Its those wide vs skinny tyres again

Submitted: Monday, Feb 06, 2006 at 11:58
ThreadID: 30475 Views:6790 Replies:22 FollowUps:40
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Okay a lot are probably saying give it a rest, tough, I thought I would provide some proof/calculations to support the wide tyre case for consideration.

Following the debate and the enormous number of follow ups and responses to post number 30344 from Troll81 on Thursday, I had a very lazy day on Friday (at work) and thought I would do some calculations on this issue.

Some basic parameters were used
A single wheel with 700mm diameter (16” rim with tyre) equates to circumference of 2200mm.
Same diameter in both cases.
Load on single tyre (vehicle divided by 4) - 500kg.
Assume sand can support load of 0.01kg/square mm.

Skinny tyre – 175mm across width of tyre
Wide Tyre – 235mm across width of tyre
Assume no bulging of side wall for both tyres (stiff side walls)
Both tyres have same pressure of 25psi (172kPa).

As a starting point lets have 1/16 of the circumference as the contact area with the sand (will probably not happen in real life) of 2200/16 = 138mm (rounded off).
Wide 235mm tyre has contact area of 235 x 138 = 32,430 square mm
Skinny 175mm tyre has 175 x 138 = 24,150square mm contact area.

Load per square mm
Wide at 500/32,430 = 0.015kg/sqmm
Skinny at 500/21,150 = 0.021kg/sqmm.
Sand can only support 0.01kg/sqmm so both will tend to sink into the sand.

How much each will sink into the sand?
For the load of 500kg and the sand able to support 0.01kg/sqmm the area at which the sand theoretically will support the load with no sinking would be 500/0.01 = 50,000sqmm.

Wide tyre at 235mm wide to have a contact area of 50,000 sqmm it would need to have (235 x 2200/50000)th of its circumference in contact with the sand, ie one 10.34th of the circumference which equates to 2200/10.34 = 213mm.
Skinny tyre the same 50,000 sqmm contact area needs to have (175 x 2200/50000)th of its circumference in contact with the sand ie one 7.7th of its circumference which equates to 2200/7.7 = 286mm.

Using some fancy trigonometry I calculate
Wide tyre would sink some 32mm into the sand
Skinny tyre will sink some 57mm into the sand.

Again allowing for the unrealistic sand that stays level around this sinking tyre the angle from the bottom of this perfectly round tyre to the edge of the sand at the tyre
Wide tyre has an angle of some 17 degrees
Skinny tyre has an angle of approx 23 degrees.

So for the wide 235mm tyre the bank of sand in front of the tyre that has sunk some 32mm into the sand would be 235 x 32 = 7,520 square mm whilst the skinny tyre at 175 mm and in at 57mm in the sand would have 175 x 57 = 9,975 square mm of sand in front of the tyre.

For the supporters of skinny over wide how can a tyre of 175mm width create less of a wave of sand in front of the tyre at the same pressure as a 235mm wide tyre on the same sand and same load when it sinks further into the sand? Which slope is easier it drive up from a standing start, 17 degrees or 23 degrees?

Okay we let down the tyre to increase the area of the tyre in contact with the sand. But in order to increase the area of the skinny tyre to match that of the wide tyre we would need to let the pressure down more to achieve a similar contact area, but at what point does the contact area not increase, buggered if I know. I think I would rather drive on sand with a wide tyre at 16-18psi than a skinny tyre at 10-12psi and have the tyre still on the rims.

In this example for the skinny tyre to match the required support area it needs to increase its contact area by over 200% whilst the wide tyre only needs to increase its contact area by some 154%, and the volume of air contained in a skinny tyre is a lot leas than for a wide tyre so where does the extra volume come from to deflate the skinny tyre to a greater amount than a wide tyre, maybe the spare tyre under the car?

So to tim_s, Shawn, Bonz, the explorer, and robak think about that whilst you try to drive those skinny tyres (less than 200mm wide tread) across the soft sand).

By the way I spent the weekend driving around the sand dunes at Beachport and Canunda with my wider tyres fitted to a Courier dual cab turbo deisel and had no issues of bogging in soft sand.

Cheers and happry driving
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Reply By: Utemad - Monday, Feb 06, 2006 at 12:19

Monday, Feb 06, 2006 at 12:19
Well I spent the weekend at Double Island Point with friends.

My Rodeo with 31in Goodyear MT/Rs
His Hilux with 205 16 Grandtreks
Both at 20psi all round

Neither of us had any trouble. He had never driven sand before and I have driven lots of sand.

= we had a great time.

However when I had the same size tyres as him I remember it being more difficult. I can't get bogged even when I try with these tyres. However I went taller and wider at the same time as well as a more aggressive tread which I would think is a negative.

As for what is wide and what is skinny, I couldn't imagine greater than 200mm being considered wide. As if a standard Jap ute comes with 205mm cross section tyres and standard bigger vehicles with 235ish wide tyres then all 4x4s must come with wide tyres. Why not draw the line at 250mm or so. This would class my 10.5in (267mm) wide tyre as wide but not a 205 16.
AnswerID: 153311

Follow Up By: Rotty - Monday, Feb 06, 2006 at 12:35

Monday, Feb 06, 2006 at 12:35
I picked the 175mm and 235mm just for convenience, the 175 more the split rim type/standard size for many utes etc and the 235 as the wider steel rims that are more aftermarket.

The issue of what is skinny and what is wide always appears to never have been defined in reponses so I just selected these as a base. The other point was the diameter, which it is not always possible to change for 4x4's unless you put low profile tyres on but that would be a waste as you then reduce clearance underneath.

