Help with Load E inflation tables

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DT installed my new set of LT285/70r17 AT3W tires today and they happen to only come in load E (118/121) range. Spoke with Falken via telephone and they advised to reference the industry standard tables. Found a pdf from the RMA entitled "Replacement Guidelines for Passenger and Light Truck tires" and figured it is likely a very applicable document. I'm a little confused though... Below is a transcription of the information, note that the load "E" only shows up at the very top of the chart, does that mean that max load is increased compared to lesser LT ratings but that inflation:load remains consistent through lesser psi ranges? Also, why would LT E tires have 118/121 index ratings listed on them if 118 is equivalent to C and 121 is equivalent to D?

Trying to understand this document for general understanding as well as future applications. DT has 44psi listed as the recommended pressuer on my current setup which jives well with what I've read anecdotally online. FWIW OE is P237/75r17 index 108S with GAWR F/R 3750lbs/3850lbs. In other words, pretty much ANY pressure listed on teh lt285/70r17 chart would be safe; just a matter of tuning ride and wear characteristics...

rigfu9.jpg



PS: If anybody knows how to attach a pdf to this thread, I would be happy to do that!
 
From Discount Tire website:

Designation Ply Rating Max. Load Carrying Air Pressure
SL (Standard Load) 4 @36 psi
XL (Extra Load) 4 @42 psi
C1 6 @50 psi
C2 6 @35 psi
D1 8 @65 psi
D2 8 @50 psi
E1 10 @80 psi
E2 10 @65 psi
F 12 @95 psi
G 14 @110 psi
 
Originally Posted By: buck91
does that mean that max load is increased compared to lesser LT ratings but that inflation:load remains consistent through lesser psi ranges

Yes

Originally Posted By: buck91
Also, why would LT E tires have 118/121 index ratings listed on them if 118 is equivalent to C and 121 is equivalent to D?

Different sizing systems. P-metric/ISO metric for load index (numbers), LT-metric/flotation for load range (letters). For example, with the 118/121 you asked about, dual assemblies are de-rated 9% to account for road crown. They're the same tire (both E), but have a different capacity.
 
Bear with me, but I'm going to break out some basic algebra here. I hope its correct!

SO given the above Load E information we can establish two data points in the format (psi, load); in other words inflation psi is X and load capacity in pounds is Y.

p1 = (35, 2105)
p2 = (40, 2315)

Using p-metric load tables from http://www.discounttire.com/dtcs/infoLoadIndex.do I find that the OE tires, rated 108, comes out to a listed 2205lbs (derate by 10% for 1985lbs per tire).

Using p1 and p2 I can create a linear equation in the y=mx+b format for:
y = 42x + 635

Plug and chug shows 1985 = 42x + 635 for x = 32.14psi. I you do no de-rate the p-metric capacity for SUV application x works out to 37.4psi (surprisingly close to the OE pressure of 38psi).


I am quite sure that these load tables are not strictly linear but I do wonder how valid this calculation is. Capcity in Pounds = 42(PSI)-635. I found that 32.14psi would be the "proper" pressure. Definitely not going to run them that low straight away, I will stick with the 44psi listed on my tire invoice... But I will DEFINITELY be watching tread wear and might even try the chalk test when the weather drys up.
 
Originally Posted By: buck91
Bear with me, but I'm going to break out some basic algebra here. I hope its correct!

SO given the above Load E information we can establish two data points in the format (psi, load); in other words inflation psi is X and load capacity in pounds is Y.

p1 = (35, 2105)
p2 = (40, 2315)

Using p-metric load tables from http://www.discounttire.com/dtcs/infoLoadIndex.do I find that the OE tires, rated 108, comes out to a listed 2205lbs (derate by 10% for 1985lbs per tire).

Using p1 and p2 I can create a linear equation in the y=mx+b format for:
y = 42x + 635

Plug and chug shows 1985 = 42x + 635 for x = 32.14psi. I you do no de-rate the p-metric capacity for SUV application x works out to 37.4psi (surprisingly close to the OE pressure of 38psi).


I am quite sure that these load tables are not strictly linear but I do wonder how valid this calculation is. Capcity in Pounds = 42(PSI)-635. I found that 32.14psi would be the "proper" pressure. Definitely not going to run them that low straight away, I will stick with the 44psi listed on my tire invoice... But I will DEFINITELY be watching tread wear and might even try the chalk test when the weather drys up.


You are pretty much correct, with one major exception: the tables start at 35psi for a good reason - no LT-rated tire is approved by its manufacturer for use below 35psi, no matter the application.

This means that even though your math shows a "correct" pressure 70-17 tire is 35psi.

HTH
 
Ah, I never knew that! Thanks, sounds like I'll stick with the plan at this point- and titrate as necessary no lower than 35psi.
 
Buck91, your wheel width can also make a difference as to how your tires wear, AND heat up under load.
In my opinion, a 10 inch wide rim is the proper wheel for your tire. Nine inch are ok, while 8 inch wide introduce quite a bit of sidewall flex, especially under load.

Quite often you see off-road guys put 33/12.50/17s on 17X8 inch wheels, only to have them wear out in no time.
Why? Because when the tire loads, the tread spreads outward, scrubbing the tread like this < >.

Tires like 33/12.50/20s are worse off because of the sidewall angle.
To optimize tire life, 33/12.50/20s should be on 10-12 inch wide wheels to minimize sidewall flex.

