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Old 10-14-2012, 11:13 AM   #71
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Two schools of thought (restated):

1. Always use the maximum pressure printed on the tire sidewall like many sources recommend; e.g., numerous trailer manufacturers (including Airstream) and tire stores (Discount Tire). Also, see repeat of Goodyear quote, below:

"It's a common practice for RV owners to lower tire pressure in their search for a smoother ride. This is not only dangerous, it's relatively ineffective, as the difference in ride quality is not significant."

Or,

2. Take a chance and gamble against the experts that lower tire pressure will provide a smoother, softer ride.


Personally, I'll risk replacing a couple of rivets, rather than pay to repair/replace wheel wells, trim, and belly pan and body sheet metal (and possibly much more extensive damage).

If we took a poll, I would guess that the people who are seeking a smoother ride are those who have not yet had a tire failure that damaged their Airstream. And, those running the maximum (or near maximum) tire pressure have probably had numerous blowouts, tread separation and other failures, and had tire-related Airstream damage repaired.
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Old 10-15-2012, 07:09 AM   #72
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Originally Posted by w7ts View Post
First of all thank you very much for this explanation. It was very helpful.

What seems to be the biggest bone of contention here right now is how to determine what pressure is appropriate when using P or LT tires on trailers. Because of the reasons you stated manufacturers don't seem to want to discuss this.

The two schools of thought here are:

1. Use the manufacturer's' PSI vs Load charts.

2. always use the maximum PSI on the tire sidewall.

Is one or the other the best practice, or is there some other method we haven't discussed?

Ken
First, let me tell you why tire manufacturers are reluctant to discuss "alternative" inflation pressures.

Every "Light" vehicle sold in the US has a vehicle tire placard that lists the original tire size and the proper inflation pressure for that size. This is required by law. This placard is specific to the vehicle manufacturer, so the inflation pressure listed there is the vehicle manufacturer's specification.

The term "Light" refers to cars, trucks under 10,000# GVW, trailers, etc.

Also, tire sizing is standardized so ALL tires of a given "size" have the same load vs inflation pressure relationship. That means it doesn't matter who manufactures the tire, the pressure listed on the vehicle tire placard is appropriate.

Some other quirks: I put the word "size" in quotes to indicate that the letters in front or behind the tire size that indicate the kind of service the tire was designed for are part of the "size". A P245/75R16 is in my world is a different "size" than an LT245/75R16.

A tire manufacturer would be foolish to specify something other than what is listed on the vehicle tire placard. There would be a whole bunch of legal liability that would open up. Besides, tire manufacturers do not conduct vehicle specific tests, so they CAN'T know how a vehicle handles with inflation pressures other than what the vehicle manufacturer specifies. The tire manufacturer CAN be sure the vehicle manufacturer tested the vehicle at the specified inflation pressure.

Also, if you calculate the load carrying capacity of the tires as specified on the vehicle tire placard, you will find them larger than the GAWR's - and if you think about it, this would make sense. The GAWR's are the maximum load the axle is designed to carry, and there is always some side to side difference, plus there should always be some reserve (unused) capacity. Nowadays you'll find that the tires have about 15% more capacity than the GAWR's.

Now we come to RV trailers. Trailer manufacturers have not done a good job of 1) Estimating the amount of stuff folks put in their trailers. 2) Folks tend to ignore the loading limits, and 3) The tires put on trailers usually do not have any reserve capacity (not a good engineering practice).

The net effect is that manufacturers of ST type tires are in a very uncomfortable position. They have the trailer manufacturers specifying a tire size that may or may not have enough load carrying capacity - and they have consumers who may or may not be careful about tire loads. The only suitable recommendation a tire manufacturer can make is to point to the vehicle tire placard for the pressure recommendation - and in EVERY case that I know of, this is the same as what is written on the sidewall of the tire - that is, the maximum.

There may be folks who will point to the tire load tables - and I would point out that it is good engineering practice to size the tire/inflation pressure to be more than the minimum needed - and MY recommendation is 15% MORE.

Now to P type tires: There is a 10% derating for use on trailers - and I would add that the 15% over-capacity in tires applies here as well.

But LT tires gets a bit more complex - and I am not going to go into this because if the the load tables are used directly (that is, without a 15% over-capacity), then this has a built in over-capacity for trailer usage.

PLEASE NOTE: The above discussion is about TRAILERS!! There are things that would apply to cars and trucks, but not everything. Be careful parsing this.

