Light truck tire aspect ratio 3

[crossframe]

I was wondering about the effect of the aspect ratio for light truck tires with respect to load capacity. Most of the discussion in other threads seems to center around performance, which is not a concern for how I drive my truck. I use it to haul things.

Apparently styling is a concern as well.

Several years ago GM & Dodge seemed to always use 75% with a wider tire, while Ford used 85 with a more narrow tire. (LT245/75R16 vs. LT235/85R16) I also notice that tractor trailers always used narrow tires, but I suspect that was more because they were mounted as duals. All this seems to changing in favor of wider tires with shorter sidewalls.

I once asked a tire shop guy which was better for load carrying. His answer was "The Michelins." The conversation continued like this:

Crossframe: "Suppose we're only talking about a single brand. Then which is better?"

Tire Shop Guy: "The Michelins."

Crossframe: "OK, let's say we're talking about two Michelins where everything is the same, except for the aspect ratio. One is a 75, and one is an 85."

Tire Shop Guy: "They're both Michelins?"

Crossframe: "Yes, both Michelins."

Tire Shop Guy: "Then they're the same."

Crossframe: "How can they be the same if they're different shapes? I realize the difference may be very small, but I don't see how they can be the same."

Tire Shop Guy: "Why do want to know that?"

This is where I just gave up

 

[GregLocock]

I agree with the tire guy. However, why not look at the load rating for the tires of interest? From what I can see at a quick glance the lower AR tire has a higher load rating.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CapriRacer]

Crossframe,

One way to look at this is that the amount of air determines the load capacity.

So a tire with more air chamber volume has more load capacity at the same pressure, and a tire with more pressure has more load capacity for the same air chamber volume.

It turns out that an LT235/85R16 has very close to the same air chamber volume to an LT245/75R16, so they have the same load capacity at the same pressure. As you can see aspect ratio by itself isn't a factor.

Light truck tires are the little brothers of the over the road truck tires - usually referred to as medium truck tires. Large truck tires are the ones used in the mining industry for haul trucks.

The trucking industry is kind of slow to react to changes because of the investment they have in equipment - in this case, rims. Wider tires take wider rims, so a given trucking company is not going to make a change unless there is a really good reason to do so. Besides, the trucks themselves would have to be redesigned for wider tires - again something that isn't going to happen without a really good reason.

A couple of other thoughts:

Talking to a guy at a tire shop is like talking to a guy who works at Jiffy Lube. These folks are pretty much clueless.

The load capacities are written on the sidewall of each tire, and a simple way to determine the load capacity is with Load Index. Spend some time at Tire Rack's web site and look at a given line of tires. That will start to fill in the gaps.

 

[crossframe]

Thanks for the replies.

The air volume concept makes sense. I guessed that the difference between the tire sizes I'd consider are practically negligible, but I figured there had to be at least an ever-so-slight difference within a given load range. I was asking from more of the geek-engineer-who-overanalyzes-everyday-items perspective.

 

[CapriRacer]

Now that you've brought up Load Ranges:

Load Range is a way to decribe - within a given tire size - the maximum load capacity of a tire.

For example: An LT235/85R16 Load Range C carries a max load of 2205 # at 50 psi, and an LT235/85R16 Load Range D carries a max load of 2623 # at 65 psi and an LT235/85R16 LR E is 3042# at 80 psi.

Notice the Load Range, max load and pressure going up together.

 

[crossframe]

"... within a given tire size..."

Does that mean that different sizes of Load Range D tires would all have different maximum loads within that Load Range?

For example, LT215/85R16D, LT225/75R16D, LT235/85R16D, and LT245/75R16D would all have slightly different maximum loads since they would presumably all contain different volumes of air, even though thay are all considered Load Range D?

 

[CapriRacer]

Bingo!!

Let me be specific:

All the following are the Load Range D single wheel rated loads and pressures:

LT215/85R16 - 2335# @ 65 psi
LT225/75R16 - 2335# @ 65 psi
LT235/85R16 - 2623# @ 65 psi
LT245/75R16 - 2623# @ 65 psi.

And an odd one for good measure:

LT325/80R16 - 3525# @ 50 psi

Notice that the first group is really 2 groups of equivalent sizes (except for rim width range!)

Also notice that the odd tire uses a lower pressure, even though it is the same Load Range. It's a long story why that is that way, but it involves trying to come up with standardized constructions.

 

[crossframe]

Well, CapriRacer, thank you vey much!

That seemingly minor question has been bugging me for a long time. A star for that!

 

[NormPeterson]

"The trucking industry is kind of slow to react to changes because of the investment they have in equipment - in this case, rims. Wider tires take wider rims, so a given trucking company is not going to make a change unless there is a really good reason to do so. Besides, the trucks themselves would have to be redesigned for wider tires - again something that isn't going to happen without a really good reason."

