Best Off Road tires for better FE, trucks need love too :)

[chillsworld]

Great article on tires for expedition use. Lots of knowledge that kind of goes against what most 4x4 folks say... And it would seem to be advice that justifies taller skinnier tires as true and capable off road choices. So for FE on a truck, less rolling resistance, less frontal resistance, less rotational mass, better fitment in wheel well (aerodynamics), less weight on suspension/drive-line components, etc etc.

Here's the link, and a few excerpts of interest for how a skinnier tire is just as good or better than a wider tire for a great deal of off road uses...

Expeditions West: Tire Selection for Expedition Travel

Understanding Off-highway Tire Performance:

Important note: 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.

The benefits of a narrow tire:

The Argument: A tall, narrow tire is a better choice for all off-highway surface conditions with the exception of soft sand, snow and soft mud that's depth exceeds 110% of the vehicles minimum ground clearance. Here is the explanation.

• Contact Pressure: Contact pressure is expressed as the vehicles curb weight distributed over the contact surface of four tires. The contract pressure is not equal to all four tire road surface contact points as the vehicles weight is not perfectly distributed. 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.

Off-highway effects of contact pressure:

Deformation- On a smooth surface (like concrete), a tire gains most of its traction by adhesion. On an irregular surface like granite and boulders, a tires contact patch will deform as a result of vertical pressure. The wider the tire, the less the rubber will deform to the surface irregularity given the same vertical pressure. The greater the deformation, the greater the tires resistance to shearing forces (spinning). This is the strongest argument to using a narrower tire.

Real world example: When climbing a ledge with a jagged surface, the narrower tire will wrap the protrusions with more contact due to the increased deformation depth. The wider tire will rest on the surface of the protrusions and will have a greater chance of spinning (shearing).




Mechanical Keying: This is the second critical benefit of a narrower tire. As the vertical load increases, so does the compression and flexing of the tires tread and rubber to the surface protrusion. A narrower tire generates greater vertical load on the rubber and the tread, increasing tread compression in conformance to the surface irregularity. A wider tire in contact with more surface conforms less, and will shear sooner than a narrow tire.

Adhesion- On a flat surface, the adhesion rate of a narrow tire and wide tire are the same, as the wider tire makes more contact (friction area), but the narrow tire generates more pressure (vertical load force). On a highly irregular surface, the higher vertical load force of the narrow tire becomes an advantage, increasing molecular bonding between the tires rubber and trail surface. That bonding becomes so great that either the vehicle moves forward, the tire tears leaving rubber on the surface, or the trail surface breaks away.

Airing Down: This is another critical concept highlighting the advantage of a narrower tire.

Traction in soft surfaces: It is a common misconception that airing down a tire for off-road traction only makes the tire contact patch wider. That is not the case. In fact, only 20% of the increased contact comes from the width. 80% of the increased contact patch comes from the tread patch becoming longer. A tall, narrow tire allows for a very long contact patch when aired down. That, coupled with the minimal frontal resistance (area), negates much of the downside to narrow tires in flotation situations. The taller tire allows for a long contact patch and still maintains good ground clearance.

Traction on rocky trails: Another common misconception is that when airing down it is the increased amount of tire on the rock (more contact patch), that allows better traction. It is not the contact patch that creates better traction, but the tires ability to conform to the surface irregularities (deformation and mechanical keying). When an aired down tire comes in contact with a rock on the trail, the tires tread collapses under the vertical and horizontal forces, causing the tire to wrap the rock, as opposed to sitting on top of it. The wrapping effect provides greater shear resistance, and in turn better traction. (Technically: the shear load is distributed over multiple planes, not just a horizontal one).

Tire spring rate: One of the great benefits of airing down a tire is improved smoothness. Less pressure allows the carcass to flex. A taller tire has greater sidewall compression, and in turn a better ride. (expressed as compressive strength=N/mm). That is why your grandma's Cadillac had such tall tires…

Negative Effects: Nothing in the world is perfect, so there are some downsides to using a narrow tire…

Stability and high speed deflection- A narrower tire (and in turn a narrower overall vehicle track width) provides less stability on the road and on cambered trails. In addition, a taller, narrow tire's sidewalls deflect more under severe turning forces, causing the inside of the tires contact patch (midline to the vehicle) to lift (roll in) from the road, increase the chance of a high shear force skid, or loss of control.

Increased potential for trail damage- A tall narrow tire has greater contact pressure, so when crossing a sensitive area like a muddy track, the tire will want to dig down until traction is found as opposed to floating on top. Make sure to air down and apply light, smooth throttle to minimize trail damage, or just turn around and save the trail from any damage at all.

