Set the parameter on the TPMS to 82 as well. When traveling, after about an hour, the system started alerting as each tire reached 99. Top pressure for a 2 1/2 hour trip was 102. The system automatically alerts when pressure increases by 20%.


Is there an issue with my tires or is 20% not a reasonable increase to alert on? I don't see a way of overriding the 20% threshhold. Should I leave my tires inflated to 82 and set the parameter slightly higher on the TPMS? I have not had the opportunity to weigh the rig and recalculate the pressure yet, and not sure if that would make that significant a difference.

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Old 10-18-2020, 07:37 PM   #2
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What size and load rating are your tires?

Not unreasonable increase at all if it was warm or hot. If you were traveling in cool weather you probably have your tires loaded above the 82 psig

Weights, load charts, safety factor is the only way to know where to determine the optimum tire psig
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Old 10-18-2020, 07:41 PM   #3
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I agree: we need a bit more info about how much you were carrying...
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Old 10-18-2020, 08:17 PM   #4
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My ACE also has 82 as the recommended PSI from which I set the +20% -10% thresholds.

I see you're in Florida.

I discovered 20% wasn't high enough on hot summer days by about 1 to 2 PSI. I reset to +25% and haven't had an alarm yet.
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Old 10-18-2020, 08:40 PM   #5
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Quote:
Originally Posted by lwmcguire View Post
What size and load rating are your tires?

Not unreasonable increase at all if it was warm or hot. If you were traveling in cool weather you probably have your tires loaded above the 82 psig

Weights, load charts, safety factor is the only way to know where to determine the optimum tire psig

Thanks for the feedback.



Tires are Goodyear G670 RV 245/70r19.5. Max load rating is 4540. GAWR on front axle is 7000 lbs and rear is 12000 lbs. Duel tires on rear axle. We don't have an excessive load as far as luggage weight is concerned, fuel was full, gray and black tanks empty and about 20 gallons in fresh water.



Temperature outside was 75 to 80 and TPMS said tires were between 94 and 100 temp.
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Old 10-18-2020, 08:43 PM   #6
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Originally Posted by 16ACE27 View Post
My ACE also has 82 as the recommended PSI from which I set the +20% -10% thresholds.

I see you're in Florida.

I discovered 20% wasn't high enough on hot summer days by about 1 to 2 PSI. I reset to +25% and haven't had an alarm yet.

Thanks. We're in CA right now on our way home. Temps are 70's. I didn't see a way to increase the parameter on the monitor. I'll have to check the instructions again. they were a little bit lacking.
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Old 10-18-2020, 08:50 PM   #7
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Originally Posted by mcpandt View Post
Thanks for the feedback.



Tires are Goodyear G670 RV 245/70r19.5. Max load rating is 4540. GAWR on front axle is 7000 lbs and rear is 12000 lbs. Duel tires on rear axle. We don't have an excessive load as far as luggage weight is concerned, fuel was full, gray and black tanks empty and about 20 gallons in fresh water.



Temperature outside was 75 to 80 and TPMS said tires were between 94 and 100 temp.
You have a nice heavy duty 14 ply tire. 82 psig is on the light side even if you were empty INMHO, I would to up to 85. Your sidewall max cold psig should be 110

I ran my F550 with similar tires at 110-115 psig but usually had 16 ply due to the heavy tandem trailers we towed often. A lot of cattle on a 32 foot trailer is a bit heavier than an RV for sure.

No issues with handling or tire wear either when running empty but you did notice the potholes
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Old 10-18-2020, 10:59 PM   #8
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Quote:
Originally Posted by mcpandt View Post
Not sure if I am posting this in the right forum but here is my question.


I am a new owner, Thor Windsport 31S, and I have just added a VESAFE TPMS. I have set the cold pressure at 82 on all tires as recommended on the safety sticker in the coach. Set the parameter on the TPMS to 82 as well. When traveling, after about an hour, the system started alerting as each tire reached 99. Top pressure for a 2 1/2 hour trip was 102. The system automatically alerts when pressure increases by 20%.