Cheers
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Follow Up By: Utemad - Monday, Feb 06, 2006 at 13:32

Monday, Feb 06, 2006 at 13:32
Which 4x4s come with 175mm wide tyres? If a Rodeo/Hilux base model is 205mm?

It is easy to fit larger diameter tyres. Just not always legal. It is sometimes easier to fit taller tyres than it is wider. Landrover Discoverys for example.

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Reply By: Member - Davoe (Widgiemooltha) - Monday, Feb 06, 2006 at 12:21

Monday, Feb 06, 2006 at 12:21
Hmm you have lost me a bit but to throw a bit of a spanner in. The 175 type tyres are usually a lower profile than a 235 and 285 tyres are often a higher profile again dpending on type etc. IMO the profile is the most important aspect as a higher profile will allow greater elongation of the tread while still having a buffer between the ground and the rim.
Still sounds like it needs some real world testing
AnswerID: 153312

Follow Up By: Rotty - Monday, Feb 06, 2006 at 12:38

Monday, Feb 06, 2006 at 12:38
The measurements were relating to what the wide of the tyre was placing on the sand, the manufacturers numbering system does make a great deal of sense to me (175/235 etc) other than I beleive this is the width of the rim or tyre.

It still comes down to the question of how low can you let a tyre down to to achieve the same area on the ground when using a skinnier tyre to match the wider tyre.
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Follow Up By: Utemad - Monday, Feb 06, 2006 at 13:39

Monday, Feb 06, 2006 at 13:39
I found that 20psi worked wonders in my 225/75 16 on my Rodeo while 20psi also works on my 31in tyres. By your theory I shouldn't need to let my 31s down as much as my 225s but 25psi in my 31s makes for hard going. So I need to go down to the same 20psi.

I witnessed a Pajero trying to snatch a bogged Pajero at Double Island this weekend. The rescue Pajero just spun his wheels and couldn't get the guy to move. However along came an 80 series and this guy retrieved the stuck Pajero without any problems.
As far as I could see the main difference was the rescue Pajero had 32psi in his 265/70 16 and the 80 series 20psi in his split rimmed 7.50x16.

As sometimes happens the theory doesn't match the practice due to the other 2452456365 variables.
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Follow Up By: Rotty - Monday, Feb 06, 2006 at 13:50

Monday, Feb 06, 2006 at 13:50
The biggest variable may have been the driver in knowing what his/her vehicle can do and how to do the recovery successfully, that is the wonder of the real world.

Over the christmas break our group do a bit of sand driving in Canunda/Beachport and we had a sand hill climb event for all members. We had a mix of deisel and petrol motors and different vehicles, some deisel drivers could hardly get 1/3 up the hill but others got over the climb, why, because they knew their vehicles capabilities and the correct gear/speed to tavckle the job, same for vehicle recovery, the 2nd bloke who came along could probably jumped into the Paj and performed a successful recovery thet tha first guy could not, why, experience and practice.

Hey its all theory anyway.
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Follow Up By: Utemad - Monday, Feb 06, 2006 at 14:55

Monday, Feb 06, 2006 at 14:55
Yes driver ability is one of the 2452456365 other variables. Considering he was the only one that got stuck and others were driving around him. I bet he enjoyed watching everyone drive around him :-)

I particularly enjoyed watching all these big cruisers and patrols revving their guts out and bogging down trying to get to where others had idled to on the Rainbow Beach side. It didn't appear that they had let their tyres down at all.
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Follow Up By: rocketsalad - Monday, Feb 06, 2006 at 15:26

Monday, Feb 06, 2006 at 15:26
Hey Utemad, we were there on friday and the cutting to the rainbow beach side was the worst ive seen in a long time. We spent most the day there on the beach pretending we knew how to fish. lol

I didnt have any problems with 265 / 70 / 16's wides at 40 psi (cause i was too lasy to let them down.) But I seen others stumble a bit there.

rs
terracan diver
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Follow Up By: Utemad - Monday, Feb 06, 2006 at 15:46

Monday, Feb 06, 2006 at 15:46
I went through that cutting on Saturday morning and it was not as dry/difficult as it has been previously. I thought it looked like it had rained recently as it appeared damp underneath although it was very dry on top. Perhaps it rained up there on Friday night? I was camped at Little Freshwater behind the dunes but it didn't rain there. Although we came back to wet tents on Saturday afternoon having been at Rainbow where it didn't rain. Go figure.

We didn't have any trouble either.

I use Staun deflators as I am lazy too.
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Follow Up By: The Explorer - Monday, Feb 06, 2006 at 19:57

Monday, Feb 06, 2006 at 19:57
"It still comes down to the question of how low can you let a tyre down to to achieve the same area on the ground when using a skinnier tyre to match the wider tyre. "

No it doesnt - as has been pointed out many times it is the LENGHT of the footprint that is paramount - Any 4WD "Skinny" tyre exceeds required 'threashold" width easily and that is why they are at least as good as wide tyres on sand. There is nothing to be gained going wider..the required width has already been reached with a "skinny" tyre. I'll say it again "skinny" tyres work fine on sand - let them down (like you have to the fat tyres) and all will be sweet.

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Reply By: Eric from Cape York Connections - Monday, Feb 06, 2006 at 12:37

Monday, Feb 06, 2006 at 12:37
You would be fun at a BBQ LOL

All the best
Eric
AnswerID: 153315

Follow Up By: Rotty - Monday, Feb 06, 2006 at 12:42

Monday, Feb 06, 2006 at 12:42
Usually try to keep the mouth shut and observe however this issue of using skinnier tyres on sand seemed crazy when I have tried both and found that the going was easier with a wider aftermarket tyre/rim combination than what the ute came with as standard.