And a couple of other thoughts:

Many folks do NOT take into account that when they weigh a vehicle that the loads are NOT evenly distributed among the tires - that there is both side to side variation and a front to rear variation. A tire doesn't know what the other tires are experiencing. It only knows what it can see - and if that tire is overloaded, it doesn't matter if its companions are not.
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Old 10-15-2012, 07:44 AM   #73
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Excellent writeup Capri.
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Old 10-15-2012, 08:42 AM   #74
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Many thanks to CapriRacer for taking the time to do this write up.
I have run P Michlens (LTX) on my 25' for the last 4.5 years. Looking at replacing them this spring so I have been reading all the tire stuff again. Probably just go back with the same tires. Trailer came with load range C tires from the factory (at least that is what the tire tag says) and that seems to be a good match. I have 5900 on the wheels when hitched and the tires carry 7940 or so at max so I have the 10% covered. I was thinking about 16" wheels, but it just does not make sense to me to put 65 psi tires and wheels on and then run them at a reduced pressure.
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Old 10-15-2012, 11:22 AM   #75
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Quote:
Originally Posted by CapriRacer View Post
..................................
But LT tires gets a bit more complex - and I am not going to go into this because if the the load tables are used directly (that is, without a 15% over-capacity), then this has a built in over-capacity for trailer usage.
.................................................. ............................................
.
I understand everything, except this paragraph. Does it say to or not to add 15%? If you add 15% is that to the load prior to entering the table, or to the pressure in the table, or does it matter?

Ken
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Old 10-15-2012, 06:19 PM   #76
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CapriRacer,
I also would like some additional clarification on the "built in over-capacity" of LT tires. Are you speaking of added material strength for the different stresses anticipated in normal use of the LT tires? (like turning and acceleration)
What kind of tires do you run on your RV trailers, or other types of trailers?
And, thank you for your informative responses!
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Old 10-15-2012, 07:29 PM   #77
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Alan, I suspect that this question is exactly like the, "how much safety margin is in the GVW, GAWR, GCWR.... numbers". I do know the answer to that one direct from my employer's engineers, but I cannot responsibly discuss it, as human nature......someone will exceed that number thinking there's more wiggle room. Not a good recipe for safety.
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Old 10-15-2012, 09:03 PM   #78
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Quote:
Originally Posted by dznf0g View Post
Alan, I suspect that this question is exactly like the, "how much safety margin is in the GVW, GAWR, GCWR.... numbers". I do know the answer to that one direct from my employer's engineers, but I cannot responsibly discuss it, as human nature......someone will exceed that number thinking there's more wiggle room. Not a good recipe for safety.
I was actually asking CapriRacer a "general" question about his statement, not anything specific with regard to LT design or capacity.

However, I am curious what he uses on his trailers, since tire design is his profession.
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Old 10-16-2012, 07:30 AM   #79
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I was hoping to avoid this discussion because lots of folks don't buy into this - but here goes:

If I design a generic tire, I can describe how it can be used based on the type of service it will be experiencing - and I can use load carrying capacity as a way to describe the differences.

Higher speeds = Lower load carrying capacity
Torque = Lower load carrying capacity
Rough roads = Lower load carrying capacity
Higher ambient temperatures = Lower load carrying capacity

If you'll notice, the difference between ST type tires and LT type tires is described in the first 3 items. That's why I say there is a built in over capacity for LT tires used in trailer service.

- BUT -

I am somewhat uncomfortable recommending that folks use LT tires in place of ST tires. Aside from the issue that LT tires don't come in sizes where you can make a direct swap (13", 14" and 15" LT tires are really scarce.), I also know I'd recommend that even LT tires have a 15% over capacity when used in LT service. Also, my experince with LT tires in trailer service indicates that there are issues there as well.

Then there's the issue of the stated load carrying capacity. It is very understandable that a tire dealer might refuse to mount tires with a lower stated load carrying capacity. I don't want folks thinking they can make this type of exchange without resistance.

I am uncomfortable because of a lack of experience and the complex nature of these kind of swaps. One example is that it is common for folks to swap ST235/80R16's for LT235/85R16's. Notice the difference in size (and I'm not talking about the letters!) That difference results in an increased load carrying capacity that I struggle to account for in my evaluations.

And lastly: What tires do I have on my trailer?