Is there an issue with my tires or is 20% not a reasonable increase to alert on? I don't see a way of overriding the 20% threshhold. Should I leave my tires inflated to 82 and set the parameter slightly higher on the TPMS? I have not had the opportunity to weigh the rig and recalculate the pressure yet, and not sure if that would make that significant a difference.
82 psi is correct for the inside back tires assuming there is 6,000 lbs on each side in the rear. The inside back will always run the hottest because they get less cooling air than the other 4 tires. To get the exact tire pressures needed you need to check your traveling weight on each corner. At least on my coach, that is 5,960 lbs on the left rear and 5,840 on the right rear. After weighing, I decided on 85 psi on the rear @ 80 degrees. On a 100 degrees day at 60 mph, I see pressures of 105 to 111 on the inside rear and 102 to 106 on the outside rear. With 2,760 lbs on the left front and 2,680 lbs on the right front I run 73 psi @ 80 degrees. The on a 100 degree day the fronts run in the 91 psi range. Tires are Sumitomo ST-718 14 ply.
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Old 10-19-2020, 03:05 PM   #9
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Something to check

My TPMS (Tire Minder) sensors can be plus or minus 3 percent of the tire pressure.

When temps change I inflate my tires to 105 PSI (a little more than inflation table re commendations) and then install the sensors. If the sensor for one tire reads 108 that is what the parameters on the system are set to. This allows 20 percent higher than 108 vs 20 percent of 105 which is lower. I haven't had an over pressure alarm since I took this approach.

My issue now is during two trips when I go up in elevation (mountains) one of my sensors (driver's side front) gives me a false rapid deflation error. It happened to me on a two lane in WV with no place to pull over and again on an interstate in the mountains of Virginia. I'm getting ready to contact Minder and have them send me a new sensor.
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Old 10-24-2020, 08:19 PM   #10
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First used my spreadsheets with the info you gave.

Your GAWR's filled in with the tire specifications gave front 88 psi and rear 82 psi.

And with other spreadsheet .
If 82 psi filled at 50 degr F, 99psi means that temp in tire is 140 degrF, wich is on the edges of what I determined it may maximaly have.
At 65 degrF filled even 157 drgr F., so to hot if no external factors.

First some questions: where the pressures front and back that high, I expect only on front?
At what temp did you fill 82 psi?
What was your speed at that moment?

My conclusions:
you have external sensors, they give pretty accurate the temperature between in and outside tire air, so you cant rely on the temp-reading. You read 94/100 degr while in real in tire 140 or 157 depending on fill temperature

If rear also 99psi overloaded.

Front certainly can do with higher pressure.
As long as you did not weigh, we have to do with the GAWR's, and my calculatons, based on your info indicates to low cold pressure.
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Old 10-26-2020, 06:22 PM   #11
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Back in my motorcycle days tire pressure for a particular load was always a hot topic of discussion. Owners' manuals typically give different tire pressures for various loads (eg, one up, two up, luggage loaded). One of the much touted ways to select the correct pressure for the load is the "4psi rule."


Pressure increases with heat (about 1 psi for every 10 degrees of temperature increase to the best of my recollection, whether the result of the weather or rolling friction). As the tire rolls down the highway, it produces heat and temperature always increases, which increases pressure, and is quite normal. However, too much of an increase in temperature and pressure is undesirable, and may indicate initial under inflation. Correct inflation can be determined by measuring the amount of pressure increase and adjusting accordingly. Those who claim to know such things state the so-called rule here:


https://www.tyrereview.com.au/tyre-advice/the-4PSI-Rule


Keep in mind, of course, that the tire can only take so much. Increasing pressure to compensate for excessive heat and pressure rise becomes dangerous if the tire is then exceeding its limits. There is a load/pressure limit for every tire. If not called upon to carry its maximum load in a particular application, the vehicle inflation recommendations will be less than the maximum inflation specified by the tire's manufacturer.
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Old 10-26-2020, 06:36 PM   #12
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Originally Posted by CHRIS1u View Post
Back in my motorcycle days tire pressure for a particular load was always a hot topic of discussion. Owners' manuals typically give different tire pressures for various loads (eg, one up, two up, luggage loaded). One of the much touted ways to select the correct pressure for the load is the "4psi rule."


