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Old 03-20-2016, 04:56 PM   #1
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Proper tire pressures & tpms

Hi,

I've got a question on tire pressure and what to beware of...

I have a 35 ft gas Class A - we're full-timing for a year. The stickers on the drivers side & engine area say ALL tires should be 80psi cold.
The tires say 100psi Max COLD.
The tires are all relatively newer

I recently got a TPMS and I realize a couple of things:

1 - I'm not sure what temp COLD is for which the tires need to be 80PSI... In some places before we leave at 9am, it's 45deg, others 60+deg. So my tires tend to be 75-82 in the morning. I wish they could all be the same PSI even if diff at diff temps, but I haven't been able to do it.

2 - the tires on my right/passenger side all go up in PSI during driving, more than the driver side. I weighed the rig left and right and they are very close to the same. Not sure why they are always increasing more on the right. For example, as I drive, the driver/left side tends to be in the low 90s PSI, while the right side is 99/100PSI.

3 - the temps of the tires react similarly, but mostly the FRONT tires seem to be highest in temps - again, rears are 90F, and fronts, closer to 100F. When I stop (city driving, or reach destination), often the tire temps shoot up to about 120F, in one case it said, 140 on one of the duallies, but when I touched the tire, it felt cold, so it seemed like the TPMS sensor was really hot from being near the middle of the tire and in the sun??

I'm not sure HOW accurate these TPMS are, but the brand is EEZRV caps (not flow thru)

I'm just wondering:
a) should I be worried about anything?

b) what alarm setting should I set for PSI, there is already a fast leak alarm, but I set it to 102 I think for PSI, and only one tire on my rt/pass side hits that.

c) what should I set for a temp monitor, I currently have it at 120F, but I have no real knowledge of what I should be setting the alarms for.

Thanks!!

Kevin
(2007 FW Hurricane, 34b (bunks))
Yes, 4-down toad (04 Saturn)
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Old 03-20-2016, 05:08 PM   #2
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Your starting point when tires are at air temp (cold) should be the mfg recommendation or a few precent higher. It is OK for tires pressure after travel to rise as tires warm and air temps warm as you are seeing. Having high pressure alarm set 25% higher than the recommended cold temp is also OK.

The max pressure on tire sidewall is when tires are at air temp (cold) and they can go much above that when warm after travel so it's OK if they rise above 100 PSI when traveling as you are seeing. I see the same thing on my Winnebago Vista Class A.
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Old 03-20-2016, 05:18 PM   #3
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The tires do not say Max pressure (unless you have car tires), they say the minimum cold pressure to support the maximum rating of the tire.
The pressure on the sidewall of a Michelin RV tire and many others is not the "Maximum" the tire should ever have (unlike car tires) it is the minimum to support the maximum rated carrying capacity of the tire.

From the Michelin RV Tire Guide:
Quote:
"If you look at the tire's sidewall, you'll see the maximum load capacity allowed for the size tire and load rating, and the minimum cold air inflation needed to carry the maximum load."
From page 6 of the GoodYear RV Tire and Care Guide:
Quote:
"How much air is enough?
The proper air inflation for your tires depends on how much your fully loaded RV or trailer weighs. Look at the sidewall of your RV tire and you’ll see the maximum load capacity for the tire size and load rating, as well as the minimum cold air inflation, needed to carry that maximum load."
From TOYO:
Quote:
Q: What are the consequences of inflating the tires to accommodate the actual loads?
A: If the inflation pressure corresponds to the actual tire load according to the tire manufacturer’s load and pressure table, the tire will be running at 100% of its rated load at that pressure. This practice may not provide sufficient safety margin. Any air pressure loss below the minimum required to carry the load can result in eventual tire failure.
But then they go ahead and publish a weight/pressure chart allowing lower pressure for RV's!!

From the August 2010 Motorhome Magazine "Tread Carefully" tire article:
Quote:
The maximum load capacity allowed for the size tire and load rating and the minimum cold air inflation needed to carry that maximum load are located on the tire’s sidewall.
From our owners manual:
Quote:
Federal law requires that the tire’s maximum load rating be molded into the sidewall of the tire.
If you look there, you will see the maximum load allowed and the cold air inflation pressure required to carry that stated maximum load. Less air pressure restricts the tire to carry a lighter load.
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Old 03-20-2016, 06:55 PM   #4
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Thanks Powercat - do you mean tire mfr, or RV mfr? I've heard that you MUST adhere to the Chassis requirement (which in my case says ALL tires at 80 psi cold)

MR D - i stand corrected. It says max load=4540 single (or 4300 dual) at 100PSI cold. I have a 20,500 rig with 7k in front (~3500# each) and 13,500 in rear (~3375# each) so I should be well below that?