The issue of defining wide and skinny never was given by others so I just used a couple of figures.
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Reply By: robak (QLD) - Monday, Feb 06, 2006 at 12:43

Monday, Feb 06, 2006 at 12:43
Rotty,

You're assuming that that 1/16 of the tyre is in contact withthe ground
Why?

You're still pushing the same arguement and I can see where you're coming from.

You're saying that front to back the same length of tyre is in contact with the ground. (for the same diameter and PSI) And the width of the tyre gives you the different footprint.

As said before, that would only be true for rigid tyres. That is, something like solid disks and as pointed out by camptain, standard tyres will do this to some degree once over 30 psi.

I am prepared to be wrong as thats the only way to lean, but to prove your point you need to explain your main assumption better.

I still think if the air inside the tyre is pushing it at 20 PSI the the pressure from the road onto the tyre must also be 20 psi. The pressures on the inside and outside MUST be equal.

R.
AnswerID: 153317

Follow Up By: Rotty - Monday, Feb 06, 2006 at 12:56

Monday, Feb 06, 2006 at 12:56
The 1/16 in contact with the ground is as you say an assumption, but in order to do some calculation you must start with a fixed point and in this case it was an arbitary value to calculate the load on a certain area with the sand being able to support a certain load. The actual portion of the diameter in contact could be 1/20 or whatever.
With the tyre at a certain pressure yes the value of the internal pressure is what the tyre walls and tread can resist, but the load of the vehicle in contact with the ground determines how much the tyre distorts not how much pressure is being placed on the ground. Yes the pressure on the ground may be at 20psi but to to carry the load the area increases to accomodate the actual load hence the distortion/bulging of the tyre.

As also stated have not allowed for bulging as this becomes very complex in actual contact areas etc so just working with a stiff side wall (does not happen in reality).
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FollowupID: 407244

Follow Up By: robak (QLD) - Monday, Feb 06, 2006 at 13:02

Monday, Feb 06, 2006 at 13:02
Perhaps I should rephrase that

You assumed that BOTH tyres have the 1/16 contact BECAUSE they have the same diameter. That's the assumption I think is wrong. That would be true for rigid tyres like on a skateboard no matter how wide they were.
R.
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Follow Up By: robak (QLD) - Monday, Feb 06, 2006 at 13:11

Monday, Feb 06, 2006 at 13:11
"Yes the pressure on the ground may be at 20psi but to to carry the load the area increases to accomodate the actual load hence the distortion/bulging of the tyre"

And thats exactly my point.

The pressure on the tyre must be 20 psi.

At points on the tyre where it is not in contact with ground the structure of the tyre creates that pressure on the air inside. Like in a balloon.
Where the tyre youches the gound, the ground "pushes" the tyre in, and creats a flat spot. The size of this flat spot has to be such that it is at 20 psi. So if your car wighs more the flat spot will be bigger.

R.
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Reply By: Snowy 3.0iTD - Monday, Feb 06, 2006 at 13:19

Monday, Feb 06, 2006 at 13:19
Rotty

I commend you on this as you have obviously given it a lot of thought, however one of your assumptions is incorrect; "As a starting point lets have 1/16th of the circumfrence as the contact area with the sand", due to Newtons thrid law "For every action there is a equal and opposite reaction".

If we have identical vehicles, running identical pressures; lets say vehicle A has wide tyres and vehicle B has skinny tyres. If A and B are identical in mass then the force they exert on each wheel is the same. Then:

Pressure (P) = Force (F)/Area (A)

Rearranging

A = F/P

We have already stated that F (vehicle mass in kg x g [9.81m/s2]) is the same, and that they are running identical tyre pressures (P) then the contact area (A) has to be the same. The shape of the contact area will vary from vehicle A to B, but the size of this area will not. The force that pushes on the inside of the tyre (tyre pressure) has to be the same as the force pushing on the outside of the tyre (contact pressure) otherwise it is defying the laws of physics. SO IN A STATIC (NON-MOVING) SITUATION THERE IS NO ADVANTAGE OF WIDE TYRES OVER SKINNY TYRES OR VICE VERSA FROM THE VIEWPOINT OF CONTACT AREA/PRESSURE. The tyre pushes down with 25psi, the ground resist with 25psi of force.

To then begin looking at the sand wave a tyre creates when its moving, how it flexes, etc, from a theoretical viewpoint, we are now talking about complex dynamics, even more complex mathematics and some rather powerful modelling software. The practical experience of a controlled experiment would be much better in this situation.

Regards

Snowy
AnswerID: 153328

Follow Up By: Rotty - Monday, Feb 06, 2006 at 14:01

Monday, Feb 06, 2006 at 14:01
I agree that the contact area will be the same, but for a narrower tyre to have the same contact area as a wider tyre the only other variable is in the amount of the circumference of the tyre is in contact with the sand and it was sand that was the point of the original discussion.

If you note in my post that I stated

"Wide tyre at 235mm wide to have a contact area of 50,000 sqmm it would need to have (235 x 2200/50000)th of its circumference in contact with the sand, ie one 10.34th of the circumference which equates to 2200/10.34 = 213mm.
Skinny tyre the same 50,000 sqmm contact area needs to have (175 x 2200/50000)th of its circumference in contact with the sand ie one 7.7th of its circumference which equates to 2200/7.7 = 286mm."

So when the tyre is deeper into the sand is not then harder to drive off?