Real world examples
The Turtle Expedition who has literally traveled around the world used a 255/85 R16 (33.3 x 10) for many thousands of miles on their full size Ford. Land Rover uses narrow 7.0 R16 XCL tires in most of their Camel Trophy events. The Rain Forest Challenge and The Trophy challenge have all been won by the aggressive Simex Trekker tire (35x11.00). Tom Sheppard often uses the 7-7.5 R16 Michelin XZL and XCL for many of his expeditions. All very narrow tires in relationship to their height…

Hope this is useful to someone


~C

[aardvarcus]

Tall skinny tires have many advantages both on-road and off-road, but they can have some disadvantages too. I have found that other features of a tire (tread pattern, sipes, compound, etcetera) are much more important than size. Also width should be considered in the context of vehicle weight and other factors.

Selection in some of these odd sizes is a big one. I wanted to get a set of the 255/85R16’s for their 33”+ diameter, but the only choices are mud terrains, or all terrains that look a lot more like mud terrains than anything else. Ended up going with 255/85R16 Cooper Discoverers, thinking they were all terrain, but they are definitely on the mud terrain side of the all-terrain spectrum.

Put them on my 2001 4x4 Chevy 2500HD, absolutely hated how they did on road. (Note truck probably weighs 7000lbs.) Swapped over to Michelin LTX M/S2 285/70R17’s, I love them on the road, and so far they have performed better than you would think off-road.

Next my 1991 Chevy 2500LD 4x4 got the 255/85R16 Cooper tires. It handled ok on-road with the tires, since this truck is probably only 4500lbs, but panic stops would send me spinning and/or into other lanes. The brakes weren’t the best, but the truck wouldn’t ever do this with the 265/75R16 Uniroyal Laredos that I had on another set of wheels and swapped back and forth.

Since I got my 2005 Tacoma, I moved the 255/85R16 Cooper tires onto a spare set of Tacoma rims, but got a set of 235/85R16 Michelin LTX M/S2 for daily driving. Love the Michelins, truck is very confident and planted on bad road conditions. Will plow through standing water on the road from torrential downpours just fine. I recently did a swapout to do some offroading, so I had the coopers on the Tacoma for a few days. I didn’t push them on road, since I already knew what to expect, but they performed ok. They handled the offroading with ease, but honestly my DD Michelins would have done ok for what I did, since there wasn’t any mud or extreme terrain.

Now for the MPG.

My next fill up for the tacoma, which was ½ DD tires and ½ Offroad tires on mostly highway was 25.1 MPG, which compared to similar tanks that included similar offroading done with the DD tires (26.8 MPG used as reference), implies that the off-road tires would have accounted for a 3.4 MPG loss for half the trip (23.4).

I had observed roughly a 3 MPG difference between the two tire sets on my 1991 Chevy, using only on road highway driving comparisons. (14 vs 17)

I saw no MPG difference between the two tire sets on my 2001 Chevy, probably because the disadvantages of the mud terrain type tread pattern were offset by the extreme weight of the new wheels and tires. Roughly 20lb wheels and 35lb tires versus 30 lb wheels and 55 lb tires is a big difference. Also when you are only getting 11-14 anyway, differences are hard to discern.

The moral of the story is: I don’t care how cool it is, I hate driving with mud terrains on pavement. Period.

Edit: Oh, and all the numbers have been adjusted for differences in diameter.

[chillsworld]

Agreed, mud terrains aren't worth it unless you see lots of mud! And there are certainly downsides to "skinny" tall tires... Especially if you drive your truck like a sports car hahaha. Tire folding is a very serious, and dangerous thing... But going with a tire that has a thicker and stronger sidewall can mitigate that to some extent.

From what I have read, the 285's are very popular in A/T for lots of expedition vehicles. The reason i like the opinions of the expedition guys, is because their trail rig is a street legal rig that gets them all over the world (including hwys and large cities). Thanks for some real world comparisons! I do think that it would be easy-ish to match mpg if one shopped around and tried their hardest to match the tire/wheel weight of the previous combo...


~C

[jamesqf]

Originally Posted by chillsworld

And there are certainly downsides to "skinny" tall tires... Especially if you drive your truck like a sports car hahaha. Tire folding is a very serious, and dangerous thing... But going with a tire that has a thicker and stronger sidewall can mitigate that to some extent.

Why not go with a tire with the same sidewall height on a larger diameter rim? (Assuming you can find them, of course.) If you look at antique cars up to say the 1930s, before there were a lot of paved roads, you see that that's what most of them have: basically skinny tires on very large diameter rims. Maybe they knew something.

[chillsworld]

Originally Posted by jamesqf

Why not go with a tire with the same sidewall height on a larger diameter rim? (Assuming you can find them, of course.) If you look at antique cars up to say the 1930s, before there were a lot of paved roads, you see that that's what most of them have: basically skinny tires on very large diameter rims. Maybe they knew something.

I could be mistaken, but I think that was more due to a lack of tire technology and the fact that super tall skinny wheels had always been used on carriages and horse drawn wagons. I would assume load bearing, and ease of clearing obstacles was priority number one.

In addition, a taller wheel leads to less sidewall... And in order to air down for driving off road, there needs to be more sidewall to allow for a greater contact patch to occur without slicing the rubber with the sides of the wheel. I believe the recommended aspect ratio for sidewall is +80% when airing down tires.


