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01-22-2015, 02:43 PM
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#1
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Senior Member
Join Date: Dec 2014
Location: San Antonio, Texas
Posts: 113
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Couple questions about tires
These may well have been asked before, but can't find any reference on here.
We have 275 80R/22.5 tires on our coach.
Any time air pressure is mentioned, it is always "cold". So, just what temp. is "cold".
Here in San Antonio, if I check the tires in the morning, the temp might be 40, but if I wait until mid afternoon, it just might be 75. So, If the pressure should be 100 psi and I set the pressure first thing in the morning, by afternoon, the pressure will be off. I have never seen a pressure chart that said "around 100 psi", so thinking they are talking a specific pressure.
On another note, those same recommendations suggest checking weight, and go so far as to suggest weighing each corner, and adjusting pressure accordingly. But, at the same time, they say to keep all tires on the same axle at the same pressure. Seems to be a bit contradictory.
And finally, do truckers adjust their tire pressure after unloading, or even after dropping half their load, then driving down the road a few hundred miles with half the weight? Having been around trucks and CDL licenses for awhile, I know checking tire pressure is certainly part of the pre-trip, but I don't recall any mention of adjusting pressure as the load changes, and certainly no mention of checking corner weight.
I ask all this because I just don't see most RVers being that focused on tire pressure. Even still, half a tank of fuel can weigh almost 500 pounds, most of which is on a single axle. Is that enough to be an issue?
I am thinking of finding a reasonable medium weight per axle, and going with that. Other than fuel load, there should not be much of a change, unless we stop at Golden Corral for lunch.
Any thoughts would certainly we welcome.
Ken Gasbarri
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01-22-2015, 02:52 PM
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#2
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Senior Member
Winnebago Owners Club Freightliner Owners Club
Join Date: Jun 2013
Location: 5 miles south of Lakeville, Mn
Posts: 3,047
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When I was a trucker I thumped the tires with my club or a hammer every morning. If they bounced the hammer back, they were good to go. We might check air pressure a couple times a year, and no, we certainly did not adjust for how much weight we were carrying.
I had trailer tires with close to a million miles on them and then they would start to blow out just from age. When that happened we would replace the whole set.
I inflate my tires to the cold setting and find that no matter what the weather is they will stay pretty close to that as they warm up during the drive. Up or down a few pounds really isn't a problem.
I have a Truckers Supply TST 507 system that I use on the Motorhome now. I check tire pressure in the morning from the drivers seat with a cup of coffee. All I am looking for is that one tire that doesn't match the rest.
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Jim and Carol Cooper with Oreo the Kitty
FAA ATC ret, VFW, AL, VVA, NRA
US Army Aviation, MACV Vietnam 65-66
2012 Journey 36M, Cummings 360hp
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01-22-2015, 04:01 PM
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#3
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Senior Member
Vintage RV Owners Club Fleetwood Owners Club iRV2 No Limits Club
Join Date: May 2014
Location: Gosnell, AR
Posts: 483
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Cold tire pressure has nothing to do with the temperature of the air. Cold tire pressure is measured before your tires have moved for the day. Hence everyone says to check them in the morning, because it is in the morning that people will begin their days travel.
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Berniece & Russell with LilBit a Netherland Dwarf rabbit
1987 Southwind, 1995 Ford F150 Supercab 4X4 toad
Life in the fast lane? No thanks! We will stop and smell the flowers at every chance.
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01-26-2015, 02:04 PM
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#4
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Senior Member
Newmar Owners Club
Join Date: Mar 2005
Location: Abbotsford, BC
Posts: 532
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Quote:
Originally Posted by DraginRat
Any time air pressure is mentioned, it is always "cold". So, just what temp. is "cold".
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As others have said, "cold" generally means that the tires haven't been run... and that the sun hasn't been on them (black tires heat up fast in the sun, so the air in the tire will heat, and expand, too... changing the pressure reading). I would assume that most tire manufacturer inflation charts were based on some "baseline" temperature... like 70 or 75 degrees, but I'm not sure what the exact number is. Most don't worry about that aspect of it and just check their tires first thing in the morning before they've been driven or heated by the sun.