The whole point is that at some point if you will have to stop on sand/beach then the skinnier tyred vehicles will sink lower into the sand and therefore would be harder to extract on moving off than would occur with a wider tyred vehicle (always going to reach a point where does not matter how wide you go no difference occurs, but do not know at what point that is).
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Reply By: desert - Monday, Feb 06, 2006 at 14:01

Monday, Feb 06, 2006 at 14:01
I agree with your calculations,Rotty for the dimensions given. The only thing I can add is this point: What if the coresponding increase in footprint due to pressure is not equal in each tyre? ie, what if the footprint of the skinny tyre LENGTHENS to a greater degree than the wide tyre increasing sideways for the same drop in pressure?
AnswerID: 153338

Follow Up By: Rotty - Monday, Feb 06, 2006 at 14:09

Monday, Feb 06, 2006 at 14:09
In all the theory I have not made any case for the width of the wider tyre increasing in width.
As with any tyre, the tyre contact length can only lengthen itself up to a certain point and no matter how much more it gets defaleted cannot make a longer footprint (unless the tread peels off totally and lays out along the track) so for both tyres being fully deflates or even partially deflated to the same pressure the wider tyre must always create a larger contact area than the skinny tyre.
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Follow Up By: desert - Monday, Feb 06, 2006 at 17:14

Monday, Feb 06, 2006 at 17:14
I'll put it another way: Assuming a wide profile tyre adds say 5mm to it's width (in bagging), a skinny may,may add 10mm to the length of the footprint, without much apparent change to it's width (bagging). So until the surface area is compared........ This will explain why some pundants stick to their theory that the wider section creates a wider wall to climb over, whereas a longer footprint will not alter the wall in front.
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Reply By: flappa - Monday, Feb 06, 2006 at 14:24

Monday, Feb 06, 2006 at 14:24
Geez , I love theory. Theory is excellent.

How about real world ?

Case: Zuk 800cc with skinny tyres. Unstopable on the beach.

Zuk 800cc with sunrayser steels and wider tyres . . . couldn't get it ONTO the beach.

Result: Chucked out bigger wheels (actually sold them to a mate with the larger 1.3l Zuk).

Reason: The build up in front of the tyres was less with the skinny tyres then the wider tyres. The lack of power in the Zuk wouldn't allow the Zuk to push away the sand.

So , which is better . . .

DEPENDS.

The Majority of modern vehicles have more then enough power to overcome the sand in front of the vehicle so fit wider tyres because they are better onroad , and in some other cases better offroad. Not many people have Sand tyres, Onroad Tyres , Mud Tyres, Rock tyres etc etc.

I found HT tyres were better in sand then AT's or MT's.

I love theory . . . getting out and trying it in the REAL world is far more fun though.
AnswerID: 153342

Follow Up By: Rotty - Monday, Feb 06, 2006 at 14:41

Monday, Feb 06, 2006 at 14:41
Flappa
The weight of the zooks is such that they tend to almost float on the surface like a butterfly anyway, adding wider tyres has probably added additional weight (more than the zook weighs originally).
Lighter vehicles tend to do better than heavy ones (except possibly the Niva, which is probably worse than a lawnmowerin sand).

Agree enjoying the real off road driving is better than reading about it.

As you say you found HT were better for you than AT, to each their own and depends on vehicle setup weight, driver experience and the other couple hundred variables (includes the handbrake that says cannot afford it because I want money for clothes etc).
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Follow Up By: Bonz (Vic) - Monday, Feb 06, 2006 at 18:49

Monday, Feb 06, 2006 at 18:49
extra weight hahahaha what a load of poppycock, the additional weight of the tyres/wheel combo over original would only be incremental. By your reckoning if it had wider tyres it would float easier.... hahahah I continue to read with interest, but this reply just got me writing
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Follow Up By: Rotty - Wednesday, Feb 08, 2006 at 08:52

Wednesday, Feb 08, 2006 at 08:52
Bonz

Maybe I needed to put some smileys at the end to indicate a funny statement about the weight of the wide wheels on a zook being more than the original weight of the zook. -)))

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Reply By: OLDMAGPIE - Monday, Feb 06, 2006 at 14:43

Monday, Feb 06, 2006 at 14:43
sounds like a lot of hot air to me cheers
AnswerID: 153350

Follow Up By: Rotty - Monday, Feb 06, 2006 at 14:50

Monday, Feb 06, 2006 at 14:50
Oldmagpie
Don't complicate matters. Adding hot air, which rises, should create less down force so if we heat up the tyres then they won't need to be let down as they make the car rise
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FollowupID: 407272

Follow Up By: OLDMAGPIE - Monday, Feb 06, 2006 at 15:25

Monday, Feb 06, 2006 at 15:25
that probably explains when i drop one, everyone rises & leaves leaving me very down, not needing much effort to force the issue.like your theory though. cheers
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Reply By: banjodog - Monday, Feb 06, 2006 at 14:48

Monday, Feb 06, 2006 at 14:48
More real world stuff (albeit many years ago) - Lada Niva was one of a handful of 4x4 that wore 175 x 16 tyres. I worked with a fellow years ago that owned a new 1994 model and he and two of his mates went to the Tip of Oz in ‘95.

Both his mates, one had a ‘Cruiser 62 series and the other a Nissan GQ, laughed at his standard tyres and said no way will you make it north of Cairns – both chipped in and bought him a new snatch strap for the trip and told him to keep it on the front seat.

Most of the time he spent up the tracks waiting for the 62 and the GQ to catch up, as both were either bogged or had a puncture – both vehicles had 31 x 10.5 x 15. Claimed he never made so many cups of tea before – had to do something to fill in the time waiting.

