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Old 05-24-2009, 09:57 PM   #1
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Tire pressure question?

Does changing from a load range c to a load range d tires effect what pressure you should run at? My 1981 25" Excella 2 weighs 3400 with a gvwr of 4500. I tow at around 4000 lbs. My owners manual recommends tire pressure at 35 psi for load range C 7.0 x 15, and 40 psi for 7.75 x15, however the vin # tag on the front of the trailer states 45 psi with 2,000 lbs. per axle max. The info in the manual may not be 100% correct since my unit is only 7' wide and the standard 25' model of that year was 8' wide. I'm now using load range d 225 75r 15's that show a max psi of 65. Based off of the weight I think I'm running well under 1,000 lbs. on each axle. Would I be safe to tow at 35 psi, or should I go higher for safety? I'd like to take as much vibration away from the airstream as possible and still tow within a safe range.

Thanks: Charlie
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Old 05-24-2009, 10:10 PM   #2
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Get the trailer on a scale and find out what the load is on each tire. Then go to the tire manufactures web site and download their load vs pressure chart. Inflate tires per chart plus 5 psi for safety factor.

I tried to put in the load/inflation chart, but it didn't work.

PM me, and I will send you an article that includes the load/weight
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Old 05-24-2009, 11:55 PM   #3
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Put your trailer tires to the max psi listed on the sidewall. The advantages you get from having the pressure a bit high significantly outweigh the risks from trying to get it 'just right' by any of the calculations folks suggest.

If you do want to trim your pressures, use a temperature gauge and reduce pressures until the tire temperatures after running an hour on the road start going up.

You should use tires properly rated for the weight they cary and that is usually a C rated tire for most Airstream trailers. A D tire produces a bit rougher ride (but I haven't noticed it on mine) and is a bit tougher tire.

An rv.net thread on this noted that Goodrich had one page suggesting weight proportioning and another for trailer tires that recommended max sidewall psi.

When the pressure is too low you will get heat and that is the major cause of failures. Putting the pressure at max sidewall rating may make the ride a tad harsher and may increase tread wear a bit in the center. Neither is a big deal unless you go overboard.

There are those who just can't stand this idea and the arguments get heated. That is sad. Consider the arguments carefully. Try it yourself.

Do keep an eye on tire temperatures as well as the hubs and don't let anomalous temperatures go without fixing the cause. This is easy to do with modern IR thermometers and a quick and easy way to tell if you have inflation problems.
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Old 05-25-2009, 09:13 AM   #4
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The brand of the tires that I have are Transmaster. I've seen these at Firestone, but had them put on at our area Airstream dealer. They say load range D max load 2540 lbs., but also say 6 ply not 8. I don't think I'd want to inflate to 65 psi with such a light trailer. I have truck tires on 2 of my vehicles which say max psi 65, but I inflate these at 35-45 depending on if I'm towing or not. I think that I will take the advise of getting it on a scale. For those who have done this where have you gone to weigh your airstream? We don't have a local Farm Bureau coop here. Do weigh stations allow this?
Assume for a minute that my Airstream is being towed at 4,000 which would be very close, and that I'm well balanced with 34% of the weight going to the airstream axles and 33% to each of the TV axles. I would then have less then 700 lbs. on each of the airstream axles if I'm correct in how the weight it distributed. If not someone please correct me.
Based off these numbers and load range D tires what PSI would be recommended? I can adjust for this once I find a scale.

Thanks: Charlie
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Old 05-25-2009, 09:23 AM   #5
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do a google for catscales, and find the one closest to you, $10 and reways $1 for 24 hours.
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Old 05-25-2009, 09:47 AM   #6
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do a google for catscales, and find the one closest to you, $10 and reways $1 for 24 hours.
Thanks: I checked but unfortunately the nearest catscale is over an hour away and in the wrong direction of my next trip. The nearest scale in route of my next trip is 375 miles so I think I'll hit that one.

The max variance of my trailer would be between 3400 -4500 lbs. Any suggestions based off of this weight range would be greatly appreciated.

Thanks: Charlie
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Old 05-25-2009, 09:50 AM   #7
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It is absolutely insane to underinflate tires. When cornering you will flex the side wall, possibly undeading them from the rim, and risk loss of control, or failure putting you family and the motoring public at risk. Not to mention liability, fuel economy. You may say "I dont corner fast", What happens if you have to avoid an accident?

