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Old 08-29-2008, 02:01 AM   #191 (permalink)
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Quote:
Originally Posted by Concrete View Post
what about changing the spec of the tire in stalled?
like putting a LT (light truck) tries on instead of P (passenger) tires

Then the rated pressures go up - so you could have 65 psi & be within spec
Would the standard passenger car rim you're mounting it on to be rated to 65 psi?

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Old 08-29-2008, 02:26 AM   #192 (permalink)
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Old 08-29-2008, 02:31 AM   #193 (permalink)
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You are correct,
The mail delivery trucks, made by Grumman, powered by GM or Ford (depends on the year) say 50 psi on the fenders.
We use GY Wrangler tires and that is the max sidewall pressure.
The P.O. specified many years ago that all tires were to be inflated to max sidewall rating.
Reasons stated were;
Too many failures from under inflation
and, get this,
Fuel economy.
It took the gubmint millions of dollars (I don't have the actual report handy)
and a team of X-spurts
and two years
to come up with this maintenance procedure.
All they had to do was ask me.
Your tax dollars at werk.
Now if we could just get them to let us align the front end properly we could end the early retirement of the steering tires.
although they get recycled and sent back to us with recap tread on them then we put them on the rear of the trucks, where they live out their life in peas and hominy. Showing NO signs of premature wear, or center tread wear before edge tread wear.
Schultz out.
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Old 08-29-2008, 10:20 AM   #194 (permalink)
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It might be time for a lesson in the difference between P metric and LT metric tires.

Small vehicles use P metric tires (or their European or Japanese equivalents). These tires are low pressure tires and this results in a need to make the rubber flexible enough to deal with the deflection required by the load / inflation curve. Low pressures also result in good ride qualities.

Larger vehicles (meaning 3/4 and 1 ton pickup trucks, vans, and in particular the Grumman trucks discussed above) use LT metric tires. These are high pressure tires ("high pressure" being a relative term). These tires tradeoff good ride qualities for increased load carrying capacity for the amount of space the tire occupies. This also makes the tire more efficient from the material / load capacity point of view. An example of this sort of tradeoff is aircraft tires where space and weight are at a premium. Some aircraft tires use over 300 psi inflation pressure.

This also results in tires that are more potentially retreadable, where P metric tires are less so because of the amount of flexing that takes place.

Which brings me to what is printed on the sidewall of tires:

There are government regulations that specify what is to be imprinted on the sidewall of tires. However, the US regulation is a little unclear. While there is agreement within the tire industry as to what the regulations say about maximum load, there is some disagreement in interpreting what pressure is supposed to be imprinted on the sidewall. Notably, the Michelin group (Michelin, Goodrich, Uniroyal) seem to think that S and T rated Standard Load passenger car tires should have 35 psi imprinted, while the rest of the industry thinks the regulations say that higher pressures should be imprinted (since it is permissible to use them.)

Hence, you will find tires that are fundamentally the same (same size / speed rating) with different max pressures listed on the sidewall.

This problem doesn’t exist for LT metric tires, so there is complete agreement within the tire industry as to what gets imprinted – both for load and pressure (if you take into account the difference in tire standards).

But there a small amount of overlap between passenger car tires and LT tires. You will find tires with the same "size" (meaning physical dimensions) of both types. The problem here is that the load carrying capacity of LT metric tires is lower than the "same size" P metric tire - due to the difference in the allowable deflection in calculating the loads. My take on this is that the tires are not interchangeable.
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Old 08-29-2008, 10:51 AM   #195 (permalink)
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Quote:
Originally Posted by metroschultz View Post
You are correct,
The mail delivery trucks, made by Grumman, powered by GM or Ford (depends on the year) say 50 psi on the fenders.
We use GY Wrangler tires and that is the max sidewall pressure.
...
It took the gubmint millions of dollars (I don't have the actual report handy)
and a team of X-spurts
and two years
to come up with this maintenance procedure.
All they had to do was ask me.
Your tax dollars at werk.
...
NO signs of premature wear, or center tread wear before edge tread wear.
Schultz out.
I knew you'd come through. Good work Mr. Schultz! An actual Gubment Study on the issue -- gold I tell ya!

Way off topic, I see some interesting USPS fleet vehicles out there. Like the Aerostar 4WD -- how are those holding up? I can't recall if a K-Car was thrown in the mix (maybe that was the FAA that still has a few).

Quote:
Originally Posted by CapriRacer View Post
Larger vehicles (meaning 3/4 and 1 ton pickup trucks, vans, and in particular the Grumman trucks discussed above) use LT metric tires.
The little Grumman runabouts that are probably in most U.S. neighborhoods (other than Rural personal vehicles) appear to be smaller than a pickup. I wonder what the GVWR is on those...

Back to Minivans -- I'm sure most people overload them. If you get 6 adults in there, the max payload is likely reached. If I remember correctly, most have passenger tires, as opposed to LT.

Anyways, I'm already sacrificing ride quality with the extra air, but it has been paying dividends. If I do haul something large in the hatchback, it would likely exceed the vehicle's recommended capacity before the tires.

I wonder how many people out on the roads are just, overloaded...

