Wide vs. Skinny

[expeditionswest]

One interesting brain teaser for ya that will have you thinking: Put an 80 on 255s at 40 psi and measure the area of the contact patch. Do the same on an 80 with 315s at 40 psi. According to your view, the 315 will put more rubber on the road. Nope. Exactly the same. Exactly the same amount of rubber touching the road. It's just a different shape.

Hi Doug... I thought you might enjoy a little discussion on this.

My work with with tire testing has not proven this to be accurate. Here is why:

If you take the same diameter tire, yet vary the width (lets use a 285/75 and a 255/85 which are very close in diameter, yet vary in width by 1.2"). The tire construct is the same (at least in the case of the BFG MT's tested), i.e. the sidewall has the same number of plies, construction, thickness, etc.

So principally, what carries the load of the vehicle in the above example?
1. The pressure inside the tire (contained by the wheel and tire and the seal created)

2. The sidewall (i.e. the tire construction).

It has been tested that a 285 and a 255 deform (elongate principally and widen marginally) at nearly the same rate (within 2%), yet the 285 is 11% wider. So the 285 does have a ~9% greater contact patch. The width of the carcass does almost nothing to support the load.

So, the contact patch of a 285/75 R16 and a 255/85 R16 are not the same area because the majority of the load is born by the sidewall and internal tire pressure.

 

[expeditionswest]

The expiditions west article says, and I quote

"For the sake of the following details, assume that the test vehicle is 5,000 lbs., and a narrow tire would be considered a 33x10.5 R15, and a wide tire would be considered a 33x12.5 R15, both run at 15psi for trail use. "

Idaho Doug is right. and all of this is wrong...

What brought you to that conclusion? Please site research... Thanks.

The diagram on the Expeditions West site is wrong, or at least incomplete, because it is only looking at one dimension of the contact patch - the width. It's ignoring the length of the contact patch. It says that the skiner tire conforms more to the spikes because it needs to to make up the difference from 3 spikes to the wide tires 5 spikes. It's not so much that the contact patches have to be wider, as is shown, it's that they get longer. They may get wider too on the skinner tire, but alot of that will have to do with the shape of the rocks (spikes in the diagram), and the construction of the tire ect. ect.

The diagram shows the depth of contact to the protrusion. The depth (deformation) is more important than the frequency (ratio). For the same reason people put sand bags in the back of their truck for snow and ice, instead of installing wide tires.

Here's another from the article that didn't sit right....

"To ease the description, let's assume that the test vehicle weights 5,000 lbs and has a perfect weight distribution. Each of the vehicles four tires would be creating 1,250 lbs. of vertical pressure on the terrain. Let's assume for the sake of this example that the vehicles tires are 10” wide, where the load and tire pressure results in a total surface area of 30 sq. inches. The total pressure per square inch (without equating the secant) would equal 40 lbs"

It's an odd way of coming to the conclusion that the tire is at 40psi..

Your statement is incorrect. The tire pressure (air pressure inside the tire) may not be 40 psi. It could even be ZERO psi for that matter. Your statement does not account for the tire construction. For example, take the same 5,000 lb. truck and fit semi tractor tires to it, with an 8,000lb load rating per tire. You could run zero psi in those tires and support the weight of the truck with almost no deformation of the carcass.

Consider Newtons third law. In the case of the tire, the opposite reaction is a combination of tire pressure AND carcass construction.

from here:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=10

This diagram shows two totally different tires and applications. One with a very short, wide carcass, and one with a narrow, very tall carcass. The intent of the tirerack page and diagram is to show the construction difference between racing tires and passenger tires, not deformation rates.

Tire talk is fun

 

[expeditionswest]

While much keyboard time has been spent discussing contact patch (which is the smallest performance benefit of a narrow tire), I wanted to make a comment about the key features of a narrow tire, so as not to diminish them.

1. Fitment: It is easier to fit a taller tire that is narrow (or similar in width to the widest tire available from the factory for that model). The goal is to gain clearance and allow for a long contact patch. A taller tire also climbs a wall easier (take a curb for example), as the angle is reduced, requiring less leverage.

2. Frontal Resistance: Critical performance benefit to narrow tires. Less mass of sand and mud needs to be pushed in front of the tire, improving performance.

