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LT Tires and How They Work

BillyHW

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I found this information really useful and I'm reposting it here. It's actually really hard to find any document that explains all of this in detail. It was originally posted here by Discount Tire:

http://forums.off-road.com/tire-tips-what-works-best/267645-lt-tires-how-they-work.html

Ever wonder when reading tire reviews with complaints about "squishy" or "slow responding" traits.... Is the tire to blame or was it the size the user decided to run? Did this driver upgrade from a stock tire with an 80psi max pressure to a larger tire with a 65psi max?

These questions pop into my mind time and time again and the answer is not always in the review.

With LT-Metric Sizing and LT-Flotation Sizing being the most popular LT Sizing you'll find on today's trucks, and by today's trucks we mean SRW 1 ton, 1/2 ton and 3/4 ton, we decided to take some time to explain the LT Tire System and focus on load carrying capacity. In this write-up we will define LOAD RANGE and LOAD INDEX and explain in detail how the two work in conjunction with each other to determine load carrying capacity. This topic can be some what confusing so bear with us to the end and if you have any questions please feel free to post them here.

Below is an example of LT-Metric Sizing and LT-Flotation Sizing. We've broken down the characters and numbers in each size to explain what they mean, which most of you already know.
LT-Metric Sizing
LT265/75R16/E 123R

LT = Light Truck Designation
265 = Section Width (mm)
75 = Aspect Ratio
R = Radial Construction (most common construction used today)
16 = Rim Diameter (inches)
E = Load Range
123 = Load Index
R (after load index) = Speed Rating


LT-Flotation Sizing
31x10.50R15LT/C 109S

31 = Tire Diameter (inches)
10.50 = Section Width (inches)
R = Radial Construction (most common construction used today)
15 = Rim Diameter (inches)
LT = Light Truck Designation
C = Load Range
109 = Load Index
S = Speed Rating
Like mentioned above, we want to specifically look at the Load Index and Load Range. Without the Load Range and Load Index, you can not determine the maximum load carrying capacity of the tire.

Q: What is LOAD RANGE?
A: LOAD RANGE (also known as the load range description) is the load range rating found on the sidewall of LT tires. It's used to determine the maximum load carrying capacity of the tire UNDER A DEFINED MAXIMUM INFLATION PRESSURE.

Q: What is LOAD INDEX?
A: LOAD INDEX is an assigned number ranging from 0 to 279 that corresponds to the LOAD-CARRYING CAPACITY of a tire.

Like most of you know, Load Range describes the ply rating of the tire. It's important to remember that these are ONLY RATINGS and not the actual number of ply's like it used to be back when tires were made of cotton belts (bias). Below are the most common Load Ranges you'll see on today's trucks.

C = 6 ply rated
D = 8 ply rated
E = 10 ply rated

Now here's where it can get a bit confusing. It's commonly assumed that C rated tires hold a maximum pressure of 50 PSI, that D rated tires hold a maximum pressure of 65 PSI, and E rated tires hold a maximum pressure of 80 PSI. This information is correct but only applies to a certain tire size range, or rather, section width.

Here's what you need to focus on.

LT tires that are 295mm (LT-Metric) in section width or 11.50" in section width (LT-Flotation) and narrower, will carry the commonly associated maximum pressure.

C = 6 ply rated and hold a maximum pressure of 50 PSI
D = 8 ply rated and hold a maximum pressure of 65 PSI
E = 10 ply rated and hold a maximum pressure of 80 PSI

Not so tricky I hope. So you're probably thinking to yourselves, what about the other sizes? And I'll tell you...

LT tires that are 305mm (LT-Metric) in section width or 12.50" (LT-Flotation) and wider, still carry the same ply rating/Load Range but hold a different maximum inflation pressure. The maximum inflation pressure on these wider sizes lose 15 PSI from the maximum inflation pressures above. The new inflation pressures on these wider sizes are as follows:

C = 6 ply rated and hold a maximum pressure of 35 PSI
D = 8 ply rated and hold a maximum pressure of 50 PSI
E = 10 ply rated and hold a maximum pressure of 65 PSI

Now what about Load Index? Like mentioned previously, Load Index is an assigned number that corresponds to the load carrying capacity. More specifically, at the maximum pressure. The assigned number or Load Index can be seen on the chart below.

