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There is a slighty tedious, but absolutely totally foolproof way to settle the tire argument/flotation issue for yourself, forever.

1) Go to a public weigh bridge , weigh each end/axles of the vehicle, loaded and unloaded.

2) On a hard smooth surface, jack up the front end of vehicle, both tires off the ground. Make a sand pad about 2cm deep, 250mm wide, by 300mm long under each tire on the ground.

3) Lower the vehicle onto the sand pads with normal road pressure in the tires and gently bounce on the front end.

4) Jack the front end into up the air again and measure the "tire contact patch", both the length and width and record it somewhere.

5) Smooth out the sand pads to erase the "contact patch".
With the front end still in the air, lower the tire pressures to minimum "off road pressure" . Lower the vehicle onto the sand pads and bounce gently.

6) Jack vehicle back into the air and measure the "tire contact patch" again and record your results.

You will find, like I did, that the contact patch has increased, only very, very slightly in width and has increased in length very dramatically.

6) Repeat the process for the rear end of the vehicle.

7) Multiply the length by width of your contact patches to obtain the total "area of flotation" for each tire, at normal "highway" pressure and "off road pressures. The two results will be your "road area" flotation" and your "off road area" flotation.

Now comes the calculations. When I did this, I used square inches rather than square centimetres.

Divide the total vehicle weight in kilos, by the total "square inches" of rubber contact patch. This will give you a kilogram per square inch of tyre contact area.

When I did this to my previous 4wd with 30" x 10 x 8 tires , 15'' rims
I found that the flotation area was about 3.5" x 8" (28 sq") for each wheel, at normal highway road pressures (32psi)

Lowering to my minimum off road pressure of 12psi, increased only 1/2" in width to 8.5" and over tripled in length to 10'' long. (85sq"")

My total vehicle weight of 2240kg divided between 4 tires at 28sq" each, gave 2240 divided by 112 sq " =20kgs of vehicle weight per square inch of tyre at highway pressures.

Total vehicle weight of 2240kg divided between 4 tires at 85sq" each, gave 2240 divided by 340 sq" = 6.58 kilos per square inch of tire at off road pressures of 12psi.


Soooooo.........lowering my tire pressures from 32psi , down to 12psi, to drive "off road" effectively tripled my flotation......

Tire pressure on the ground surface reduced from 20kg per square inch down to 6.5kgs per square inch.

(for comparison, a 6 foot man of about 85 kilos , wearing a size 10 boot, places a pressure on the ground of about 5kg per sq inch. I know, I did this to myself also......

Other advantages of lower pressure

3 times the number of tire lugs in contact with the ground surface gave vastly better traction.

Rolling over obstacles like rocks and logs was vastly improved and vastly more comfortable, due to the tire conforming to the shape of the obstacle better, therefor again allowing better traction and impact/shock absorbtion.

Any disadvantages when running ?....

Two slight ones that I noticed.

I had to run the tires on the narrowest rim size suitable for the tire, in order to protect my rims(mags) from damage, when on rocky trails and to prevent the bead from dislodging(causing deflation) at lowest off road pressures.

I always had to pump up the tires when returning to the highway, 12 psi on the highway can quickly cause overheating in the sidewalls and potential blowouts at highway speeds. I bought an onboard compressor

Michael

Interesting read. I would like to say I would do it with my car but I am too lazy. Hopefully some one with a liberty will take the time to do it so I have a rough idea of what mine will be like.

Michael_va wrote:There is a slighty tedious, but absolutely totally foolproof way to settle the tire argument/flotation issue for yourself, forever.
Michael

It is close, but makes some simplifications, as does my analysis below.
The advantage of your method as opposed to simpler methods is it partially accounts for tyre stiffness.

An engineering perspective:

On a hard surface (one which does not deform under vehicle tyre pressure), and ignoring tyre stiffness:

Pressure on the ground equals the tyre pressure.