With this thread in mind I've been noticing that, very specifically, ready-mix concrete trucks are already using single much wider tires than is usual for trucks hauling other products and running the conventional dual arrangement. So I'm wondering if this is the early stage of change or if it's something unique about concrete hauling that makes it a more favorable choice there than elsewhere (with possible implications for the LT tire selection process).


Norm

 

[GregLocock]

Ever followed a duallie with a half brick wedged in the gap? The lawsuits would encourage me to change.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CapriRacer]

Norm,

"Super Singles" have been around for decades, but their acceptance has been slow for the reasons mentioned earlier.

There are places where Super Singles are commonplace. The major advantage is reduced weight, and that translates to more cargo capacity. However, some government regulations severely restrict their usage, citing increased damage to the road surface.

I suspect that the reason ready-mix trucks have adopted Super Singles is that this allows the payload to be carried lower (the frames rails are further apart) in addition to the increased payload.

As far as LT's are concern, the changeover would involve a redesign of the truck, and this is not something that seems to have a lot of momentum. Don't forget that the front tires on duallies are the same size as the rears, and I'm sure this has a lot to do with the resistance to this type of change.

 

[danlindberg]

Capri,

You should write a book about tires.

I noticed your comments from above and the part about chamber volume don't make sense to me, the higher pressure/higher load part does.

Please explain.
I always "thought" that it was the contact patch Area times Pressure that determined the Load rating, with wider/larger dia tires having higher contact patches and resulting higher load ratings.

"So a tire with more air chamber volume has more load capacity at the same pressure, and a tire with more pressure has more load capacity for the same air chamber volume."

As for the comment about wider singles on over-the-road trucks, recently I read in a design rag that some fleets are experimenting with single, wide tires/rims on their trucks/trailers, with the hope of reduced rolling friction and fuel cost.

Dan

 

[NormPeterson]

Odd. I'd always heard that tire load capacity is a function of the volume of air that it held. I think there's a related FAQ somewhere.

Many people try to relate contact patch area and pressure to load, and/or load capacity. Trouble is, unit loading over the contact patch is far from uniform (it varies over perhaps a 2:1 range) due to tread stiffnesses. And this is further complicated by tread void area.

I've been thinking a bit on the notion of "super singles" over the last few months. They may be better suited for short haul usage, as you still have a safety net of sorts should one tire of a pair fail.

Only a few days ago during a highway trip, I did happen to watch part of the failure sequence of a forward inboard right side trailer tire on a tractor-trailer (a little smoke, then lots of smoke, then you could clearly see a freestanding inner wheel flange with no sidewall contacting it whatsoever). It's by pure coincidence that this thread has been resurrected today, on my first day back. Anyway, I would think that having three good tires out of four (in this particular case) would leave you better off than having only one out of two. BTW, the driver did make it to the next exit without further problems.


Norm

 

[CapriRacer]

Dan,

I noticed your comments from above and the part about chamber volume don't make sense to me, the higher pressure/higher load part does.

Please explain.

I'm not sure I can except to say that is just the way it works

I always "thought" that it was the contact patch Area times Pressure that determined the Load rating, with wider/larger dia tires having higher contact patches and resulting higher load ratings.

"So a tire with more air chamber volume has more load capacity at the same pressure, and a tire with more pressure has more load capacity for the same air chamber volume."

From time to time I'll encounter someone who thinks that tires are designed with the contact patch first and the rest of the tire is just a consequence. It's the other way around. The most important thing about a tire is its ability to support a load.

As for the comment about wider singles on over-the-road trucks, recently I read in a design rag that some fleets are experimenting with single, wide tires/rims on their trucks/trailers, with the hope of reduced rolling friction and fuel cost.

I think this is more what reporters can get their heads around, than the actual reason. Once a trucking fleet has invested in rims and axles, it's those things that you mentioned that they focus in on. And while fuel economy is important, I think the reason trucking fleets have been slow to adopt super singles is the severe cost investment that has to be made.


Norm,

......Anyway, I would think that having three good tires out of four (in this particular case) would leave you better off than having only one out of two......

I really don't think that fleet owners consider that. It might be something that comes up in the discussion when you talk to the drivers, and it might be something that a fleet owner might mention when he justifies why he didn't make the changeover, but in the big scheme of things, I don't believe that it really enters into the equation when a changeover is being considered.

 

[tireguru]

Mixers using super singles on rear axles is largely a regional thing. Mixers have needed large tires on the front axle for quite a while, but then had no way to use the casings (nervous about running retreads on the steer axle).

Going to wide base tires on the rear allows them to simplify their tire inventories and use the casings that they generate from their steer axles.