3. Less weight: Rotating mass is reduced, improving acceleration and braking EDIT: Reciprocating mass is also reduced, lowering shock heat, providing better suspension control and helping to reduce hop (for the same suspension).

4. Less road resistance: We can all appreciate improved fuel economy



And, for those you who still don't believe me, here are some folks with PHD after their name:

University of Nebraska Tire Testing: (These test are in dirt, which make the results very applicable to our environment).

Addressing the "contact patch is the same argument"

Leonard Bashford, NU Test Laboratory:
"Wider tires were expected to have more pulling power because their footprints are larger."

"If the only objective is to pull, a narrower tire seems to be better," "It's not at all what we expected."

"We looked only at tractive performance," Bashford says. "Logic tells you that the wider tire should perform better. You've got more lug in the ground for more bite and more pull."

But the test data show that, on both untilled and tilled wheat stubble, the narrow tires out-performed the wider units in both tractive efficiency (TE) and dynamic traction ratio (DTR). TE is a measurement of how efficiently a tire converts axle horsepower to drawbar horsepower. DTR is the pull/weight ratio that measures how much weight a tractor can pull compared to its weight.


So, what is the best tire size for our Toyota's?


It depends


What do I use, that provides the best compromise for my expedition vehicle?
255/85 R16 in a mud terrain

I am currently working on specifying an ice and snow tire for my Arctic Ocean expedition this winter. Now that is fun research...

 

[spartan]

Scott - thanks for sharing your knowledge on this subject. Your comments are well made and researched. For the record I agree with you.

There was a recent thread on stuffing 315 75 16's under a stock 80. I called BS and made jest of the subject for purposes of function and was promptly subjected to a flaming barrage for daring to call a stock 80 with 315's a mall cruiser.

Do you remember the 34 9.5 15's BFG AT tire? I used those throughout the 1980's with great results.

I am now considering the Interco SST 35 10.5 16.5 tires for my Ford Superduty. Have you ever tried them?

 

[expeditionswest]

There was a recent thread on stuffing 315 75 16's under a stock 80. I called BS and made jest of the subject for purposes of function and was promptly subjected to a flaming barrage for daring to call a stock 80 with 315's a mall cruiser.

Most guys love their trucks as much as their mom's. So, unfortunately, heated (not always logical) discussions almost always result, and flaming is common.

"never make fun of a man's truck, dog or mom; and in that order"

One of the challenges these guys face is a lack of narrow availability over 33" diameter. Mostly because the military needs everything produced right now (XZL's, etc.) and lack of DOT approval for the SIMEX and similar brands.

So, to get a 35" tall tire on an 80 for rock crawling, the choices are limited in radials. Tires from most manufacturers get wider to help improve stability on the road (reduce liability). Most vehicles spend 90% of their time on the street. If you look at technical terrain competitions like the Rainforest Challenge and Outback Challenge in AU, the most common tire is the narrow SIMEX trekkers.

Everything is a compromise, unfortunately; usually only cured with loads of cash.

I am now considering the Interco SST 35 10.5 16.5 tires for my Ford Superduty. Have you ever tried them?

I have tested those tires and like them. An aggressive solution in a radial tire. Street performance suffers though (noise, cornering deflection, etc.)

 

[FirstToy]

There was a recent thread on stuffing 315 75 16's under a stock 80. I called BS and made jest of the subject for purposes of function and was promptly subjected to a flaming barrage for daring to call a stock 80 with 315's a mall cruiser.

you stated incorrect information in that thread, and then deleted all your posts.


Expeditionwest,
your links are not working, but I'm interested to read it.

A couple things, while I'm no expert for sure- this is how I think of it, I am not saying this is fact- but adding to discussion

-the narrower you go, the contact patch elongates and narrows
-wider tires have a more "squareish" contact patch
-at the same pressure, a slightly narrower tire actually has more rolling resistance b/c of the greater contact patch. Obviously, this goes away when the size difference is greater.

-I tend to visualize the skinnier tire as you would a high heel shoe; a incredible amount of psi on it's contact patch- enough to punch a hole thru metal, thus great grip.