Road contact area equals vehicle weight divided by tyre pressure.

So halving the tyre pressure means double the road contact area.

e.g. a 3000 lb vehicle with 30 psi tyres will have a contact area of 100 square inches.
With just 15 psi in the tyres, the contact area increases to 200 square inches.

Soft ground is where is gets complicated.

If the surface (the ground) deforms when pressed with a tyre, it means that the natural state of the surface cannot support the applied pressure.
The applied pressure will be up to, but cannot be more than the tyre air pressure.

Case 1.
The ground deforms at 20 psi and
Deformed ground can still only support 20 psi
Tyre pressure equals 30 psi
Vehicle weight equals 3000 lb
Contact area required at 20 psi = 3000/20 = 150 sq in
So the tyres will retain their shape but will sink into the ground until the total contact area equals 150 sq in.
That sinking feeling!!!

Case 2.
The ground deforms at 20 psi.
Tyre pressure equals 15 psi
Vehicle weight equals 3000 lb
Contact area required at 15 psi = 3000/15 = 200 sq in
So (a) the ground does not deform, and (b) the tyres deform until the total contact area equals 200 sq in.
Flotation!!!

Case 3 - (soft sand)
The undeformed ground will support negligible pressure.
The more the ground is deformed, the more pressure it will support.
Tyre pressure equals 30 psi
The tyres will sink into the ground until the ground is able to support 30 psi.

Case 4 - (soft sand)
Same as case 3 but with tyre pressure lowered to 15 psi
The tyres will sink into the ground until the ground is able to support 15 psi.

Comparing case 4 to 3, "flotation" is being achieved by virtue of the tyres not sinking as far into the sand.
Is it half the distance for half the pressure?
I don't know; it depends on the properties of the sand.
Perhaps someone else might know.

Also cases 3 and 4 are highly simplified because some support is being provided by the "non-contact" sections of the tyre.

Dale.

Great post Dale.....and just when I thought flotation was simple.......

Once apon a time, I sat on top of the 42 mile crossing "big sand hill" on the SA Coorong, with a cold beer thinking I was pretty cool having got up the "big one" on my first attempt, with my 32' Desert Duelers at 15 psi and 351 cleveland roaring in my truck.

Then around the bottom of the hill came 4 yahoos in an old HG Holden station wagon. 1 yahoo was driving, three were on the tail gate yelling and bouncing. The HG had 7inch mags and big fat street rubber radials let down so to close to "dead flat" it was amazing they didn't loose the bead seals. It had a 307 chevy engine, a tri matic auto and a rear limited slip diff. They scaled the sandhill like it wasnt there and dissapeared out onto the beach. That hill was stopping and bogging lots of lesser prepared 4wd's.

That destroyed my smug "you gotta have "real power" and "4wd" illusions forever. Needless to say, there were Subie's out on the beach doing it easy. And they spent one quarter of my fuel bill getting there. They really added salt to the wounds......lol...I decided to go study this "flotation stuff" a bit.

From an engineering perspective, can you rationalize tread pattern effecting any real change on flotation.?

Michael_va wrote:From an engineering perspective, can you rationalize tread pattern effecting any real change on flotation.?

I was thinking about that last night when I wrote the previous reply. In short, I am not sure I can. However some thoughts/comments:

(a) Does a chunky-tread tyre deform as much on a hard surface as does a slick? It has less rubber on the ground than does the slick, so if the contact area was still to be, say, 100 sq in, then you would have to say it would.

