Why skinny tires

[Esmi]

Yes, you sort of lost me after this part:

It is as simple as understanding the coefficient of friction (COF), which is (Ff = Cf x Fv).

. . .

My everyday condition is steep dirt, either with or without gravel and dry grass on top of it. My experience is that I need to engage 4X4 much more often in my new truck with wide tires than I did in my old truck with skinny little bones. Could be a multitude of other things (Ford now vs. GMC then, or greater willingness to brush-stripe the side of the old truck, or quien sabe).

 

[Martinjmpr]

Perhaps the question should be reversed: Why fat tires? Back in the olden days (i.e. the 70's) fat tires on 4x4s were fairly rare here in CO. Now they're quite common. Has 4 wheeling changed, or is it likely that the "fat tire" craze is simply a fashion trend? Seemed to me people used to 'wheel skinny-tired Jeeps, Broncos, Blazers and Land Cruisers just fine in the days before fat tires became the de facto standard for off-road vehicles.

Don't get me wrong, there are places where fat tires make sense - the high desert, any areas of large sand, where flotation is key, are great terrain for fat tires. But otherwise - you're pushing a lot of weight that, depending on the terrain, may or may not be adding anything of value.

But here's the thing: The modern 4x4 "culture" evolved in Southern California in the 60's and 70's, and often times, equipment that serves a purpose in that terrain is put onto vehicles in other parts of the country simply because it looks cool.

Note that the military - where many of our most beloved 4x4s were first used - has been using skinny tires for years. They're starting to move towards fatter tires on some vehicles, but then again that may reflect more the simple fact that we are doing more military operations in the desert than we used to.

 

[Scott Brady]

ignorant statement (the show truck part).

Ignorant - no. What would make my statement ignorant? SEMA is filled with big, fat tires for appearance only. They have no place on an exploration vehicle. I am happy that your 33x13.5 tires worked for you, but that has much more to do with the tread pattern than the width. Mud requires a balance of Fv and tread pattern.

I specifically mention the tundra buggy, as there are time when maximum flotation is the only attribute required. Icelandic trucks are the same. The primary goal is to reduce contact pressure and distribute the truck's weight over the largest area possible. Again, these are highly specialized environments, like competitive rock crawling on manmade courses, which has nothing to do with distributing the trucks weight over the largest area, and has everything to do with adhesion and deformation.

 

[spencyg]

I think I'd like to hear from somebody who has actually run a 285x75-16 along side an equivalently "treaded" (?) 255x85x16 in equivalently weighted and arranged vehicles and run them side by side in the same terrain. I certainly follow the physics lesson Scott has assembled and it is very good. As with all things though, theory and practice sometimes diverge somewhere along the way. I agree that a wider tire has a higher rolling resistance and higher windage. I also agree that a wider tire for similar diameters will be heavier. We all know this effects suspension dynamics with unsprung weight and the function of dampening.

The only reason any of us diverge from a stock tire however is capability off the road, and this is where I am a little more skeptical. There is absolutly a higher point load pressure on the tread when you're air'ed up, but all this discussion includes the fact that we air down, and airing down is undeniably beneficial in all terrains for traction. This is the case because you increase your footprint, therefore increasing the amount of tread in contact with the surface (and allowing the tread to conform to the surface), and DECREASING your point loading. Yes, an aired down 255 will have a larger point loading than an aired down 285, but is it that significant? I'm not questioning the fact that there is a difference when going to a smaller tire, but I'd like to see some real-world examples of "with this tire I couldn't do this, but when I went to this other tire I could...".

Possibly an upcoming feature in OJ?



Spence

 

[Redline]

I agree with you, and think popularity, fad, and the So. Cal. desert & Baja influence has much to do with the the trend toward wider tires in recent decades (can their be any doubt about this?). This wide influence has now morphed into the wide tire AND ‘tall wheel’ craze that we have today. Tires with very little sidewall on cars and trucks that are not 'high-performance' on-road vehicles, but general purpose vehicles that would be much better served by a shorter wheel and more sidewall (ride, flex, traction, etc.).

I have some fairly old 4x4 books, from the mid-80s. My memory is that their is text about the 'recent' introduction of wider tires and wheels in speciality sizes for 4x4s. And as you state, I remember it being in reference to the desert southwest environment.

snip..............

But here's the thing: The modern 4x4 "culture" evolved in Southern California in the 60's and 70's, and often times, equipment that serves a purpose in that terrain is put onto vehicles in other parts of the country simply because it looks cool.

Note that the military - where many of our most beloved 4x4s were first used - has been using skinny tires for years. They're starting to move towards fatter tires on some vehicles, but then again that may reflect more the simple fact that we are doing more military operations in the desert than we used to.