- A wider tire distributes the weight out over the surface. In irregular terrain, I think a wider tire "grabs" and "conforms" to the irregularities in terrain. While applying less psi for the given contact patch, it will grip better b/c it can "grab" where the skinnier tire cannot.

-so going over rocks, for example, the skinnier tire will have better grip in it's contact patch. If that contact patch has no traction, you are stuck.
-The wider tire may also loose that grip but b/c it's wider, can grab a rock or ledge next to it, thus making it over. So, I tend to think in real-world wheeling the wider is better.

- on sand, I tend to think wider is better b/c it allows you to float on top of the sand instead of cutting into it. Spread the weight out and float.

-On a straight line pull (like towing or pulling someone out), I tend to think the narrower tire will grip better b/c the shape of the contact patch is in line w/ the wheels.

I did a exaggerated doodle of the contact patch below

 

[expeditionswest]

Expeditionwest,
your links are not working, but I'm interested to read it.

I just checked the linky, and it worky

A couple things, while I'm no expert for sure- this is how I think of it, I am not saying this is fact- but adding to discussion.

I love the doodle by the way

-the narrower you go, the contact patch elongates and narrows.

The contact patch does narrow; however, elongation is a byproduct of load, carcass structure and internal air pressure.

I.E.-

Increase the load, contact lengthens (and widens slightly)
Decrease the carcass sidewall strength, contact lengthens
Decrease the air pressure in the tire/wheel, contact lengthens

-wider tires have a more "squareish" contact patch.

right on

-at the same pressure, a slightly narrower tire actually has more rolling resistance b/c of the greater contact patch. Obviously, this goes away when the size difference is greater.

Not really. resistance is highest as the tire first comes in contact with the road surface. So the wider the contact, the more resistance.

A long contact starts at a high resistance, normalizes and then lifts from the surface. It is the width that is the greatest resistance factor to the road surface and wind.

- A wider tire distributes the weight out over the surface. In irregular terrain, I think a wider tire "grabs" and "conforms" to the irregularities in terrain. While applying less psi for the given contact patch, it will grip better b/c it can "grab" where the skinnier tire cannot.

Wide and narrow tires perform two different functions (essentially). The wider tire provides more flotation because there is less pressure per square inch. So, with less pressure, there is less conformation (deformation).

-so going over rocks, for example, the skinnier tire will have better grip in it's contact patch. If that contact patch has no traction, you are stuck.
-The wider tire may also loose that grip but b/c it's wider, can grab a rock or ledge next to it, thus making it over. So, I tend to think in real-world wheeling the wider is better.

Please add more detail if possible to the above statement. Thanks. (not quite sure of the conclusion you were drawing)

- on sand, I tend to think wider is better b/c it allows you to float on top of the sand instead of cutting into it. Spread the weight out and float.

For the most part that is true. A 33x12.5 will have better flotation than a 33x9.5. You can overcome this by fitting a taller, yet narrow tire with the same lift required to fit a shorter/wider one. Then, the tall/narrow tire can be aired down more, creating more flotation while presenting less frontal resistance.

Much of the flotation gain of a wide tire in sand is lost to frontal resistance. This is important for those of us without big HP motors (Toyota's in general) as power is required to overcome the resistance.

 

[FirstToy]

sorry it's just quicker for me to draw it-

I mean even tho theoretically, the narrow tire has more traction in it's contact patch, a wider tire has more chance to grip and perform in the real world.

In my doodle, the narrow tire could be on a climb and find a rut where it has no traction. On the same line, the wider tire can make it.

 

[expeditionswest]

sorry it's just quicker for me to draw it-

I mean even tho theoretically, the narrow tire has more traction in it's contact patch, a wider tire has more chance to grip and perform in the real world.

In my doodle, the narrow tire could be on a climb and find a rut where it has no traction. On the same line, the wider tire can make it.

oooh, even more cool drawings

In your example, it would depend. Is the surface in the notch the same as the sides? If the notch is smooth or even slick, and the sides are rough and chunky then yes, the example on the right would be a better traction situation.

But fortunately, we can pick different lines to take advantage of the surface friction coefficient and the deformation of a narrower tire.