However, my guess is that the rubber lugs are more like legs on a table, with the deformation being similar to if the lugs weren’t there. If that is correct, the actual ground pressure is higher than the tyre pressure - proportional to the reduced contact area. On that basis…

(b) On a softer surface, (thinking mud tyres or tractor tyres) the higher lug pressure will cause the ground to deform under the pressure, with the tyre possibly sinking in to the depth of the tread. At that depth the surface area increases significantly, so the ground pressure reduces accordingly and sinking might stop.
If we now think tractor tyre with its huge tall skinny lugs, it should relatively easily sink into soft ground. Traction is achieved by virtue of the huge amount of soil which would need to be pushed/moved in order to achieve wheel spin. (The tread design of tractor tyres makes them very directional. In one rotation direction, the tread tends to bight into the ground and can provide incredible traction. In the other rotation direction, the tread tends to come out of the ground when under load and it results in relatively poor traction. I don’t know what the forward/reverse traction difference is but I would not be surprised if it was five times or more in ideal soil conditions.)

(c) Mud tyres. I have no experience with them, but my guess is they work by using the higher lug pressure to penetrate the soft mud and grip the harder ground underneath. If the soft mud is deeper than the lugs, wheel spin will very quickly remove that soft mud. i.e. it digs itself in until it reaches a firmer base.

A road tyre is useless in such situations because the tiny tread-gaps quickly fill with mud and won't clear, so one is left with what is effectively a slick. At that point the method by which a tractor tyre achieves traction ceases to exist - totally. Surface pressure will be tyre pressure so it won't sink any more than any other tyre, but it has no means by which to achieve tractive force.

(d) Sand. Again I have minimal informative experience so much of what follows is just theoretical thoughts.
Tractor tyres. These tend to perform very well on sand, particularly when the majority or the weight is over the driven wheels. They always sink in such that the lugs are fully into the sand. I suspect the sheer size of the tyres (width and rolling diameter) compared to the tractor weight leads to a relatively low ground pressure when on sand. Traction is still relatively good because of the large volume of sand which would need to be moved to achieve wheel spin.
Road tyres. These are mostly lug, not much gap so the pressure increase on the lugs is small and lug penetration is minimal. Also the tread depth is small (almost negligible against a tractor tyre). Sand is fine so the tread will have little difficulty is getting some grip and will stay clean (unlike in mud). Because the volume of sand captured in the tread is small, the traction is small and the amount of sand moved with wheel spin is also small.
Mud tyres. Not a lot different to tractor tyres in concept, and those are good in sand; so why are mud tyres not chosen for sand use? I don't know, maybe it is partly driver related.

Consider this: when the traction limit is reached, wheel spin begins and sand removal starts to take place. If the driver has a 351 cleveland and likes using the loud pedal (who doesn't) there is plenty of excess power to spin those wheels. A slick tyre will remove the sand relatively slowly but with a mud tyre and its sand removal ability, how long do you think it would take to burry it to the axles? Probably a lot less time than it takes for you to take your foot off the pedal if you had it planted.

This reasoning leads me to think that bigger lugged tyres could perform well in sand against traditional sand tyres from a traction point of view, but beware the loud pedal.

Now another factor: tyre construction. On a hard surface, a tyre changes its contact surface area according to its air pressure - by deforming the side walls to allow more tread to contact the ground.

A good offroad tyre will have strong side walls to resist puncture. There is also often a very squarish profile at the tread-sidewall interface - with nice strong lugs to bite into the hard ground or grab the rocks. That corner can be much thicker than any other part of the tyre. When the tyre pressure is lowered for sand work, the tyre will tend to maintain its squarishness at the tread edge, limiting any surface area gain due to sidewall bulge. This will also result in the tyre sitting a little deeper in the sand.

Conversely a soft road-orientated tyre will probably have a much weaker sidewall and more rounded, softer lugging at the tread edge. At low pressures, it will more easily deform at the tread edge to create a useful sidewall bulge.

How does all that sound (assuming you made it this far without getting stuck)?

Great Discussion Dale,

OK...here I am able to add to the discussion I hope.

was thinking about that last night when I wrote the previous reply. In short, I am not sure I can. However some thoughts/comments:

(a) Does a chunky-tread tyre deform as much on a hard surface as does a slick? It has less rubber on the ground than does the slick, so if the contact area was still to be, say, 100 sq in, then you would have to say it would.

