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Old 07-10-2007, 09:14 AM   #1
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Desert Driving

All of the threads concerning proper tire pressure got me to wondering about the correct tire pressure when driving through our desert areas.

If I am driving I 40 from Williams, AZ to Barstow, CA in the middle of the summer, I can expect to encounter extreme heat. Since I can expect the heat to cause my tires' pressure on both the TV and the trailer to expand because of the heat, should I lower the tires' pressure before I cross the desert? If you think, as I do, that I should, then by how much?

Both trailer and TV each weigh 9,000# with the wd hitch, so my three trailer axels aren't carrying a max load. I wouldn't plan to travel greater than 60 mph. I'm thinking that I can reduce my tire pressure by at least 5# from their maxium rating. What do y'all think.

________Tom
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Old 07-10-2007, 09:18 AM   #2
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Hi Tom,

Too low tire pressure actually leads to higher heat buildup and greater chance of tire failure. Remember the Ford/Firestone problems a few years ago? I would leave them the same as you keep them in San Antonio.

Love the hill country. Used to live near Wimberley.

Vaughan
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Old 07-10-2007, 09:29 AM   #3
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Use the PSI # on Tire Sidewall

Lower tire air pressure than reccommended by manufacturer leads to higher internal temperatures and can lead to tire failure. I would (cold inflate) to sidewall maximum PSI. You should be fine with this. Also note the DOT code located on the sidewall. It, in code form , tells how old the tire actually is. Example DOT 3803= a tire manufactured in the 38th week of 2003. I would not trust a tire older than 4 years old IMHO.
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Old 07-10-2007, 09:36 AM   #4
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Quote:
Originally Posted by vswingfield
Hi Tom,

Too low tire pressure actually leads to higher heat buildup and greater chance of tire failure. Remember the Ford/Firestone problems a few years ago? I would leave them the same as you keep them in San Antonio.

Love the hill country. Used to live near Wimberley.

Vaughan
Right now, I carry the maximum pressure (65psi). So, if I lower them 5# to 60 psi, is that too low? I remember the under inflation fiasco that firestone/ford had, but isn't there a reasonable range of psi that can be used? Also, how much should I expect the tire pressure to increase? Is over inflation as serious as under inflation?

The forecast for Bullhead City,AZ (think Mojave Desert here) is 110*. That's not exactly a temperature I'm used to driving in, or living in for that matter.

_______Tom
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Old 07-10-2007, 09:47 AM   #5
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I know I ought to look in the books before I run headlong into this post, but what the heck...

Tire pressure doesn't vary on hot days as much as you'd think, since the increase in gas pressure is based on the ratio of temperatures in degrees Kelvin, not degrees F. So if you go out in the morning and it's 65 degrees F, the tire is already at 538 degrees Kelvin (absolute zero being -473 F, or 0 Kelvin). When you're zooming across the Mojave where the air is 115 F and the tires are at 140 F (my estimate from putting the palm thermometer on a hot tire), they're at 613 Kelvin. Tire pressure (assuming constant volume) would be 613/538=1.14 times the orginal pressure (pressure is up maybe 9 psi). That's not a lot different from a normal day when the tires would be at 115 F (588/538=1.09, or about 6 psi).

Now, let's take the alternative situation. You've reduced the pressure 5 psi and you get through the Mojave and you forget/don't have a chance to put air back in and you wind up watching the elephant seals at San Simeon before heading up the coastal highway, where it's going to be 65 F all day. You're now running on seriously underinflated tires. I don't know about you, but in my single axle Caravel I'm always running within about 15% of max load capability on my tires. I don't know the precise calculation, but if you reduce trailer tire pressure by 10 psi, from 60 down to 50, you've probably reduced highway speed load carrying cability by half. A 5 psi reduction is likely a 20% reduction in safe carrying capacity. That would kill the tires on a Caravel.

I think the best plan is to always run trailer tires at their max load capacity pressure, usually 60 or 65 psi.

