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Old 08-26-2013, 10:01 AM   #43
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Thanks everyone for all the insights on this. It all gets real in the morning as we head out on our first full day on the road.

After much thought, we've decided NOT to take along our spare tire. Its maximum cold pressure is only 35 PSI, so I'm not sure how much help it would be even if we tried to use it in place of a failed 65 PSI tire.

I get that limping along with only 3 tires might cause those to also fail, but in that case, we'll plan to replace the whole set, keep the best one as a spare, and then we WILL have a 65 PSI spare for the next time. Hopefully by then we'll have figured out a good way and place to keep it until needed.

Thanks for the warning about shear pressure when backing. We'll be extra careful about that, avoiding sharp turns whenever possible.

Based on our experience watching bike tires pumped to the maximum pop on hot days, we'll start out with the pressure a bit below maximum, as we are expecting hot days initially. We're also aiming for 55 MPH tops on the road, both for mileage and for tire health.

We'll update this once we know how it all turned out...
"Its maximum cold pressure is only 35 PSI"
Looks like you have a Passenger type tire as a spare. You might want to check what the tire says. If you do have a Pxxx/yyR15 the Max load on the tire needs to be reduced by 10% for any trailer application.
What was the original size & type on your unit? Did the previous owner upgrade the ground tires to ST type tires or did they skimp and save money by buying a lower cost Passenger type tire for a spare?
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Old 08-26-2013, 10:10 AM   #44
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Keep in mind that a bicycle tire really can't be compared since the construction is not the same. Your tires on your tow vehicle and trailer are designed to be able to accommodate heat generated on the road and max. cold temperature instructions account for potential pressures increases that occur once rolling.

The only time I truly make adjustments on tire pressures on the road is when we are changing climates. In those cases I may reduce or increase my pressures to get them back to max inflation recommendations. Note however I only do this when the tires are cold. Never when they are warm from running or from direct sun exposure.

For example sitting in my driveway, the morning sun hits the tires on the street side. It's not unusual to see a 2-4 lb difference from side to side of the trailer. So one side in the shade will be sitting at 80 psi, and the sun exposed side could be 84. I will not adjust the pressure to match the curb side since the pressures will even out once we start rolling.

Jack
Good job. You understand the correct way to set your pressure. Basically the rule of thumb is 2% for every 10F change so you have proof of the heating of tires done by the sun.
Don't forget in addition to increased inflation pressure tire temperature increase accelerates the aging rate of tires. Older tires have less flexible rubber which is one of the two primary causes of failure.
That's why I advocate and use WHITE covers. I published, in my blog, the results of a test June 16, 2011 "Tire Covers - Do they do any good?" You might be interested is seeing just hot hot unprotected tires get.
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Old 08-26-2013, 10:16 AM   #45
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That only applies if you load your Airstream with an extra 2 or 3 thousand pounds of concrete.
Sorry but the Inter-Ply Shear forces occur whenever a round tire is loaded. Yes the forces increase with load but these forces run about 24% higher in tandem axle trailer application than an identical tire under the same load & inflation would experience in a TV or motorhome. This is one of the main reasons TT tires do not last as long as TV or passenger car or MH tires do.

The shear breaks down the rubber bonds at the molecular level. The force creates cracks and causes the microscopic cracks to grow with every mile. Tires do not repair themselves so the cracks simply continue to grow.

Yes lower load will lower these forces. So will increasing inflation. BUT you can never make them zero unless you get to zero load.
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Old 08-26-2013, 11:46 PM   #46
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The shear breaks down the rubber bonds at the molecular level. The force creates cracks and causes the microscopic cracks to grow with every mile. Tires do not repair themselves so the cracks simply continue to grow.

Yes lower load will lower these forces. So will increasing inflation. BUT you can never make them zero unless you get to zero load.
Would you say that happens to all tires in a tandem axle trailer configuration regardless of tire design?

BA
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Old 08-27-2013, 06:18 AM   #47
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It's called worrying too much....
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Old 08-27-2013, 06:58 AM   #48
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It's called worrying too much....
It's called educating yourself to the realities. Knowledge is a great thing.
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Old 08-27-2013, 07:06 AM   #49
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Good job. You understand the correct way to set your pressure. Basically the rule of thumb is 2% for every 10F change so you have proof of the heating of tires done by the sun.
Don't forget in addition to increased inflation pressure tire temperature increase accelerates the aging rate of tires. Older tires have less flexible rubber which is one of the two primary causes of failure.
That's why I advocate and use WHITE covers. I published, in my blog, the results of a test June 16, 2011 "Tire Covers - Do they do any good?" You might be interested is seeing just hot hot unprotected tires get.
It's 2%?....I had always heard 1%. That explains why I see more pressure rise on my TPMS than I expected when comparing tire temps to pressure rise. But I don't see as much (about 1%) on the truck tires?????

Truck has same LT tires as trailer. Rear axle of TV is lighter than an individual trailer axle, front (diesel) roughly the same as trailer axle. Trailer is tandem and at last weigh was 7600# on the ground total. 75 psi in truck and trailer.

Why more rise on the trailer? Less sophisticated suspension? More squirm and wiggling around on trailer axles? (straight midwest interstate travel)

Pretty generalized questions, but I'd like your input.
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Old 08-28-2013, 06:12 AM   #50
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.........Why more rise on the trailer?.....

If you don't mind, I'll try to answer your question.

The rise in pressure is going to be dependent on the load - so more load = more pressure rise.

