Why is smaller better

Submitted: Thursday, Feb 02, 2006 at 09:21
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Hi Guys

I was having a conversation with my Dad the other day about some beach driving tyre pressures and tyre sizes and then I told him the skinnier the tire the better for beach driving and that's why the beach touring companies have skinny tyres. He started arguing the point that a big tire can be deflated more thus having a bigger area that it covers on the ground. I then started telling him about a post I read on here that explained the reason why smaller is better but I could not for the life of me remember what was said and I can't find the post. Please help
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Reply By: Rotty - Thursday, Feb 02, 2006 at 09:28

Thursday, Feb 02, 2006 at 09:28
Sorry Troll 81 your father is right, the wider the footprint the better the tyre "floats" over the sand as the load is spread over a wider area. The skinny tyres (sometimes called chees cutters) have a small footprint and thus the load per square centimetre/foot is greater so places a greater force on the sand and hence more prone to sinking into the sand and leads to greater force required to drive. Keep the skinny tyres for mud.
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Follow Up By: Member - Troll 81 (QLD) - Thursday, Feb 02, 2006 at 09:40

Thursday, Feb 02, 2006 at 09:40
I totally agree with the wider the tyre the better I run big tyres but someone here gave a good explanation of why the skinny ones work better on the sand...unless I read something wrong
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Follow Up By: tim_s - Thursday, Feb 02, 2006 at 09:46

Thursday, Feb 02, 2006 at 09:46
The width of the tyre has no relationship to the contact area with the sand or road. If the pressure is 20 psi, and the weight per tyre is 1000 pounds (sorry for the imperial units), then the contact area MUST be 1000/20 = 50 square inches. Increase the pressure to 30 psi, the area reduces to 1000/30 = 33.3 square inches. Now for a skinny tyre to flatten to such an area requires much more bending of the tyre than a wide one (this causes heat and an increase in pressure - this is the reason that race cars use fat tyres - to keep the heat down). But this does not really answer your question.

I always thought that wide tyres were worse in the sand. All tyres in soft sand push a wave of sand ahead of them. This wave is obviously alot wider for wide tyres than skinny and therefore skinny are better. Perhaps since wider tyres do not deflect as much, they do not increase in pressure as much (due to temperature increase) and retain a lower pressure.

Just my thoughts

Tim
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Follow Up By: Shawn - Thursday, Feb 02, 2006 at 10:00

Thursday, Feb 02, 2006 at 10:00
I could be wrong here but I was always led to believe that wider doesn't always mean a better footprint.
It would be the case if the the 'cheese cutter' and the wider tyre had the same rolling diameter.
All tyres push a wall of sand in front and wider tyres have to push against a larger sand wall compared to a 'cheese cutter'.
The old Model T Fords could drive over sand on their narrow tyres because of the footprint they (and similar cars) had. Large wheels with tyres that had an extremely large rolling diameter although a narrow track. They placed a large footprint on the ground to drive well over sand with only a narrow sand wall to push against.
So narrow tyres (with a larger rolling diameter) are better to float over the sand as it is uses less effort to push against the sand wall the tyre produces.
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Follow Up By: Rotty - Thursday, Feb 02, 2006 at 10:31

Thursday, Feb 02, 2006 at 10:31
"The width of the tyre has no relationship to the contact area with the sand or road. If the pressure is 20 psi, and the weight per tyre is 1000 pounds (sorry for the imperial units), then the contact area MUST be 1000/20 = 50 square inches. Increase the pressure to 30 psi, the area reduces to 1000/30 = 33.3 square inches."

Tim you have used flawed calculations in your explanation. From an pure engineering aspect on load calculations the tyre pressure is related to the vehicle weight and the area of footprint purely because the tyre can flex and change its area of support.

To explain the change in load across an area consider this
A load of 4000kg is supported on four blocks each 100mm x 100mm cross section area (total of 4,000 sq millimetres) so that load per squre millimetre would be 1kg per square millimetre. Now reduce the area of these four blocks to 50 x 50 millimetres cross sectio area (equals 10,000 square millimetres) so now the load per sqaure millimetre becomes 4kg per square millimetre. Now place these two different loads onto a soft surface (eg sand) and observe what happens. I beleive that the 4kg/sq mm will sink a gretaer amount than the 1kg/sq mm. So the tyre width does have a relationship to the contact area with the sand/surface/road etc. Whilst moving across the sand maybe the naroower footprint may be better but what happens when you stop in soft sand, I would rathet be placing less pressure per sq mm on the sand to allow for an easier departure.
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Follow Up By: tim_s - Thursday, Feb 02, 2006 at 10:53

Thursday, Feb 02, 2006 at 10:53
Rotty

Your calculations are flawed, not mine. To start with, 4 blocks 100x100mm totals 40,000 sq mm and results in a pressure of 0.1kg/sq mm. For 4 50x50mm blocks the pressure is 0.4kg/sq mm. Leaving that aside (you show the same, but with wrong numbers), you are probably correct that the smaller blocks will sink in further. However, this is not an accurate analogy to tyres. As you stated yourself in your post, tyres can and do deflect and this occurs whether the car is stationary or not. Thus the pressure on the sand is the same for a given weight and tyre pressure, irrespective of the tyre width. The blocks can not deform and therefore exert different pressures and therefore will probably sink further. In relation to your observed footprint in soft sand, perhaps you are confusing shape and size. The width of the tyre will definitely have a big effect on the shape of the footprint, but I stand by my original statement that it can not affect the size.