Assuming the rubber compounds used in construction of the two tread patterns are identical. The mud radials are often of a at stiffer compound to All terrain/sand radials.

However, my guess is that the rubber lugs are more like legs on a table, with the deformation being similar to if the lugs weren’t there. If that is correct, the actual ground pressure is higher than the tyre pressure - proportional to the reduced contact area. On that basis…

(b) On a softer surface, (thinking mud tyres) the higher lug pressure will cause the ground to deform under the pressure, with the tyre possibly sinking in to the depth of the tread. At that depth the surface area increases significantly, so the ground pressure reduces accordingly and sinking might stop.

Correct, a set of 32" x 12.5 x 15 Desert duelers sat on top of the beach sand leaving a tread imprint approximatly 6mm shallower than a set of the same size Mud Terrain Radials.

If we now think tractor tyre with its huge tall skinny lugs, it should relatively easily sink into soft ground. Traction is achieved by virtue of the huge amount of soil which would need to be pushed/moved in order to achieve wheel spin. (The tread design of tractor tyres makes them very directional. In one rotation direction, the tread tends to bight into the ground and can provide incredible traction. In the other rotation direction, the tread tends to come out of the ground when under load and it results in relatively poor traction. I don’t know what the forward/reverse traction difference is but I would not be surprised if it was five times or more in ideal soil conditions.)

(c) Mud tyres. I have no experience with them, but my guess is they work by using the higher lug pressure to penetrate the soft mud and grip the harder ground underneath. If the soft mud is deeper than the lugs, wheel spin will very quickly remove that soft mud. i.e. it digs itself in until it reaches a firmer base.

Correct for relatively narrow mud tyres.
The narrow tires dug down to a firmer base providing traction, provided they had enough sidewall height and rim height to reach the firmer base before the vehicle under body contacted the mud providing massive suction/friction resistance that could not be overcome by the available propulsion.

A road tyre is useless in such situations because the tiny tread-gaps quickly fill with mud and won't clear, so one is left with what is effectively a slick. At that point the method by which a tractor tyre achieves traction ceases to exist - totally. Surface pressure will be tyre pressure so it won't sink any more than any other tyre, but it has no means by which to achieve tractive force.

Exactly correct, my Bridgestone Desert Duelers became effectively useless, in the grey sticky "river flats" mud. 4wd makes little difference when wearing "slicks"

The second advantage of the mud tread, is the ability to throw out the mud and self clean as the tire rotates. Larger lug spacing allows dislodging the mud load.
Consider the option of wider mud tires. The wider tires do not sink so quickly through the mud to firmer ground, therefore providing less available traction. I found this apparent disadvantage was actually an advantage. My vehicle had "flotation" which prevented the under body contacting the mud in all but impossible conditions. Propulsion, I assume, was being provided by the "paddle effect" of the tires lugs.The ability of the tread, to spin out the mud and self clean, became critical in the deep stuff.
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(d) Sand. Again I have minimal informative experience so much of what follows is just theoretical thoughts.
Tractor tyres. These tend to perform very well on sand, particularly when the majority or the weight is over the driven wheels. They always sink in such that the lugs are fully into the sand. I suspect the sheer size of the tyres (width and rolling diameter) compared to the tractor weight leads to a relatively low ground pressure when on sand. Traction is still relatively good because of the large volume of sand which would need to be moved to achieve wheel spin.
Road tyres. These are mostly lug, not much gap so the pressure increase on the lugs is small and lug penetration is minimal. Also the tread depth is small (almost negligible against a tractor tyre). Sand is fine so the tread will have little difficulty is getting some grip and will stay clean (unlike in mud). Because the volume of sand captured in the tread is small, the traction is small and the amount of sand moved with wheel spin is also small.
Mud tyres. Not a lot different to tractor tyres in concept, and those are good in sand; so why are mud tyres not chosen for sand use? I don't know, maybe it is partly driver related.

