Need Tire Education..

[Dkevdog]

Ok so I'm not the sharpest tool in the shed. I have 16x8 aftermarket rims, generic mud/snow tires. They tires are 265/75/16 D . They are rated for 3012 Lbs at 65 psi.



I've been told not to worry about "E" rated tires because the larger 265 "D" tires can handle 3000+ lbs. I looked at some commercial BFG TA's that were 235/85/16 E and they are only rated for 3042Lbs. If that's the case, then why are my current tires "D" rated?? And could I run the 235/85/16 tires on a 16x8 rim?? Whould the hieght be the same as a 265/75? The 265's rub a bit in the turns and I really don't need the width. But I do like the Hieght.



I'm confused .

And is it me? or are most tire shop guys even more confused than I am!!!!???



Kev

 

[jsnyder]

Kev,



"E" means it is a 10 ply, "D" is an 8 ply. A tire of the same size will have different load ratings in E vs. D. Comparing a 235/85 to a 265/75 is apples and oranges, because the 265/75 is wider and thus spreads the weight out over a larger area. That is why it can handle more weight.



A 235/85 and 265/75 are very close to the same height, within a 1/2 inch I believe. I may be mistaken, but I don't think you can mount 235/85's on an 8" rim? There is some slight variation in track width among different brands of tires. You could shop around and find one that is a little narrower if you dont' want them to rub. Coopers seem to run narrow.



It's kind of comical the different answers you can get from different tire shops about the same tires .



Jason

 

[MJawsman]

Technically speaking it's the air chamber space that holds the weight not the footprint of the tire. The 265 D tires hold as puch as 235 E tires because of air space. That doesn't mean that a really big C tire with a similar weight rating is a good idea, as the air pressure may not be sufficient to prevent tire "squirm" under load.



Just as an FYI, the big 66" or 73" monster truck tires run an average of 3-6 psi while a bicycle might run 150 psi. Pressure is good to a point, but really has nothing to do with capacity. It's all about the volume.



My humble opinion, run the 265's in D or E range and be happy!

 

[jlccc]

Originally posted by MJawsman

Technically speaking it's the air chamber space that holds the weight not the footprint of the tire. The 265 D tires hold as puch as 235 E tires because of air space.



My humble opinion, run the 265's in D or E range and be happy!

You are mistaken. It's not "air space" but footprint and sidewall construction. Did you know you can get a pretty good estimate of the weight on a tire by multiplying the area of the "contact patch" and the air pressure inside the tire?



Wider tires have a larger contact patch and can carry the same load at a reduced pressure. For example, a friend of mine has some 295 tires on his truck and they are rated load range D. My truck has the factory 265 load range E tires. Guess what, his tires are rated to carry as much or slightly more than my tires. The reason his tires only carry a D rating is because they can only be aired up to 65psi whereas a load range E tire can generally be aired up to 80psi.



Volume has nothing to do with it, except that volume increases as the tire gets wider, assuming two tires of equal height.

 

[RustyJC]

As I recall from the tire rating chart in our 1996 3500 (which came with D-rated tires), the load rating for a given size D or E tire was the same up to 65 PSIG - that is, the chart (which gave tire load capacities at various air pressures) showed the D-rated and E-rated 215/85R-16's both had the same load rating at 65 PSIG, but the load chart for the E-rated tire kept going up to 80 PSIG while the chart for the D-rated tire ended at 65 PSIG.



To reiterate, the difference is that the E-rated tire, due to its construction, was rated for its maximum load at 80 PSIG, not 65 PSIG like the D-rated tire. Therefore, a primary factor in the higher load rating of the E-rated tire is its ability to run at higher air pressures than the same size D-rated tire.



As the tires go up in size, the load rating curve just shifts up. Thus, it's quite possible to find larger D-rated tires that would have the same or higher load rating at 65 PSIG than smaller E-rated tires at 80 PSIG. You're just playing with two (2) variables - tire size and air pressure.



Rusty

 

[jmeehan]

I'm running 235/85R-16's on the factory aluminum rim. I've been told the factory aluminum rim is 8"s wide, though I haven't measured it for myself. The tires mount fine on that wide of a rim and I don't get any rubbing, but I don't do any off-roading with my truck either.

You could always have one mounted and see if you like the looks.

And yes, all tire shop guys are confusing (confused?), that's why I try to do my own research, then I just tell them what I want.

The National Tire Company web site, natltire.com, has some good charts that'll give you tire sizes in inches for comparison.

