Thread: Wide tyres on a SIII

The ground pressure will, for all practical purposes, be the same as the tyre pressure (the sidewall stiffness will add some pressure, but for most real life situations this is negligible.

So on this basis, the width should make no difference as long as the tyre can operate at the necessary pressure, although fitting/not fitting existing ruts may be a factor. I can't see how diameter can make any significant direct difference either, although it will make a very sigificant indirect difference - larger diameter means better ground clearance, and the difference between dragging the diff through the mud and not doing this really does make a difference.

John

Interesting thread. My brother and I watched a lot of 4WD's trying to get up Big Red near Birdsville. They were all running wide tyres, which may have been standard on some of their vehicles, and taking long runups with heaps of power. Some made it, some didn't.
My brother (who has been around Landies much more than me) instructed me to drop the pressure on my SIII's tyres to about 12 PSI, which I did, and then we just drove up it in low range second. She pinked a bit near the top when I gave it full throttle and I was going to reach for another gear but brother stopped me and we chugged over the top no problems. Lots of people took photos as we appeared at the top <smug grin>.
My brother commented that it wasn't so much the vehicle, but the tyre pressure and how you drove it that was important. He was critical of those using lots of power as some were just "burying" their wheels. Me, I'm just a newbie at this, but I was pretty pleased, as you can well imagine....
By the way, we had to drive up the back of Big Red as the front is under about 10' of water as far as the eye can see in both directions. They call this the Simpson Desert?

Coop
BVille BR1.jpg

Originally Posted by JDNSW

The ground pressure will, for all practical purposes, be the same as the tyre pressure (the sidewall stiffness will add some pressure, but for most real life situations this is negligible.

No so. A while back I saw a study that someone did of contact patch size on flat ground for different tyres at the same pressure. There were huge differences.

n.b. contact patch was measured as total surface area directly in contact with the ground.

Originally Posted by isuzurover

No so. A while back I saw a study that someone did of contact patch size on flat ground for different tyres at the same pressure. There were huge differences.

But this will depend on sidewall and belt stiffness rather than width. To the extent that this is due to the belt stiffness, larger diameter will reduce the effect, but I would be very sceptical whether the practical difference (say 29" - 32") is significant. Do you have a link for the study?

Another point is that on the face of it, everything being equal, a 15" tyre will dislodge from the rim more easily than a 15", as the tangential load on the bead is greater for the same torque.

My personal experience with wide tyres was in desert operations in the sixties - all available tyres then were 100% section, so some of the wide tyres had smaller wheels - and they came off the rims if pressure was lowered enough to be usable. Standard tyres (7.50 on the small vehicles) were just as effective, but when radials became available they were much more effective, mainly because lower pressures could be used without destroying the tyres fairly rapidly.

John

Originally Posted by JDNSW

Do you have a link for the study?

Can't find the particular study (done with 4x4 tyres) atm, but here is another:
Fact or Fiction? Tire contact patch and air pressure.

Originally Posted by JDNSW

The ground pressure will, for all practical purposes, be the same as the tyre pressure (the sidewall stiffness will add some pressure, but for most real life situations this is negligible.

So on this basis, the width should make no difference as long as the tyre can operate at the necessary pressure, although fitting/not fitting existing ruts may be a factor. I can't see how diameter can make any significant direct difference either, although it will make a very sigificant indirect difference - larger diameter means better ground clearance, and the difference between dragging the diff through the mud and not doing this really does make a difference.

John

its the lead in angle that counts, think of it like an approach angle.

try this..

take small socket and look at it from the round end and place it ontop of a sponge and push it in say 5mm notice theres quite a steep angle at the point of contact on the sponge in relation to the circumference of the socket.

Now try it with a larger socket.

Whats happening is an effect of "The angle of the tangent at the point of contact and that ties into rolling resistance. Heres how it works.

We all know that any line that has the circumference of a circle touching anywhere along its mid point is doing so at right angles to the radius of the circle at the point of contact. Where the wheel makes initial contact to the ground this same geometric concept is in effect. If we assume that the weight of the wheel is always directly down (ignoring hill climbing and motion at this stage) as the wheel sinks into the ground the angle of the point of contact is increased, quickly for the smaller diameter wheel and less so for the larger one.

Thats why generally a taller tyre travels better throught mud and sand (ignoring the extra ground clearance) Its expending less energy trying to get ontop of the sand/mud.

there are other factors that come into play such as the length of the contact patch and how much it "stretches" as the tyre pressure is lowered (sidewall flex permitting, a licorice strip is going to run out of sidewall clearance long before it gets any decent tread lenght increase) The rolling resistance from a narrow leading edge compared to a longer one and the construction of the tyres.

A larger diameter tyre also has the ability to produce a longer footprint.

Originally Posted by isuzurover

Can't find the particular study (done with 4x4 tyres) atm, but here is another:
Fact or Fiction? Tire contact patch and air pressure.

Obviously in this example, the structure of the tyre is supporting a substantial part of the weight. Whether this is true of the tyres we actually use I do not know, but suspect it is not necessarily the case. Worth noting that in the example however, the front and rear tyres, presumably with similar construction, show little difference in ground pressure for changes in width.

John