Not only was the Niva more economical but could go further without assistance and had no tyre issues the whole trip. I believe both his mates soon changed back to the standard splits with 7.50 x 16 once back home.

The snatch strap did get used but not by the Niva.

AnswerID: 153351

Follow Up By: Utemad - Monday, Feb 06, 2006 at 15:01

Monday, Feb 06, 2006 at 15:01
They probably had cheap and nasty 31in tyres. There must be a big difference in tyre construction when an MT/R costs $240 odd dollars and a Kmart special can cost around $140. Plus the narrower track of the Niva would have kept it away from the stakes on the side of the track.

More of the other 2452456365 variables :-)

I always save my smart arse comments about other vehicles until we get home safely or they are actually having trouble. Can save face that way.
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Reply By: chump_boy - Monday, Feb 06, 2006 at 14:57

Monday, Feb 06, 2006 at 14:57
I can't help myself - I just have to wade into this one.

I think everyone is only looking at half the equation here.....

There are 2 sides. One is Newtons Law. The other is tyre stiffness.

As pressure increases, Newtons Law comes more into play.

As pressure decreases, Tyre design and Stiffness comes more into play.

When I had a sierra, i had pretty big tyres on it. I had a flat one day, but the tyre were strong enough to hold the car up, with no pressure....

I think the equation you are all looking for goes something like:

CONTACT AREA = FORCE divided by ( PRESSURE + STIFFNESS FACTOR)

The stiffness factor is determined through experimentation at very low pressures. It is set up so that at zero pressure, and a given force, area would be a maximum. As pressure increases, it becomes less of a factor. Also, a higher stiffness factor would equal a lesser contact area.

This stiffness factor would also help show how much of an angle the front edge of a tyre would be at, as a higher stiffness factor would mean a higher circular radius, therefore less of a percieved angle of approach. The equation would go along the lines of:

ANGLE OF ATTACK is proportional to 1 over STIFFNESS FACTOR.

And finally, an equation needs to be developed that links the two, such that:

TYRE ABILITY = CONTACT AREA + MATERIAL PROPERTY FACTOR times ANGLE OF ATTACK times WIDTH OF TYRE.

Material Property Factor would depend on the material. Obviously concrete would have a value of zero, but sand would have a large value, determined through experimentation. The softer the material being driven on, the higher the value.

This doesn't take into account things like friction or available torque, and would only work for flat surfaces like roads, sand and flat mud. Things like rock climbing ability could be determined using the stiffness factor, but the equations would be slightly different.

LOL - I can be such an engineer somedays......

Chump
AnswerID: 153353

Follow Up By: Rotty - Monday, Feb 06, 2006 at 15:01

Monday, Feb 06, 2006 at 15:01
What is that stuff you are drinking to come up with that? It almost makes sense.

We are all getting way too serious!!!!!!!!!!!!!
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FollowupID: 407280

Reply By: Member - Phil G (SA) - Monday, Feb 06, 2006 at 14:59

Monday, Feb 06, 2006 at 14:59
Thanks for the numbers Rotty!

Can you redo them all. This time:
#1 Use a 195 or 235 wide tyre as the skinnies, as these are the widths of the 750R16 and 235/85R16 tyres commonly used on base model Cruisers and Patrols.
#2 Use the 265 or 285 wide tyres as the fatties.
#3 Factor in the tyre's profile. The skinny 750R16 have a profile of 100% while the 265/70 obviously have a 70% profile. (A high profile tyre has the ability to get a longer footprint when aired down)
#4 Factor in tyre diameter - as increasing the diameter is one of the most effective ways to increase floatation on sand, as the footprint is always longer.

Then post all the new results to the forum, and not to me :-)))

Cheers
phil
AnswerID: 153354

Follow Up By: Rotty - Monday, Feb 06, 2006 at 15:07

Monday, Feb 06, 2006 at 15:07
Phil
Is next week okay for the direct response?
Suddenly got busy at work!
Don't drive a Pooootrol or a Cruiser so cannot help on those.

I am now too tyred and need a rest out in the sand. If I could get one of them government grants maybe I could produce a fifteen page report for the forum - cost $200,000, any donations gratefully received to my retyrement fund.
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Follow Up By: friar - Monday, Feb 06, 2006 at 16:51

Monday, Feb 06, 2006 at 16:51
Hi Rotty
I am with Phil G on this one I run 215/85r16 on the hilux which are bigger in diameter, than standard tyres but the same width ,which in my opinion perform better in sand because of the longer track they put down, the day they make a set of tyres you dont have to deflate for sand I will be the first to buy them,skinney or wide.
Friar
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FollowupID: 407306

Follow Up By: Member - Phil G (SA) - Monday, Feb 06, 2006 at 21:50

Monday, Feb 06, 2006 at 21:50
Hi Rotty,
hehehhe
I was just mucking around - I got lost on the maths, but understand where you're coming from - but you got a few brains ticking, so well done!

Cheers
Phil
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FollowupID: 407392

Reply By: robak (QLD) - Monday, Feb 06, 2006 at 15:41

Monday, Feb 06, 2006 at 15:41
Rotty

I've re read your theory and theres some things I agree with and others not so.

I feel some assumptions are wrong.

1. That 1/16 which i mentioned before,
2. To "Assume sand can support load of 0.01kg/square mm." is just not realistic. That would never be a constant, but would increase the further you sink and the sand as it compacts. Otherwise you would just keep sinking, right?