I'm still confused a bit. It seems that it should be 80PSI by ford? and then that rating chart, which I don't have for less than max weight.

The tire is an off brand - but I'e given the max load stamp above.

Thanks!
Kevin
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Old 03-20-2016, 08:43 PM   #5
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Kevin,

I set my TPMS temp alarm to 157F. This is the temp recommended by TST TPMS manufacturer. Tires on hot roads will get near those temps but usually not over. Set your pressure per the tire manufacturers load chart as referred to "cold" or ambient temp. So if it is 75F outside then the TPMS should indicate 75 give or take a few degrees. (Sun or shade) Air your tires to the load of your coach per the manufacturers charts. Most of these are standard per tire size versus load applied. If your front axle is 7k pounds, 3.5k per wheel with 245/70R19.5 tires then Toyo recommends 85 psi per tire. Michelin is pretty much the same but they may differ depending on the style. Michelin offers 3 styles on this size truck tire. Note that the higher the pressure in the tire for the same load (85 psi vs 95 psi @ 7k axle) the rougher the ride will be.

So, after all that see what EEZRV says to set the temp alarm to and see what your tire manufacturer recommends for your coaches weight. ou should find this info on their websites. Once you have this input into your TPMS and tires enjoy the ride unless the alarm goes off.

Oh, one final thought. If your tires are between 5 and 7 years old, no matter tread depth, replace them. Don't cheat on RV tires. Get tires from reputable manufacturers, Michelin, Toyo, etc. (I do not recommend Goodyear as I have never has a good tire from them on anything) One blow out could cause thousands of dollars damage to your coach.
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Old 03-20-2016, 10:35 PM   #6
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The pressure molded into the tire sidewall is the minimum pressure required to carry the tire's maximum rated load.

The pressure indicated on the RV manufacturer's placard (usually placed near the driver's seat) is the minimum pressure required to carry the axles' maximum weight, also indicated on the same placard.

Many people will say that one needs to weigh each corner of your rig, and run the pressure that is determined based on the tire manufacturer's weight and load tables. This is generally a good idea, but if you talk to pretty much any tire manufacturer, they will refer you to their tables and then state that, notwithstanding their tables, the best option is to use the RV manufacturer's rating.

With that in mind I use my RV manufacturer's recommendations, which is 95 psi in front and 90 psi out back. With those pressures I can safely load up my RV to the actual GAWR and know that the tires will not be overloaded. The 4-corner weights of my RV indicate that I could be running 75 psi up front and 70 psi in the rear. Those pressures will safely carry the weight as measured, but if I added any more cargo then I'd be overloaded.

Now let's look at the TPMS question: I know my actual weight, and the minumum pressure I need to carry that weight. If the pressure drops below that minimum then the tire is overloaded, so I set the minimum pressure on the TPMS for that pressure: 75 up front and 70 in the rear. The tires are actually running 95 psi up front and 90 psi out back. The pressure molded into the tire sidewall indicates the load rating of the tire and my tires state 110 psi. Figuring some expansion for heat I set the maximum TPMS pressure at 130 psi. For the maximum temperature I believe that TST presets that for the temperature at which the rubber in the tire starts to break down - 180 degrees if I recall.

In the 2 years I've been running the TPMS system I've never seen a tire temperature over 120 degrees - it usually sits about 10 degrees higher than the ambient air temperature, winter or summer.
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Old 03-21-2016, 12:15 AM   #7
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kevin, both mr d and alan made it clear - it is minimum psi to carry the max load.

as you probably already know, the #1 cause of tire failure is under inflation. if you pay attention to those highway semi trucks, their tires are really full. i haven't seen anyone has modestly flat area at the very bottom kissing the road surface. they are really round that tell me they have been pumped to their full recommended psi.

on the sticker inside my coach, it says 120psi front and 110psi rear. i do exactly that to 120psi at front and more than that at 115psi for rear. it gets good fuel mileage, drives great and most importantly, safe on tires.
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Old 03-21-2016, 12:49 AM   #8
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Quote:
Originally Posted by kevinkevin View Post
Hi,

I've got a question on tire pressure and what to beware of...

