Tire width: the long and the short of it.
Posted: 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.
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.
Point of case was the river trip last year, I got stuck on the flat sand.
I do a lot of sand driving, and will form an opinion soon 