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Old 10-17-2012, 06:16 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.

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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.

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........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.

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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.

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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.

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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.

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........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>)

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........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, 06:58 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, 07:06 AM   #83
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We're lucky to have these contributions. Thanks again, CapriRacer.
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Old 10-17-2012, 07:24 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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Old 10-17-2012, 09:44 AM   #85
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Good read!
What about tread design in this discussion as some tires seem to have more of an open tread versus a more closed tread (some treads hold onto rocks longer and shoot them out a higher velocity) ...
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Old 10-17-2012, 02:16 PM   #86
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Again, I hope people forgive me for truncating and parsing.


Quote:
Originally Posted by dznf0g View Post
This is from 2012 Silverado Owner Manual (see orange highlight):.......

The recommended pressure is the minimum air pressure needed to support the vehicle's maximum load carrying capacity.

.............


This is sort of correct, but it glosses over something that I think shouldn't be glossed over.

Calculate the load carrying capacity of the tires at the placard pressure and compare that to the GAWR's. You'll find there is about a 15% over capacity in the tires.

So that statement is sort of right, but not exactly accurate.

Quote:
Originally Posted by dznf0g View Post
........Personally, if a am at GAWR, I inflate to max sidewall.........
And this might present problems because different tire manufacturers have different ways of stating what appears on the sidewall. Net effect, you could wind up with completely different pressures depending on the tire manufacturer - and that difference isn't backed up with anything factual.


Quote:
Originally Posted by dznf0g View Post
........If I am below, I kind of look at the difference between max and recommended and balance that to where I am on load. .........
Well, the math doesn't work as I explained above.

Quote:
Originally Posted by dznf0g View Post
........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).......
My rule of thumb is for P metric tires: 10% buildup (~3 psi): OK - 10% to 15% (3 to 5 psi): monitor the situation, 15% (5 psi) buildup: add air!!

Quote:
Originally Posted by dznf0g View Post
........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? .....
See earlier comment on buildup.


Quote:
Originally Posted by dznf0g View Post
........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...........
Are you sure about this? This might be true for oversized tires - and some LT tires would qualify as over sized - but LT tires in the same "size" (meaning dimensions) need to use 15 psi more to get the same load carrying capacity.
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Old 10-17-2012, 02:46 PM   #87
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I don't know how to parse your parses.......so to speak.
This is sort of correct, but it glosses over something that I think shouldn't be glossed over.

Calculate the load carrying capacity of the tires at the placard pressure and compare that to the GAWR's. You'll find there is about a 15% over capacity in the tires.


So that statement is sort of right, but not exactly accurate.


Relative to your "sort of correct" in the owner manual, I stake my life that it IS correct. GM is the only mfr that tests EVERY tire offered in EVERY application offered for load and durability AND requires a TPC stamping the sidewall. (Don't confuse TPC with testing of different pressures. It is all a part of a very exhaustive testing and approval process.) Not trying to get argumentative, as your rules aren't wrong...may just be a generalization whereas GM tests to the specific application when a placard recommendation is made and it's pressure relationship to that specific tire(s) Sometimes there are several suppliers to the same model configuration. 15% is apparently the MINIMUM safety margin GM is willing to accept.

And this might present problems because different tire manufacturers have different ways of stating what appears on the sidewall. Net effect, you could wind up with completely different pressures depending on the tire manufacturer - and that difference isn't backed up with anything factual.

I was only referring to my current situation with the AS. I wasn't clear at all on that....and really haven't come to that conclusion on that as of yet. I do think a 7 - 9psi rise at 70psi cold indicates not enough air.

Well, the math doesn't work as I explained above.

That's sorta my point. The math ISN't working with the PSI rises I am seeing in the AS with ~2000 lbs on each tire....compared to the chart (which I don't place a lot of confidence in) or the max load on the sidewall.

See earlier comment on buildup.

But the Michelin LTs on the AS are not behaving at all like the Michelin LTs on the truck relative to load vs. pressure buildup. This IS the issue for me. I find it baffling...and no I do not have any brake or bearing issues and all 4 are similar in behavior.

Are you sure about this? This might be true for oversized tires - and some LT tires would qualify as over sized - but LT tires in the same "size" (meaning dimensions) need to use 15 psi more to get the same load carrying capacity.

I'll have to ask them.....They may be running at 50psi. IIRC placard on a 1500 is 35psi.
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Old 10-17-2012, 05:53 PM   #88
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Drop back 10 and punt! My head hurts....I want to get back on track with Capri's post #72 and leave out all the other stuff I started wandering off to.

