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Checking tire pressures
#1

So I got to thinking about tire pressures.....

What temperature do you check your tires at?
What temperature do you check your tires at?  I know the books all say to check your tires when they are cool.  But what exactly does that mean?  My cool (in central Alabama) maybe 70F while our friends in the ozarks may wake up to 5F and think what a nice day.  That range of temperatures surely has an affect on the tire pressures.

So I did a little research....
   

This chart came from Michelin and reflects the general belief that air pressure should be adjusted by 2% for each 10F change of temperature.  (Increase temperature for lower temperatures).  (I suspect that these pressures at the cold end are far in excess of what the rims can hold)

So to answer my own question, I think that the charts (see below) that relate pressure to load carrying capacity on tires assume 65F.  So, if you are close to 65F in the morning, go for it.  If the temps are much higher or lower, adjust accordingly.

How much Pressure?
   

Of course you have to know what your coach weighs to use this chart, but the point is that the tires are spec'd for a range of pressures.   This implies some sort of sweet spot.  Too much pressure and your ride is rough and its not good for the tires.  Too little and the ride is better but you risk a blowout.

I have decided on 115 lbs (front), 110lbs (drives) and 100lbs (tag).  

ps.  I used to run 85 lbs in tag (this was from the michelin chart, but I bumped it up to 100 and I think the coach rides smoother.  Your milage may vary.

How old is your air gauge?

I have three manual air gauges and a TPMS on my coach.  Would a reasonable person assume they all would match?  Well....THEY DONT.  All the manual gauges are good quality but with slightly different designs.  I tested all the tires with all four devices.  I found a maximum difference of 2lbs between the lowest and the highest.  I know 2lbs is not a lot but it could be just the difference between a low tire and a blowout.

Why is this important?  What if your gauge is reading much lower than reality.  How would we know?  I ended up with two manual gauges reading about the same and the other manual gauge and the TPMS reading about the same.  (Again they were all pretty close).

I read a recommendation that we use a new gauge to verify our old gauge....continue using the old gauge and only get the new gauge out to verify the old gauge.  (keeping the new gauge in a very protected and safe spot...Like not in your toolbox).  It would be fun at the next rally to test our tire gauges and see how much they vary.

Finally, I am going to print some cards that will allow me to keep track of the tire pressures over time.  I might put the temperature / pressure conversion chart on the back.  Let me know if you are interested and I will print some extras.

Cheers,
bill

Bill Johnson
Birmingham, Alabama
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#2

Good information Bill. Michelin Truck Tire Service Manual states that:

"Ambient temperature will affect the pressure within the tire. For every 10-degree temperature change pressure readings will change between 1 and 2 pounds per square inch (psi). Consider this when checking pressures. Check all tires when cold at least 3 hours after the vehicle has stopped. Never bleed air from hot tires."

The chart you quote would indicate a larger change than that, more like 3 psi but the incremental change does seem to be a little greater at temperatures I hope to never experience with my Newell (-20 to -50). I am assuming that at higher temperature 100-120 degrees F, the pressure change is closer to the 2 psi per 10 degree temperature change.

To further complicate the matter, another factor in tire pressure is altitude. According to the Michelin Truck Tire Service Manual:

"Additionally, altitude can have a slight affect on pressure. For every 1,000 foot increase in altitude above sea level, pressure will increase approximately 1/2 psi. For example, a tire inflated to 100 psi at sea level will read slightly over 102 psi in Denver, Colorado."

Michael Day
1992 Newell 43.5' #281
NewellOwner.com
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#3

The tire companies have designed their tires to safely and reliably carry a load and to ride reasonably well. The air pressure in the tire is what supports the load going down the road. The combination of PSIG tire pressure and the square inches of road contact determine the load carrying of the tire. For instance, on a tire supporting 6000 pounds, at 100psig there would need to be 60 square inches of road contact. (60 in squared X 100 lbs/in squared = 6000 lbs)

Tires are round and must flatten in order to produce the needed contact surface to carry the load. Flattening requires flexing of the tires and flexing generates heat. Tires are designed so that the flexing needed for developing the weight supporting contact surface will not generate heat injurious to the tire. Increasing the air pressure, for a given weight supported, reduces the contact surface, reduces heat generation, reduces ride softness, and reduces maximum braking capability. Reducing tire pressure , for a given supported weight, increases tire temperature, increases steering squirreliness, increases ride softness, and increases tire wear. Finding the right balance is the job of the tire designer and then, having made their decisions they can tell us how much air pressure is needed to produce the contact surface for supporting the weight.

As users all that above is the easy part; for a given tire, so much weight requires so much pressure. Now how do we take into account the effects of ambient temperature. We are directed to measure the pressure only when the tires have been resting for some time, usually an hour or two. Why? The manufacturer knows they will heat up as we drive, and they know about how much they will heat up above ambient temperature. Their tables are designed to create the designed surface area while driving, when the tires have heated as much as they would expect above the ambient temperature. Their expectation is that you will set the pressure using their tables and with the tires cooled to ambient. When first driving the tires will be underinflated and generating extra heat, that is OK because that extra generation will dissipate as the tires approach the increased pressure that is expected. Then everything will be as intended.

We only need to concern ourselves with the 65 degree table above is if we are setting tire air pressure in a setting with a temperature significantly different from the one in which we will be driving, say, in a 65 degree garage and pulling out and driving in minus 20 degrees.

If we set the pressure in our tires according to the load tables and make any adjustments when the tires are at ambient temperature to where we will be driving we will be OK. Unless we start in Montana in February and drive to Palm Springs there is usually little reason to worry about changing pressures along the road. However, I consider it good personal practice to measure tire temps when I stop and occasionally back that up with pressure checks.

