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Old 09-02-2013, 08:09 PM   #21
Irlpguy
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quote:Originally posted by moutard2

I'm not questioning anything that has been stated here however I am curious as to the source of the "facts" as outlined in the posts particularly the post that states that there is NO reserve capacity.
Personally I believe that there must be some kind of built in "reserve" because each tire experiences different loading factors as we motor down the highway. Have you ever hit an expansion joint or other road hazard that caused your trailer to shift onto one side or the other? I would think that this movement would cause a significant load increase on one side thus the need for a "reserve" capacity. Don't shoot .... I'm just thinking.
Ha Ha I am not shooting and I am glad you are thinking. Here is what I have researched and found on the NHTSA website: Specifically tires.

http://www.nhtsa.gov/Laws-Regs

Also specifically rules on this website:

http://www.nhtsa.gov/cars/rules/ruli...dex.html#ref74 In particular the reference to part D. Tire Selection Criteria/De-Rating of P-metric Tires You have to read down a good number of paragraphs to the final ruling.

It is these references to tire reserve load that some have used a play on words to suggest an LT tire has a reserve capacity where an ST tire does not.

Unless I misunderstand this ruling after reading it over and over then what I determine it means is exactly as it states:

For passenger cars and for non-passenger car vehicles equipped with LT tires, the final rule requires that the vehicle normal load be based on 94% of load rating at the vehicle's placard pressure. Therefore, vehicle manufacturers will be required to insure that the tire reserve load corresponds with the tire's load carrying capabilities when the tire is inflated to the vehicle manufacturers recommended cold tire inflation pressure rather than the tire manufacturer's maximum cold inflation pressure shown on the tire sidewall. The 94% figure was chosen to approximate closely the load reserve that results from the current requirement of 88% based of load rating at the tire's maximum inflation pressure.

It refers to the tires reserve load capacity when it is inflated to the pressure specified by the vehicle manufacturer, rather than what is on the side of the tire as it did originally. The load reserve is built by the vehicle manufacturer because they are required to use only 94% of the tire capacity. There is no mention in all of these rulings that state the LT tire itself has more (or less) reserve capacity than an ST tire.

The difference and confusion is that trailer manufacturers do not have to use this 94% reserve when they rate the load capacity of the trailer, they can use 100% of the tires rating and put that on the placard on the trailer.

The original play on words that has been floating around this and other forums was originally posted by a fellow by the name of Barry Smith on Allexperts.com and further crap has been added by others. It has been debunked by others who have challenged those that promoted the so called "reserve capacity" theory to provide written backup of their claims. That to date has not happened, because there is none.

P and LT Tires cannot be compared to ST tires other than the fact that they both have the same DOT requirements stamped on them. They are built for a completely different application.

The LT tire that is DOT rated for a load of 3042 lbs cannot safely carry more than that. By only using 94% of that capacity a "reserve capacity" is created on those passenger vehicles as specified in the ruling.

By the same token an ST tire DOT rated at 3402 lbs cannot safely carry more than that. The difference is RV manufacturers do not have to reduce the DOT rated capacity which leaves no "reserve capacity".

There is literally hours of reading in these documents: some a bit hard to understand and somewhat confusing but these rules are what the legal profession would research and use in any lawsuit. Here is another site referencing sticker information regulations I found to be of interest.
http://lemon.onecle.com/fmvss-standard-no-110/

The "real" reserve capacity that we need to worry about on our trailers is the safe spread we have between what the tire is DOT rated to carry and what our load is likely to be at it's maximum, the greater the difference in those numbers the more reserve/safety we have.

Once again I will state I am absolutely no expert on tires and do not claim to be one. This is how I and others have interpreted the NHTSA rulings with respect to load reserve/capacity. Read some or none of this material, agree or disagree we are all free to do that.
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Old 09-02-2013, 08:25 PM   #22
Irlpguy
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Moutard2, just to comment on hitting an expansion joint or other road hazard. When we are running on tires that are basically at their maximum load capacity and something like this happens damage can occur but not cause a blowout to the trailer tire at that time, by the same token it can happen on a LT or P tire as well. Alone it might not be the straw that breaks the camels back.

I believe that we can unknowingly or knowingly hit something causing some damage, this damage may not manifest itself until quite some time later when we have driven many miles. I also believe that overloading and under inflation which both cause heat to be one of the greatest reason our ST tires fail. Speed is another factor.

Just thinking also, but do you think perhaps we could get a road hazard guarantee on our ST tires.

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Old 09-03-2013, 07:32 AM   #23
moutard2
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Quote:
quote:Originally posted by Irlpguy

Moutard2, just to comment on hitting an expansion joint or other road hazard. When we are running on tires that are basically at their maximum load capacity and something like this happens damage can occur but not cause a blowout to the trailer tire at that time, by the same token it can happen on a LT or P tire as well. Alone it might not be the straw that breaks the camels back.
Perhaps the expansion joint was not the best example. I wasn't trying to illustrate an example of when damage could occur but rather one where load (and thus weight) shift could occur. If my left rear trailer tire was running at maximum inflation and at a scale weight of maximum capacity and I hit a pothole on that side, I would think that there might be some kind of loading on the left side tires due to inertia, gravity, etc. and the load on those tires might then be at maximum plus something. Without any "reserve" capacity in terms of tire construction, material, etc., a catastophe could occur. I dunno ... maybe my thought process is way out in left field on this but we see examples of reserve capacities built into just about everything these days to allow for the "human" factor of pushing everything to the limit. OK, next topic!!
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Old 09-03-2013, 07:34 AM   #24
moutard2
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Quote:
quote:Originally posted by Irlpguy

Moutard2, just to comment on hitting an expansion joint or other road hazard. When we are running on tires that are basically at their maximum load capacity and something like this happens damage can occur but not cause a blowout to the trailer tire at that time, by the same token it can happen on a LT or P tire as well. Alone it might not be the straw that breaks the camels back.
Perhaps the expansion joint was not the best example. I wasn't trying to illustrate an example of when damage could occur but rather one where load (and thus weight) shift could occur. If my left rear trailer tire was running at maximum inflation and at a scale weight of maximum capacity and I hit a pothole on that side, I would think that there might be some kind of loading on the left side tires due to inertia, gravity, etc. and the load on those tires might then be at maximum plus something. Without any "reserve" capacity in terms of tire construction, material, etc., a catastophe could occur. I dunno ... maybe my thought process is way out in left field on this but we see examples of reserve capacities built into just about everything these days to allow for the "human" factor of pushing everything to the limit. OK, next topic!!
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Old 09-03-2013, 08:00 AM   #25
Irlpguy
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There are documented cases of damage to rims in such a scenario with no apparent damage to the tire, most often however the tire is damaged and in some cases I guess the extra weight or inertia will cause the tire to fail. How much extra on any given tire would be hard to determine.

There are all kinds of tests done on passenger tires, but few if any on ST tires that I have been able to find. Perhaps an LT tire will take more abuse than a ST I do not know. I do know the DOT load rating on both is what is supposed to be the Maximum at the specified pressure.

I would just like to see some documentation either from a legal requirement or tire manufacturers requirement that says the LT has more reserve capacity than an ST tire.

Tires will be a never ending debate, yes next topic.

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Old 09-03-2013, 08:17 AM   #26
Irlpguy
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For anyone interested here is a great site that gives all kinds of information on Passenger tires, nothing on trailer tires however it is interesting.

http://www.safercar.gov/Tire

If you look at "tire aging" for example the very last paragraph states that over 11,000 tow away crashes occur every year related to tire issues. That figure may not include trailers as the site is related to passenger type tires.
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