Here's what I understand from post 72.

LTs may have some built in overcapacity to Capri's explanation of the 15% stuff. For this post I'm going to ignore that for my ultimate ending dilemma and question.

As I understand.....here's my calculated process.

LT225/75/16 tires
GVWR of AS is 10,000#
GAWR is 5000# each
since a minimum 10 of GVWR must be TW, "practical" GAWR is 4500# (2250# per tire)

If I were a trailer mfr, installing this tire on this trailer I would look at the chart, add my 15% load margin and find 2588# and recommend 75 or 80 psi.

Is that correct?

If so, then let's back up to actual loads on my particular AS. Last time over the scale I was at 7600# on the axles combined. (for this exercise, lets assume all tires are as close to equally loaded as possible). Now I know I've gained a little weight since my last weigh, so to make it easy, let's say 8000#

So to find a proper pressure for my load:

Add 15% to 2000# = 2300#. Go to the chart, find 2300# for the LT tire size and get a pressure of 65psi.

And this isn't considering the "built in overcapacity" you mention.

This is the question haunting me.....I have been running 65psi and have had what I believe to be too great of a psi increase over cold set pressure. As I mentioned above, it has been consistently 7 - 9 psi over cold ambient. (I have taken into account sun, wind direction, and normal ambient temp rise throughout the day when noting this. Meanwhile all the various TVs, some with LTs and some with P tires, inflated to placard pressures see a very consistent 4 psi rise.

This implies to me that I need more pressure. Thus far in that experiment I have only tried 70psi, with no difference in pressure rise. And yet, buy your explanation in post 72 (I know...you said LTs are more complex) I am some 15% overcompensating, due to the built in 15% of an LT. Plus I artificially upped my actual load. If I did the honest numbers without the extra 15%, I should be running at 50 psi. What would mu pressure rise be at 50psi????? I think I'd potentially damage tires if I were to experiment with that.

I can't believe that something isn't wrong here. We have had so many explanations of the charts that it reminds me of tax tables.....oh wait.....Govt. involved in both. Forgot.

Capri, can you clarify my specific example? It may help a lot of us understand.
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Old 10-17-2012, 06:16 PM   #89
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I put 16" michelin Lt E rated tires on and run them at 65 psi. Running PSI is 4 PSI higher. I ran my D rated ST tires at 60 PSI and 4 psi higher at running speed.

Michelin charts for ST and LT tires don't say run at max psi but give psi to weight limits. I would follow that over what Discount tires says any day... IMO.. I have never had a blow out.
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Old 10-17-2012, 07:23 PM   #90
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dznf0gjust wanted to tell you on my TV (Chevy Avalanche with stock 17" tires) while towing my tires increase 6psi when inflated to 34, 36, 38, 40, 42 or the sidewall max of 44psi. The starting pressure has not made any difference in the rise in psi.
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Old 10-17-2012, 07:45 PM   #91
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Quote:
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I put 16" michelin Lt E rated tires on and run them at 65 psi. Running PSI is 4 PSI higher. I ran my D rated ST tires at 60 PSI and 4 psi higher at running speed.

Michelin charts for ST and LT tires don't say run at max psi but give psi to weight limits. I would follow that over what Discount tires says any day... IMO.. I have never had a blow out.
But what is your weight on the axles? I believe you're a lot lighter at 65psi and 4 psi rise. That's what I am trying to get to, and I think it'll take 80 psi at 8000#s on the axles to get there. That is way off the chart spec for my tire, weight and speed, as I explained above.
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Old 10-17-2012, 07:49 PM   #92
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Quote:
Originally Posted by Wazbro View Post
dznf0gjust wanted to tell you on my TV (Chevy Avalanche with stock 17" tires) while towing my tires increase 6psi when inflated to 34, 36, 38, 40, 42 or the sidewall max of 44psi. The starting pressure has not made any difference in the rise in psi.
I don't doubt you, but that has not been my experience, in the norm. There are always highs and lows, and I haven't played with raising and lowering in the TV while empty or loaded (IOW, I raise to to tow at one setting and lower it to door jamb when trip is over. Depending on vehicle...2500s I don't even raise it. 1500s I do.) It doesn't surprise me that you don't see additional lowering of the rise above door jamb pressure. That tells me that the mfr has got the jamb pressure for the corresponding GAW correct. Where we differ is the amount of the rise. For me, it is virtually always a 4psi rise. Sun load can raise it another 1 or 2 on one side, and twisty roads will too, higher speed will too. But I live in the midwest. All my comments and measurements are on pretty flat, straight interstates at 63 mph. Should've stated those parameters as well above.
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Old 10-17-2012, 09:35 PM   #93
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Quote:
Originally Posted by dznf0g View Post
This is from 2012 Silverado Owner Manual (see orange highlight)
I read the things you posted from the Silverado Owner's Manual. I have been driving 1500 Chevy trucks for the last 30 years or more. I have about 1,000 miles on my first 2500. (always drove Fords before) What I noticed early on about the tires on the Chevys, especially on the 4 wheel drives, was the tires always were worn more on the outer edges before the end of the tread life at the center of the tire. Balance or alignment was not the problem. After many sets worn this way over the years, on the advice of a friend, I started adding about 5 PSI above what was on the door placard. (still below the max psi on the sidewall) Once I started doing this I always got even wear and an extra 5 or 10,000 miles on the tires. The ride was better too. I'm a believer in this, and will continue to do so.
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Old 10-17-2012, 10:00 PM   #94
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Quote:
Originally Posted by dznf0g

