How does this work

I'l take my chances with the petite little chick thanks Roachie!! LOL!!!

Roachie, you'd prefer the petite little chick anyday. especially if she was wearing a short skirt.

Only a banker could remember the size of a one cent piece!

Pete

Reduce the tyre pressure and the footprint is wider and has more of a wave of sand to pull over

Bonz

Reduce the tyre pressure and the footprint is longer (you can not make a tyre wider)

Richard

Thanx Richard from what I have observed the lower pressure tyre is both wider (maybe 10% and heaps longer (up to 30%) I know what u mean

Sorry Bonz, but I think your "10%" increase in width is being VERY generous, whereas the 30% increase in length is nearer to the mark. The tyre might "appear" to be somewhat wider, but the vast majority of that apparent increase in width comes from the bulge in the sidewall......only a tiny bit of it (like 2% tops) is transferred to the ground as increased width.
Just my opinion.......

When I was on Fraser I did this measurement of width, measured the tyre track at 35 PSI then at 15 PSI it was noticeably wider and measure around 10-15% wider I havent got the actual measurements, also measured the depth of the track and that was shallower by about 30% too as I recall.

Bonzarelli

Admit it, you were observing and measuring something else entirely. (a leetle thing maybe)

In a past 4WD monthly this subject was done to death and to prove its length not width that increases they put a 4WD on glass stands/floor.

Photographed , measured and marked the tread footprint.

Lowered the pressure to 18psi or thereabouts and applied the aformentioned procedure.

Result: tread footprint lengthened. No increase in width except for a visual ballooning of the sidewall.

Glass is flat and hard as steel. Does any one recommend letting tyres down to drive on steel plate?

Sand is 'fluid' and most tyre footprints in sand are a good 100mm deep. I dry sand it can go deeper than that.

Did they try various aluminium dinghey's on the same rig to show how they sit in the water? That is a flawed test.

I think it's a bit of both but clearly in favour of the length. Bagging out is bagging out IMO.

V8Diesel

Your knowledge, experience and in depth appreciation of this subject is obviously second to none.

However in the interest of clarity and non obfuscated comment , you really must address your replies/thoughts on this subject to the author of the article in the subject 4WD monthly, who writes under the Pseudonym of - none.

Interesting theory..... the length of the footprint is definitely the 'driving' factor but I think Bonz is on to something; in soft sand the bulge in the sidewall will make contact with the sand due to the 'v' shape of tyre track in the sand (as opposed to the glass experiment). Whether that extra contact is pushing through soft sand or aiding in flotation is another matter ;-)))

I

Mr Bware

I feel that in the interest of fairness I must bring to your attention that the only thing this BONZ character is on - is a 'higher plane of awreness'

He is in that/this state due to the fact he is currently tooling around in TD 100 series.

The effect of same is like your archtypical English gentlemen taking delivery of a branch new Roller or Bently.

They and he just drool, stare and babble like idiots.

Whilst he is in this besotted state, take whatever he says with the proverbial grain of salt.

There will be may pics of this state of affairs in weeks to come after the OBC 07 where he will displaying his and the 100series prowess in the outback, where it is tough, rough and mighty unclean, but not for a 100 series mean machine.

Lucy, for the life of me I can't understand this 'footprint length' argument only. We all know the length of footprint is by far the main factor (driving on 12 ply crossplies in that gutless desert sand tells me this) as I said, it but to say the bagging out of wide soft sidewalled tubeless radials in sand does NOTHING AT ALL because you've seen it on a sheet of glass just doesn't add up to me. Just having the 'U' shape must help. They also help when turning to run up and over the wall of sand as well.

The tracks look different for starters. That would tell me something. The tread width will remain the same when deflated (derrr) but the sidewalls most certainly DO get wider and DO come into contact with the sand. To say they don't do anything at all because you've seen them on a sheet of glass is a bit silly IMHO. Ever seen a camel foot (not toe;-)) It's a good visual aid but not in a 'real world' context. I reckon you want to 'float' over the top, not eat into it.

Here's the litmus test. I'll let my tyres down to 15psi and then run over your clenched fist with the sidewall and see if it does 'nothing at all'. I'm off 4x4'ing for two days next weekend along the WA coast and will be running my near bald road pattern fatties at low pressure for the trip because they seem to work well.

Anyway, I guess if it was in a 4x4 mag it must be the gospel so let's just agree to disagree. I'll make my own mind up. Either way it works, so in the long run who cares. More important things to worry about.

V8

Its like this.

When you air a tyre up or down the Dimensions do not change and by that I mean a 31in tyre does not suddenly become a 28in tyre or a 35in tyre just because I change the presures.

And this is more evident in the with - a 10in width tyre from tread block edge to tread block edge always remains 10in. Changing the air up or down will never ever make it a 12in width tyre.

(now having said all of that, there is a fraction of a change in Dia due to a bet of stretch but nothing to get excited about.

Sooooooooooooooooo! the only thing that changes is SHAPE.

This is the key to it all.

