Good answers to tire pressure questions

[davidbrady]

www.bridgestonetrucktire.com


It's clear from the answers that there's no simple set-it-and-forget-it tire pressure setting. This is illustrated very well when setting in one temperature extreme and driving to another. Owner's must always monitor their pressures and deflate and inflate as necessary depending upon ambient conditions.

[pgchin]

Good find David,
Yep I check pressures with a gauge EVERY time I roll, EVERY DAY, and adjust as necessary. I also use the IR gun to check for anomalies every 2-3 hours. While "hanging" around last week getting the bird new "sneakers", I had a chance to see about a dozen "road kill" take offs the crews brought in. You would be AMAZED how the INSIDE of the tire was alligatored and the rubber was separating from the belts, outside, the tire looked new!!!!!! My takeoffs were 8 years old and looked new both outside and inside........FWIW I also "cover them". The first set of 3 sets I did not cover and only got 4-5 years before they started cracking, the last set covered lasted 8........... I also roll the bus EVERY 2 weeks even when not in use........you know the drill about rolling tires, chemical release, blah, blah, blah..............

[davidbrady]

Good plan Pete, keeping 'em rolling is key! I'm envisioning a bus treadmill! ROTFL :-)

The author does a good job of illustrating how setting tire pressure is a compromise. Manufacturers know that the tire pressure will build from friction heat while driving, and pressure will also increase or decrease due to ambient temperatures and altitudes. So what do they recommend? The recommend a compromise setting that kinda works across the usual ambient and altitude swings that a driver is likely to see during a days travels, but they correct it by insisting that driver's check and adjust their pressures every day when the tires are in a "cold" condition.

What the manufacturers really want is a consistent tire sidewall deflection. The author points this out with its reference to tire cross section. Tire engineers don't care about internal tire pressure, they care about tire sidewall deflection. Too little deflection can be corrected by decreasing the tire's internal pressure, to much by increasing the pressure. It's the tire's deflection that controls all the properties that the tire engineer is looking for from slip angles, to grip, to tire temperature, to promoting the swirl of chemical release, to ride quality, to longevity, to puncture resistance, to scuff resistance - the list goes on. An incorrect deflection will swing the balance of these properties. We could do away with tire pressure gauges if we could measure deflection accurately.

The author also implies that a tire inflated to the proper deflection (pressure) in a cold condition will be slightly overinflated when hot, but he says that this is okay and that the tire is constructed to handle these pressure increases. In other words, it's non-ideal. In an ideal world, if the tire has the proper deflection in a cold condition, then as the tire heats up pressure could in theory be bled off to restore the optimal sidewall deflection. I'm imagining an active tire inflation control system that adds and bleeds air during a days travels all the while maintaining the perfect sidewall deflection!

[pgchin]

David,
Yeah I agree, I believe sidewall deflection really is the key item to watch having done exactly that in the last two sets. I can't speak for the new Michelin set yet but on the other two sets I had, for every 10 degrees F of temp, I lost 6lbs of air at "cold" status. So, technically, if I set my pressure here in Florida at 85 degree's F, and drive to Michigan, where it is say, 60 degrees F, I just lost 15 lbs of air! I am now severely under inflated! This actually happened to me one summer very early on in the learning curve! Now you know why I check every morning I roll!
FWIW anecdotal data, the new Michelin's ride SO SMOOTH AND SOFT, compared to my other brand, I thought they were under inflated and kept checking the pressures! This years walk about will be a new learning experience again! The other brand was a steel belt design.

FYI for those of you on 8.25's like me:
http://fleetowner.com/question/proper-rim-size

I had the same issues this fella had. When I went to try and buy other brands ( 4 different brands) from the guy I used to use, the computer told him I needed 9" rims??????? HUH??? He even called one of the brand reps and they said, yep we used to spec 12r's on 8.25 but we no longer do????? HUH??? SO finally I said the heck with it and went with Michelin who DO spec 12r's on 8.25........ Just relating my trials and tribulations, your experiences and decisions will differ! :-)

[patticake]

Talking about "problems" with air pressure differences, think about an airplane.
When I was in the Air Force, we had to check the air pressure(on B-52's) before every flight. They had to be exact. As a kid, I never thought much about it, until my crew chief told me why(other than safety). He said, basically, think about the changes the tire goes through from the ground to high altitude and back, plus, when upon landing, they are not rotating at all, until they touch the runway.
Gives a new respect to what a tire goes through in it's life.