Jon Kabbe
1993 coach 337 with Civic towed
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#4

Thanks Michael, That's why I posted....its not always clear. The chart came from Michelin as well. Note I said 2% not 2 lbs. The altitude adjustment seems to be pretty insignificant to me.

Bill Johnson
Birmingham, Alabama
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#5

On the issue of variations in readings of tire pressure gauges, this summer Tom and I compared our tire gauges in Ruidoso, NM. Between our 3 tire gauges and his new TPMS readings, we had variances of about 8 psi (lowest to highest readings) at 110 psi as I recall. So compared to temperature and altitude changes, individual tire pressure gauges still seem to be the biggest variable.

Coming back from Montana this summer, I had an elevation drop of over 5,000' between southern Colorado and Austin for a decreased tire pressure of roughly 3 psi due to altitude. Temperatures changes were actually not much different but in the opposite direction as it was still pretty warm in Colorado in early September.

Michael Day
1992 Newell 43.5' #281
NewellOwner.com
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#6

The analysis engineed in me is getting turned loose for a moment. The idea that tire pressure increases with altitude ain't necessarily so, to quote a song from a musical. It is not true if the pressure is measured in absolute terms, that is the pressure above a pure vacuum. It is true if it is measured as pressure above ambient pressure. So, while the absolute pressure, which uses absolute zero pressure as the reference, stays the same as the tire gains altitude, the pressure sensed by our gauges, which use ambient as the reference, increases only because the reference ambient pressure decreases in absolute terms.

In terms of the performance of our tires, only the gauge pressure, which uses ambient reference pressure, matters since that pressure is the one that determines the size of the contact between the tire and the road. Thankfully, that simplifies our task.

Personally, a gauge that was off by more than a couple pounds would bother me, even if I knew by how much it was off. I wouldn't be confident I was making the correction in the right direction on the typical fuzzy minded morning checks, and going the wrong way with the correction doubles the error. With a TPMS I wouldn't worry so much. I would set the alarms in moments of great clarity (supposing my fuzziness doesn't falsely lead me to think it's clarity) having accurately corrected for sensor errors, assuming the errors are consistent, and listen for alarms and avoid looking at the pressures.

Nah, I would look, I would mess up the mental calculation, I would presume I set the alarms wrong, and I would be so distracted driving I would head butt a semi and probably die. Sooo, the only thing to work for me would be to get the sensors reporting reliably and not get distracted. Hey, that's my personal illness, just like wanting to make sure we are talking the right pressure. I admit my OCD seems to have served me better than it has my significant others.

Jon Kabbe
1993 coach 337 with Civic towed
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#7

Well said, Jon!

I think OCD is warranted with tires!

My TPMS went into alarm on the way home last trip. It said I was 50 lbs low in one of the drive tires. I dove for the shoulder and sat there for a few minutes (in heavy traffic). It stayed the same. I sought some counsel and was advised to limp to the next exit where I could safely examine the situation.

When I got to the truck stop at the next exit, I measured the tire (and all the others). It was fine! When I got back in the coach and looked at the TPMS, it said all was fine. I yelled at it....."What do you mean its fine now?????!!!! What about a little while ago???!!!!" The TPMS just looked back at me and said...."Tire Pressures OK".

bill

Bill Johnson
Birmingham, Alabama
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#8

TPMS has been on my list of wants for the Newell. I "bit the big one" and put all new Michelins on it 2 years ago. Since we rarely travel a long way from home and usually in the early fall I have just been vigilant on the tire pressures. I recently found a 2 year old TST system with 7 sensors on Craigslist for $140. The seller was a very nice gentlemen about an hour drive from my home. I got a great deal and made a friend who travels in a Type C motorhome! Now I need one more sensor and probably a repeater. No hurry since the Newell is in storage until next spring.

1993 Newell (316) 45' 8V92,towing an Imperial open trailer or RnR custom built enclosed trailer. FMCA#232958 '67 Airstream Overlander 27' '67GTO,'76TransAm,'52Chevy panel, 2000 Corvette "Lingenfelter"modified, '23 Grand Cherokee.
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#9

I won't do a sales pitch again for TPMS, but I am a believer.

Relative to changes in ambient, here is how I use the TPMS. During our summer trip we saw morning ambient temps of 85 to 35 on travel days. I could see the tires were lower in the lower ambients obviously but I didn't touch the pressures. What I did do is compare axles, i.e were the two fronts still equal, four drives still equal, and tags still equal? I felt this was sufficient to tell me if the lower pressure was due to a leak or lower ambient pressure.

Richard and Rhonda Entrekin
99 Newell, 512
Maverick Hybrid Toad
Inverness, FL (when we're home Cool )
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#10

I think the problem with adjusting tire pressures due to either temp or altitude comes with where are you going? Are you staying at the same altitude for a day or more of driving? Or will the temperature rise to 80 or higher while driving?
I've never adjusted my pressures due to altitude or temp and have found low pressures would climb to the acceptable ranges after an hour or so of driving. But if I had raised the press then they could be over inflated once they warm up or I descend to a lower altitude. These are all readings using a TPMS.
Richard has a good idea to compare axles, just be aware if the sun is warming up the tires on one side the pressure may be higher due to the increased temp.

Forest & Cindy Olivier
1987 log cabin
2011 Roadtrek C210P
PO 1999 Foretravel 36'
1998 Newell 45' #486 

1993 Newell 39' #337 
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