These guys seem to have a grip (no pun intended) on off-road tire 'n wheel combinations. www.customoffsets.com
 
The 2nd half of my opinion is the volume of air, of your wheel and tire combination.
Theoretically because of the greater air volume of your 285/70/17s, should be able to carry more load at the same air pressure than 285/60/20s.
If you can figure out the approximate volume of air in your 17s, at 85 psi with 10 inch wide wheels, how much weight could they carry?

The answer is more than your wheel bearings and axle.

Moving up in wheel width and tire size, the pressure goes down for a given load capacity.

Out on a limb, 40/13.50/17s on 12 inch wide wheels, will not require high pressure to pack a heavy load.
 
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A couple of thoughts:

1) There is a quirk in the LT load tables. It was felt that there ought to be a point where people ought to move out of the LT type tire and into medium truck tires. The way those LT tires were going was much too open ended.

So the powers that be decided that NEW entries to the LT load table would stop at 3195# (1450KG). Why that value? That was about the point where trucks crossed over to 10,000# GVW and pretty much everyone in the vehicle business and the government say that puts the truck into a different category and the rules are different for those kinds of vehicles.

When did this happen? I don't know, but it has been a while.

And here is where the quirk really stands out: If the table already had entries that exceeded 3195# (1450KG), then they left them. (The general principle is that you can't undo something that's already been done.)

But that doesn't prevent people from "inventing" entries on the table - and a Load Range E LT285/70R17 is just that. Many tire manufacturers "invented" parts of the table, but they (as a general rule) comply with the 3195# (1450kg) limitation.

2) The load curve is indeed a curve. There is a formula, but please don't make me look it up and try to write it. It involves a odd power figure and those require some fancy formatting to get it to come out right. Plus, it's hard to find and I may have lost access to the formula.

But for a curve, the load curve is pretty linear and you can interpolate between data points. I would urge caution when using the last point as the above explanation should inform you that that point might not be part of the curve, but an arbitrary value.

I would also urge caution extrapolating - for the very same reason.

Also, while the curve is fairly linear, it doesn't look like it intersects the zero,zero point. (That is zero pressure = zero load). What the curve looks like is that it will intersect zero pressure at some positive load value - which makes sense in that even a completely deflated tire should be able to carry SOME load.
 
Originally Posted By: buck91
DT installed my new set of LT285/70r17 AT3W tires today and they happen to only come in load E (118/121) range.
You mentioned the 118 and 121 but got no answer ... I believe the 118 is for dual tire installations, 121 is for single. Capri will correct if I'm wrong.
 
Originally Posted By: CapriRacer
A couple of thoughts:

1) There is a quirk in the LT load tables. It was felt that there ought to be a point where people ought to move out of the LT type tire and into medium truck tires. The way those LT tires were going was much too open ended.

So the powers that be decided that NEW entries to the LT load table would stop at 3195# (1450KG). Why that value? That was about the point where trucks crossed over to 10,000# GVW and pretty much everyone in the vehicle business and the government say that puts the truck into a different category and the rules are different for those kinds of vehicles.

When did this happen? I don't know, but it has been a while.

And here is where the quirk really stands out: If the table already had entries that exceeded 3195# (1450KG), then they left them. (The general principle is that you can't undo something that's already been done.)

But that doesn't prevent people from "inventing" entries on the table - and a Load Range E LT285/70R17 is just that. Many tire manufacturers "invented" parts of the table, but they (as a general rule) comply with the 3195# (1450kg) limitation.

2) The load curve is indeed a curve. There is a formula, but please don't make me look it up and try to write it. It involves a odd power figure and those require some fancy formatting to get it to come out right. Plus, it's hard to find and I may have lost access to the formula.

But for a curve, the load curve is pretty linear and you can interpolate between data points. I would urge caution when using the last point as the above explanation should inform you that that point might not be part of the curve, but an arbitrary value.

I would also urge caution extrapolating - for the very same reason.

Also, while the curve is fairly linear, it doesn't look like it intersects the zero,zero point. (That is zero pressure = zero load). What the curve looks like is that it will intersect zero pressure at some positive load value - which makes sense in that even a completely deflated tire should be able to carry SOME load.



As always, great info - thanks for posting.

For the visual learners in the crowd (that would be me) here is a graph of the table for the LT285/70-17 tire in question:

TirePressure_08AUG16_zpsgxooqvum.jpg


Pleasse ignore the "Recommended Pressure" notes as they are specific to my application - 2013 Toyota Land Cruiser.

HTH
 
Originally Posted By: userfriendly
Buck91, your wheel width can also make a difference as to how your tires wear, AND heat up under load.
In my opinion, a 10 inch wide rim is the proper wheel for your tire. Nine inch are ok, while 8 inch wide introduce quite a bit of sidewall flex, especially under load.


Toyota disagrees with you when they recommend an 8-inch wheel (17 x 8 x 50) for use with the LT285/70-17 tires:

TRDpoopSheet1_17MAR16_zpsz1nrluhz.jpg


TRDwheelTireSize_01AUG16_zps1idk5hbn.jpg


HTH
 
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Same with the Ram Rebel. Big tires on narrow rims for side bite & off-road traction. Not so good for long tire life packing heavy loads.
An option for ether one is BFG MT or Cooper's Maxx 255/80/17 (10x33) or KO2 34/10.50/17s @ 10x33.5.
 
Go with what rides nice. If you inflate them to their rated pressure you will not like the ride. The only time to put more air in them than the door sticker is if you are hauling or pulling a heavy load in the summer or it rides squirmy.
 
I got onto BFG's website and used their contact page to get a load/pressure chart from them on my LT275/70R18 KO2's. They emailed me a copy of the page from their guide and gives me the proper pressures to run at various axle loadings. Pressures from 35 psi to 80 psi.


 
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