Trailer Tire Inflation and Outside Temps

[Dreamsend]

Okay, I admit I'm a naughty RVer.  I've had an EEZ Tire TPMS system on my trailer tires since the beginning of travels, but was not actually using the monitor.  :-[  Mostly cause I thought I had screwed up registering the last sensor to the monitor, but . . anyway, finally got around to turning on the monitor and checking tire pressures. Seems to be reading things okay.  I've been parked in a CG for 35+ days.  All were too low I think!  In April, the trailer was in to a dealer for an overhead fan install, and I ask them to put all the tires at 80 psi which is spec.  Maybe they didn't know how to get the locking sensors off and on without deflating things too much?  I did give them the removal tool, and demonstrated, but, , ,

I had a small portable compressor with me - one I'd had for 3-4 years in the S&B.  Seemed to work, but then on the second tire, no love.  Went to ACE late afternoon and purchased a new compressor. Short side story = Checked the monitor reading and first tire was still too low so I removed the sensor to use new compressor, and behold, the valve stem was stuck open.  Too late, so this morning I went back and got a valve stem repair kit!  Every RVer trailer should have one of these.  Replaced the valve stem just fine and when I plugged in the new compressor to continue airing up, the hose blew off the unit.  SO - back to ACE to return the broken thing.  Meanwhile went online to see if nearby ACE or Home Depot in another city had a useful compressor. NOT!@!  Ended up getting what I think is a nice unit at O'Reilly's which is across the street from the campground.  I've aired up the problem tire and the TPMS monitor reads 80 lbs.

So, after all that wordy intro, here's my question.  The TPMS is showing me temps of 97 and 99 degrees.  The tire pressures are 80, 78, 76, and 79 at those temps.

Now I always thought the recommended psi was supposed to be on "cold" tires.  But then, what's cold?  50 degrees, 65? 72? 

I suspect I'll get differing opinions, but to what degree do I inflate? and at what temps?  I know there is some play here as of course tires heat up on the road and it also still depends on ambient air temps.

Did I mention I was scheduled to leave here to day and head to Grand Canyon.  Needless to say, all the running around and waiting on compressors to inflate, repairs, and testing results etc. put me way too far behind for my liking.  So, I'm just hanging around here for another day.  To compensate, I'm going out to a really good Mexican eats place to treat myself.

Given that, I'll have time in the early morning to check the monitor again when it's cooler and inflate as needed.  I'd just like some advice on "what is needed".  The low pressure warning is set at 10% (72 psi) on the TPMS system.

Any thoughts or advice?

Linda

 

[SeilerBird]

I have always aired up my tires without thinking about the temperature.

 

[Gizmo100]

IMHO..........

Cold pressure is equal to the normal air temperature. If your in an area that has a extreme temperatures then you could adjust a few PSI accordingly.

 

[Back2PA]

"Cold" doesn't mean a certain temperature, it means they haven't been driven on that day, or for many hours. In the morning I'd inflate them to sidewall temp. Your first morning in a warmer climate, check the pressures and adjust accordingly (i.e., if you leave somewhere where it was 50 in the morning and inflated tires to sidewall pressure, and you travel to somewhere where it's 90 in the morning, As Gizmo100 suggests, you'll likely need to let a little air out. Do not adjust pressures based on the tires heating up from driving.

 

[TheBar]

Gizmo is right, it is the normal or ambient temperature of the outside air. This means if you check your tires right after driving 100 miles it will be several lbs higher. You never let air out of your tires to lower the pressure when the tire is hot. That can cause the tire to fail because it will run even hotter. 80 lbs of pressure is 80 lbs whether the air temperature is 20 or 100. But Back2PA is also right airing up one morning at low ambient temp and the next day the air temp is higher you may need to let some out. The tire is built to maintain its profile and rated weight limit at 80 lbs pressure. You don't air up to the maximum rated air pressure of the tire, you go by what the trailer or motorhome manufacturer recommends. A few lbs difference won't hurt the tires but it can affect handling. Under inflated tires can be mushy but over inflated tires can over react to rough road surfaces giving too much feedback to your steering wheel. Tires are part of the shock absorber system and are expected to absorb part of the road irregularities according to how the suspension is tuned. Many people add heavier shocks, sway bars, and suspension gadgets when really all they may need is the right air pressure and correct alignment.

 

[Arch Hoagland]

According to Bridgestone cold is:

Tires are considered cold when the vehicle has been parked for three hours or more with the tire in the shade.

 

[FastEagle]

The low pressure warning is set at 10% (72 psi) on the TPMS system.

Any thoughts or advice?