By lowering the presure the 'footprint lengthens' The width does not alter one iota.

Raise the presure and you can change the footprint to a 'ball bearing' size

Footprint = the tread area that contacts the earths surface be whatever it maybe.

Now for the side walls.

YES! these suckers do ballon out when the presure is lowered an straighten up when the presure is raised.

However on sand that is hot, dry and powdery the side wall shape does 3 fourths of nothing except push the sand out of the way.

If the sand is wet and stable like some beaches or the Simpson then you don't have to lower the presure in the first place (Most times - or just a bit)

The only time that the side walls come into play is in really deep mud and thats only for front tyres where you can turn them L/R. L/R to try ( Its esoteric as to whether it works or not) and increase the traction.

So there you have it - shape is the answer not dimesion change.

Hope this gives some clarity on the subject.

Yes it would be like letting the air out of your inflatable boat to make it float better

;-)

V8

I think I may have a better way of explaining the theory of weight displacement via the wheels.

Imagine an army personnel carrier weighing 8-10tonnes on four wheels and tyres at 80psi attempting to drive on a beach, hot dry bottomless sane etc etc.

The sucker is going to sink to its guts.

However - replace those tyres with tracks and that mother just keeps laying its own track.

Back to your 4WD . Lower the presure and you lengthen the foot print anin so doing create a minutature 'track' (actually four of them).

So the lower the presure the longer the 'tracks' and the vehicle just creates 'own base' to roll along on.

There! piece of p i s s really.

Lucy, I am fully au fait with the longer footprint and have no argument with it as you will see if you actually read what I have WRITTEN, not what you think I am saying. I fully understand the logic and principal of it, have no argument with it and have also acknowledged that the tread is a fixed width. I do get it, you are preaching to the converted.

BUT.......

I do not agree with the assertion that the bulging sidewall has no impact on floatation or general 'manners' in soft sand. As I said before, it would only stand to reason the 'U' shape would have to assist the tyre in not 'biting in' as you yourself said by "pushing the sand out of the way". A foot long tread with sharp vertical sides clearly would not be as desirable as a broad rounded bulging sidewall to displace the sand. Floatation is what makes it happen.

Richard, how is the boat relevant to this discussion apart from the fact we are talking about air?

Once again, I urge you all to go back and re-read what I've actually written, not what you think I am saying.

Anyway, as I mentioned before, who cares? - it works and that's all that matters.

V8

Up until about 6 years ago I always thought it was the 'ballooning' of the side wall that was the cause and effect so to speak secret of sand driving.

However, those thoughts of mine were corrected as the years went on during which I became involved with the extreme 4WD Outback Challenge, ATECO competitions were a number of the competitors are tyre importers in their everyday jobs.

By asking them a zillion questions about tyres, pressures, bead locking, puncture fixing, rim straightening ets etc I now believe it is the tread lengthening that is the said secret.

By looking at the ballooning of the tyres I to would have thought there was some 'gain' there.

Maybe there is and maybe there isn't , I just think 'length not width' and everything is just fine and dandy.

I must say however that no one has every impressed me and taught me at the same time something I would never have believed, unless I saw it and that was:

Re seating the beads with a tad of shellite inside the tyre, trail up over tyre and 3 metres away, valve out, light shellite trail with a match, duck down, BOOM and fully reseated tyre on the rim.

Very impressive indeedy - if not with a degree of danger.

May be read a couple of Len's early book pre 60's he tryed wide tyres and went back to narrow

Lucy, I repeat - I am fully conversant with the lengthening of the tread and concur. At no stage have I disagreed that this is not the main factor at play. All along my point has been the ballooning of a sidewall in a wide radial must have a beneficial effect and a sheet of glass is clearly not a fair representation of sand.

Richard, I am probably the most ardent and vocal advocate of split rims and crossply tyres for true 'off road' aka bush bashing on this forum. I am aware of Len's work and I worked professionally doing much the same stuff myself ie: NO tracks whatsoever and taking compass readings and bashing through the bush in the remote and often uninhabited regions of eastern WA. This was my full time daily occupation. Len used DC3 wheels from an old WW2 Dakota aeroplane from memory, not a modern day wide radial with a non agressive tread pattern.

I have driven many off road km's in desert sand and agree that skinny crossplies are fine for the job as I mentioned in my earlier post. This is how I knew the 'secret' of footprint. I have also made many posts advising against using big agressive mud type tyres for desert work.

Anyway I think we got our wires crossed a little here. I was making a point and you blokes were arguing a different one. I still invite any one to let me run over their fist with a sidewall and tell me it does nothing.

All good, have fun!

sand doesnt really compact anyway this is why they use it as bedding for pavers. Imagine using clay/dirt they would soon sink etc all over the place

Robin,

"We would all agree that narrow tyres (of same diameter/pressure)
sink further into the ground"

This has been debated ad nauseam on every 4wd forum known. From what I have gathered and experienced ( to a small extent ) is it's more a matter of the LENGTH of the footprint NOT the width. The longer the footprint, created by lowering the tyre pressure, the better the going in sand. Width is virtually immaterial.