First, I am not an RV'er. I have a trailer that I used to haul my racecar around. It has P metric tires that I applied the 15% rule and the 10% derating and they are over sized at that. I checked my tires and the pressures before every tow. That means twice every race weekend. At worst I was towing 200 miles (I can only think of 2 exceptions) and the total mileage per year was only a couple thousand. I don't think folks should point to my experience as an example.
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Old 10-16-2012, 10:31 AM   #80
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Correct Pressure

Every pneumatic tire equipped conveyance that I have owned or operated has a manufacture specified tire inflation. The door jam of all cars requires a sticker showing the inflation pressure appropriate for the load in the vehicle. My BMW motorcycle shows 36 psi front and rear normally and 36/42 with a passenger. Neither tire has a maximum pressure on the sidewall matching these recommendations. This is the case with my Pontiac G8 and Mercedes C-300, and every recent car I can remember.
Racing bicycles use very high pressure tires. Michelin has recommendations for inflation pressure reflecting rider weight. Their 700/23c race tires list a maximum pressure of 135 psi and a recommended inflation of 110-115 depending on rider weight. Rolling resistance tests have shown inflating to maximum rated pressure increases it. This is because the tire will 'bounce' over road surface irregularities rather than flex to follow the contour of the pavement. This 'bounce' causes greater heat build up.
How does this relate to trailer tires? If you inflate the tire to the maximum when the load does no require it, you may be generating more heat within the tire than at a lower pressure. This may seem counter intuitive but the data back this up in other tire testing. Maxxis provides a load chart. Why doesn't Airstream? Who knows. My guess is that the lawyers have become involved so much in liability, that we have been dumbed down as vehicle owners. If you recommend your trailer tires be inflated to the maximum, then the owner can't overload them if staying within the trailer's load limits. Why confuse the driver? Whenever I have had tires serviced or replaced on my cars, the dealer always inflates to the maximum sidewall number--never what the manufacturer of the car specifies. That doesn't make that the best, but covers them from possible under inflation if the vehicle is fully loaded. The DOT requires a load/tire inflation table be highly visible. Why would that be required by law if it isn't to be used?
Others have posted articles explaining how ST trailer tires have stiffer sidewalls to withstand high loads and pivoting issues with tandem axle trailers. Why the 65 mph speed rating? I would like to know. It may have been an industry agreement for a testing standard. But, given the construction of ST tires, it clearly is a heat issue when traveling over this speed. This may be the primary cause of tire failure more than inflation pressure variation. ( I have also observed that my trailer tires get far hotter from a long mountain descent than at any other time. This comes from braking action that heats the wheel which in turn heats the tire. Since I switched to disc brakes, my wheels don't get hot anymore on a long descent). Using an LT tire may be the solution for travel in excess of 65 mph. Maybe this is why Airstream is experimenting with it on the Eddie Bauer. They had to switch to 16" wheels in order to have enough load capacity. I'm assuming that the tandem axle 'skidding' isn't a problem with an LT tire (maybe for a P rated passenger tire it is) or Airstream wouldn't be using it. ST tires are built with greater UV resistance and sidewall strength to resist deformation when stored for long periods of time. If you don't treat your tires to those conditions, maybe LT tires are the way to go. But, because of the smaller diameter of the 225/70 LT tires, I'm staying with a Maxxis ST, E rated tire this time around. I'm also not ready to spend money on 16" wheels, and to research whether or not they pose clearance issues. Wish me luck.
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Old 10-17-2012, 07:16 AM   #81
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I hope people will forgive me for truncating and parsing the post. I just want it to be clear which parts I am responding to.

Quote:
Originally Posted by rp709 View Post
........Racing bicycles use very high pressure tires.....
Yes, but bicycles don't have a suspension - except for the tires. That changes things significantly.
Quote:
Originally Posted by rp709 View Post
........Rolling resistance tests have shown inflating to maximum rated pressure increases it........
I hope you are not saying that RR increases with increased inflation pressure. EVERY thing I have ever seen says the opposite.

Quote:
Originally Posted by rp709 View Post
........This is because the tire will 'bounce' over road surface irregularities rather than flex to follow the contour of the pavement. This 'bounce' causes greater heat build up.........
Again, bicycles don't have a suspension, and properly designed suspensions don't bounce, so this isn't an issue for cars and trucks. But I think that increased inflation results increased RR even factoring in surface irregularities.

Quote:
Originally Posted by rp709 View Post
........How does this relate to trailer tires? If you inflate the tire to the maximum when the load does no require it, you may be generating more heat within the tire than at a lower pressure........
No, I don't think that is true. Higher pressure = lower heat build up.