In general:
And I am not saying (nor have said) that narrow tires will always be the best solution for every off-road situation. In fact, my paper is intended for the expedition traveler, who sees many different environment in one trip. Sand, mud, rock, lots of road miles, etc. Fuel economy is also critical to my needs, so is a lower COG, etc.

There are some conditions where a skinny tire is the worst thing to have (really deep snow or mud for example).

It is like anything else. It all bells out, with 10% of the environments requiring a really wide tire, 80% of the environments favoring a 9.5-10.5" tire (for the average 5,000 lb. truck) and 10% that would favor even narrower.

Just buy what works for your needs.

 

[FirstToy]

I'll have to look at the skinny tires- the 80 a couple pages back is running them...I remember someone w/ a nice 60 posted them a while back (Razor?)... something to think about...

 

[spartan]

Actually I posted correct information in that thread and gave up on argueing with those who feel that 315's on a stock 80 are appropriate.

I then deleted my posts because it's really hard to reason with anyone regarding their truck, dog or mother as Scott points out.

This thread is functionally addressing the issue of tire size for an 80 series Landcruiser rather than lamely asserting the merit(s) of stuffing tires that are too large into your wheel wells for the sake of having " big " meats.

I'm not here to argue. I live, share, listen and learn.

 

[Nay]

oooh, even more cool drawings

In your example, it would depend. Is the surface in the notch the same as the sides? If the notch is smooth or even slick, and the sides are rough and chunky then yes, the example on the right would be a better traction situation.

But fortunately, we can pick different lines to take advantage of the surface friction coefficient and the deformation of a narrower tire.

I think where this debate gets lost is the concept that harder core wheeling is about forward traction. Much like driving in very bad weather, forward traction is the lesser issue for your tires. What is critical and only available from the tires is lateral traction. Even if we take for fact that a narrower tire creates more of a pull contact patch, that isn't your only goal in rock crawling - you want that forward pull, but you cannot sacrifice good lateral traction in order to get it. Lots of ways to go forward better (tire diameter, suspension articulation, gearing, lockers), but only one way to keep the lateral: your tires.

The most common serious situation I see offroad (and have experienced myself plenty of times), is the failure of a lightly loaded front end to hold lateral traction on steep obstacle climbs. When you have serious conditions and are using front and rear lockers and low gears, you are maximizing driveline forward traction, really independent of the tires. What you are relying on from your tires is two things: 1) to get the body up and over the rocks so you don't get hung up, and 2) to hold lateral traction so you stay on your line. Wider tires are better for both. Now this doesn't mean that forward pull from the tires isn't important, but when you spin all four you can typically line back up and reevaluate. When you go sideways bad things often happen.

This is why most of us rock crawlers aren't stampeding to 9.5" wide tires. Many of us have tried them and I doubt many are going back. We build forward traction into every element of our builds, and going too narrow with small lugs just helps all of that forward ability work against you in excessive side slip, which is always sudden and always at exactly the wrong time and always in the worst conditions.

I guess it all depends on what you consider to be performance and failure to perform. I don't ever mind not being able to go forward offroad because there are other ways to go forward (a little back up and bump, winch, strap, back up and take the bypass, or just head home). I do mind being sideways on a major obstacle close to rolling, especially when that roll could be fatal.

Failure in a serious fashion that causes damage and/or need for extended and complicated extraction is so often lateral in rock crawling (forward gets the nod for breakage). Which may very well be why we find a balance of forward/lateral traction in the 12.5" width. After all, years ago the 33x12.5 was the baseline tire, now it is the 35x12.5 and the 37x12.5 has gained great favor in the hardcore market. What you notice is that despite a major increase in diameter we continue to favor 12.5" wide.

There must be something to it that just doesn't play out on paper. Either that or despite the tens of thousands put into rigs to create extreme modifications, we just can't pick the right tires and nobody has figured it out.

Nay

 

[Grench]

OK, for those of us who are wanting to go with narrow tires...

255/85R16 are there from a few places. Enough that they can be had. So, if you want 33" and narrow, you're set.

Now, for those of us who want a narrow 35" tire and live in the US, what is there? Does anyone make a 35" narrow for sale in the US?