 

[Chris85xlt]

My old Bronco i had before was built mainly for the bottomless mud pit up in Azusa Canyon (Definitely not an expedition vehicle). I've ran from 37x12.5 tires to 40x17 and hands down the worn out 40x17 did the best in the mud. They also did very well in the sand dunes compare to all my past 12.5" wide tires. The wider tires just had globs of floatation but the downside was how horrible it was to drive on the street. Terrible gas mileage and it liked to wander on the highway.

Now with the jeep on 255's it has great manners on the road and pretty decent mileage on the highway. Last time i checked was 21mpg.

 

[jeepmedic46]

I tried to find the article on the website, but couldn't find it.

 

[Redline]

Fireside Chat or General Veh. Mods

Not that it really matters, I enjoy the debate/conversation regardless of the location, but couldn't this thread be in the general vehicle mods section instead of fireside chat?

Seems like tires/tire choice it one of the most regularly asked and researched questions.

 

[spencyg]

I tried to find the article on the website, but couldn't find it.

http://www.expeditionswest.com/research/white_papers/tire_selection_rev1.html

 

[Antichrist]

I have some fairly old 4x4 books, from the mid-80s. My memory is that their is text about the 'recent' introduction of wider tires and wheels in speciality sizes for 4x4s.

In nearly all of the 4WD mags I've saved from the the mid 70's wide tires were all the rage. They were seriously promoted.
FWIW I've always run 7.00x15 or 7.50x16's on all my Land Rovers. I tried a set of something like 30.5x10x15 on my Lightweight, but didn't like them all that much.

 

[jeepmedic46]

Thank you, It is a excellent write up.:ylsmoke:

 

[James86004]

Something I really do not understand is low-profile mud tires!

There is a very interesting discussion about why racecars went to wide tires in the 1960s in one of Carroll Smith's "To Win" books. I think it was Engineer to Win. Fascinating series, by the way.

 

[RHINO]

i see it from both sides, having two rigs set up differently. mind you my wide tires are only 37-12.5, thats not real wide for its height, but its still 12.5 against about 9 on the other rig.

for my tech related reply,,,, my thoughts are that both tires have merits in all conditions, i run the same terrain with both rigs and i honestly dont see much difference in the end result. the skinnier tires dig much quicker and either find a bottom or get weight to the other tires, where the wider tires tend to float a bit more and dont need to dig as often. i believe if you know how to drive you'll avoid much potential problems before wide vs narrow even matters.
as for the airdown debate of wide and skinny, again i dont think it matters as much as it gets made out to. scott posted up some good stuff about the the length of tread being more important, well no matter how wide a tire is if you air down you lengthen the tread, the obvious difference is flotation, the goal is not to spin but if you do to find forward traction.
i dont play much in mud so my example is geared to sand. the example of arctic trucks is a perfect one, you find the same scenario in the sahara and flotation would be so much better than the way everybody does it now. the problem is you need a specialty rig, much easier to do in iceland where your staying in the specialty environ, i think the whole skinny debate in sand would be blown out of the water if it was economical to build a specialty rig for it. skinny is a compromise in sand for the expected hard pack and tarmac the same rig will be spending much time on.

now for my chat related reply,,,, i think skinny tires have stayed in fashion with the explorer types mostly because the rigs stay closer to stock, so the smaller lighter tires really do account for less wear, better fitment and so on. i think wide tires have stayed in fashion with recreation guys because those rigs stray from stock and are played with more. i do like the merits of a skinny tire for a rig that sees much travel on hardpack and benefits from the MPG increase.

 

[Scott Brady]

It is just about being reasonable on width.

The benefit of an 85% aspect ratio tire is that it is taller, but likely the same width that came from the factory, therefore easier to fit in the wheel well, on the stock rim, better turning radius with the same off-set, etc.

There is a reason 90% of the tires fitted to modern 4wds are a 75% aspect ratio tire on a 16" rim.

Is the 255/75 R17 (85% equiv. for a 16) picked by the Jeep engineers completely at random?

We can have a whole other discussion about airing down, but the common misconception is that airing down is only for "contact patch". There are three reasons to air down. 1. Ride quality (a big deal for us, given corrugations, etc.) 2. Flotation (in sand, mud, snow, all surfaces with a low COF) 3. Deformation (the ability of the carcass to deform and wrap around rocks, ledges, tree roots, etc.). To say that you are airing down to increase "contact patch" overemphasizes that benefit, as most surfaces we deal with are low adhesion scenarios. Moab being one of the few exceptions.

 

[Scott Brady]

I love that photo