Pressure increases with heat (about 1 psi for every 10 degrees of temperature increase to the best of my recollection, whether the result of the weather or rolling friction). As the tire rolls down the highway, it produces heat and temperature always increases, which increases pressure, and is quite normal. However, too much of an increase in temperature and pressure is undesirable, and may indicate initial under inflation. Correct inflation can be determined by measuring the amount of pressure increase and adjusting accordingly. Those who claim to know such things state the so-called rule here:


https://www.tyrereview.com.au/tyre-advice/the-4PSI-Rule


Keep in mind, of course, that the tire can only take so much. Increasing pressure to compensate for excessive heat and pressure rise becomes dangerous if the tire is then exceeding its limits. There is a load/pressure limit for every tire. If not called upon to carry its maximum load in a particular application, the vehicle inflation recommendations will be less than the maximum inflation specified by the tire's manufacturer.
One of the very few tire post I can't take some issue with

Great post!
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Old 10-26-2020, 07:20 PM   #13
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@CRIS1u( so no long quote again.

Differences between motor and RV.
Motor pressures in the range of 35 psi, and tires made for 99mph and more. then 4 psi rule OK.
RV often 80 psi or higher .
Then ,to get same temp in tire, the pressure rises more, about 10% is 8 psi for 80 psi.
And the 22.5 and 19.5 inch max speed 75mph .
Then temp in tire rises more at same speed and with that the pressure , so more then 10% pressure rising over 120 psi, so even 15 psi can be alright.

Then motors loading varies more, because driver and codriver and load, is bigger part of total weight.
RV, if you weigh them once per axle-end, the weights stay practically the same trough the years. So one pressure setting can do.

Then I read about motors with tmps, that calculate back the pressure to 65 degr F, so even totally warmed up with 140 degrF in tire, still gives the fi 35 psi , and not the 4 psi higher.

To get back to the original question, motortires mostly have a comfortable reserve in maximum load. RV often have tires to the limits.
So RV can have warm pressures higher then max allowed given on sidewall.
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Old 10-26-2020, 07:38 PM   #14
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I think we can all agree that heating of the tire causes the internal pressure to increase. The heat is the result of the tire's flexing and friction against the surface (road). The greater the tire's deflection the more heat is generated. Likewise the greater the force applied to the tire in moving the vehicle (acceleration/deceleration - vertically or horizontally) will increase tire temperature. Tire flexing is greater with the load and speed. So the higher the cold pressure, the less pressure rise (delta P) for a given load.

The maximum tire pressure listed on the tire's side is not an estimate but an actual computation based on carrying the rated load at the rated speed on a standard day and not damage the tire from overheating.
Lastly most tire pressure tables use ideal gas laws for the pressure rise predictions. Usually there are no corrections for water vapor in the tire's air and that value can vary greatly. The reason that nitrogen is recommended for airing up the tires is it contains no water vapor. The water vapor problem can be explained this way. Assume you use a small potable compressor to completely air up a tire at 70 degrees and 80% humidity to 100 psi. That night the temperature drops to 25 degrees and and ice forms on the inside of the tire. The internal pressure of the tire will decrease by about about 1 psi per 5 degrees F or the gauge should read 91 psi. Checking the tire pressure (with gloves on) it reads 84 degrees because the water vapor holding up the tire before is now ice.
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Old 10-26-2020, 08:06 PM   #15
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Tire Pressure

What the hell is a VESAFE TPMS ?

Alajeu
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Old 10-26-2020, 08:24 PM   #16
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@ Beau388

At 70 degrF from the 100psi overpressure, is 114.5 psi absolute pressure the part of the gas-shaped water can maximally be 0.36 psi.
So at 25 degrF the 91 psi can maximally go down to 90.64 psi and not 84
At 25 degr F partial pressure of watergas is 0.06.
So 100% humidity then is 0.06 psi for part of the water as gas.

I read these values from a spreadsheet I once found, and I added the coloms for psi and degr F.

Liquid water takes about 1/1000 of the space as water as gas. Ice probably a bit more space , say 1/950 .
So practically no space .
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Old 10-28-2020, 02:23 PM   #17
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https://www.goodyeartrucktires.com/p...dinflation.pdf

Page 6 describes the correct tire pressures for your tires. Weight and configuration (single vs dual) make a huge difference. Make sure you are starting off with the correct cold tire pressure to begin with then adjust your sensor after that.
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