I agree with this though:
"Wide tyre at 235mm wide to have a contact area of 50,000 sqmm it would need to have (235 x 2200/50000)th of its circumference in contact with the sand, ie one 10.34th of the circumference which equates to 2200/10.34 = 213mm.
Skinny tyre the same 50,000 sqmm contact area needs to have (175 x 2200/50000)th of its circumference in contact with the sand ie one 7.7th of its circumference which equates to 2200/7.7 = 286mm."
Or in simpler terms
50000/235= 213
50000/175= 286
and i belive they would be at the same PSI

But since they have the same surface area (and therefore pressure on the sand because P=F/A) wouldn't they sink the SAME distance.

Let me elaborate:

You have a surfce area of 50 000sqmm and a force of 5000N (per tyre)
From year nine science:
Pressure = Force over an area
F = 5000N
A = 50 000sqmm
P= F/A
P = 5000N / 50 000 sqmm
P= 1 newton per 10 sqmm

If the pressure is the same, how can one set of tyres sink further then another?

And that's why I think this is wrong:
"Using some fancy trigonometry I calculate
Wide tyre would sink some 32mm into the sand
Skinny tyre will sink some 57mm into the sand."

R.

AnswerID: 153361

Follow Up By: Rotty - Monday, Feb 06, 2006 at 16:28

Monday, Feb 06, 2006 at 16:28
Robak
As initially stated some assumptions were made and one was the bearing capacity of the sand, I just used a figure for the sake of the exercise, yes the sand would compact underneath as the load to be carried meets the bearing capacity of the sand.

The point that seems to be hard for some to grasp is that in order for two width tyres to achieve the same area of support is for one of them to be lower into the sand. Yes, the area supported is the same for both tyres but when one is 235mm wide and one is 175mm wide, so the other variable then becomes the circumference of the tyre that makes up the area calculation.
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FollowupID: 407302

Follow Up By: robak (QLD) - Monday, Feb 06, 2006 at 17:04

Monday, Feb 06, 2006 at 17:04
aaaah....
I think I get what you're saying.

What you're saying is that, because the wheel is round, the required surface contact is achieved by the wheel sinking in the sand which increases the area. Right?
Then the wheel that sinks further has more difficulty in driving out of the hole that it is in.

Well that would make sense for tyres that are at very high PSI like about 40 or higher. ( or rigid wheels like on the skateboard) That is, where the wheels are so stiff that they don't get a flat spot on the bottom. However when your tyres are at low PSI, this increase in area is achieved through the flat spot you get where the tyre touches the sand.

Think about it. there are two ways of increasing the contact between the tyre and sand
1. letting the tyre sink into the sand (your theory)
2. increasing the flat spot. (my theory)

BTW I don't necessarily think skinny tyres are better.
That is because - As you had shown;
To achieve the SAME contact area, wider tyres will have a shorter but wider footprint.
Skinny tyres will have a longer but narrower front to back footprint.

The skinny tyre will ALWAYS have a longer footrpint for the same contact area.

As you decrease air pressure the lenght of the footprint increases. As the lenght of the footprint increases, the flat spot of the tyre moves closer to the rim. (ie the car is lowered)

For the same pressure (and contact area) the narrower tyre will have a longer footprint and therefore the flat spot will be closer to the rim then on a wide tyre.

At a the crtitical PSI the narrow tyre's flat spot will hit the rim while the fat tyres flat spot will still be some distance away because the wider tyre has a shorter footprint.

THAT is why fat tyres are better. They can be deflated more.

A tyre width of 300 will give you a footprint thats twice as large as a 150 because the 300 will be able to be deflated to 5 PSI while the skinny can only be deflated to 10 PSI.

Does that make sense?

R.
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FollowupID: 407310

Follow Up By: robak (QLD) - Tuesday, Feb 07, 2006 at 09:52

Tuesday, Feb 07, 2006 at 09:52
Rotty

I ws thinking about your theory last night and I believe you have in part shown us why deflating your tyres helps us with driving on sand, regardless of whether they are skinny or wide.

R.
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FollowupID: 407470

Follow Up By: Rotty - Wednesday, Feb 08, 2006 at 08:57

Wednesday, Feb 08, 2006 at 08:57
Robak

Exactly, but each of us has our own opinion over wide vs skinny; Waeco vs engel; etc etc.

As some recent 4wd training that I attended recently said stop, and think and tread lightly in our actions. At least I have made some people think about why we deflate our tyres.

Always an interesting read of posts on this forum.

Cheers
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FollowupID: 407693

Reply By: Scoey - Monday, Feb 06, 2006 at 16:57

Monday, Feb 06, 2006 at 16:57
Here's a hot tip...

Use the tyres you like, at the pressures that work for you and I'll continue doing likewise! How's that for a simple calculation? ;-)

Cheers
Scoey.
AnswerID: 153380

Follow Up By: gramps - Monday, Feb 06, 2006 at 17:06

Monday, Feb 06, 2006 at 17:06
Agree with you Scoey.

Skinnies worked in the Simpson, wides worked in the Simpson. Not overly interested in beach driving at the moment but from anecdotal evidence here and other sources, I can't see a problem.

Regards
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FollowupID: 407314

Follow Up By: Scoey - Monday, Feb 06, 2006 at 17:39

Monday, Feb 06, 2006 at 17:39
Yeah it all seems like a lot of muckin' around for every bugger to keep doing exactlywhat they have been doing for years! Don't reckon too many people will go ".....ohhhhhh, so if I divide the amount of lugs on my all terrains by the average number smarties in a fun size pack it's obvious that skinny tyres will allow me to sit 6.5mm higher in the sand. I'm gonna o get some new rubber!" haha! It does help keep the cobwebs outta the brain but I guess! Pigs to that but!