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Old 08-29-2008, 10:06 PM   #196 (permalink)
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Quote:
there a small amount of overlap between passenger car tires and LT tires.
Yep - here is one list that directly compares the sizes - presumably for making the swap.
(thought it might be a handy link for this discussion)
Tire Tech Information - Diameter Comparison of Light Truck Tire Sizes


Quote:
The problem here is that the load carrying capacity of LT metric tires is lower than the "same size" P metric tire - due to the difference in the allowable deflection in calculating the loads.
Are you saying this because we are still expected to put 32 psi in them?
Because this article is all about not converting from LT to P rated tires
exactly because the P tires are lower rated.
CanadianDriver: Advice - Light Truck vs Passenger car tires: what you should know


Quote:
My take on this is that the tires are not interchangeable.
you have lots of information in your post but not much on this opinion
what are the issues with this?
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Old 08-30-2008, 05:59 AM   #197 (permalink)
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I'm going to shorten this for clarity:

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

you have lots of information in your post but not much on this opinion
what are the issues with this?
Here's a comparison of the load curves for the "same size" P metric to LT metric. (excerpted from 2005 TRA Yearbook)

Pressure......P245/75R16...........LT245/75R16
...(psi).....Max Load (pounds)....Max Load (pounds)

...30...........1914....................... ---
...35...........2065 (SL)................ 1700
...40........... --- ....................... 1865
...45........... --- ....................... 2030
...50........... --- ....................... 2205 (C)
...55........... --- ....................... 2335
...60........... --- ....................... 2480
...65........... --- ....................... 2623 (D)
...70........... --- ....................... 2765
...75........... --- ....................... 2900
...80........... --- ....................... 3042 (E)


Notes:

1) The letters in the parantheses are the load range and are placed next to the maximum load (and corresponding pressure) for that load range.

Put another way: An LT245/75R16 LR C has a max load of 2205 # and it occurs at 50 psi - and 50 psi is the max usable pressure.

2) I've applied the 10% load reduction for "P metric tires used in LT application" to make the comparison "apples to apples".

3) The "---" in the table means the yearbook doesn't have an entry for that pressure - meaning the table either doesn't go that high for P metric or that low for LT metric. I take that to mean those pressure are not usable.

There are some notes under the published load tables that allow higher pressures than listed in the table above. Because of the way the governmental regulations read, this note is usually what governs what gets imprinted on the sidewall - and as I said in the previous post, there is not 100% agreement within the tire industry about passenger car tires.

What you should get out of this comparison is that for the same load, the LT metric tire requires a higher pressure, but the maximum load carrying capacity is higher for an LT metric tire (at a higher pressure).

From all this, I conclude that P metric and LT metric tires are not interchangeable.
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Last edited by CapriRacer; 08-30-2008 at 06:14 AM..
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Old 08-30-2008, 09:32 AM   #198 (permalink)
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Quote:
Originally Posted by cfg83 View Post
trikkonceptz -
I think you are talking about this?

Aquachannel tyres.
Car Bibles : The Wheel and Tyre Bible Page 1 of 2

CarloSW2
These were called Goodyear Aquatreads. I had these tires and they were superb in the rain. I know without a doubt these tires saved my life in a downpour once in Connecticut. I did not have a wear problem, but I put them on my mother's large car and they didn't last 12,000 miles. They were discontinued after a few years.
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Old 08-30-2008, 02:52 PM   #199 (permalink)
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Quote:
P metric and LT metric tires are not interchangeable.
this page says otherwise
(load PDF named Replacement Guidelines for Passenger and Light Truck Tires)

CapriRacer,

I appreciate the data, but when you say
Quote:
The "---" in the table means the yearbook doesn't have an entry for that pressure - meaning the table either doesn't go that high for P metric or that low for LT metric. I take that to mean those pressure are not usable.
followed by:
Quote:
There are some notes under the published load tables that allow higher pressures than listed in the table above.
it makes it look like you are just showing the data that supports your point
(just an observation from the other end of the text - please don't take it personal)

Also from an industry that loves the aftermaket and the mod community
and gives us dangers like this...

and this


it is hard to imagine over inflating a bit
or upgrading to a professional grade tire to up pressures
is anywhere near as signifigant as either of these

Ecomodders are for the most part careful people
please help us quantify the risk and not just say it is impossible
Thanks,
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Old 08-31-2008, 06:03 AM   #200 (permalink)
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Quote:
Originally Posted by Concrete View Post
this page says otherwise
(load PDF named Replacement Guidelines for Passenger and Light Truck Tires)

CapriRacer,

I appreciate the data, but when you say

.....missing quote.....

followed by:

.....missing quote.....

it makes it look like you are just showing the data that supports your point
(just an observation from the other end of the text - please don't take it personal)

..................
Note: I truncated the end of the post.

I wondered if anyone was going to pick up on that and decided not to address that issue in the post. But since it has been brought up:

The notes at the end of the load table are labeled "permissible". This means something less than "recommended" (having gone through a similar discussion in the process of issuing another industry publication). These notes also come with stipulations as to when they are "permitted".

But my point about the difference in load carrying capacity remains. The table clearly shows they have different characteristics.

As has been pointed out, the RMA publication says you can swap P metrics with LT metrics: (the link above didn't work, but here's a link to the page so you can download the publication):

https://www.rma.org/publications/tir...TOKEN=85722324

If you were actually try to do what the publication suggests, you'll find there is a very small window where this is even possible - and that assumes that the rim is capable of handling the higher pressure - and most of the time it isn't - so for practical purposes, you can't interchange the 2 (except in some rare circumstances).

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