Now I'll duck for cover as the 'fat tyre brigade' launch another jihad against we skinny tyre users :)))))))

Hi Al

Yes we agree - thats why I included (of same diameter/pressure) in my reply
to prevent those questions and keep answer simple.

Carrying my analogy further a larger diameter(or lower pressure) and consequent footprint increase makes angle of approach to the "step" shallower as you don't sink down , and hence easier also.

Robin Miller

I understand the above comments, this would definitely work on a tyre being driven,
But a trailer is a dead weight, I just thought less tyre tread less friction.

I remember a few years ago trying to move one of my trailers in the front yard, it's tyres were deflated somewhat, because of lack of use over time, trying to move it was hard, after giving up, I pumped up the tyres and moving it around was significantly easier.

Another theory I have is would it be harder to pull a fridge on a trolley with the tyre pressure up or down?

I await your thoughts.

Cheers Steve.

Robin,

No worries, understand what you're getting at.

Steve,

My thoughts would be that the trailer, without deflating the tyres, would still be 'digging in' no matter fat/skinny tyres. Dragging would be no different to driving.

I'm with you Gramps on the skinny ones.
One could argue that the wider the trailer tyre, the more lateral resistance.

cheers
T.Ribs

Whether the tyre is driven or not makes no difference.

On a hard surface less contact area means less friction no question, but on sand the physics are totally changed, it's more like a boat where the further you can get the boat out of the water the less drag. In sand the more contact area the less sinking, hence less drag.

It's just like walking in sand, get yourself some big old school surfer joe thongs and you walk right across the top but barefoot your sinking in and working twice as hard.

Why not do the same experiment with your trailer on sand, or maybe with a bicycle wheel to make it easier.

So do you think this is relevant to tyre size.

EG: If you have a 750 /16 on a split rim, you would lower the pressure to say 18 psi to give you a wider foot print.

Hypothetically this then gives you a foot print the same size as my 265/75/ 16s do I now drop my tyre pressure or are they the same.

I'm a think outside the Square type of bloke.

Cheers Steve.

Steve,

You've missed the point. It's NOT the width of the footprint that counts, it's the LENGTH.

Well this is when things go to the another level of complication. My best guess is a reduced pressure skinny would have less drag than a high pressure fat with the same contact area. My reasoning is reduced pressures also make your tyre more flexible and more able to make a flatter contact area on the bottom. This means you are reducing the effect of trying to drive over the little hill of sand formed in front of the tyre. Say the tyre is at extremely high pressure and doesn’t deform at all eg is circular at the bottom, then you are constantly driving up more of a hill.

The other important thing is when reducing pressures you are actually increasing the length of the footprint more than the width. It’s an interesting experiment to measure the length of you footprint at different pressures by sliding a sheet of paper or card front and back of you tyre and measuring the distance between. You can actually almost double the length of the footprint going from say 40psi down to 12.

Would be an interesting experiment to measure the force required to pull a trailer with different size tyres and pressures. It's not as straight forward as it first appears.

Sorry Gramps

I missed your first post, All is understood and making perfect sense.

Cheers Steve.

Steve,

No worries, I had a devil of a time coming to grips with it while watching the replies to a similar post a while ago. It's one of those classics on this forum like Toyota v Nissan, Coopers v BFG, etc etc etc :))))

A tyre acting on sand is very similar to a vessel on water, i.e. they have a flotations based on the WEIGHT they are carrying, so that weight displaces an equal amount of sand so you either have a wide low lip of sand to push thru or a higher slimmer lip of sand to push thru. When Dr Karl did his famous corrugation experiments he deduced that the effort to overcome the wide and low was equal to the thin and higher lip, all things being equal. QED

Gramps

You are the man.

Terang Teror

You are a dud

Hahahahaha give him heaps Lucy :))))

READ THE POST AGAIN hehehehe I KNEW the fishing would be good on this one....... and it caught the biggest trout, rofl the Lucy BigMouth Trout hehehehe

I said.......When Dr Karl did his famous corrugation experiments he deduced that the effort to overcome the wide and low was equal to the thin and higher lip, all things being equal. QED

.....was equal to.......all things being equal. it is not possible for this comment to be wrong

Gramps

I know I shouldn't talk about this, but it really is obvious to one and all that BONZ went forth and multiplied last night.

Half his luck!

LOL frolicking was he hahahahahaha please, no more. The disturbing mental images could consign one to the home for the bewildered :)))))

Lucy I dont want to een BEGIN to understand how you KNEW THAT

Hi All

It also depends on how the trailer tracks behind the towing vehicle.

Cheers

Daza

ha ha

all general statements are false

When they got stuck, which they inevitably did, they didn't use a 4Wd to pull them out, they used a 4LD

That's a 4 Legged Drive - a horse, or two big blokes :)

Hey Des, No doubt you can remember the days of the T model really well.....YOU WERE THERE .......AND you had a drivers licence.....hahahaha. Exactly how many christmases have you seen mate????? hahahaha

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