Quote:
Originally Posted by DraginRat
On another note, those same recommendations suggest checking weight, and go so far as to suggest weighing each corner, and adjusting pressure accordingly. But, at the same time, they say to keep all tires on the same axle at the same pressure. Seems to be a bit contradictory
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The idea is this: weigh all corners of your coach, then for each axle, inflate all tires on that axle to the pressure needed to carry the largest weight measured for any tire on that axle. So, for instance, if you weighed the two sides of your front axle as 6,800 pounds for the driver's side and 7,000 pounds for the passenger side, you'd inflate BOTH tires to the pressure needed to carry the 7,000 pounds so that they are equal. Many advocate adding 5-10psi to whatever pressure your chart says for the weight... as a safety margin to account for variation due to different loading conditions, etc.
This video might help some, too:
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John
2005 Newmar MADP
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01-26-2015, 05:49 PM
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#5
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Senior Member
Freightliner Owners Club
Join Date: Apr 2002
Location: Pensacola
Posts: 2,728
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Quote:
Originally Posted by DraginRat
I am thinking of finding a reasonable medium weight per axle, and going with that. Other than fuel load, there should not be much of a change, unless we stop at Golden Corral for lunch.
Any thoughts would certainly we welcome.
Ken Gasbarri
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The tire pressure charts give the minimum air pressure for the tire to carry that particular load. Adjust your pressure to at least enough to support the maximum weight you have at the time. I also usually add 5psi to the chart pressure as a cushion for air lost checking the pressure or other variables...
Over-inflation causes a rough ride, under-inflation can damage the tire.. A tire run 20% underinflated is considered to have been run flat and needs to be inspected or replaced.
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Hooligan, Pensacola, Fl -U.S. Coast Guard 1956-1985
2016 Thor Siesta Sprinter 24ST diesel -1972 Moto Guzzi
2008 Suzuki Grand Vitara TOAD
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01-29-2015, 06:40 AM
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#6
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Senior Member
Join Date: Oct 2011
Posts: 124
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I believe 65 to 67 degrees F is best ambient to set air pressure and then I check with my TST 507 in the am with my coffee before moving on.
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Steve Corsetti
2010 Monaco Diplomat
St Petersburg, FL
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01-30-2015, 06:48 PM
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#7
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Senior Member
Join Date: Aug 2001
Location: North America somewhere
Posts: 30,982
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Cold inflation pressure means having driven less than a mile after sitting for 8 hours. I forget which tire mfgr's literature that came from.
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2000 Winnebago Ultimate Freedom USQ40JD , ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG 11B5MX,Infantry retired;Good Sam Life member,FMCA. " My fellow Americans, ask not what your country can do for you, ask what you can do for your country. John F. Kennedy
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01-31-2015, 09:52 PM
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#8
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Senior Member
Newmar Owners Club Freightliner Owners Club
Join Date: May 2012
Location: Today? Clayton, North Carolina
Posts: 5,093
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Both temp and pressure will increase above nominal as the tires are driven. Whether you start from an ambient of 40 degrees or 75 degrees doesn't matter-- it is all accounted for in the manufacturer's calculations.
A good source of information is Roger Marble, sometimes on this forum. He is a retired tire engineer. He also runs a blog under the name "tireman9". Check it.
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John and Diane (RIP Lincoln, 21 FEB 22) RVM103 NHSO
Fulltimers since June, 2012
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element
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02-03-2015, 03:23 PM
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#9
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Senior Member
Tiffin Owners Club
Join Date: Oct 2010
Location: Granger, TX
Posts: 595
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I agree with the above: it's in the table numbers somewhere as long as you haven't driven the coach. FYI standard atmospheric temp is 59F. That never shows up in the charts I have (Michelin). Always check before departure, but if the tires in the sun are a couple lbs higher, no problem as long as the lower side meets minimums. Don't chase the difference. Just don't go low!
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02-03-2015, 04:29 PM
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#10
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Registered User
Join Date: Feb 2013
Posts: 472
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Never have i heard so much about nothing... Cold means not driven. Not refrigerated, not iced down, not only on a freezing winter morning.
Cold. like it's been sitting all night. Check the air and drive the wheels off. This isn't a NASCAR race.
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