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Old 05-25-2009, 03:28 PM   #1
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Correct Tire Pressure

Around the campfire this weekend the subject of tire pressure came up. 2 of the RVers had Newmars the same size as mine. Both said they run all 8 of thier tires at 120#, that is the max pressure stamped on the MICHELIN 305 tire. that is what they run in all 8.. I stated that that I run mine with what the plackard says that NEWMAR put beside the driver seat..(I think if memory serves me) 85# in front..85# on main axle, 75# on the tag.. I was told that is too low.. Run um all 120#, run cooler, better milage etc. What is the thoughts of other NEWMAR owners of like coashes......Vacman
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Old 05-25-2009, 04:25 PM   #2
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truckers love to run at max shown on tire..Rv'ers like to run at min needed for safety to get a better ride..so first weigh your coach..4 positions best..front and rear axles separate at minimum, then check your tire pressure chart from the tire manufacturer. Set the tire for that weight and pressure..I like to add 5# per just in case...I also had a 4 point weight done and know I'm 1,000# higher on right rear..so adjust by using that as the weight for both sides. Higher than needed pressure may save fuel and run cooler and may also handle less well in an emergency as not as much of the tire is in contact with the road. Key is knowing your weights, I weigh my coach every year on the first trip out with it fully loaded. I will now put on my fire proof suit and wait for the discussion as nothing raises the discussion level higher than tire brands and tire pressures..
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Old 05-25-2009, 05:23 PM   #3
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Quote:
I will now put on my fire proof suit and wait for the discussion as nothing raises the discussion level higher than tire brands and tire pressures..
You can take your fire suit off, as you are 100% correct. Well....except for the extra 5 psi. No tire manufacturer recommends "extra air".

Get individual wheel weights and set your tire pressures according to the tire manufacturers charts.

Too high of a pressure results in uneven tire wear, reduced traction, and harsher ride.

That's the facts Jack!
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Old 05-25-2009, 05:50 PM   #4
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Weigh it and set to the tire manufacturer tables... PERIOD. NO EXCEPTIONS. No fire suit needed.
Why this has to be so religious I'll never understand. There is NO religion needed, just a weight and a tire table from the manufacturer!

I run a full tilt EX on 315 x 22.5 and after a six corner weighing I run 120 FRONT and the min of 85 on the 6 rears as that is the design pressure for the weight I carry.
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Old 05-25-2009, 06:20 PM   #5
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The all upper case (shouting) and absolutism in this matter is indeed a need for a fire suit.

If you want to accommodate error in tire pressures, you want to err on the high side. There aren't any significant risks going up to the max sidewall PSI. There is significant risk in having the pressures too low. (context is an RV with proper tires)

The major cause of tire failure is low tire pressure. That causes flex and that creates heat.

There are many factors to consider as well. Measuring pressure is fairly easy and it is easy to get consistent measures. This is not the case for weight measures, especially individual tire measures. Mfg tables are for a particular set of circumstances with certain assumptions and are not intended as more than a general guide so they need 'interpretation' as well and that can be a source of error.

Perhaps one easy way to tell if you don't have enough air in your tires is to keep an eye on the temperature with an IR thermometer. If they run hot (over 140F) then add air.

There has been a good synopsis of the pros and cons here and those should serve much better than shouting absolutes, IMHO.
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Old 05-25-2009, 07:58 PM   #6
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Back to my original post: As an owner, should I put any value in the tire pressure that Nwmar has posted on the placard beside my driver seat?
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Old 05-25-2009, 08:08 PM   #7
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vac = Put another way, your actual loaded weighing and the pressure from the tire tables will override the Newmar label, or any other label that is not provided by the tire manufacturer for the specific load carried. In the absence of either an accurate weight or manufacturer tire tables the next best estimate will be the Newmar (OEM) label (assuming it has sufficient inclusion for user loaded conditions - it already has wet loading built in) and the least best estimate is to inflate to the sidewall pressure as that will be the max capacity for that tire (and then hope that your actual load does not exceed even that).
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Old 05-25-2009, 08:35 PM   #8
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Regarding the placard posted in the coach, it could mean anything IMHO. When I purchased my coach Goodyear was on strike so I got Michelin tires at no additional cost but it was an option. So the placard in my coach states I should carry 120 PSI in the front and 85 PSI in the drive axle tires. The stated PSI was for the GY tires and not the Michelins so the placard is of no use. After weighing my coach, the Michelin tire pressure chart states to use 105 PSI in the steer axle tires and 85 PSI in the drive axle tires. I've adjusted the PSI cold in the morning and can check it with my TPMS. About one hour on the road the sunny side front tires are running 116 to 118 PSI and the rears 96 to 98 PSI, with temps of 125 on the shaded side and 135 on the sunny side. So this would reflect a heat related increase of 12% on the steer axle tires and 14% on the drive axle. This said, I'm curious why a person would increase the cold base pressure 5 PSI when it would increase much more than that by running down the road.
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Old 05-25-2009, 08:40 PM   #9
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Another thing that comes to mind is if you inflate your tires to max PSI stated on the tire, i.e. 120 PSI, and the heat related increase is 14%, then your running pressure in all tires is approximately 137 PSI. My Alcoa aluminum rims state max tire pressure should not exceed 120 PSI. So how much safety factor is built into the Alcoa rims?
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Old 05-25-2009, 10:05 PM   #10
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You can take your fire suit off, as you are 100% correct. Well....except for the extra 5 psi. No tire manufacturer recommends "extra air".
I get my motorhome weighed by wheel position every year and add 5 pounds to the Michelin chart. Here is the reason. If Linda packs more stuff in the motorhome I don't have to add a couple pounds more air. And for every 10 degree drop in temperature - tire pressure will drop 1 pound. So when I set the pressure in sunny Florida and drive to Indiana where the temperature is 50 degrees cooler I don't have to reset the pressure every morning during the drive.

At a Spartan rally in Michigan two engineers from Michelin gave a seminar and I asked about the "5 pounds for safety" and they said that is a good idea. Later I bought new Michelin tires and they were a new load range that was not in Michelin's charts. I email Michelin and they emailed me a chart for my new tires. They told me to weigh the motorhome by wheel position and use their new chart, "plus add 5 psi for safety."

As for running 120 psi or the max in all tires that is not a good idea. The ride will be rougher, blow outs from potholes will increase, there is less contact with the road so driving on wet roads can be dangerous, the steering can become squirmy, and the tires will wear quicker in the middle of the thread.
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Old 05-25-2009, 10:19 PM   #11
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I live in a small town, and the only place around that has a scales is the local feed mill. Would that be ok to use by getting 3 measurements? Front axle,dual axle, and the single axle tag.
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Old 05-26-2009, 01:03 AM   #12
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The increase in pressure from heat when running is considered in the max psi rating. Tire pressures ratings and recommendations are for ambient temperature conditions.
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Old 05-26-2009, 08:07 AM   #13
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Correct air pressure by the weight you carry I undestand but how accurate are the tire pressure gauges on the market for air pressure in 120 psi range ?
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Old 05-26-2009, 08:51 AM   #14
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Originally Posted by VACMAN View Post
I live in a small town, and the only place around that has a scales is the local feed mill. Would that be ok to use by getting 3 measurements? Front axle,dual axle, and the single axle tag.
That would be a good start. Is there any way you could just put one side of the rig on the scales? You could get the three left side weights that way (for instance), then subtract from the full axle to get the other side.

joe
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