 

[MJawsman]

Originally posted by jlccc





You are mistaken. It's not "air space" but footprint and sidewall construction. Did you know you can get a pretty good estimate of the weight on a tire by multiplying the area of the "contact patch" and the air pressure inside the tire?



Wider tires have a larger contact patch and can carry the same load at a reduced pressure. For example, a friend of mine has some 295 tires on his truck and they are rated load range D. My truck has the factory 265 load range E tires. Guess what, his tires are rated to carry as much or slightly more than my tires. The reason his tires only carry a D rating is because they can only be aired up to 65psi whereas a load range E tire can generally be aired up to 80psi.



Volume has nothing to do with it, except that volume increases as the tire gets wider, assuming two tires of equal height.

I'll concur that sidewall construction has a great deal to do with it. From what I've been taught, "sidewall" construction is actually a misnomer. It's actually bead construction. The sidewalls of two 265 tires D & E are the same with regard to how many belts they have. But an E range tire has a reinforced bead to handle the additional air pressure.

The 295 tires you speak of have a significantly higher air volume than the 265 tires and hence carry exactly the same amount of weight at a lower pressure. I guess my point is whether or not you call it contact area or air chamber volume, the numbers come out the same when comparing tires of equal height. The biger tire will always have a higher weight capacity if load range (air pressure) and height are the same.

 

[jlccc]

Originally posted by MJawsman





I guess my point is whether or not you call it contact area or air chamber volume, the numbers come out the same when comparing tires of equal height. The biger tire will always have a higher weight capacity if load range (air pressure) and height are the same.

I agree with this. To illustrate why it's contact area and not "air chamber volume" consider two tires with the same tread width but different sidewall heights. Let's assume a 265 70R16 and a 265 85R16. (These tire sizes are hypothetical and may not even exist but they serve to illustrate the point). Since the 85 profile tire has a higher volume, can it carry a higher load? No, its contact area is almost identical and, assuming similar construction, the load rating would be the same.

 

[MJawsman]

I agree with this. To illustrate why it's contact area and not "air chamber volume" consider two tires with the same tread width but different sidewall heights. Let's assume a 265 70R16 and a 265 85R16. (These tire sizes are hypothetical and may not even exist but they serve to illustrate the point). Since the 85 profile tire has a higher volume, can it carry a higher load? No, its contact area is almost identical and, assuming similar construction, the load rating would be the same. [/B]

According to the BF Goodrich All Terrain KO spec sheet, an LT265/70R16 load range D tire has a max capacity of 2835 lbs at 65 PSI. The LT265/75R16 load range D tire has a max load of 3042 lbs at 65 PSI. Even though the contact patch is the same, this tire has a higher profile or sidewall height, it carries a higher volume of air at max pressure and therefore carries more weight. Perhaps this is why I was taught that it's the volume of air not contact area that carries a load.



As a side note, I enjoy good spirited discussions!

 

[RustyJC]

Just got the following data for a Michelin XPS series light truck tire off the Tire Rack website. See how it fits your various theories:



LT215/85R16E - 2680 lbs @ 80 PSIG



LT225/75R16E - 2680 lbs @ 80 PSIG



LT235/85R16E - 3042 lbs @ 80 PSIG



Rusty

 

[jlccc]

Originally posted by MJawsman





According to the BF Goodrich All Terrain KO spec sheet, an LT265/70R16 load range D tire has a max capacity of 2835 lbs at 65 PSI. The LT265/75R16 load range D tire has a max load of 3042 lbs at 65 PSI. Even though the contact patch is the same, this tire has a higher profile or sidewall height, it carries a higher volume of air at max pressure and therefore carries more weight. Perhaps this is why I was taught that it's the volume of air not contact area that carries a load.

That's interesting data for sure. I wish we could find two tires of similar construction with equal volumes but different widths and heights. The narrower tire would have the taller sidewall and the wider tire the shorter sidewall. Something like that would throw some light on things.



A simple physics construct, called a "free body diagram" answers this question easily--it's contact area, not volume. But I'm content to agree to disagree without there being any hard feelings.

 

[RustyJC]

Originally posted by jlccc





That's interesting data for sure. I wish we could find two tires of similar construction with equal volumes but different widths and heights. The narrower tire would have the taller sidewall and the wider tire the shorter sidewall. Something like that would throw some light on things.

Check the data on the Michelin XPS LT215/85R16E and LT225/75R16E immediately above your post. Both of these tires are rated for a maximum load of 2680 lbs at 80 PSIG.



