Why skinny tires

[Antichrist]

People get skinny tires for the same reason people get wide tires- for looks.

Not always. Series Land Rovers had relativey narrow rims as stock, and because of the larger studs and different stud spacing it was harder to find wider rims. I bought the tires that would fit, which were skinny and they worked well for me. I never felt a need to spend money on wider rims so I've just always used tall skinny tires. They've always done the job for me.

 

[Scott Brady]

Never answered my question-
what size tires did you and Pasquale run in the arctic and south america? what do you run on your personal vehicles?

Tacoma: 255/85 for Central America and most of its 70,000 miles. A 33.5x11.5 for the Arctic, only because that was the size available with the softer compound and glass rating.

Land Rover Defender: 235/85 R16

Land Rover Discovery: 245/75 R16

EarthRoamer Jeep: 285/75 R17 KM2 (11.5 wide), only because the 255/80 R17 was not available

G-wagen: Stock size. 265/75. Will upgrade to 255/85

Overland JK: 255/80 R17

 

[Scott Brady]

those are tractor tires on a wheat field with a FARM TRACTOR and 46" wheels!
we might as well look at bicycle and moto tire tests

even in that article it says
"On a firm soil surface you won't see as much difference in performance between tires. As soils become more moist, or mellow, you can expect wider tires to perform better.

"If you farm with several tractors, it makes sense to equip them with different size tires, 18.4s for row crops and 20.8s and larger for primary tillage work," Brodbeck says. "The 710s run at half the tire pressure of the 18.4R46s and the wider, lower-pressure tire is going to work better in loose, wetter soils and cause less compaction.

The important point is that the narrow tires were slightly better, and in other cases they state you will not see much of a difference. SO, if there is a nominal difference in traction, then all of the other benefits of narrower tires become the reason to decide one way or the other.

Look at it from that perspective. If narrow and wide are about the same overall because trail conditions are so variable, then why use a heavier, wider tire? What would the benefit be for our typical application?

 

[Spur]

On my new FJ55 (that will hopefully be completed in the next year, full resto, diesel, locking differentials, modernization, etc) 35s x 12.5's x 17's this time is what I decided. They are metricl BFG AT's that are "305s" so a hair narrow technically but are basically 35 x 12.5 more or less. This is if I want to be able to drive to Moab or other areas and back, get decent mileage with the AT's (25 mpg with my diesel again), but be able to hopefully do a trail like Pritchett Canyon there which is among some of the hardest trails in Moab. My other consideration is 255 85 R 16 which is a tall skinny that is about 34 x 10.5. They make an E rated version as well.

Great post. Thanks.

255 85 R 16 looks like a really interesting size. I looked it up on BFG's website and couldn't find it in a TA KO. Do you have a link to the tire you're thinking of?

 

[RHINO]

scott i found your dream rig dude:smiley_drive:

http://www.swfloridaonline.com/images/swampold1.jpg

 

[michaelgroves]

More PPSI (Pounds Per Square Inch)

To the extent that a tyre is a bag of air supporting a load on a hard surface, the size of the contact area of any tyre (on a given vehicle), will be identical, for any given pressure. A load of 1000kg on a tyre at 2 bar (i.e. 2kgf/cm2) will put down 500 square centimetres, and at 1 bar, it will put down 1000 square centimetres, regardless of whether the tyre is skinny or fat.

That's a bit theoretical, but it makes a good starting point for identifying what determines any real-world discrepancy from that. I suggest that the discrepancy is small. My reasoning is that there are only two things supporting the load: - the air under pressure, and the stiffness of the tyre carcass. How much contribution does the latter make? I don't know if there's likely to be a significant systematic difference between the stiffness of wide versus narrow tyres (as opposed to brands/tread patterns/load ratings), but for wide tyres to have bigger footprints in general, they would have to be much, much stiffer.