~C

[jamesqf]

Originally Posted by chillsworld

In addition, a taller wheel leads to less sidewall...

Yes, that was the point: if the problem is tire folding due to sidewall height, then you could reduce or eliminate the problem by going to a larger rim with a shorter sidewall tire to keep the same diameter.(Assuming of course that you can find manufacturers that make the necessary sizes.) But if that causes other problems, then you either have to work out the optimum combination, or pick one over the other.

Then too, with a larger tire diameter, you'd get a larger contact patch. In the extreme, consider a tracked vehicle, which has the same contact patch as a tire many yards in diameter.

[brucey]

I think we should message interco and ask them when they're making the Super Swamper with LRR technology.

[CapriRacer]

Originally Posted by chillsworld

I could be mistaken, but I think that was more due to a lack of tire technology and the fact that super tall skinny wheels had always been used on carriages and horse drawn wagons. I would assume load bearing, and ease of clearing obstacles was priority number one.........

You are correct that tire technology had not reached the point where low aspect ratios were common - or even possible.

For practical purposes, plain jane bias tires can not be produced with an aspect ratio lower than about 75. If you add a belt (as in bias belted tires) lower aspect ratios become possible - but those weren't invented until decades after the invention of the automobile - and once you've started down a path, it's hard to introduce radical changes.

[redneck]

.

If you look at antique cars up to say the 1930s, before there were a lot of paved roads, you see that that's what most of them have: basically skinny tires on very large diameter rims. Maybe they knew something.

https://www.youtube.com/watch?v=nq2jY1trxqg

[chillsworld]

Originally Posted by jamesqf

Yes, that was the point: if the problem is tire folding due to sidewall height, then you could reduce or eliminate the problem by going to a larger rim with a shorter sidewall tire to keep the same diameter.(Assuming of course that you can find manufacturers that make the necessary sizes.) But if that causes other problems, then you either have to work out the optimum combination, or pick one over the other.

Then too, with a larger tire diameter, you'd get a larger contact patch. In the extreme, consider a tracked vehicle, which has the same contact patch as a tire many yards in diameter.

The goal is to have a taller sidewall, decreasing the sidewall defeats the purpose of going with the taller skinnier tire. The large sidewall is what allows for airing down, it allows the rubber to flatten and conform to the substrate both front to back and side to side. "Tires with an aspect ratio of 50 or lower (like a 255/50 R 16) have very low volume of air and shouldn't be aired down much" (http://4x4abc.com/jeep101/soft-sand-deflate.html)

Example:

Two tracks on sand, same size tire, one at 32psi a one at 10psi. Do you see the difference? One sinks and one floats. This is due to the ability to air down and increase surface area both in length and in width... With length being greater than width (you can see it's barely wider than the fully aired tire).





Another issue, is that a larger wheel often means more weight than a smaller wheel with a taller tire... At least when it comes to AT and MT tires on trucks. You also have to consider the greater cost of larger wheels and corresponding rubber. Here is a side by side comparison between 15" and 17" wheels with I believe 37" rubber... I tried to find one with more likely 33" tires, but I couldn't.

Comparison
15-Inch............17-Inch
Ply Rating 6-ply (Load Range C) ...10-ply (Load Range E)
Load Rating (lb) 2,910 @ 35 psi... 4,080 @ 65 psi
Pit Bull Tire Weight (lb, ea.) 88... 90
Allied Rock8 Rim Weight (lb, ea.) 34... 46
Overall Weight Per Corner (lb) 122... 136
Pit Bull Tire Retail Cost (ea.) $399... $450
Allied Rock8 Retail Cost (ea.) $217... $308
Overall Retail Cost Per Corner $616... $758

From: Picking The Right Size Wheel For Your Truck - Rim Pickins
Follow us: @fwmag on Twitter | fourwheelermag on Facebook

As far as a comparison with a tracked vehicle, that's not even a remotely fair comparison. A tracked vehicle would be similar to having 4 or 5 smaller wheels on the vehicle, not having two larger wheels.

Lets take a look,

30psi


15psi


7psi


A tire capable of going to 10 or less PSI will most likely double it's contact patch, to accomplish that without airing down (or only airing down to about 20psi)... Well, you would need one seriously massive (tall) tire. I hope that shows that your tracked vehicle comparison is actually the reverse. An aired down tire is comparable to a track... A taller inflated tire is simply a taller tire.

The main issue, is choosing a tire which is capable of airing down and then also behaving as you would expect it to on the road. My statement about sidewalls folding over was a disclaimer regarding on road driving habits/techniques. If you drive your truck like a sports car, hitting twisty roads, death turns, etc, then you should get a lower aspect ration side wall and anticipate a reduced ability to air down on the trail (or so I would assume). Then again, if you are an ecomodder... You shouldn't be driving like a bat out of hell anyway


~C