Cheers
Scoey
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FollowupID: 407317

Follow Up By: gramps - Monday, Feb 06, 2006 at 18:02

Monday, Feb 06, 2006 at 18:02
Yep. Appreciate the effort the guys have put in but in the end they have as much chance of a definitive answer as the other old faithfulls:

Toyota v Nissan
Engel v Waeco

etc. etc. etc.
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FollowupID: 407325

Reply By: russ36 - Monday, Feb 06, 2006 at 18:34

Monday, Feb 06, 2006 at 18:34
on my first 4wd i have replaced the original 16 inch skinnies with 15 inch road type 9 inch wide radials with not aggressive tread in an attempt to stay legal and to avoid burying deeper when partially bogged...i am keen to learn from others knowledge on the issue of skinny vs wide, but tired after a hard day at work, and slightly numbed from a can of bourbon that i just smashed down, so , can someone with no mathethmatics prowess and 20 yrs experience of sand driving please interupt us , and just give us the straight facts on what works on the beach?
AnswerID: 153403

Follow Up By: Eddy - Monday, Feb 06, 2006 at 18:56

Monday, Feb 06, 2006 at 18:56
I worked for a time on morton Island in the early eighties as a mechanic servicing the accos & 11a landies at the resort.

Ran about 10psi and as wide as possible for maximum floatation.
We also had a falcon plumbers ute running around the sand on wide tyres.

If you are making a bow wave in the sand that you can't climb over you are going too fast or too slow!

If you stop,you should reverse out in your tyre tracks & find a firmer bit of sand.

Never did see a tractor that worked in soft gound on skinny tyres.
Thats why they use track laying vehicles for maximum footprint to weight area.

Just my observations.

I think sand driving is also an aquired art.
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FollowupID: 407339

Reply By: Wayne (NSW) - Monday, Feb 06, 2006 at 18:56

Monday, Feb 06, 2006 at 18:56
Rotty,

There were two vehicles at Stockton beach on Sunday. One was a late model Hilux ute, 3.4lt petrol, auto with standard road tyres and with a esky in the back.
The other is 75 Series diesel with after market turbo,manual, mud tyres (285x75x16) weights in at 3t+++. Both let the tyres down to 18psi.

What vehicle do you think would be more suited to driving on sand?

The bottom line is it all gets back to the driver and how often the driver drives on sand. Knowing what tyre pressure to run will only come with experience and weather they are skinny or fat makes very little difference.

Good theory but I would hate to make that calculation every time a vehicle went on sand.

Wayne
AnswerID: 153409

Follow Up By: Member - Davoe (Widgiemooltha) - Friday, Feb 10, 2006 at 02:48

Friday, Feb 10, 2006 at 02:48
Sorry for the late answer but definitly the Hilux - they rock in sand I guess it is the lighter weight. Having said that probably neither had too much problem although 18 sounds a bit high - Would be alright for medium/hard sand
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FollowupID: 408148

Reply By: HJ60-2H - Monday, Feb 06, 2006 at 19:00

Monday, Feb 06, 2006 at 19:00
Real world is that they both work. I have used skiinies and wides on Stockton, Fraser ( to Sandy Cape etc) and lots of bush tracks.

The diameter changes so the gearing changes so the revs change so the way you drive needs to change a little between the 2 types of tyres.

Personally I found the the wider tyres are better because the gearing "feels" better. Though it doesnt worry me which set of tyres I have on when I go to Stockton as I can drive all over the place with confidence.

If the choice between skinnies and wide tyres makes a difference to you in sand as to where you can and cant go then you need more practise!!
AnswerID: 153412

Reply By: The Explorer - Monday, Feb 06, 2006 at 20:43

Monday, Feb 06, 2006 at 20:43
Firstly well done - I am impressed by your maths (honestly) - but a lot of theory as you state e.g. "will probably not happen in real life, theoretically, buggered if I know, allowing for the unrealistic..."

Secondly both my cars have tyres ~200mm wide (based on measuring spare) and I have no problems on sand (apart from having no low range on RAV!!) – what am I doing wrong? Maybe soft sand in WA is not as soft as ‘theory” world…so I don’t need to “think about that” while driving on the sand...Anyway...

Main problem I see with your calcs is that (as I have mentioned previously) - the threshold contact area required is exceeded by a “skinny" tyres (deflated as required) and therefore it doesn’t have to be deflated more to match the TOTAL area of contact of a wider tyre – so in reality they don’t sink further into the sand as your calcs suggest and as your argument requires.

Drive from tyres is front to rear - this is where you need to get sufficient contact area to stop sand slipping (ie combine frictional forces of sand grains) which is what will cause you to get bogged - someone correct me if I’m wrong !! A skinny tyre already exceeds the required width for this. All you need is length - let your tyres down (fat or skinny) – contact length increases - AWAY you go - as proven every day by thousands…read the posts of those that do it on a continual basis – don’t deny it happens. Ive seen it – I do it!!.

If you want to argue what is “better” (narrow/wide) there are two many other variables to get real answer. Some narrower tyres will be better than some wider ones and visa a versa – (not to mention what vehicle, sand conditions etc) so don’t bother – just get out there, wide or skinny, and do it. If you have a problem it’s not because your tyres are less than or equal to 200mm wide.