*Please note that this chart only goes up to 150 as most LT truck tires today do not exceed 128

Pressure.jpg


So lets take the Load Index 123 for example. This Load Index tells us that the corresponding load carrying capacity is 3417lbs at maximum pressure. Now it doesn't show what the maximum pressure is so we now have to go look at the Load Range on the tire we are looking at.

Taking the example of LT-Metric Sizing listed earlier (LT265/75R16/E 123R). We know the tire is 10 ply rated and is narrower than 295mm in section width which tells us that the maximum inflation pressure is 80 PSI. By using the Load Index chart we know the 123 Load Index carries 3417lbs at maximum pressure. So from what we have discussed, we now know that the LT265/75R16/E 123R tire carries a maximum load of 3417lbs. @ 80 PSI.

Now taking the example of LT-Flotation Sizing listed earlier (31x10.50R15LT/C 109S), we know the tire is 6 ply rated and is narrower than 11.50" in section width which tells us that the maximum inflation pressure is 50 PSI. By using the Load Index chart we know the 109 Load Index carries 2271lbs at maximum pressure. With this information we now know the 31x10.50R15LT/C 109S tire carries a maximum load of 2271lbs @ 50 PSI.

Here's where it get's fun.

A lot of people think that Load Range E tires will always carry more weight than the lower ply rated/Load Range tires. This is not always the case. Let me show you an example.

A common OE (original equipment) LT tire size is LT245/70R17/E 119S. By using what we have learned above we know that this LT tire size carries 2998lbs @ 80 PSI. Now lets look at an LT315/70R17/D 121T which is a common plus size LT tire. Again, from what we have learned above we know this tire carries 3197lbs @ 50 PSI. So even though the OE tire we used for an example is E rated with a maximum pressure of 80 PSI, the plus size D rated tire carries more weight at a lower pressure. In other words, LOAD RANGE E doesn't always mean the tire can carry more weight at maximum pressure when compared to LOAD RANGE D at maximum pressure.

Here's where your application and uses come in.

What does this mean to you if you are upgrading from the stock E Load Rated tire with an 80psi max to a new, larger, E rated tire with a 65psi max What if your upgrading to a larger size that is D load rated?

If you do not tow much it may not matter as long as your new tire carries equal to, if not a higher, load index number. The new tire will still be able to carry at least the same weight at 65psi as your stock tire would at 80psi. The only real difference is the way the vehicle may handle under load. If you are accustomed to towing with 80 psi in your tires and now you are only able to tow with 65psi, your vehicles response can feel squishy or lazy and you can experience a bit more "roll" as the vehicles weight shifts in turns.

How do you tell if the E Load Rated tire you are looking to buy has a 65psi or 80psi max?

On our website we indicate all 80psi E Load Rated tires as "E1" and all 65psi E Load Rated tires as "E2" in the tire size. Same thing goes for "D" and "C" Load Rated tires.
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BillyHW

BillyHW

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Here's another great article:

http://www.trucktrend.com/how-to/towing/1407-understanding-tire-load-ratings/

Understanding Tire Load Ratings

People have been riding on air since John Dunlop invented the pneumatic tire in 1887. The average motorist might think that his tires’ rubber and steel belts support the vehicles’ weight, but air pressure actually carries the load.
The greater the tire’s internal air volume, the more weight it can theoretically support. But bigger doesn’t always mean greater load capacity. Keep reading to find out why.

The goal here is help you match the tire’s load-carrying capacity (LCC) to your vehicle, particularly for hauling and towing. Dennis “Doctor Tire” Franklin, a longtime tire-retailer trainer, consultant to Interco, and a second-generation tire scholar (his dad designed the original light-truck flotation tire, the Armstrong Tru-Trac), helped us debunk common misconceptions about load ranges and carrying capacities. The focus here is on trucks that have single rear tires.

The Three Rules
1.
A tire’s LCC is determined by its operating temperature.
2. The higher the speed, the more heat—and the lower the LCC.
3. Weight also increases heat, decreasing LCC.