But in any case, I don't think stiffness plays a very big part in footprint size, except perhaps at the extremes. It seems to me it's an easy hypothesis to test: take a wheel with no air in the tyre, and press it downwards until it has a given footprint size (say 1000 cm2 - the same deformation as a heavyish truck on tyres at 1 bar). Note how many kg of force it required to do so. I'd guess less than 50kg, for most tyres? In which case, my conclusion would be that the tyre's construction is carrying 50kg, whenever it's deformed to that extent. So for most expedition trucks of 3 or 4 tonnes, the actual tyre structure "supports" only 5 or 10% of the load - the rest of the support is air pressure. This, to my mind, lays to rest any idea that wider tyres have a significantly bigger contact area than narrower ones.

[Of course, bigger tyres in general - wider or taller - have the advantage that any given amount of deformation takes place over a bigger expanse of rubber, so it's possible to run bigger tyres at lower pressures without excessive bulging. But in principle, the size of the tyre plays no part in its contact area. Footprint size is overwhelmingly a function of pressure versus load.]

Footprint shape is a different kettle of fish, and for most lower-speed and off-road conditions, I think a long narrow footprint is somewhat preferable to a short wide one. The clincher is that for any given vehicle, it's usually possible to fit taller tyres, if you go skinnier. Within reason, I'd choose a narrow tyre rather than a wide one, but the benefits of tall tyres vastly outweigh their disadvantages, IMO.

I keep intending to do some measurements of various tyre footprints under controlled loads and pressures, but haven't ever got around to it. So in the meantime, I welcome any counter-arguments, or pointing out of flaws in my arguments!

 

[jh504]

I have a question that kind of goes along with this topic. The military fills their tires with an expanding foam type stuff to keep them from going flat in combat. I used some expanding foam to fill my lawnmower tires that I couldnt keep air in. Does anyone know what the comparable PSI would be like with filled tires? Anyone had any experience with these tires in an offroad setting?

 

[Martinjmpr]

I have a question that kind of goes along with this topic. The military fills their tires with an expanding foam type stuff to keep them from going flat in combat. I used some expanding foam to fill my lawnmower tires that I couldnt keep air in. Does anyone know what the comparable PSI would be like with filled tires? Anyone had any experience with these tires in an offroad setting?

Uh....is this something new? Every military vehicle I drove or rode in had plain old air in the tires. The HMMWV M1025 and M1026s (Hatchback, lightly armored patrol vehicles) I had in the FRY originally had the bias-ply tires with the metal "run flat" ring inside. While we were there, our motor pool replaced them with radial tires that had a rubber run-flat ring.

Other than that I'm not aware of any special compounds or foam in tires unless that is a recent development, maybe in up-armored HMMWVs or MRAPs.

 

[JamesDowning]

I'm enjoying this discussion!

However, I have a couple things to add.

First, the coefficient of friction isn't affected by the contact patch size, nor does it affect the normal force. Tire contact patch theoretically has nothing to do with the available friction at the tire. This theory is much easier to see on pavement though.

There are two reasons you see wider tires on performance vehicles. First, is wear. If a vehicle is driven hard, the tires wear quickly, and with a wider tire, there is more rubber there to wear down. Think of extreme cases... a very thin tire is going to wear down much quicker than a wide cylinder of a tire.

The second reason has to do with shear. As you know, when someone does a burnout, it leaves black marks. The tire is actually shearing away, or tearing, as Scott mentioned. With a larger contact patch, there is more material that must shear, and the shear force is spread over a greater cross section, thus making it harder for a tire to tear away, and thus makes sense for drags or street racing.

Our terrain is different though, as we can actually bite into our terrain. This leaves the terrain to shear. So again, if you are in a situation where you are spinning tires and kicking up terrain (sand, mud, snow (not ICE)), you could possibly benefit from a wider tire (or just a greater contact patch). Think of a 2WD baja vehicle or a Rally vehicle.

To me it seems to come down to a difference of speed. If you are traveling at high speeds, or traveling in lots of deep mud, sand, or snow, then a wider tire could be beneficial... However, the main expedition mode of travel is generally slower and over a varied terrain, which allows us to run a slightly narrower tire.

