XL Tire Load Rating

[userfriendly]

Well I'm glad we got all of that out of the way in one post.
Now I'm going to have to un-learn everything I know about tires too.
When I'm either driving fast, carrying a heavy load or both, and stop for any reason, I'll check how hot the tires are running.
I believe that tires heat up because of flexing.

When trying to figure out how a tire and wheel combination will act on a given vehicle,

I look at the ratio of the wheel/tire size first.

Wider wheels on a given tire size reduce flex, increase ride height and load radius, heat up less and therefore have a higher load rating than the same tire on a narrow wheel.
So there you have it. Wider wheels increase the load rating of a given tire.

"Old Dad's Tire Advice" : stick with me kid and you'll learn a thing or two.

 

[CapriRacer]

Originally Posted By: used_0il
Well I'm glad we got all of that out of the way in one post.
Now I'm going to have to un-learn everything I know about tires too.
When I'm either driving fast, carrying a heavy load or both, and stop for any reason, I'll check how hot the tires are running.
I believe that tires heat up because of flexing.

When trying to figure out how a tire and wheel combination will act on a given vehicle,

I look at the ratio of the wheel/tire size first.

Wider wheels on a given tire size reduce flex....

Wider wheels will tend to stiffen the tire laterally, but it doesn't change the radial stiffness very much - which is why autocrossers tend to use wider wheels.

Originally Posted By: used_0il
.....increase ride height and load radius, heat up less and therefore have a higher load rating than the same tire on a narrow wheel......


Sorry, but No, to all those things.

Originally Posted By: used_0il
......So there you have it. Wider wheels increase the load rating of a given tire.......


If I accept your premise, what you said would be true, but unfortunately, Physics trumps your premise. Wider wheels (within the range published) has very little affect on things - which is why they publish an allowable range. Go outside that range and some very strange things happen.

Just a thought: If wider wheels give more load carrying capacity - and more load carrying capacity causes less tire failures (for a given load), wouldn't car manufacturers supply wheels as wide as possible? Wouldn't this be an obvious place for an "expert" to make a lot of money suing car manufacturers for supplying wheels that were less safe than they could be?

 

[userfriendly]

The experts at GM put 275/55/20 tires on 20X 8.5 wheels up until last year and 20X9 this year on their 1500 pick-ups along with the Z71 off-road suspension package.

The 265/60/20s offa my 2500 fit perfectly on the 1500 with no rubbing.

275/65/20s went on the 2500.
Who wants like new P rated 275/55/20s?
I would put those on 10 inch wide wheels. not 8.5s, and certainly not on an off-road application.

You said that the wheel width in brackets (8) was the wheel width that the dimensions were determined.

If narrower wheels did not reduce inflated dimensions and air volume, and wider wheels did not increase inflated dimensions and air volume, why would it be necessary to put any of the approved wheel choices in brackets?

Since air is what holds your vehicle up, more is better.

If you don't believe me, go and let the air out of your tires and see what happens

 

[CapriRacer]

Originally Posted By: used_0il
........You said that the wheel width in brackets (8) was the wheel width that the dimensions were determined.

If narrower wheels did not reduce inflated dimensions and air volume, and wider wheels did not increase inflated dimensions and air volume, why would it be necessary to put any of the approved wheel choices in brackets?......


Sorry, but I didn't say that narrow wheels don't reduce inflated dimensions - and I didn't say that wider wheels don't increase inflated dimensions. In fact it has a name: The 40% rule. The change in rim width from the nominal (measuring) rim width results in a 40% change in tire width in the same direction. So a tire mounted on a 1" wider wheel, increases in width 0.4".

However, the diameter does not change. (OK, maybe a teeny bit, but not enough to matter)

Originally Posted By: used_0il
........Since air is what holds your vehicle up, more is better......


I could get into a side argument on this, and I've even written up a thing that covers this:

Barry's Tire Tech - Air or Tire?

but let's postpone that argument for later.

Where I think you are having problems is that the rule of thumb that "more air - that is, either more pressure or a larger interior volume - results in a larger load carrying capacity". That is true up to a point. Where it falls apart is in the details of how the load carrying capacity is determined.

Find a tire load table. Here's one:

Barry's Tire Tech - TRA Page

Notice that there are different tire sizes and different inflation pressures - and you can pick out a combination to get the load carrying capacity.

What you may not know is that EVERY load on that table is at the same deflection - that is, the difference in height between an unloaded tire and a loaded tire. It's been a long time since I had to deal with the formula itself, but if I remember correctly, the "deflection" is calculated as a percent of the section height.

What is interesting is that within the range of allowable rim widths, this deflection hardly varies at all - and certainly not enough to worry about. It certainly isn't the difference between having a tire overloaded (or not!)

To try to put this in perspective, NONE of the tire standardizing organizations makes any provision for changes in rim width vs load carrying capacity. Interestingly, ETRTO (European Tyre and Rim Technical Organization) has some notes about camber angle.

Does this help?

 

[userfriendly]

Everything you say helps.

There was never a problem with a tire's interior air volume until "they" started putting 18 and 20 inch tires on vehicles that had an off-road as well as an on-road application.

The new 1500 GMCs and Chevys will not accept a 16 inch wheel, and a 17 inch wheel just barely clears the tie-rod ends.

If you hit a rock and bend
a wheel with a very small clearance, steering could be lost.

Our 1500 came with 275/55/20s on 20x8.5 wheels (this year is 20x9).

This particular truck sees 1,000 miles of gravel road a month.

The quick solution was to put the 265/60/20 tires from my 2500 on the 1500.

My invented air volume formula indicated that the new tire size could carry the 1500 with 2 lbs lower pressure than the lower profile 275/55/20.