[cmillsap]

(06-21-2013 09:08)pgchin Wrote:  Good find David,
Yep I check pressures with a gauge EVERY time I roll, EVERY DAY, and adjust as necessary. I also use the IR gun to check for anomalies every 2-3 hours. While "hanging" around last week getting the bird new "sneakers", I had a chance to see about a dozen "road kill" take offs the crews brought in. You would be AMAZED how the INSIDE of the tire was alligatored and the rubber was separating from the belts, outside, the tire looked new!!!!!! My takeoffs were 8 years old and looked new both outside and inside........FWIW I also "cover them". The first set of 3 sets I did not cover and only got 4-5 years before they started cracking, the last set covered lasted 8........... I also roll the bus EVERY 2 weeks even when not in use........you know the drill about rolling tires, chemical release, blah, blah, blah..............

God bless you, Pete,

I would venture a guess that 9 out of 10 owners/drivers do not gauge their tires everyday, much less IR them for temps. Further, not many people are going to adjust their tire pressures based on the ambient. I think the tire manufacturers use about 60*F to rate their tires.

I must confess that I don't gauge my tires everyday. I gauge the tires before the first day out and about every 5 days on a trip but I do a visual and feel the tires for excessive temp every time I stop. Often I will substitute a mallet to bang on the tires that I keep by the entry door but not each time I stop.

Man! I don't want to be checking tire pressures every day!
This is the reason why so many are resorting to tire monitors. They are great as long as they function properly. As you know, they are not failsafe. I had a set of TPMS on my former rig, they drove me crazy. But some people have had good results using them with few problems.

You can set the pressure and temperature parameters and they will tell you if you have a tire problem. Again, that's if they work properly all the time.

[folivier]

I use a tire monitor system on my coach. It's not perfect but helps mainly if a blowout does occur hopefully it will give me a heads up before it happens so I can prepare for it.
But I also check my tire temps everytime I stop with an IR gun. Don't always adjust for pressure due to ambient temp. Whenever I check my pressures some tires have been in the sun so will be 5# or so higher than the shaded ones so I take that into account if I adjust my tires. If they are correct in the afternoon and temp. drops overnight I won't add pressure.

[davidbrady]

(06-21-2013 20:34)patticake Wrote:  Talking about "problems" with air pressure differences, think about an airplane.

Steve,

A few weeks ago my 6 year old had a cross-over cub scouts ceremony at the Pisgah Astronomical Research Institute (PARI) here in the Great Smokey Mtns of Western North Carolina. PARI has a museum and amongst the displays were several spare parts from the space shuttle program. They had a Michelin tire used in the main landing gear on display. I think the price of the tire from Michelin was something like $6000.00 (per tire). The tires were used once and then replaced. Here's some additional fun facts:

[tcandt]

(06-21-2013 12:03)pgchin Wrote:  David,
Yeah I agree, I believe sidewall deflection really is the key item to watch having done exactly that in the last two sets. I can't speak for the new Michelin set yet but on the other two sets I had, for every 10 degrees F of temp, I lost 6lbs of air at "cold" status. So, technically, if I set my pressure here in Florida at 85 degree's F, and drive to Michigan, where it is say, 60 degrees F, I just lost 15 lbs of air! I am now severely under inflated! This actually happened to me one summer very early on in the learning curve! Now you know why I check every morning I roll!
FWIW anecdotal data, the new Michelin's ride SO SMOOTH AND SOFT, compared to my other brand, I thought they were under inflated and kept checking the pressures! This years walk about will be a new learning experience again! The other brand was a steel belt design.

FYI for those of you on 8.25's like me:
http://fleetowner.com/question/proper-rim-size

I had the same issues this fella had. When I went to try and buy other brands ( 4 different brands) from the guy I used to use, the computer told him I needed 9" rims??????? HUH??? He even called one of the brand reps and they said, yep we used to spec 12r's on 8.25 but we no longer do????? HUH??? SO finally I said the heck with it and went with Michelin who DO spec 12r's on 8.25........ Just relating my trials and tribulations, your experiences and decisions will differ! :-)

When I switched to Michelins, I did because I swapped my BMW Continentals for Michelins and had the same experience. Michelins on '95 Pretty Penny had the same result over the Bridgestones they replaced. Quieter, smoother and greater traction on curves.