It seems to me that a narrower tire is always beneficial due to fit, aerodynamics, rolling resistance, steering force, etc... but narrow is all relative. It seems the rule of thumb is to go as narrow as possible for your specific application/terrain.

The alternative side is exactly as stated in posts above... if you go too narrow, you either have to lose sidewall, or lose stability. A tall narrow tire will experience much more waddle than a wider tire. Along the same school of thought... a tall narrow tire is also much more susceptible to popping beads when under a side load (turning or off camber).

So for every application there seems to be a happy medium between too tall and too narrow.

At least thats my point of view.

 

[R_Lefebvre]

First, the coefficient of friction isn't affected by the contact patch size, nor does it affect the normal force. Tire contact patch theoretically has nothing to do with the available friction at the tire. This theory is much easier to see on pavement though.

I'm sorry, but this is wrong. The Cf is affected the contact patch pressure (related to size). This is very well supported by virtually all vehicle performance engineering sources.

There are two reasons you see wider tires on performance vehicles. First, is wear. If a vehicle is driven hard, the tires wear quickly, and with a wider tire, there is more rubber there to wear down. Think of extreme cases... a very thin tire is going to wear down much quicker than a wide cylinder of a tire.

The second reason has to do with shear. As you know, when someone does a burnout, it leaves black marks. The tire is actually shearing away, or tearing, as Scott mentioned. With a larger contact patch, there is more material that must shear, and the shear force is spread over a greater cross section, thus making it harder for a tire to tear away, and thus makes sense for drags or street racing.

These are secondary benefits. The main benefit is traction, and tire temperature. Tire temperature goes hand in hand with your other 2 points, however.

you could possibly benefit from a wider tire (or just a greater contact patch). Think of a 2WD baja vehicle or a Rally vehicle.

Rally cars typically don't have very wide tires, relatively speaking. Their gravel tires are narrower than their tarmac tires, and the ice tires are narrower still. In fact, I first learned the term "pizza cutters" in reference to rally ice tires. In 1999 I won an ice racing championship in a fwd VW with 155/85/13 Blizzaks, beating out some AWD vehicles even, because they couldn't get narrow tires for their wheels.

Any discussion which compares different tires in different widths, and comparing relative "stability" should keep in mind the differences in sidewall stiffness amongst different tires. It's a pretty big factor when talking about stability. You can't say "I tried a wide BFG KM, and a narrow Goodyear M/T, and the narrow tire was less "stable", so in conclusion, narrow tires are less stable."

 

[jh504]

Uh....is this something new? Every military vehicle I drove or rode in had plain old air in the tires. The HMMWV M1025 and M1026s (Hatchback, lightly armored patrol vehicles) I had in the FRY originally had the bias-ply tires with the metal "run flat" ring inside. While we were there, our motor pool replaced them with radial tires that had a rubber run-flat ring.

Other than that I'm not aware of any special compounds or foam in tires unless that is a recent development, maybe in up-armored HMMWVs or MRAPs.

The first I had heard of it was a few months ago when I was pricing some 37" michelins that came off of some type of armored vehicles in Iraq. I am sorry I cant give you anymore specific information than that. These tires were cheaper because they were filled with foam to keep them from being shot out. It sounded like something that the guys in Iraq took upon themselves to do, so they didnt get caught out in the cold. The surplus guy was getting a lot of these "filled" tires in.

 

[zimm]

The important point is that the narrow tires were slightly better, and in other cases they state you will not see much of a difference. SO, if there is a nominal difference in traction, then all of the other benefits of narrower tires become the reason to decide one way or the other.