But what is your weight on the axles? I believe you're a lot lighter at 65psi and 4 psi rise. That's what I am trying to get to, and I think it'll take 80 psi at 8000#s on the axles to get there. That is way off the chart spec for my tire, weight and speed, as I explained above.
I'm running around 4900-5000 dry. So yes well under the max rating. The 4psi raise I figure they must take into account even if you start at the max psi and go over. I don't want to run max psi as it is overkill for the weight of my AS. And may beat it up going down the road. I don't think you can hurt anything to much running them at max psi, but if your AS is well under the max rating of the tires you will give the AS a hard ride.
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Old 10-17-2012, 11:44 PM   #95
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As I was checking the tire pressure prior to our last trip I noticed. Imprinted on the rim. Max Pressure 70#. So, in my opinion, one should check the specs on the rims used.
I have been running 55#'s in the tires for well over 10,000 miles without a problem. Don't plan to change.
My trailer is 38 years old and has no signs of rivets popping inside or out. Nothing shakes lose, tires don't run hot. The last 2700 mile trip completed just days ago. Not a single pound of air was lost in any of the 4 tires.
I keep my speed at 60 MPH. Averaged 13.2 MPG. With a 5.9 Ltr Dodge 3/4 ton Club Cab, Long Box pickup. Traveling thru the mountains of Wyoming, Colorado, Utah and Arizona. The total weight of the truck and trailer was 11,600#.
I use air bags on the truck and do not use an equalizer hitch.
While I have no proof, I feel an equalizer hitch, if not set up correctly. It may cause excessive damage to tires on the trailer. Consider the pressure on the side walls when making a sharp turn. When the equalizer hitch restricts the up and down movement at the ball and the anti sway system is trying to hold the trailer straight behind the TV. So in my way of thinking, as you turn a corner, you are literally trying to swing the trailer like a baseball bat. If the horizontal pivot point is restricted by the anti sway system, the tires are under excessive side wall stress as the trailer is forced sideways in a turn. I feel the same is true when the vertical pivot point is restricted. If you are on an uneven surface while making a turn, one tire or the other is under enormous pressure because of the sidewall stress placed on it. Just my humble opinion.
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Old 10-18-2012, 04:26 AM   #96
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Again, forgive me for parsing.

BTW, the way you do this is to use the quote type of reply and add /quote in brackets where you want the quote to end and your comments to start. Then add the "quote= whatever" in brackets, that is in the beginning of the first quote.
Quote:
Originally Posted by dznf0g View Post
Relative to your "sort of correct" in the owner manual, I stake my life that it IS correct. GM is the only mfr that tests EVERY tire offered in EVERY application offered for load and durability AND requires a TPC stamping the sidewall......
No, that is not correct. Don't confuse the TPC marking with GM doing the testing. First, other vehicle manufacturers also have markings, they just don't crow about it.

For example, every Ford tire has to have a qualifcation number on it. BMW's have to have a star, Porsche's have to have an "N", etc.

Second, EVERY vehicle manufacturer requires certain tests, but they are usually done by the tire manufacturer and the data is suppklied to the vehicle manufacturer.

- BUT -

My point was that when the tire load carrying capacity is calculated it is LARGER than the minimum needed to carry the load (accoprding to the GAWR's.) So it is technically stated incorrewectly - although it could be argued that that this value is what GM considers the minimum.