Linda

A Cold tire is one that has sit idle for at least 3 hours.

That low pressure warning needs to be set at 80 PSI.

Here is a hypothetical. Tire size; ST235/80R16 LRE at 80 PSI provides 3420# of load capacity. At 72 PSI it has dropped to 3184# of load capacity.

 

[grashley]

IMHO..........

Cold pressure is equal to the normal air temperature. If your in an area that has a extreme temperatures then you could adjust a few PSI accordingly.

Agreed.  The actual temp will change from day to day, but the temp is the temp.  Simply adjust pressure first thing in the morning to the chosen pressure. 
NEVER release pressure to adjust tires except for the morning check.
Pressure does not to be exact.  Anything from 79 to 82 is close enough.
Set the TPMS low alarm to 75 psi to avoid nuisance alarms.
Set TPMS high alarm to 95 psi.  If you get nuisance alarms, raise it to just above the observed pressure.  If no alarms, lower the set point a couple pounds.

On my FW, tires are rated at 3960# at 85 psi.  They are only carrying a bit less than 3000#.  I inflate to 75 psi.

 

[Dreamsend]

That low pressure warning needs to be set at 80 PSI.
at 80 PSI provides 3420# of load capacity. At 72 PSI it has dropped to 3184# of load capacity.

That;s a good point, but setting the alarm at 80 psi isn't very practical either.  In my case, I think I'm good.  Per the trailer manufacture, with whom I discussed weights and loads several times, the trailer tires are actually rated at 11,000 lbs capacity, but they rate the overall trailer maximum weight capacity at 10,400 lbs. (I just assumed this max was due to some other structural component) In addition, I'm still at around 16% excess capacity (based on the 10,400), so I'm not too concerned I'll exceed the tire's capabilities.

Just for grins, in your example, if the tire is inflated to 84 psi, does the load capacity increase?  Just wondering.

Linda

 

[Dreamsend]

Great replies, and I think I got it!  Makes complete sense -- inflate to trailer manufacturer's posted specs (on side of trailer) when tires are at ambient temp for not less than 3 hours, ignore any changes due to driving, and then adjust pressure next day (or when temps vary drastically) for any changes in ambient temps that result in more than 2-3 lbs of pressure change.  Perfect!  I've been watching the TPMS monitor as temp cools now in the evening and sun goes down, and it looks like, generally, that the pressure drops by about 2 psi for a 10 degree drop in temp.  That's an early observation.

I"ll check early tomorrow before sun hits the one side, and when temps will be about 30 degrees lower than this afternoon.  Pretty sure I'll need to add 5-6 lbs to each tire, which with the new pump #3 is pretty easy.  I feel so much better about managing the tires now, so once again, thanks to you all for your generous advice. 

Best to all
Linda

 

[Ernie n Tara]

Also remember the phrase "in the shade". I've seen sun side tires run 3-4 psi higher than those in the shade.

Ernie

 

[xrated]

Probably, a better description for the term "cold" would be Ambient Air Temperature.  In other words, only set your tire pressure when they have reached or is at the ambient temperature of your location.  That obviously cannot happen if a tire is in direct sunlight, or you have been driving on them.

 

[dave54]

Pressures will increase as you drive down the road and the tires heat up.  Fact of life, happens every day.  The tire manufacturers know this and is already factored into their recommendations.  Don't adjust during the day for temperature fluctuations.

 

[BruceinFL]

No, if the tire press is higher than 80 (your tires are rated at 80 psi cold max), the max load does not increase above that shown on the sidewall.

 

[FastEagle]

That;s a good point, but setting the alarm at 80 psi isn't very practical either.  In my case, I think I'm good.  Per the trailer manufacture, with whom I discussed weights and loads several times, the trailer tires are actually rated at 11,000 lbs capacity, but they rate the overall trailer maximum weight capacity at 10,400 lbs. (I just assumed this max was due to some other structural component) In addition, I'm still at around 16% excess capacity (based on the 10,400), so I'm not too concerned I'll exceed the tire's capabilities.

Just for grins, in your example, if the tire is inflated to 84 psi, does the load capacity increase? Just wondering.

Linda

No. But that depends on the tire's maximum load range. If a load range E tire is inflated to a cold 84 PSI there is no load capacity increase. It's just an over inflated tire with stiffer sidewalls and is probably going to wear the center treads out faster than the rest.

It the increase to 84 PSI is caused because the tire is being used on the highway, it's just a normal thermal condition and no adjustments are recommended or necessary. 

 

[grashley]

Increasing pressure to 84 is as FastEagle said.  However, lowering cold pressure to 70 psi will REDUCE carrying capacity.