 

[FirstToy]

That's a good q uestion gench- like a 285 width 35" tire?

Lots of nice info to consider the possibilities. Certainly, there are some good benefits to both sides.

 

[sethro]

35" narrow options...

Depends. If you want a 'road worthy/DD/ able to drive to Rubicon while being able to hear your cellphone ring' tire, I haven't found any options.
There's stuff available in the swamper style (q78, etc.) but I'm building my do-it-all/daily driver/expedition rig, and those tread patterns are so much less efficient for the hours I will spend on the road getting to the trailhead.

I've been looking online for a few days now, and it's become pretty frustrating...

I'm probably going to go with a 35x12.5 trxus MT on my soon to be 4bt/700r4/part time 4wd 1994 80 series. I love the way tirediron's 80 looks, but them's some BIIIIIG tires. Sure looks cool though.

cool,

seth

 

[tarbe]

We certainly could use a 285/85 option for the 6,000 lb expedition 80s...I wonder if anyone is listening out there?

 

[stumpy]

so after reading this entire thread, i am still confused about one thing that i think scott might be best to explain-

how is it that, if inflated to equal tire pressure, i.e. 40 psi, a wide tire and a narrow tire can have different footprint AREAS? i understand they will have different SHAPES, that makes perfect sense. but area and shape are different. what does carcass strength have anything to do with anything? ground pressure is ground pressure is ground pressure, isnt it? it doesnt make sense to me that sidewall strength can support more than the air pressure inside the tire. the air must support the tire at the same "strength" as the sidewall, otherwise there would be less ground pressure at the center of the tire, requiring the footprint to elongate.

if, in the real world, a tire footprint does not have equal pressure across the entire area of the footprint, the total ground pressure would still have to be the integral of all the pressures on all the very small square inches. assuming a constant internal air pressure for all four tires of either narrow or wide dimension, the ground pressure for a 5000 lb vehicle must still be 5000 lbs regardless of total footprint area and regardless of shape. if the pressure on each square inch of tire changes, so must the area of the footprint.

that just seems like basic physics to me, so i am confused as to how i am wrong in this logic...

 

[expeditionswest]

There are a few concepts at work here, so I will do my best to address them individually.

how is it that, if inflated to equal tire pressure, i.e. 40 psi, a wide tire and a narrow tire can have different footprint AREAS? i understand they will have different SHAPES, that makes perfect sense. but area and shape are different.

Two tires of the same carcass construction, yet different widths will have different footprints with the same pressure. The shapes will also have very close to the same length. However, the wider tire will have a greater width dimension.

Let me try to explain this in different terms by removing the air component for a moment. Think of a solid rubber wheel: One that is 1" wide, and one that is 20" wide. The rubber is solid, and has no measurable deflection. The 20" wide wheel will have 20 times the area in contact with the ground compared to the 1".

Now lets reintroduce air, and go into detail on this: Air pressure inside the tire is only ONE component of what gives a tire shape and load capacity. The other is the tire itself. Have you ever tried deflating an E rated Super Swamper bias ply tire? On a light vehicle, the carcass wont even really start to deform until single digit numbers. This is the critical point. On that E rated Swamper, what is doing most of the job in supporting the Jeep? The carcass. Don't forget that the sidewalls are integral in supporting the weight of the vehicle. Where a passenger rated, single ply tire will start to deform much sooner as it relies more on air to support the vehicle.

Area and shape are different. A wide and narrow tire of the same construction must have different areas and different shapes.

Think again in the extreme. Do you believe that a 39x18 and 38x11 Bogger will have the same total area in contact with the ground? It is physically impossible, as they are within less than 10% of the load rating of each other, and within 5 psi of max pressure.

Pictures for reference:
18" width

11" width

what does carcass strength have anything to do with anything?

Carcass strength and air pressure is what supports the load (vehicle and payload). Without the carcass, air has nothing to contain it.

Look at the side of a 255/85 and a 285/75. The sidewall is the same height. Now think of those tires without a rim; just resting against the garage wall. The tires maintain form because of the carcass construction. The heavier the load rating, the more they maintain form under load. Carcass' do not stretch (much), so if the air pressure in both is the same, and the only difference is width, why would the narrower tire elongate significantly more than the wide one? It wouldn't.

ground pressure is ground pressure is ground pressure, isn't it?