Construction Considerations
Logic suggests that the tire’s internal air volume is always proportional to its LCC. This isn’t necessarily the case. Tire construction affects heat generation and dissipation, which determines operating temperature, which establishes LCC. (A tire’s normal highway operating temperature is up to 200 degrees F.)
When comparing tires that are the same size/same internal static air volume but have different load ranges, one with a higher load range at higher air pressures actually may have a lower LCC. For example, a P275/65R18 can have a 2,500-pound maximum LCC at 35 psi, whereas an LT275/65R18/C (six-ply rated) might only have a 1,948-pound capacity at its maximum 50 psi. The difference: P versus LT.
“The only things P-metric and LT tires have in common is that they’re black and round,” Dennis Franklin says. “LT fitments have more rubber: heavier plies, a deeper tread, and a stronger bead bundle. More rubber means greater weight or mass, which generates and retains more heat.” See Rule 1 above.
The LT designations (LT-metric, LT-flotation, and LT-numeric) all carry load ratings. The maximum load, designated by a load range letter, is molded into the tire’s sidewall. In contrast, P-metric sizes aren’t load-rated. The tire industry has established that, when used in light-truck applications, P-metric sizes lose 9 percent of their LCC (0.909 to be exact). An example using a common F-150 OE fitment:
P255/65R17 = max 2,205 lb @ 35 psi (sidewall rating) x 0.909 = 2,004 lb actual LCC on a light-truck
As a safety margin, an additional 20 percent reserve load capacity is deducted from all tires to determine the constant working load (WL). Using the 255/65R17LT example above:
2,004 lb x 0.80 = 1,606 lb WL x 2 tires = 3,206 lb per axle
Deduct the approximately 1,800 pounds the F-150 rearend weighs from the tires’ 3,206-pound WL capacity. That leaves 1,406 pounds of combined rear towing/hauling weight that this truck can safely, constantly support at its 35 psi recommended maximum inflation up to its maximum rated speed.

Load Range: LT-Metric Vs. LT-Flotation Vs. LT-Numeric
A tire’s load range letter is an attempt to equate radial tires’ carrying strength in old bias-ply terms. Air pressure is the functional indicator: Higher-load tires accept higher maximum pressures. Also, a single mono steel ply, or one-ply, is stronger than multiple cotton-cord plies from the early bias-ply days. Further, a single steel ply can actually support more weight than multiple plies; less mass and less friction means less heat, ergo greater load capacity.

Wheelers often wonder why some mud-tire sizes aren’t available in Load Range E. The answer is tire construction. Flotation sizes have even more mass and softer sidewalls than the often-narrower closest LT-metric or LT-numeric sizes, so they generate and retain more heat.
Flotation tires are built to flex—great for conforming to uneven terrain, but less stable when supporting loads on the road. They require higher air pressure than LT fitments to support comparable weight. This torpedoes the flotational philosophy of airing down for a wide, amoeboid footprint.
Borrowing an example from Toyo’s catalog, an ’06 HUMMER H2 with an OE LT315/70R17 has a maximum LCC of 2,595 pounds at 37 psi per tire. For a “plus 0” (same wheel diameter) swap to a 35x12.50R17 flotation tire, the industry-standard TRA Load & Inflation Table (available on Toyo’s and other tire manufacturers’ websites) helps determine proper air pressure for the required LCC. According to the table, the 35x12.50R17 supports 2,581 pounds at 39 psi (slightly under OE spec), so the table-recommended pressure is 40 psi—good for 2,625 pounds.
Further, the smaller the sidewall’s static load radius (measured from the axle’s center to the ground), the greater its load-carrying capacity. Low-profile sports car tires corner flat. Similarly, an LT-metric truck tire on a “plus size” 20-inch wheel has less mass, dissipates heat better, accepts higher pressure, and has a higher LCC than a same-diameter, same-width tire on a 17-inch wheel. Flotation-tire sidewall flex comes at the expense of strength and LCC. Wider wheels also increase the functional LCC by stiffening the sidewalls, allowing for a cooler running assembly.

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There's more at the link. Read the whole thing.
 
 



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