Zep (shields up)
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Old 07-10-2007, 09:55 AM   #6
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Rivet The Pressure is On

Quote:
Originally Posted by Stream 1529
Right now, I carry the maximum pressure (65psi). So, if I lower them 5# to 60 psi, is that too low? I remember the under inflation fiasco that firestone/ford had, but isn't there a reasonable range of psi that can be used? Also, how much should I expect the tire pressure to increase? Is over inflation as serious as under inflation?

The forecast for Bullhead City,AZ (think Mojave Desert here) is 110*. That's not exactly a temperature I'm used to driving in, or living in for that matter.

_______Tom
Hi Tom,

I doubt that running 60 psi would give you problems; after all, you have 6 tires to share the load. Myself, I would still air all 6 of mine to 65. But then again, I have bicycle tires that will hold 220.

Vaughan
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Old 07-10-2007, 10:05 AM   #7
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It is not necessary and not recommended to lower a tire's cold inflation pressure in anticipation of high ambient air temps. Nothing wrong with inflating to 60 psi on a 65 max psi tire. Just don't do it for the high air temp reason. The tire manufacturers have factored into their design criteria, temperature extremes. Pressure will rise between 5-10 F in high ambient conditions. We have a GMC truck with tire pressure monitors. It helps pass the time checking the pressure. The pressure also drops in extreme cold conditions. Biggest thing you can do to reduce vehicle/trailer stress at high ambient temp is to slow down. Go 55 instead of 60 for increased safety margin.
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Old 07-10-2007, 10:11 AM   #8
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I went and found an article on tire pressure, The Tires . There it says that the psi will increase by 1# for every 10* variance. So, if I leave from Williams, AZ this morning where it's 65*, and hit the Mojave where it's 110* the psi would increase by 4# just from ambient temperrature increases. This doesn't factor in friction. So, I guess that being 4# overinflated wouldn't be excessive.

________Tom
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Old 07-10-2007, 10:32 AM   #9
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elevation causes the greatest tire pressure changes...

Here's what I have found. If you set your PSI at your home elevation above sea level in the cool of the morning and travel up to Flagstaff and Williams (above 6K feet) and check your PSI in the cool of the next morning it will be low. Don't do anything! By the time you get to Barstow and sea level it will be close to normal again. If you're realy concerned get one of those laser/IR hand held temperature sensors and check tire temps at every rest stop.
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Old 07-10-2007, 11:24 AM   #10
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Stream 1529,....Let us know how it goes as I will be doing the same thing next week. Excursion v-10 towing 1994 34ft. from Tucumcari to Las Vegas on I-40. I'm putting on 6 new LT's but am concerned about the heat @ 60 mph.
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Old 07-10-2007, 11:47 AM   #11
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Quote:
Originally Posted by Bing-Bing
Stream 1529,....Let us know how it goes as I will be doing the same thing next week. Excursion v-10 towing 1994 34ft. from Tucumcari to Las Vegas on I-40. I'm putting on 6 new LT's but am concerned about the heat @ 60 mph.
Bing, I think Bobkelly's suggestion of slowing down to 55mph will be the answer for both of us. Your tow should be lighter than mine with the same TV. I plan to check my pressure in the afternoon of the day before I leave Williams, and inflate to 63#. I'll end the trip in Apple Valley, CA and reinflate to 65# before I move on. If I monitor the pressure once or twice inbetween everything should be fine. All of my tires are less than two years old.

You'll get warm between Gallup and Winslow, and then again after Williams. You could take a break in Gallup and Winslow, and then again at the Hoover Dam. That should give your tires some time to cool down, and for you to check their pressure. Have fun, and be safe.

____Tom
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Old 07-11-2007, 10:13 AM   #12
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IMHO you will want to do everything you would normally do before a trip -- Although the desert is hot, if you have your tire pressure where it's supposed to be, the desert heat won't have enough affect to worry about. However, the one thing you ALWAYS need to do is carry plenty of water. During the summer months I use this as my tow vehicle Seriously, be sure to take water.
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