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Old 05-24-2014, 09:06 AM   #1
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Running Pressures and Temperatures

I recently installed 16" LT225/75-16 MS/2s on Sendel wheels. My home compressor wouldn't go to 80 psi so I set them at 75 psi which I now believe is too high. When I get home I am going to drop the pressure to something more appropriate for the load, probably around 45 or 50 psi based on the Michelin load tables. I know this may be controversial, but my TV weighs about the same as my TT and the LT tires on it say max pressure 80 psi on the sidewall but Ford says to run them at 40 psi.

On the 200 mile run to our holiday weekend I noticed the curbside running hotter by 5 or so degrees and 5 psi higher when they were set at the same pressure and read the same temperature, or maybe even roadside was a little hotter due to sun heating, in the driveway.

I'm guessing this is due to different weight loading due to either weight distribution in the trailer or crowning of the road putting more weight on the curb side. Any other guesses as to what might be the cause? What are others with TPMS seeing driving down the road?

I'm not too concerned as the pressures went from 75 to 85 on the highest tire, and the temps went to 113 at the highest. My Dill manual says it alarms at 30% over pressure or about 98 psi or 176 degrees so I am well inside those limits.
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Old 05-24-2014, 09:49 PM   #2
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Regarding plan to lower my inflation pressure.....

Thanks to Roger (tireman9) and others I have seen the light and it all starts to make sense. ASI recommends 65psi for the GYMs. That gives over 10,000 pounds of capacity for four tires on a trailer with a GVWR of 6300# and a tongue weight somewhere north of 700#, or about 5600# on two axles rated at 3000# each. Why would they do that? I have seen recommendations of 15% margin if you don't know the load on each wheel, but the part I was missing was Roger's identification of as much as 24% increase due to stresses unique to tandem axle trailers. Taking both margins on top of my estimated axle weight gives me close to 4000# per tire. So I'm guessing that ASI has put some additional margin in there to get to their recommendation of 65 psi.

Regardless of this guessing, there have also been postings identifying what I think is a DOT requirement on tire dealers to provide a replacement inflation label when tires of a size different than those specified on the vehicle's placard are fitted. This new label must specify the inflation pressure that will result in the new tires having at least the same load capacity as the original tires. It seems that means that I should be running my M/S-2s right where I have them, at 75 psi.
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Old 05-24-2014, 10:06 PM   #3
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Someplace on the forum are some great articles about PSI and load. One of the things I picked up on was the difference in how the tires handle the load as a single axle or dual axle. My takeaway was that in dual axle set ups you should run nearly max pressure. So the placard pressures start to make sense.

On my 25FC the sticker says 65# for the GYM's. The load charts for the max weight of the trailer are something like 45#s. I'll defer to the factory and the strong information about the forces being different on a dual axle.
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Old 05-25-2014, 08:25 AM   #4
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Rule of thumb:

Normal pressure buildup should be no more than 10%. The number doesn't matter - it's the percent of the starting pressure.

If you are above 10%, then you should consider using a tire with more load carrying capacity - and in this case, I mean going higher in inflation pressure.

If you are above 15% - EVER - then you need to do something immediately.

So, Al and Missy: 5 psi is over that 10%, so you need to start with a higher pressure. I'm guessing you need 50 psi. See what that does.
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Old 05-26-2014, 10:18 AM   #5
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I'm actually running 75 psi

Quote:
Originally Posted by CapriRacer View Post
Rule of thumb:

Normal pressure buildup should be no more than 10%. The number doesn't matter - it's the percent of the starting pressure.

If you are above 10%, then you should consider using a tire with more load carrying capacity - and in this case, I mean going higher in inflation pressure.

If you are above 15% - EVER - then you need to do something immediately.

So, Al and Missy: 5 psi is over that 10%, so you need to start with a higher pressure. I'm guessing you need 50 psi. See what that does.
Thanks for the suggestion, but I was only planning to run 45-50 psi. The 10 and 15% guidelines are helpful though. The 4-5 psi I saw were well under 10% of my 75 psi pressure, so I think I'm good. Now all I have to do is figure out how I managed to get the spare programmed into the Dill receiver instead of the left rear.

Al
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2002 Classic 30 Slideout
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2013 Dodge 2500 Laramie 4x4 Megacab Cummins
2001 Safari 25 RB Twin (Gone, but not forgotten)
WBCCI 1322, TAC FL-39, AIR 82265

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Old 05-28-2014, 03:00 PM   #6
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When I set pressures I saw a 1# PSI variance at 60# on the sensors. It was sundown and tires were close in temp per pyrometer.

Next morning, same variance but PSI was now 56-57#.. Still a 1# variance. When rolling, temps came up after a short time and temps rose.

Sunny side tires ran hotter. But I did not see more than 5 degree above ambient air temp on any sensor. Pressures rose from our "normal ambient" temp pressure of 60# to 68# which is greater than 10%.. But still under the tire "cold pressure" spec. So well within run envelope.
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