Quote:
Originally Posted by rp709 View Post
........ This may seem counter intuitive but the data back this up in other tire testing.........
You are absolutely going to have to show the data.

Quote:
Originally Posted by rp709 View Post
........ Maxxis provides a load chart. Why doesn't Airstream? Who knows..........
Because Airstream has determined that the pressure they put on the vehicle tire placard is adequate for all the conditions they specify - that is from curb weight to GVW.

Quote:
Originally Posted by rp709 View Post
........Whenever I have had tires serviced or replaced on my cars, the dealer always inflates to the maximum sidewall number--never what the manufacturer of the car specifies. That doesn't make that the best, but covers them from possible under inflation if the vehicle is fully loaded......
Actually that creates a legal liability for the dealer because he is not following the vehicle manufacturer's specification. If something bad happens the dealer would be on the hook.

Quote:
Originally Posted by rp709 View Post
........The DOT requires a load/tire inflation table be highly visible........
No, it doesn't. There is a law that says the vehicle has to have a tire placard, but the placard only has to list the original tire size and the inflation pressure. Usually the pressure is a single number which might be different front to rear, with no indication that it should be different depending on the load. (Remember I said usually. There are vehicle that do, but they are more the exception than the rule>)

Quote:
Originally Posted by rp709 View Post
........Why the 65 mph speed rating?.....
As I explained above higher speeds = lower load carrying capacity. ST tires are specified with a load carrying capacity AND a speed limitation.
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Old 10-17-2012, 07:58 AM   #82
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My experience and training agrees with everything CapriRacer says in post 81....except the dealer liability thing. I don't think there's actual liability there, but it is not the right thing to do for the customer nor the vehicle.

Relative to the placard, Capri is right, HOWEVER, many MFRs will have verbiage (vague at best) about tire and axle loading IN THE OWNER MANUAL.
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Old 10-17-2012, 08:06 AM   #83
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We're lucky to have these contributions. Thanks again, CapriRacer.
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Old 10-17-2012, 08:24 AM   #84
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This is from 2012 Silverado Owner Manual (see orange highlight):

Tire Pressure
Tires need the correct amount of air pressure to operate effectively.
Notice: Neither tire underinflation nor overinflation is good. Underinflated tires, or tires that do not have enough air, can result in:
Tire overloading and overheating which could lead to a blowout.
Premature or irregular wear.
Poor handling.
Reduced fuel economy.
Overinflated tires, or tires that have too much air, can result in:
Unusual wear.
Poor handling.
Rough ride.
Needless damage from road hazards.
The Tire and Loading Information label on the vehicle indicates the original equipment tires and the correct cold tire inflation pressures. The recommended pressure is the minimum air pressure needed to support the vehicle's maximum load carrying capacity.
For additional information regarding how much weight the vehicle can carry, and an example of the Tire and Loading Information label, see Vehicle Load Limits . How the vehicle is loaded affects vehicle handling and ride comfort. Never load the vehicle with more weight than it was designed to carry.


So, the pressure on the placard is what GM recommends based on their "tire load chart" and some other limiting factor in the RGAWR figure basically divided by two. (max load per tire for that particular application) NOTE the word MINIMUM in the orange highlighted section. That's the vague part.


Personally, if a am at GAWR, I inflate to max sidewall. If I am below, I kind of look at the difference between max and recommended and balance that to where I am on load. Then keep an eye on temps. I am looking for that 4 - 5 psi rise from cold tire to hot. (correlates to 40 - 50* rise in temp)


This works well for TV....I just can't quite find the same answer for AS with LT tires. I see an 8 - 9* rise at 65, and 70psi....no difference. I am going to try 80psi for my last trip of the season and see if they run any cooler. I am not sure if 8 - 9* psi rise is too much or not (as it relates to temp).


Comment, Capri?


EDIT: I should qualify my TV load/pressure statement with: This assumes tires which are matched to the TV. For example, I have a customer who puts LT tires on their 1500 Silverados due to the off road surfaces they run, which are very abusive to P tires. Running anything above placard pressure is completely unnecessary in this situation, as the RGAWR and GVW are so far below the tire carrying capability and they rarely carry more than a few hundred pounds.
This situation is more similar to many of us ASers, BUT I am unconvinced that TV scenarios = AS scenarios. Thus my attempt to answer with temps vs pressures. HOWEVER, what I find, with 8000 lbs on my four LTs will likely be very different than some of you with different ASes.
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