Community Forums

EMD_Driver · 04-08-2012, 04:26 PM

I found this on another forum and thought it might be worth the read...

I copied this from another thread about LT vs ST tires:
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Portion of a post by Mike Mitchell, NuWa (HitchHiker) CEO in a discussion regarding trailer tires on the NuWa Owner's Forum.

As we banter about regarding tire types and loading, I believe that we are finally starting to understand a few important things.

I have asked many times for someone to explain how a ST tire can be rated to carry more weight than a LT tire in a similar size, without a good answer.

The answer lies in what is called reserve capacity. To quote from Trailer Parts Superstore and this same statement exist on just about every tire site:

HEAVY DUTY 'LT' TRUCK / TRAILER TIRES
'LT' signifies the tire is a "Light Truck/Trailer" series that can be used on trailers that are capable of carrying heavy cargo such as equipment trailers.

If a tire size begins with 'LT' it signifies the tire is a "Light Truck-metric" size that was designed to be used on trailers that are capable of carrying heavy cargo or tow vehicles. Tires branded with the "LT" designation are designed to provide substantial reserve capacity to accept the additional stresses of carrying heavy cargo.

So what is reserve capacity? It is capacity beyond the rating of the tire, capacity that is held in reserve. This reserve capacity comes from the heavy-duty sidewall of the LT type tires. LT's rank at the top of the list when we look at P, ST and LT tires.

Now I finally have an answer to how a ST tire can be rated to carry more weight than a LT tire of similar size.

The ratings of ST tires infringe into the reserve capacity of the tire. This is double bad, because the design of the ST gives us a tire with less reserve capacity to start with as it has a lighter sidewall to start with as most ST tires are much lighter than their LT counterparts.

To quote one tire site:
"Put a different way, the load carrying capacity of an ST tire is 20% greater than an LT tire. Since durability is strictly a long term issue - and the results of a tire failure on a trailer are much less life threatening than on a truck - the folks that set up these load / inflation pressure relationships allow a greater......ah......let's call it load intensity."

There it is in print to be read. They make a calculated decision to give the ST tire a higher load rating because a failure is less life threatening.

I have on a number of occasions pointed out the weight difference between the different tires and have been told that does not matter. Well it does matter. The rubber in the average tire only makes up around 40 some percent of its weight, the rest is in the steel belts, gum strips, steel beads, and the carcass plies. The remaining 60 or so percent of the stuff in a tire is what builds in the reserve capacity.

So to review again, here are some weights:
1. Michelin XPS RIB LT235/85R16 LRE (rated to 3042lbs) Weight 55.41
2. Goodyear G614 LT235/85R16 LRG (rated to 3750lbs) Weight 57.5
3. Bridgestone Duravis R250 LT235/85R16 LRE(rated to 3042lbs) Weight 60
4. BFG Commercial TA LT235/85R16 LRE(rated to 3042lbs) Weight 44.44
5. Uniroyal Laredo HD/H LT235/85R16 LRE(rated to 3042lbs) Weight 44.44
6. GY Marathon ST235/80R16 LRE(rated to 3420lbs) Weight 35.4

So which tires on the list have the most reserve capacity? Well that is not a completely simple answer, as one of the tires is a G rate 110 lb tire and the rest are LRE at 80lb inflation. So if we disregard the G614, then the Michelin XPS RIB and the Bridgestone Duravis R250 due to their all-steel ply construction will have the most reserve capacity inherent in their construction. The twin Commercial TA and Laredo will be next and the Marathon would have little or no reserve capacity available because it was used up in its higher load rating, AND because of it's much lighter construction it had much less inherent reserve capacity to start with.

So what have we learn from this?

I think that the first thing that we learned was that a LT tire can be used at or near it max rated loading without having issues, as they built with "substantial reserve capacity to accept the additional stresses of carrying heavy cargo".

The second thing we may have learned is why ST tires are failing on mid to larger 5th wheels, in that they do not have inherent reserve capacity beyond that rated max loading. Again this is because they have less reserve capacity to start with and their greater "load intensity" used up any reserve capacity that might have been available.

Now, here is an interesting bit of information. I just called Maxxis Tech Line and asked the weights for two tires.

ST235/80R16 LRD 3000 lb rating at 65 lbs of air weights 38.58
ST235/80R16 LRE 3420 lb rating at 80 lbs of air weights 43.43

What??? The Maxxis load range E tire weights almost the same as the Commercial TA?? This is a ST tire that has heavier construction than the GY Marathon at 35.4 lbs. So it has more inherent reserve capacity due to its heavier construction.

Those that claimed its virtues maybe did not know why it was a better ST tire than some of the others, but there it is! It is a heavier built tire with more reserve capacity.