Please elaborate, thanks.

it doesnt make sense to me that sidewall strength can support more than the air pressure inside the tire. the air must support the tire at the same "strength" as the sidewall, otherwise there would be less ground pressure at the center of the tire, requiring the footprint to elongate.

Think of the example above. Just a naked tire carcass with no wheel and no pressure. It maintains its shape, and will even take a load on top of it without crushing. Once you combine a tire, a wheel (making a chamber) and air pressure the load capacity increases significantly. The narrower tire and wheel combo have the same internal pressure as the wide one, only the volume changes. So why would the narrower tire be subject to greater elongation than the wider one with the same load? It wouldn't.

if, in the real world, a tire footprint does not have equal pressure across the entire area of the footprint, the total ground pressure would still have to be the integral of all the pressures on all the very small square inches.

Sorry, I don't quite follow you here. Please provide more detail, thanks

assuming a constant internal air pressure for all four tires of either narrow or wide dimension, the ground pressure for a 5000 lb vehicle must still be 5000 lbs regardless of total footprint area and regardless of shape.

Yes, true. Vertical load is vertical load. What changes between narrow and wide tires is how much load there is per square inch.

if the pressure on each square inch of tire changes, so must the area of the footprint.

The footprint will change, which will change the ground pressure for each square inch.

Hope this helps... I am off to dream of Diesel Troopy's (with narrow tires of course ;p )

 

[expeditionswest]

Even if we take for fact that a narrower tire creates more of a pull contact patch, that isn't your only goal in rock crawling - you want that forward pull, but you cannot sacrifice good lateral traction in order to get it.

Sorry Nay, I missed your post. No Troopy dreams for me quite yet...

You bring up an interesting observation, regarding lateral traction. That might warrant some testing...

In the meantime:

1. My article was not intended for "rock crawling", but for the expedition traveler (which requires a balance). The values of a narrow tire is much more important for what my requirements are than the potential (as this has not been tested) of greater lateral traction.

2. The research I have conducted and the resources I use for data do prove the lateral traction advantages of a wide tire (deflection, roll, scrub, heat control, etc.), but these are all under high speed tests on highly tractive surfaces (CF greater than 1). It will be interesting to test these theories in a very slow, technical trail environment with a CF of much less than one.

3. Lateral traction is affected by suspension dynamics and design, gearing, lockers, etc. Just think of the design effort that goes into a race car, to allow it to turn fast... A vehicle with a front locker engaged will favor understeer.

I will say, that in my experience, lateral traction on technical terrain has never been observed as an issue for narrow tires. I have run most of the well known 4 rated trails in the west with a 10.5" wide tire.

 

[spartan]

In terms of availabilty for a true 35 10.5, the Super Swamper SSR is it:

SSR-41R 35x10.50R16LT

Tread depth 21/32
Tread width 9.0
O.D.Height 35.3
Center Section 11.1
Rim 16x7-9
Air 65
Load 3415

Only tire I've been able to find that actually fits the bill. As Scott points out in an earlier post however, this isn't really a functional on road, expedition type tire.

I have been using 295 75 16 tires for several years now and prefer them to the 35 12.5 and especially 33 12.5 tires that I have used in the past. The 33's always seemed to spec at more like 32 or less which was undesirable. While the 35's seemed to usually be in the 34.5 range.

The 295's seem to be at least an inch taller than the 285's across the board depending on the brand yet only 10MM wider. As in 285's, height varies. The 295's are a relatively new size therefore up until recently only came in two flavors: BFG AT's and Nitto TG's. There have been two recent additions that I am considering: Hankook DynaPro MT RT03 which seems to be a blend between an AT / MT tire and the General Grabber AT 2.

The General Grabber AT 2 is rated to be a 33.5 inch tall tire with an 11.6 inch section width. At about $130 apiece it's a hard tire to pass on.

If I can get a set of the Hankook's siped I might try them as well.

I'll pass on another set of Nittos - 30K mile tire. I'll also pass on the BFG's - expesive and they stink on wet pavement.

If only they made the AT REVO in a 285 95 16 size.