Right on. Driving 110 kilometres along the Coorong breach, to the Murray Mouth, using 32 inch Desert Duelers and doing the exact same trip again on 32 inch Mud Terrains, I found that the Mud tires, out performed the Desert Duelers very easily. The difference in performance was so noticable, I have chosen Mud Radials ever since, even for major sand driving.

It is a common misconception that mud tires don't perform in the sand. WRONG!!!! when you need forward propulsion, they beat of closed tread tires any day.

Consider this: when the traction limit is reached, wheel spin begins and sand removal starts to take place. If the driver has a 351 cleveland and likes using the loud pedal (who doesn't) there is plenty of excess power to spin those wheels. A slick tyre will remove the sand relatively slowly but with a mud tyre and its sand removal ability, how long do you think it would take to burry it to the axles? Probably a lot less time than it takes for you to take your foot off the pedal if you had it planted.

This reasoning leads me to think that bigger lugged tyres could perform well in sand against traditional sand tyres from a traction point of view, but beware the loud pedal.

Agreed......exactely my findings. My mud radials easily out performed the "sand tires" in heavy beach sand, so long as I eased on and off the throttle. Mind you, with a hot 351 cleveland under my foot, hanging the ocassional "rooster tail" out the back, was an irrestible temptation.

Now another factor: tyre construction. On a hard surface, a tyre changes its contact surface area according to its air pressure - by deforming the side walls to allow more tread to contact the ground.

A good offroad tyre will have strong side walls to resist puncture. There is also often a very squarish profile at the tread-sidewall interface - with nice strong lugs to bite into the hard ground or grab the rocks. That corner can be much thicker than any other part of the tyre. When the tyre pressure is lowered for sand work, the tyre will tend to maintain its squarishness at the tread edge, limiting any surface area gain due to sidewall bulge. This will also result in the tyre sitting a little deeper in the sand.

True, sidewalls are also potentially a source of traction if the tread is continued down the sidewall. High performance mud tires often have side wall tread. It does not effectively support vehicle load, but may provide propulsion by the "paddle effect" when the vehicle sinks into the ground surface to include the side walls, as typically happens in deeper mud. This advantage would also extend to deep soft sand conditions as well.

Conversely a soft road-orientated tyre will probably have a much weaker sidewall and more rounded, softer lugging at the tread edge. At low pressures, it will more easily deform at the tread edge to create a useful sidewall bulge.

No....here I disagree. Sidewall bulge, for flotation, is extremely limited and sidewalls are a major weakness for staking, even in sand, there are obstacles that may damage.Sidewall bulge (radials) provides minimal increase in contact area....I measured only a half inch increase when deflating from highway pressures to sand driving pressures.

How does all that sound (assuming you made it this far without getting stuck)?

Sounds fine to me, after 10 sets of sand, all terrain and mud terrain tires on the same vechicle, my choice is for two sets of rims, one set with el-cheapo radial tires for everyday driving and a second wide tall set of mud terrains for all serious sand and mud, off road work. For me, the as wide as possible, tall as possible, mud radials, out perform the sand/all terrains, in all off road driving conditions.

They do have one major disadvantage that must be planned for though.

Because they take you a lot lot further into the nasty stuff before you get stuck, when you do finally get stuck, you are "really Stuck" like, your stuck up to your as######..... and nothing short of recovery gear and other vehicles will be of assistance.... Oh well, the joys of off roading"

Now for the lift kit and tall wide mud terrain radials for the Subie.

I wonder if Vic road transport would let me slide a 351 clevo into my great little Subie MY fun wagon.

Michael

Yes, it has been a great discussion. I have learnt a lot from the analysis (cleared the cobwebs out of the brain) and your input has been very informative as well.

As for the sidewall bulge bit that you disagreed with, I was trying to find reasons why one style might perform better than another. If it's bull***t then it's bull***t.

Dale.

good reading guys.

pretty good information there