The side wall inflation reccomendations arent grasped out of thin air. They come with engineers time, and tests. They also effect the tire warranty.

If you buy truck tires. Put them at there sidewall reccomended P.S.I. Same with trailer tires. If indoubt contact tire M.F.G. customer service.
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Old 05-25-2009, 10:10 AM   #8
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I'm trying hard not to open a can of worms here. If I take my truck to my dealer with lt tires they don't inflate to 65 psi just because that's the max recommended pressure. I'm trusting that there is a range inside of the max allowable psi that is "safe". Airstream also has recommended psi's inside the max that varies by trailer weight according to my manual. To follow Airstream, Ford, and Chrysler recommendations is not under inflating the tire.
Based off what Airstream recommends I'd be safe at between 40-45 psi but that's on the older tire 7x15 tires. I'm just trying to find what I can operate at on the newer tires.

Thanks: Charlie


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It is absolutely insane to underinflate tires. When cornering you will flex the side wall, possibly undeading them from the rim, and risk loss of control, or failure putting you family and the motoring public at risk. Not to mention liability, fuel economy. You may say "I dont corner fast", What happens if you have to avoid an accident?

The side wall inflation reccomendations arent grasped out of thin air. They come with engineers time, and tests. They also effect the tire warranty.

If you buy truck tires. Put them at there sidewall reccomended P.S.I. Same with trailer tires. If indoubt contact tire M.F.G. customer service.
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Old 05-25-2009, 10:39 AM   #9
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Based on the load/pressure chart, ST 225-75 R15, will carry 1600# at 30 psi. With four tires to share the load, at 30 psi you can carry the whole trailer and then some if your 4000# weight is any way near correct.

Jim
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Old 05-25-2009, 12:17 PM   #10
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Quote:
Originally Posted by Pacerized View Post
I'm trying hard not to open a can of worms here. If I take my truck to my dealer with lt tires they don't inflate to 65 psi just because that's the max recommended pressure. I'm trusting that there is a range inside of the max allowable psi that is "safe". Airstream also has recommended psi's inside the max that varies by trailer weight according to my manual. To follow Airstream, Ford, and Chrysler recommendations is not under inflating the tire.
Based off what Airstream recommends I'd be safe at between 40-45 psi but that's on the older tire 7x15 tires. I'm just trying to find what I can operate at on the newer tires.

Thanks: Charlie
It makes sense to run the air pressure for the weight carried, just as is done on cars and trucks. My AS weight is 4900 loaded, so I use 40 lbs, same tires you are using. Still good to weigh trailer, and check heat often.
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Old 05-25-2009, 09:46 PM   #11
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Quote:
Originally Posted by jimmickle View Post
Based on the load/pressure chart, ST 225-75 R15, will carry 1600# at 30 psi. With four tires to share the load, at 30 psi you can carry the whole trailer and then some if your 4000# weight is any way near correct.

Jim

Thanks Jim, and Roger for answering my question.
I thought I'd be safe at a lower psi with the new tires then with the old load range C tires but I wasn't sure. I don't think I'd ever feel comfortable going as low as 30 psi even if it made sense based off the weight. I think I'll lower to 45 until I get to a scale, then perhaps come down to 35-40 if my sidewalls stay cool and the scale confirms everything.
Charlie
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Old 05-25-2009, 09:56 PM   #12
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You will be fine running 35 psi with the weight you are talking. At 65 psi you will beat your trailer for no reason. The max PSI on the sidewall is just that, max load & speed. If you drive a prudent speed( 60-65 mph is prudent to me) and are running a low weight it makes no sense at all to inflate to max PSI. Adios, John
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Old 05-26-2009, 12:58 AM   #13
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re " At 65 psi you will beat your trailer for no reason." -that's FUD mongering.

When I inflate my tires to max sidewall I get better handling and the lack of movement of things left on counters or tables indicates the trailer is doing just fine.

What doesn't make sense is thinking you can weigh a trailer and get something you can use. Trailer loads change as the tanks are used and in the normal course of events as you live in it. Weights only make sense in static terms and laboratory conditions, not in the real world.
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Old 05-26-2009, 08:32 AM   #14
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Check for a local recycler or scrap yard or truckstop they usually have scales and most are state certified because the buy and sell over that scale.
I would inflate tires to what ever the sidewall says is max. They only problem I have had is with my TV tires being low on pressure and causing cupping and I did have to replace them before their time because I didn't watch the pressures recommended by the manufacturer.
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