I think it is safe to say that given the "load intensity" of trailer tires, their pressure rise would routinely be higher.
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Old 08-28-2013, 09:37 AM   #51
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Would you say that happens to all tires in a tandem axle trailer configuration regardless of tire design?

BA
Inter-ply shear happens in all tires. No matter the size, brand or application. The problem with tandem axle trailers is that the forces are exaggerated because the tires are actually dragged around corners (not just tight corners but any turn)
The tighter the turn the larger the force. The lower the inflation, with constant load the higher the force. The higher the load, with constant inflation, the higher the force.

Some tires may have construction features that help lower these forces but no tire has zero inter-ply shear when in use.
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Old 08-28-2013, 10:28 AM   #52
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It's 2%?....I had always heard 1%. That explains why I see more pressure rise on my TPMS than I expected when comparing tire temps to pressure rise. But I don't see as much (about 1%) on the truck tires?????

Truck has same LT tires as trailer. Rear axle of TV is lighter than an individual trailer axle, front (diesel) roughly the same as trailer axle. Trailer is tandem and at last weigh was 7600# on the ground total. 75 psi in truck and trailer.

Why more rise on the trailer? Less sophisticated suspension? More squirm and wiggling around on trailer axles? (straight midwest interstate travel)

Pretty generalized questions, but I'd like your input.
Note I said "rule of thumb" is 2%. I believe it actually works out to 1.8% BUT that is based on dry gas and theoretical Gas Law properties. I also note that TPMS are reasonably accurate but their prime purpose is to provide warning of loss not to provide actual temperature and pressure to lab standards. I have published test results on my blog and find TMPS are acceptable for general usage but they are not reporting the actual temperature of the tire or the gas used to inflate tires as there is a cooling affect of the valve stems in moving air.

I think you will see a larger increase in temperature of trailer tires than TV tires due to higher levels of flexing of the tire structure. Plus different compounds generate different levels of heat and low heat generation compounds are more expensive. Are the tires on the TT same brand & age? Do they have the same tread depth? (more tread depth is more heat). Same cold inflation? Same actual tire load?
All these seemingly little things add up.
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Old 08-28-2013, 10:49 AM   #53
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Note I said "rule of thumb" is 2%. I believe it actually works out to 1.8% BUT that is based on dry gas and theoretical Gas Law properties. I also note that TPMS are reasonably accurate but their prime purpose is to provide warning of loss not to provide actual temperature and pressure to lab standards. I have published test results on my blog and find TMPS are acceptable for general usage but they are not reporting the actual temperature of the tire or the gas used to inflate tires as there is a cooling affect of the valve stems in moving air.

I think you will see a larger increase in temperature of trailer tires than TV tires due to higher levels of flexing of the tire structure. Plus different compounds generate different levels of heat and low heat generation compounds are more expensive. Are the tires on the TT same brand & age? Do they have the same tread depth? (more tread depth is more heat). Same cold inflation? Same actual tire load?
All these seemingly little things add up.
Thanks, I should've noted I was using pressure rise off the TPMS and temps utilizing an infrared thermometer, pointed between the tread blocks at the shoulder of the carcass. Same brand of tires, trailer 2 years old, truck 6 month old (not by born on date...should check) truck 12k and trailer 6k miles. No tread depth measurements. Front truck axle is pretty darn close to each trailer axle load....but the front trucks are out in clean air too.
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Old 09-05-2013, 05:42 PM   #54
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FWIW, we settled on 59 PSI for our 65 PSI cold limit Marathon tires, and they did fine on our recent 2,000 mile trip from Illinois to California by way of Texas. I was impressed that they held their pressure the entire way, and with outside temperatures over 100 degrees on black surface roads much of the way, I'm glad we didn't push the pressure limit.

I inspected each tire at each stop, and was surprised how cool they ran on even the hottest days. All I had to do was pick out an occasional piece of embedded gravel in the tread.

If I end up doing such trips often, I'll get a tire pressure monitoring system, but it wouldn't have had much to say this time.
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Old 09-06-2013, 10:30 AM   #55
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FWIW, we settled on 59 PSI for our 65 PSI cold limit Marathon tires, and they did fine on our recent 2,000 mile trip from Illinois to California by way of Texas. I was impressed that they held their pressure the entire way, and with outside temperatures over 100 degrees on black surface roads much of the way, I'm glad we didn't push the pressure limit.

I inspected each tire at each stop, and was surprised how cool they ran on even the hottest days. All I had to do was pick out an occasional piece of embedded gravel in the tread.

If I end up doing such trips often, I'll get a tire pressure monitoring system, but it wouldn't have had much to say this time.
Sorry to see you have decided not to lower the interply shear forces. Hope things work out for you in the future. Simply being able to go 2,000 miles 10% low does not mean you have not done internal structural damage to your tires. BUT that is your choice.
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Old 01-11-2014, 12:36 AM   #56
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FWIW, I was reading the original manual for our '71 Safari, and finally noticed that Airstream officially recommended 60 PSI for our tires in it. That's also what was recommended to us by CGTrailers (cgtrailer.com) when we asked them about it after having some work done there.

After pulling the trailer through over a dozen miles of some horridly bumpy dirt roads in Mexico recently, we realized there's also a case for lowering air pressure on such "roads", to avoid destroying everything inside the trailer (a front side window, in this case), so dropped it to 55PSI on the return trip with no further damage.

We also had CG Trailers add a spare tire, just in case. It cleverly mounts under the front of the trailer.
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