Increasing the wheel width to 9.0 would make another one pound difference.

We'll take it.

Because of the tight clearance of the 17 inch option and the possible hazard with mud, snow, sticks and stones wedging in between the wheel and steering, 18 inch tires is our only other option that I would feel safe off-road with.

When using my invented calculations, I came up with a 265/70/18 tire on a 18x9 wheel having slightly more interior air volume than a 275/65/18.

The difference made about 2 psi reduction in air pressure possible.

We'll take that too. The only problem was that there was not a single decent 3 ply sidewall tire in the 265/70/18 size until the Toyo CT last month and the new KOs this month were announced.

I figured that either of the 18 inch sizes could carry the 1500 with about 10 less psi than the original 275/55/20.

I've never had my 2500 over 9,600 lbs, and the 1500 maxes out at around 7,600, so there is redundant load capacity with any of the tires, stock or otherwise, especially with 275/65/20s on the 2500.

What I am trying to accomplish is finding a 18 and 20 inch wheel and tire combination that will fit the stock height truck without rubbing the fenders and carry the 1500 loaded, with no more than 36 psi of air in the tires.

While figuring all of the above, it sure looked like very slight increase in air volume from a slightly wider wheel raised the carrying capacity of a tire slightly, or the increase in volume would allow the same tire to carry the same weight with a one or two pound reduction in air pressure.

When I first posted, I stated that XL tires were not necessarily of a better construction than the standard load models.

I assumed that by looking at tires on the rack. Some XL and some not, and all looking like garbage to me. It wasn't until looking at the higher speed rated tires, the difference could usually be seen, felt in sidewall stiffness and weighed more.

 

[CapriRacer]

Originally Posted By: used_0il
.......The quick solution was to put the 265/60/20 tires from my 2500 on the 1500.

My invented air volume formula indicated that the new tire size could carry the 1500 with 2 lbs lower pressure than the lower profile 275/55/20.......


Let's see if my method comes up with the same value:

The first thing that jumps out at me is the difference in tire type. The OE size is a P275/55R20 - and according Tire Guides is supposed to be inflated to 35 psi. The other tire size only comes in an LT version - Load Range E.

The load carrying capacity of the OE tire with the placard inflation pressure (35 psi) is 2185#. In order to carry the same load, an LT265R20 needs to be inflated to 42 psi.

Mmmmmm. That's a different number.

If I assume that there would be no difference in tire type (P type to P type), I come up with 27 psi - again a different number.

So what is your invented formula? It doesn't seem to be getting a reasonable answer.

 

[userfriendly]

I figure out the approximate air volume of one tire compared to the other and not whether the tire is P,XL, or LT rated.

Since most of the E rated tires are 80 psi, they follow a pattern of X amount of volume provides X amount of load capacity.

XL tires at 50 psi follow a psi/load rule and so do P rated at 35 psi/load.

From that I could roughly calculate how much volume at what air pressure was needed to carry a given load.

It was not hard to find a pattern that indicated that a tire and wheel combination that a large volume, could carry a given weight with less air pressure than a tire and wheel combination that had a small volume.

For this exercise, I ignored the difference duty application of the tires and stuck to volume/pressure/load only for an apples to apples comparison.

For performance and steering response, or how quickly a tire responds to driver input, I thought up a flex ratio formula.

That formula also has a trend, which for street driven vehicles does not exceed a given value.

My guess the reason why is that a large low profile tire on a wheel that is close to 100% of the tire section causes driver fatigue, over steering and accidents.

For example, every time I got my Mercedes back from my daughter, there was 65 lbs of air in the tires.

She just kept adding air until the car responded like a C63 AMG DTM instead of a C240.

Ever since then I've gotten into the 88-95% rule of tire/wheel width, just to keep the kiddies happy.
Kids don't want traction, they want to drift.

The flex ratio I use is not much more complicated than drawing an X from flange to outside tread and a couple more inputs.

Low profile tires on wide wheels flex very little.

Especially at 65 psi.

 

[CapriRacer]

Originally Posted By: used_0il
I figure out the approximate air volume of one tire compared to the other and not whether the tire is P,XL, or LT rated.....


I was looking for something a little more detailed.

Like are you assuming the tire is oval in cross section, with the width known and the aspect ratio giving you the height, then figuring out the air volume based on the toroidal shape?

Originally Posted By: used_0il
.....Since most of the E rated tires are 80 psi, they follow a pattern of X amount of volume provides X amount of load capacity.......


Nope, that's not true. While the inflation pressure vs load carrying capacity is fairly linear in the region we use it, it is NOT a direct proportion - that is, a straight line fit doesn't go through the original. I demonstrate that here:

Barry's Tire Tech - Load Tables

Originally Posted By: used_0il
.....XL tires at 50 psi follow a psi/load rule and so do P rated at 35 psi/load........


Ah ..... Mmmmmmm ...... 2 thoughts:

First, XL tires would be 41 psi, and 2, no, they don't follow that rule either.

Originally Posted By: used_0il
.....From that I could roughly calculate how much volume at what air pressure was needed to carry a given load.......


So I see at least one difference. Let's discuss this before we move on.

 

[userfriendly]

You came up with 42 psi and I came up with 39.7 psi with the same tire on an 8.5 instead of a 7.5 inch wide wheel.

But now I know why light trucks designed for off-road use come with low profile 20 inch "P" rated tires.

The good thing is that I learned something.

 

[CapriRacer]

Used_Oil,

I didn't point out, but when you use a P type tire in a light truck service (defined as use on a pickup, van, or trailer), you have to derate the load carrying capacity by a factor of 1.1.