Think about that Rotty....while watching all those skinny tyred cars driving around in the sand:)

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

Lifetime Member
My Profile  My Blog  My Position  Send Message
Moderator

AnswerID: 153439

Reply By: D-Jack - Monday, Feb 06, 2006 at 20:44

Monday, Feb 06, 2006 at 20:44
Here's a new abbreviation to add to the current list, it applies to all you guys who are still debating this matter, and especially you Rotty! See if you can guess what it is!!

YGWTMTOYH!!!

D-Jack
AnswerID: 153440

Follow Up By: Rotty - Wednesday, Feb 08, 2006 at 09:08

Wednesday, Feb 08, 2006 at 09:08
D-Jack
Okay you got a bite.
I could give you a lot of abbreviations as well that may not mean a thing to most people.

Being an open forum we are all entitled to post our opinions on all sorts of issues, if you do not like these discussions do not reply with vague abbreviations that may only have a meaning that you understand.

We may not all agree on every opinion on these posts but at least some have put a bit of thought into this and many other issues.

Cheers
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FollowupID: 407699

Reply By: Member - Collyn R (WA) - Monday, Feb 06, 2006 at 21:17

Monday, Feb 06, 2006 at 21:17
Why toss all the theories and wild numbers around when you readily measure the respective contact areas?

Pump tyres to 35 psi . Jack up a wheel and apply a coating of dirty black something that washes off. Lower tyre onto a sheet of white paper.

Let tyres down to 20 psi. Do the same again. Compare the footprints.

You are likely to find that the overall footprint of a skinny tyre of the SAME LOAD CARRYING CAPACITY as a wide tyre will generally have a much larger (mostly extended) footprint.

Why - because the wide one does not bag out at tread level but about halfway up the tyre wall.
Collyn Rivers

AnswerID: 153454

Reply By: Member - Troll 81 (QLD) - Tuesday, Feb 07, 2006 at 11:55

Tuesday, Feb 07, 2006 at 11:55
What have I started here...........LOL
AnswerID: 153592

Follow Up By: gramps - Wednesday, Feb 08, 2006 at 08:54

Wednesday, Feb 08, 2006 at 08:54
The evil that men do lives long after them ....... etc etc etc :)))))))))))
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FollowupID: 407692

Reply By: Rotty - Wednesday, Feb 08, 2006 at 08:45

Wednesday, Feb 08, 2006 at 08:45
The lack of responses from me yesterday was because I was away from work.

Hey don't blame me for the original discussion that was Troll 81 the last time, I just thought i would put my thought into the forum, and as always there is going to be a wide difference of opinion as there is about engel vs Waeco; toyota vs nissan vs all others.

I am still happy with what I drive and yes I do get into the sand driving at least every couple of weekends.

Cheers.
AnswerID: 153791

Follow Up By: The Explorer - Wednesday, Feb 08, 2006 at 10:59

Wednesday, Feb 08, 2006 at 10:59
Good stuff - no blame to anyone - its all good fun - and an ideal opportunity to brush up on some mathematics and spelling (bugger the grammar).
Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

Lifetime Member
My Profile  My Blog  My Position  Send Message
Moderator

0
FollowupID: 407713

Reply By: 2TH DR - Monday, Feb 27, 2006 at 13:18

Monday, Feb 27, 2006 at 13:18
Phew

After reading all the posts, I gotta say my brain is full, but I am still somewhat confused and the issue I am researching has not been addressed.

I have a 4.5l 100 Series cruiser. It runs on 265/75 Cooper ATRs and on the sand @ Stockton, I've never had a problem when down at about 18psi. Am currently planning a trip down the Canning (North to South) and have been told by someone that running skinnies will reduce the fuel consumption, because of the smaller hill to push

Is the fuel consumption reason enough to warrant a change, or are there oither reasons to go split-rims instead of tubeless on the CSR?

Thx in advance

2TH DR
AnswerID: 157295

Follow Up By: Rotty - Monday, Feb 27, 2006 at 13:42

Monday, Feb 27, 2006 at 13:42
2TH DR
Cannot help regarding the Canning conditions or sand types.

I think you need to consider how much weight you will have in the vehicle as to economy on normal road and this would also be relevant on the Canning as anywhere off road.

I have heard many stories regarding different tyre and rim types and it ususally comes across that split rims are easier to replace the tyre however with the advent of manufactured tyre removers this would probably be less of an issue.

Even though the Canning is almost becoming a busy freeway you need to be pretty well independant in regrads to fuel, tyres etc as just about everything out there is more expensive.

In regards to a smaller"hill" of sand in front of skinny tyres as opposed to the "hill" in front of wider(normal street tyres such as those you indicated) I do not believe that is possible as the skinneir tyre must sink lower into the sand to compensate for load bearing area to match wider tyres. With a tyre deeper into the sand it must take more effort to move forwards/out of the depression.

As always it appears that you need to have at least two vehicles going and if they both have similar wheel sizes then it is easier to spread the load in relation to spares, fuel, water etc.

I am going up through the centre from Millicent (SA) to Darwin in Late May for three weeks and am still finding out info regarding NT. Went to Caravan and camping show in Adelaide on weekend and got even more info to sift through on NT.

Enjoy your trip.

Cheers
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FollowupID: 411537

Follow Up By: 2TH DR - Monday, Feb 27, 2006 at 19:59

Monday, Feb 27, 2006 at 19:59
Rotty

Thanks for that.

It'll be a reasonable convoy, so there will be bead breakers etc at hand. I am only 100 series, so will probably need 2nd spare wheel, At this stage I am inclined to stick to tubeless, the 10-ply sidewalls on Coopers should give protection (to some degree) against sidewall damage and they will be more compatible with the normal use of the vehicle.

2TH DR.

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FollowupID: 411610

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