Rusty

 

[MJawsman]

I'm sure there are tire physicists that could throw a million formulas as to how it's figured, and my education came from tire companies (not engineers) over the course of many years. Not all of them agreed either, but certainly I have no hard feelings about any opinion someone may have. After all, that's why we live in this great country and drive awesome trucks!! Oo.

 

[jlccc]

One last effort

I'll make one last effort to convince you and then whether you believe me or not is up to you.



Consider a tire that has a load of 3000 lbs on it. That tire will flatten on the bottom (where it contacts the road) until the product of its contact area and tire pressure, exactly balance the 3000 lb load.



For example, a tire with only 30 psi inside will flatten out until there is a contact area of 100 sq. in. (since 30psi x 100 sq in = 3000 lb).



The same tire with 60 psi of pressure only requires 50 sq. in. of contact area.



Now consider two tires, each carrying the SAME 3000 lb load and inflated to the SAME 30 psi pressure, but of DIFFERENT widths. The tread and sidewall of the narrower tire will have to flex more in order to flatten out to the same contact area (100 sq in this case) than will the wider tire.



Incidentally, this flexing is what causes tires to heat up when they are underinflated. Eventually the tire gets hot enough that it gets soft and you have a blowout. (ala the Ford Explorer with its 24 psi tire pressure recommendation)



In the example above, the narrower tire could be inflated to 45 psi and then only 67 sq in of contact area would be required to support the load. This higher pressure keeps the tire from deforming as much (it stays more round and doesn't flatten out as much on the bottom) which helps keep the tire from overheating.



Notice how volume didn't come into play at all?

 

[jlccc]

Originally posted by RustyJC





Check the data on the Michelin XPS LT215/85R16E and LT225/75R16E immediately above your post. Both of these tires are rated for a maximum load of 2680 lbs at 80 PSIG.



Rusty

I saw it before I posted, but notice that I said "equal volumes". Are you certain that the two tires you named above have equal internal volumes?

 

[jlccc]

Originally posted by MJawsman

Just as an FYI, the big 66" or 73" monster truck tires run an average of 3-6 psi while a bicycle might run 150 psi. Pressure is good to a point, but really has nothing to do with capacity. It's all about the volume.

These monster truck tires have a contact patch that's probably over 1500 sq. in. At 6psi, each tire is capable of supporting 9000 lbs. Not volume, contact area. ;-)

My humble opinion, run the 265's in D or E range and be happy!

I heartily agree with this recommendation.

 

[RustyJC]

Originally posted by jlccc

I saw it before I posted, but notice that I said "equal volumes". Are you certain that the two tires you named above have equal internal volumes?

Not having the detail drawings, I can't be certain of anything. Running a thumbnail calculation, however, treating the tire profile as a rectangle, the 215/85 has an external cross-sectional area of 39291 square mm while the 225/75 has an external cross-sectional area of 37969 square mm, a variance of 3. 37%.



By the way, I agree that the sums of the products of the tire pressures times the individual contact patch areas will equal the laden weight of the vehicle. The only points I ever tried to raise were that (1. ) a larger load range D tire at 65 PSIG could well have the same weight rating as a smaller load range E tire at 80 PSIG and (2. ) the weight ratings of the same size load range D and E tires were the same at a given pressure - the load range E tire has a higher weight rating because it can operate at higher inflation pressures than the load range D tire.

 

[jlccc]

Originally posted by RustyJC

Running a thumbnail calculation, however, treating the tire profile as a rectangle, the 215/85 has an external cross-sectional area of 39291 square mm while the 225/75 has an external cross-sectional area of 37969 square mm, a variance of 3. 37%.

The percentage difference in width is (225/215-1)=4. 65% which might theoretically equate to as much as a 125lb difference in carrying capacity. Maybe they thought this was too small to worry about. I don't know.

 

[jlccc]

Originally posted by RustyJC

By the way, I agree that the sums of the products of the tire pressures times the individual contact patch areas will equal the laden weight of the vehicle. The only points I ever tried to raise were that (1. ) a larger load range D tire at 65 PSIG could well have the same weight rating as a smaller load range E tire at 80 PSIG and (2. ) the weight ratings of the same size load range D and E tires were the same at a given pressure - the load range E tire has a higher weight rating because it can operate at higher inflation pressures than the load range D tire.

I agree with you 100% on both issues. Most of my comments were directed towards MJAWSMAN.

 

[jlccc]

Originally posted by MJawsman

The bigger tire will always have a higher weight capacity if load range (air pressure) and height are the same.

When you say 'bigger', I assume you mean 'wider' since you have said that both tires are the same height. Otherwise, I agree completely with your statement.