Quote:
Originally Posted by dznf0g View Post
....... (Don't confuse TPC with testing of different pressures. It is all a part of a very exhaustive testing and approval process.) Not trying to get argumentative, as your rules aren't wrong...may just be a generalization whereas GM tests to the specific application when a placard recommendation is made and it's pressure relationship to that specific tire(s) Sometimes there are several suppliers to the same model configuration. 15% is apparently the MINIMUM safety margin GM is willing to accept.......
GM's process is no more exhaustive than FORD's is (other vehicle manufacturers, too!). I know this as first hand info as I used to conduct the testing for OE qualifcations. The actual tests are different - as are the goals and the net result - but every vehicle manufacturer is doing model specific tests where applicable - like ride and handling approvals.

And I think you'll find Ford uses that 15% as do other vehicle manufacturers.

So GM is NOT unique in any of this regard.

Quote:
Originally Posted by dznf0g View Post
.......I was only referring to my current situation with the AS. I wasn't clear at all on that....and really haven't come to that conclusion on that as of yet. I do think a 7 - 9psi rise at 70psi cold indicates not enough air. .....
I have an Excel chart using the ideal gas formula that calculates the temperature rise based on the pressure rise - and the pecent rule works. 10% - whether you start at 30 psi (and get a 3 psi buildyp) or at 70 psi (and get a 7 psi buildup) results in very simlar temperature buildups.

Quote:
Originally Posted by dznf0g View Post
........

That's sorta my point. The math ISN't working with the PSI rises I am seeing in the AS with ~2000 lbs on each tire....compared to the chart (which I don't place a lot of confidence in) or the max load on the sidewall......
No, no, no! The "math" has to do with the load carrying capacity.

The load tables for P metric tires peak at 35 psi, but some tires say 35 psi max, some say 44 psi, and some say 51 psi. So the "math" you were using doesn't work consistently.

[QUOTE=dznf0g;1216642].......But the Michelin LTs on the AS are not behaving at all like the Michelin LTs on the truck relative to load vs. pressure buildup. This IS the issue for me. I find it baffling...and no I do not have any brake or bearing issues and all 4 are similar in behavior.../quote]

Remember the load has to be the same to get the same pressure buildup - PLUS, this has to do with heat generation and tires do vary in this regard.
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Old 10-18-2012, 05:45 AM   #97
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Quote:
Originally Posted by dznf0g View Post
Drop back 10 and punt! My head hurts....I want to get back on track with Capri's post #72 and leave out all the other stuff I started wandering off to.

Here's what I understand from post 72.

LTs may have some built in overcapacity to Capri's explanation of the 15% stuff. For this post I'm going to ignore that for my ultimate ending dilemma and question.

As I understand.....here's my calculated process.

LT225/75/16 tires
GVWR of AS is 10,000#
GAWR is 5000# each
since a minimum 10 of GVWR must be TW, "practical" GAWR is 4500# (2250# per tire)

If I were a trailer mfr, installing this tire on this trailer I would look at the chart, add my 15% load margin and find 2588# and recommend 75 or 80 psi.

Is that correct?......
I am going to assume you did the math right, but your methodology is correct.

Quote:
Originally Posted by dznf0g View Post
...........If so, then let's back up to actual loads on my particular AS. Last time over the scale I was at 7600# on the axles combined. (for this exercise, lets assume all tires are as close to equally loaded as possible). Now I know I've gained a little weight since my last weigh, so to make it easy, let's say 8000#

So to find a proper pressure for my load:

Add 15% to 2000# = 2300#. Go to the chart, find 2300# for the LT tire size and get a pressure of 65psi.

And this isn't considering the "built in overcapacity" you mention.

This is the question haunting me.....I have been running 65psi and have had what I believe to be too great of a psi increase over cold set pressure. As I mentioned above, it has been consistently 7 - 9 psi over cold ambient. (I have taken into account sun, wind direction, and normal ambient temp rise throughout the day when noting this. Meanwhile all the various TVs, some with LTs and some with P tires, inflated to placard pressures see a very consistent 4 psi rise.

This implies to me that I need more pressure. Thus far in that experiment I have only tried 70psi, with no difference in pressure rise. And yet, buy your explanation in post 72 (I know...you said LTs are more complex) I am some 15% overcompensating, due to the built in 15% of an LT. Plus I artificially upped my actual load. If I did the honest numbers without the extra 15%, I should be running at 50 psi. What would mu pressure rise be at 50psi????? I think I'd potentially damage tires if I were to experiment with that.