So as one chooses a replacement tire or is asking for an upgrade on a new trailer please get educated on where the reserve capacity exist. Is it inherent in the tire you choose or do you have to factor it into the weight rating of the tire you choose.

SmokeyWren · 04-09-2012, 02:42 PM

Mike Mitchell may be a CEO of a big company, but he apparently NOT an engineer specializing in tire engineering.

Tire weight capacity and load/inflation data for all tires sold in the USA are developed by a bunch of engineers called the Tire and Rim Association (TRA). Pure science and engineering, without a hint of marketing or money-making influence. The load/inflation tables, speed ratings and load ratings they develop apply to all brands of tires sold in the USA. So if they say an ST235/80R16 tire has a higher load rating than an LT235/85R16E, then you can believe it, regardless of what a trailer manufacturer says about it.

Commercial tires are designed for four different uses, and the use defines the construction of the tire. Those four uses are steer axle, drive axle, trailer axle, or all-wheel positions.

Generally, the all-wheel-position truck tires cost more than the same size tire for a specific axle use. For example, one excellent all-position truck tire is the Michelin XPS Rib. It is the only 16-inch Michelin tire included on the Michelin truck website, and is rated an all-wheel-position truck tire - which means it is a trailer tire as well as a steer axle or drive axle tire. The other Michelin LT tires are not rated for any position of a truck, so they are not suitable as trailer tires.

Most ST tires are limited to 65 MPH speed. The advantage of the XPS Rib is it is has a speed rating of "Q" = 99 MPH. I often exceed 65 MPH when towing, but rarely exceed 99 MPH, even when passing on a 2-lane highway.

If you dig around on the Goodyear website, you'll find that Goodyear makes a tire that is almost identical to the Michelin XPS. All-steel construction. Recapable. Rated as an all-position truck tire. And it costs an arm and a leg and your first-born son, just like the XPS Rib. And it comes only in the same sizes as the XPS Rib =
LT215/85R16/E
LT225/75R16/E
LT235/85R16/E
LT245/75R16/E

That's it. If you want a different size, then you'll have to choose a different tire.
http://www.michelintruck.com/micheli...o.do?tread=XPS RIB

When it comes to ST tires, I've had some really good service from Cooper and Maxxis ST tires, and I've had some awful service from Goodyear Marathon trailer tires. I haven't run the expensive XPS rib on my trailers yet, mainly because all 4 of my trailers have 14" or 15" wheels, not 16". I wore out two sets of XPS on a '99.5 F-250 PowerStroke tow vehicle. They lasted more than twice as long as the almost-as-costly BFGoodrich AllTerrain.

JIMNLIN · 04-09-2012, 08:55 PM

You can find more ST vs LT info on this click link http://http://www.rv.net/forum/index...d/25786030.cfm
by CapriRacer (Bary Smith) a actual tire engineer who has ST tire experience. Many tire engineers may work for a tire company that doesn't manufacture or sell a ST tire.

The LT can be used in a multi position however its best to check with the tire manufacture as they may only recommend only one or two of their LT tire lines for trailer service.
Example is I did a 1-800 to BFG and asked which LT tire line they recommended for a 11200 lb tandam axle RV trailer. The responce came from Michelin and gave the BFG Commercial T/A all season and the sister tire from Uniroyal Lerado HD-H a thumbs up and of course the XPS Rib tire. Michelin owns BFG and Uniroyal.

I come from a haulers background (private and commercial) and have owned a RV of some type since the early '60's. As a hauler the ST simply don't make the grade operating at max loads at interstate speeds so naturally I don't use them on any trailer type.

On the flip side is a poster that had a 6k-7k trailer with STs that ran 4k miles in six years. They worked fine for his situation.

taken · 04-10-2012, 04:47 PM

That is a very interesting writeup and seems to make a lot of sense. I really wanted to replace the Towmax bombs on our new rig before our first big trip in June, but it seemed the only viable choice was the Maxxis as we have 7k axles and didn't want to be on underrated XPS's. I guess they wouldn't be underrated at all.....

SmokeyWren · 04-10-2012, 06:11 PM

Quote:

Originally Posted by JIMNLIN

You can find more ST vs LT info on this click link http://http://www.rv.net/forum/index...d/25786030.cfm

Jim, your link doesn't work. Notice it begins with two sets of "http:"

Here is a link that should work:
RV.Net Open Roads Forum: Revived Trailer Tire Thread (formerly on the 5th Wheel Forum

That is a recent 80-page thread in a forum on RV.net.

JIMNLIN · 04-10-2012, 06:46 PM

Thaks Smokey. Thats the clicky I tried to post. Right now its cooled off and hasn't had a comment for several days.
If anyone is interested you can read more of Barry Smiths ST vs LT comments/recommendations on Allexperts.com.