Look at it from that perspective. If narrow and wide are about the same overall because trail conditions are so variable, then why use a heavier, wider tire? What would the benefit be for our typical application?

youre putting too much weight on this. pulling with a tractor is not the same dynamic as rolling over a surface.

having spent time on the family farm, and using a tractor... what you are seeing in the study is the effect of soil compression and tread depth. the tilled and untilled test came out relatively the same because tilled soil will recompress. fat tires that distribute evenly over a surface, instead of being narrow and rounded, wont compress the soil at the center line of the tire.

in an suv, i think you youre better off leaning towards a bit of float. first, youre much lighter, and you dont need the same soil "strength" to push you forward, and second youll break a lot less soil surface.... tread lightly!

 

[jh504]

in an suv, i think you youre better off leaning towards a bit of float. first, youre much lighter, and you dont need the same soil "strength" to push you forward, and second youll break a lot less soil surface.... tread lightly!

I was thinking of this earlier. It seems like a fat tire would be the tread lightly tire of choice.

 

[UrbanCummin]

Just my 2cents but I think that fat vs skinny tire debate also could be classified as the expo (skinny) vs off road (fat) debate. A lot of the expo trucks I have seen on this site are very close to stock for their capability, usually just a rear locker and a slightly bigger more agressive tire. Most of the updgrades are for convieince rather then performance.

While the fat tire crowd seems to go harder off road, deeper mud, more rocks etc. These people seem to favor fatter tires as they offer a lot more flotation, which you really need in mud. Oh and if your 33x9.5 is finding solid ground under that mud, then that is some pretty shallow and easy to traverse mud. I've been wheeling for years and done many trail runs, wide tires help a lot off road, in almost every wheeling situation.

Also the expo crowd seems to have a lot of people that consider almost any driving to a far location to be an expedition. It is logical that these people prefer a narrow tire because of the improved street handleing.

I gotta run to work, will post further on this latter.

P.S. climate and terrain will greatly affect your tire needs, hard packed dirt needs much less width then snow/ sand or mud. I live in a relatively dry area, but even here the mud in the forests can easily overwelm a 44" bogger.

 

[JamesDowning]

I'm sorry, but this is wrong. The Cf is affected the contact patch pressure (related to size). This is very well supported by virtually all vehicle performance engineering sources.

I have never seen dry friction related to contact patch size. The universal equation for dry friction is Friction = mu * N. Mu (coefficient of friction) is related only to the materials that are in direct contact, and N is the force (not pressure) acting normal to the contact surface. Contact patch size doesn't play into dry friction.

However, on loose dirt, mud, sand, and snow, contact patch size does matter because you are shearing the surface in which you are traveling. In this case, the frictional component under the tire becomes fluid friction (or viscous drag), which is much more complex. Think of the drag related to pulling an inflatable raft through the water. The dynamics are similar for a tire that is rotating faster than the vehicle is moving.

So, thats why terrain plays a huge role... your tires could be acting in dry friction with your terrain, or you could be constantly shearing your terrain... it all plays into what tire you may prefer.

These are secondary benefits. The main benefit is traction, and tire temperature. Tire temperature goes hand in hand with your other 2 points, however.

We essentially agree here, its a matter if what we feel is the primary benefit.

Rally cars typically don't have very wide tires, relatively speaking. Their gravel tires are narrower than their tarmac tires, and the ice tires are narrower still. In fact, I first learned the term "pizza cutters" in reference to rally ice tires. In 1999 I won an ice racing championship in a fwd VW with 155/85/13 Blizzaks, beating out some AWD vehicles even, because they couldn't get narrow tires for their wheels.

Rally vehicles have relatively wide tires for their vehicle weights. Granted, for dirt racing, they stick to somewhere around 215 width, but that's pushing it for their vehicles. When the terrain is being sheared due to high tire speeds, fluid friction (viscous drag) comes into play. In most applications (vehicle airflow design), you want to minimize fluid friction (drag). When it comes to rally tires, they want to maximize the fluid friction in order to provide more propulsion.

Ice, is a different story altogether. I agree that super skinny tires are the way to go here, and as I understand it, its to maximize the force on each individual ice studs. You don't want float in that circumstance, you want to dig.