Tire width: the long and the short of it.

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Tire width: the long and the short of it.

Post by Peter Connan » 10 Mar 2019 11:10

This is something that we as 4x4 drivers discuss a lot, and probably think about a lot too, but there is a lot of disinformation and widely different opinions being bandied about. I decided to add a little bit (just a little) of science to the mix.

Of course, as usual, this is a matter of compromise, and different situations require different approaches. However, the starting point of the whole discussion has to be the question of flotation, and that a wider tire has more flotation than a narrow tire.

This is unfortunately a myth.
The thing is that a tire automatically adjusts the size of it's contact patch with the ground on the basis of it's internal pressure and the load it's carrying.
The stiffness of the carcass (strength of the sidewall) also has some effect, but for most pressures this effect is relatively small, but basically the contact patch can be calculated on the hand of the two factors named above.

But because of the construction of a modern tire, the contact patch doesn't really become any wider as the tire deforms, it basically just becomes longer.

Thus, if you run the same pressure on the same weight of vehicle, the contact patch of the tire has basically the same area irrespective of tire width.
This means that on a wider tire, the contact patch, being wider, is also shorter.

Thus, on a soft surface, a vehicle with narrower tires is not going to sink in any deeper than a vehicle with wider tires, assuming the same tire pressures and weight. What is going to happen is that the vehicle with narrower tires will have longer contact patches.

So let's look at the situation in a bit more detail on the easiest surface to deal with: soft sand.

On a soft surface the tire will always sink in a little bit. Thus, as you drive, you are constantly "climbing out of a hole" just a little bit. Thus, a narrower tire is "digging" a narrower groove, thus wasting a little bit less energy. On virgin sand, this is not a huge deal because if your tire pressures are low enough, the amount you are sinking is pretty small and thus not a lot of energy is lost. But if you are driving in somebody else's tracks, the effect gets greater with every passing vehicle, especially since at the edges of the grooves the sand tends to fall back in, and a car with narrower tires than those that have come before will be lass affected by this than one with wider tires.
But to create the longer contact patch means that there is more tire deformation. The tire bulges more, and as a result the ground clearance is reduced a little bit. If the middle-man is so tall that this loss of ground clearance means that you are dragging your diffs, you will probably lose more energy than you gained.

Just to put a number on it, at 0.8 bar, a pretty wide tire (12.5" or 305mm nominal) loaded with 750kg lowers by about 40mm from the nominal size, but a much narrower tire (255mm or 10.5" nominal) lowers by about 60mm, or 20mm or 30% more. These calculations were done using the actual width of the contact patch of tires I have here, not the nominal width.

Thus, to my mind a tire on the narrow side of the spectrum is better in sand that one on the wide end of the spectrum. I have had both extremes on my car, and have done trips to the same areas of the Kalahari on both. Now I haven't managed to get stuck in sand yet, but on the trip with narrower tires I had far fewer issues with the car getting hot than what I experienced with the wider tyres. I thus conclude that the car was having to work less hard with the narrower tyres. I know this is not conclusive, because the trips were several years apart, but still.
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Re: Tire width: the long and the short of it.

Post by Peter Connan » 10 Mar 2019 11:26

So what about mud?

To my mind, there are basically two types of mud, and these behave completely differently. The less common but far trickier is clay. This tends to form a crust, a hard(ish) surface on top of soft gloop extending downward an unknown but (in terms of the ground clearance we have available at any rate) effectively infinite distance. Think Makgadigadi salt pans.

On this stuff, once you break through the crust, you are effectively stuck and buggered. I don't think tire width makes much difference here. Pressures are important, and a larger diameter and a higher sidewall may help (as this is what controls how large you can make the contact patch by deflating) but in terms of the modifications you can effectively do to a car which must still function well on tar, it probably doesn't make much difference either. The best thing you can do is to keep your vehicle as light as possible, or even better, just avoid the stuff.

The other type of mud is a sloppy layer with a reasonably firm base. On this stuff, what you actually want to do is to cut through the slip and get traction on the firmer base.
Once again, the narrower tire has a better chance.

The only example I can quote is the forum trip to Botswana this time last year. Now a couple of guys got stuck (many of them repeatedly). I believe this had nothing to do wit tire width but everything with tire diameter. Basically, everybody who got stuck was lying with their diffs on the middelman. And the guys who got stuck almost all had smaller diameter tires than the guys who didn't. There was just one exception, but in that case the car that got stuck was towing a second car...

However, it was clear (to me anyway) that, on my narrow tires, I was able to stay on the middelman better than the guys with the wider tires. The other guys were starting each pool on the middelman, but usually slipping off into the ruts quite quickly...
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Re: Tire width: the long and the short of it.

Post by Peter Connan » 10 Mar 2019 11:32

So what about rocks I hear you ask (well, in my mind anyway)... :imsorry:

I doubt tire width makes much difference here, at least not to traction. Where I do see a difference is that with a narrower tire it seems to be easier to avoid sidewall damage. But keep in mind that the difference between the widest common size and the narrowest common size is only about 30mm per side, so not massive. Driver skill and the use of a skilled spotter probably remains the most important aspect here too.
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Re: Tire width: the long and the short of it.

Post by Peter Connan » 10 Mar 2019 11:49

And lastly of course: roads.

Here, wider probably means better traction most of the time, but more friction and thus worse fuel consumption. You pays your money etc...
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Re: Tire width: the long and the short of it.

Post by martyn » 10 Mar 2019 15:48

Peter, very true in 99%0 of the cases. But I am the exception, I get stuck on a tar road with any type of tyre. :redface: Point of case was the river trip last year, I got stuck on the flat sand.
Proof, the driver makes a massive difference.
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Re: Tire width: the long and the short of it.

Post by NoPressure » 10 Mar 2019 18:48

Thanks Peter

As some know I have some tall and skinnies on my new (old) patrol. I have thought about putting skinnies on previous vehicles of mine, but could never get myself to pull the trigger. Went for a bit of a river drive over the weekend, I haven't to date got stuck with any vehicle in the river, but with harder than normal pressures (2.3bar) it handled the thicker sandy parts well. It might just be the patrol. I did have the first plonk on the h260 pumpkin - now I know where it is atleast :rolling: I do a lot of sand driving, and will form an opinion soon :lol:

I must add that it is ridiculous how difficult it is to find a pair of tall and skinnies...just tall easy, just skinny easy. Furthermore, there is a YouTube clip where they drive over a metal rod welded 90degrees on a base plate. Then they drive over it with HT, AT and MT tyres at different pressures - it's rather informative.

What are the skinniest 35's one gets?

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Re: Tire width: the long and the short of it.

Post by Kagiso II » 10 Mar 2019 22:05

I agree with Peter's views here -- if you page back some years ago we had similar discussion [I drove a Hard body then and Dwergie wife had a Terrano .. Both ran on 245 tyres ]
My view has always been, the wider the tyre, the higher the rolling resistance it has to overcome to move forward - Therefore, on a 235 I will deflate to 1 bar up in Kgoudom .. on a 265 about 1,2 to 1,5 and on the 285's I have it on 1,8 to 2 bar --
I would LOVE to get a set of 235 / 85 x 16's on my vehicles
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