I can't believe that something isn't wrong here. We have had so many explanations of the charts that it reminds me of tax tables.....oh wait.....Govt. involved in both. Forgot.

Capri, can you clarify my specific example? It may help a lot of us understand.
First, I am NOT a fan of using anything but what appears on the vehicle tire placard. Mostly because I think vehicle manufacturers are in the best position to know how their vehicle handles.

I am also not a fan of folks using the load tables the tire manufacturers publish - partially because most folks don't know how to do this correctly and partially because it doesn't matter. Minimums are minimums and there are any terrible things that happen if you are somewhat over the minimum.

But your explanation is correctly done, so it's hard to argue with.
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Old 10-18-2012, 05:54 AM   #98
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Originally Posted by A W Warn View Post
I read the things you posted from the Silverado Owner's Manual. I have been driving 1500 Chevy trucks for the last 30 years or more. I have about 1,000 miles on my first 2500. (always drove Fords before) What I noticed early on about the tires on the Chevys, especially on the 4 wheel drives, was the tires always were worn more on the outer edges before the end of the tread life at the center of the tire. Balance or alignment was not the problem. After many sets worn this way over the years, on the advice of a friend, I started adding about 5 PSI above what was on the door placard. (still below the max psi on the sidewall) Once I started doing this I always got even wear and an extra 5 or 10,000 miles on the tires. The ride was better too. I'm a believer in this, and will continue to do so.
You've got to be careful here:

First is that the 15% over capacity is a relatively recent phenomenon. My best estimate is that vehicle manufacturers completed their conversion by 2008. Prior to that there was a mixed bag - especially on tow vehicles.

The second thing is that if tires are worn on one shoulder and not the other, that DOES indicate an alignment problem. Please note that "alignment problem" includes vehicle alignments "in spec" - meaning that the spec is wrong.

One of the major problems in trying to sort this all out is that things change over time. Vehicle suspensions change, tires change and without doing A-B-A testing (changing one thing at a time) it is hard to be sure what is causing an issue.
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Old 10-18-2012, 06:55 AM   #99
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I'm running around 4900-5000 dry. So yes well under the max rating. The 4psi raise I figure they must take into account even if you start at the max psi and go over. I don't want to run max psi as it is overkill for the weight of my AS. And may beat it up going down the road. I don't think you can hurt anything to much running them at max psi, but if your AS is well under the max rating of the tires you will give the AS a hard ride.
I agree Jason, I believe (what I am trying to figure out) is that there is a proper pressure rise, for each tire size/brand/type, where that rise will be 4 PSI, (again, under my climate, geography, and speed).

My THEORY, and only a theory at this point, is that the automakers do EXTENSIVE validation testing on tires and the behavior we can note with TPMS vs. load. vs speed, etc., we should be able to translate over to the AS where we are deviating form OEM tires and be able to answer these nagging proper pressure questions. (How's that for a run on sentance?)

I think that (especially those of you with lighter rigs) can find that 4 psi mark, and reduce pressure till you see a rise greater than that, then back up to the pressure where you just see 4 psi (or your number for your climate, speed.....that you normally see on your properly inflated TV). At least you will be running the temps that an OEM automaker has deemed good to go. That's a lot more than we have to go on with our AS questions.

I just haven't found it yet on my rig...and I am not sure why, based on the info above.
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Old 10-18-2012, 07:22 AM   #100
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I have an Excel chart using the ideal gas formula that calculates the temperature rise based on the pressure rise - and the pecent rule works. 10% - whether you start at 30 psi (and get a 3 psi buildyp) or at 70 psi (and get a 7 psi buildup) results in very simlar temperature buildups.


OK, I agree with your RELEVANT TO THIS DISCUSSION parts, (won't argue the other stuff, other than to say, I underwent significant training during the Ford/Firestone thing, and, at least at that time, there were SIGNIFICANT differences in the way MFRS tested, "certified" and selected tires.)

I am, however, confused by this statement. I had always understood, and TireRack states (for whatever that is worth) that the correlation is 1psi = 10 degrees, irrespective of the starting pressure.

Are you saying that if I find a pressure where I see (consistently) 7 psi rise at 70psi cold, that I will be running the same temperature as a theoretical tire seeing a 3psi rise at 30 psi. (disregard construction variables, etc., just based on gas thoery)

Tire Tech Information - Air Pressure, Temperature Fluctuations
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