I have a 35 ft gas Class A - we're full-timing for a year. The stickers on the drivers side & engine area say ALL tires should be 80psi cold.
The tires say 100psi Max COLD.
The tires are all relatively newer

I recently got a TPMS and I realize a couple of things:

1 - I'm not sure what temp COLD is for which the tires need to be 80PSI... In some places before we leave at 9am, it's 45deg, others 60+deg. So my tires tend to be 75-82 in the morning. I wish they could all be the same PSI even if diff at diff temps, but I haven't been able to do it.

2 - the tires on my right/passenger side all go up in PSI during driving, more than the driver side. I weighed the rig left and right and they are very close to the same. Not sure why they are always increasing more on the right. For example, as I drive, the driver/left side tends to be in the low 90s PSI, while the right side is 99/100PSI.

3 - the temps of the tires react similarly, but mostly the FRONT tires seem to be highest in temps - again, rears are 90F, and fronts, closer to 100F. When I stop (city driving, or reach destination), often the tire temps shoot up to about 120F, in one case it said, 140 on one of the duallies, but when I touched the tire, it felt cold, so it seemed like the TPMS sensor was really hot from being near the middle of the tire and in the sun??

I'm not sure HOW accurate these TPMS are, but the brand is EEZRV caps (not flow thru)

I'm just wondering:
a) should I be worried about anything?

b) what alarm setting should I set for PSI, there is already a fast leak alarm, but I set it to 102 I think for PSI, and only one tire on my rt/pass side hits that.

c) what should I set for a temp monitor, I currently have it at 120F, but I have no real knowledge of what I should be setting the alarms for.

Thanks!!

Kevin
(2007 FW Hurricane, 34b (bunks))
Yes, 4-down toad (04 Saturn)
Re: #2. I talked to the Michelin Factory Rep last week with the same question. He told me that higher pressures and temps on the right side tires are caused by the crown in the road. The MH is tilted slightly to the right as you are driving due to the crown. That puts more weight on the right side causing higher temps and higher pressures.
Re: #3. I too have higher temps on the right front tire especially when I have come to an extended stop. I think it is due to engine exhaust heat being sensed by the valve sensors. It clears to normal temp once I get moving again.
My TPMS valve sensors are external and are not only sensing the temp of the air in the tire, but also ambient air temps. You will often notice that the temps on the sunny side of the MH are higher than the shady side. You will also sense road temp, axle temp, brake heating, etc.
I recently used an infrared thermometer to measure external tire and brake temps. I found that after hard braking, city driving, the external tire temps were a bit lower than read on the TPMS, but the wheel temps were higher due to braking. My conclusion was that the TPMS was reading ambient temps.
For the reasons stated above, I mostly monitor tire pressures and take the temps with a grain of salt.
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Old 03-21-2016, 08:33 AM   #9
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ajpaero ,

thanks for that - That was one of my theories as well, the slight lean to the right, since I'm generally in the right lane. But, I wasn't sure since it's not always corrected when I drive in the left lane for a spell.

as for #3, yes, exactly - I do notice the temps on sunny side are higher and also the ambient temps related to those things. Sometimes though, it's the driver front side that shoots up, but yesterday, the rear inner dually shot to 140 - which I suspect was not the actual tire.

Thanks!
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Old 03-21-2016, 09:08 AM   #10
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You have gotten alot of good info so far, and seem to be getting there

One other thing that could be making the tires seem different is obviously different weights and pressures,
but also different make of tires
as well as size of tires
(and yes, an older tire will be marginally smaller than a new tire)

and alignment of the rv - if the tire is 'scrubbing' and not running straight and true, it will be hotter... good luck !
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Old 03-21-2016, 05:42 PM   #11
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You have fallen victim to the TPMS information overload syndrome. First time users tend to try to fit the tire pressures into a specific window. After a few years of use you will realize how much fluctuation there can be. I sometimes think I was better off when I checked the pressures once a month with a gauge and thumped them every morning before departing. That's all the OTR truckers do. There was less to worry about. Now, I take the readings with a grain of salt and watch for any developing trends. With my system, I went for over a year without adding or releasing any pressure and was never below the minimum cold psi and never close to the alert temps.
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