Tim
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Follow Up By: Member - Stephen M (NSW) - Thursday, Feb 02, 2006 at 11:12

Thursday, Feb 02, 2006 at 11:12
Have a look in last months 4x4 monthly magazine and they have done a test using ink on the tread pattern then rolling accross paper at different pressures quit interesting. Also did testing on sand (beaches etc) Regards Steve M
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Follow Up By: robak (QLD) - Thursday, Feb 02, 2006 at 12:18

Thursday, Feb 02, 2006 at 12:18
Guys

also try this post

R.

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Reply By: Rotty - Thursday, Feb 02, 2006 at 09:59

Thursday, Feb 02, 2006 at 09:59
"The width of the tyre has no relationship to the contact area with the sand or road."
" Now for a skinny tyre to flatten to such an area requires much more bending of the tyre than a wide one (this causes heat and an increase in pressure - this is the reason that race cars use fat tyres - to keep the heat down)."

"skinny are better."

Tim I do not know how much sand driving you do but around Beachport/Robe we have plenty of sand and if you are crazy enough to go go into the dunes with skinny tyres then expect to make your vehicle work harder as when you stop the footprint created will be smaller and as previously explained there will be a greater tendency to sink into the sand. As for racing cars using wider tyres to keep heat down I think in my opinion you need to pay closer attention to the commentators on car racing broadcasts in that the cars need heat in the tyres to make the tyres sticky as they use a totally different compound and these cars often start with as low as 10psi in the tyres because they do heat up from the flexing of the side walls.

I have tried skinny tyres and wide tyres in sand and I would definitely use the wider tyres in sand at all times in preference to skinny tyres.
AnswerID: 152464

Follow Up By: tim_s - Thursday, Feb 02, 2006 at 10:16

Thursday, Feb 02, 2006 at 10:16
Perhaps some of my comments were a little on the conjecture side (eg race cars – perhaps using skinny tyres would heat up too much??), however it is an irrefutable law of physics that the contact area of a tyre is dependant only on the tyre pressure and weight on the tyre. It is has absolutely no bearing on the width of the tyre. Bear in mind, that this is concerns footprint only and not sand driving ability. I am not sure how you measured your “footprint” and compared it to other width tyres, but your comment “as when you stop the footprint created will be smaller (with skinny tyres)” simply defies the laws of physics. Perhaps the answer involves a somewhat complex set of interacting variables, all of which add up to the correct answer.
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Follow Up By: Member - George (WA) - Thursday, Feb 02, 2006 at 15:18

Thursday, Feb 02, 2006 at 15:18
We do a lot of beach/dune driving. Not concerned with all the technicallities, just experience. Totally agree with Tim_s explanation. In sand use wide tyres, the wider the better
For what it is worth
Cheers
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Follow Up By: Bonz (Vic) - Thursday, Feb 02, 2006 at 18:07

Thursday, Feb 02, 2006 at 18:07
Guuys a skinnier tyre like a 235/85 will give a longer footprint and a larger area footprint than a wider tyre and also less rolling resistance, so skinnier isnt better but within limits I'd rather a thinner tyre than a very wide one.

I recall a test years ago the mags did the the thinner tyre had the larger footprint, it was simply longer
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Reply By: Turbo Diesel - Thursday, Feb 02, 2006 at 10:11

Thursday, Feb 02, 2006 at 10:11
TROLL

The theories go either way, however letting your tyres down is all about increasing the track of the the tyre in other words it has nothing to do with the width. In super soft sand it is alot harder for a motor to rotate a large or wide tyre as opposed to a thinner tyre. For example talk to the rangers on Fraser island, one had a hilux that he ran fats on but his cruiser (company car) had skinnys. The hilux he said was always struggling, changed the tyres over that is put skinnys 23575R16 on the hilux, it was then unstoppable easy to drive and better fuel economy. Traction means everything with tyres, good traction means your 4wd is going forward. The army still uses skinny tyres for their rovers, they also have placards that state what tyre pressure to run in sand mud etc. I remember our old rovers always went better and drove alott easier with skinnys rather than fats. We run 23585R16 on the cruiser td 100 series and they work a treat. I have never seen someone driving in sand with skinnys and low tyre pressure bogged.
AnswerID: 152467

Reply By: Member - Troll 81 (QLD) - Thursday, Feb 02, 2006 at 10:22

Thursday, Feb 02, 2006 at 10:22
Thanks for the responses so far and it looks like skinny is better then for Sand and Beach work
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Follow Up By: crfan - Thursday, Feb 02, 2006 at 23:31

Thursday, Feb 02, 2006 at 23:31
I don't have to have mumbo jumbo. if you get a VW bettle drive down the beach you get bogged. stick a set of wide tyres on the back you you will go places most 4wd's can't .
It comes down to what you are doing on the beach ie:high speed low speed towing .or conditions ie:soft sand, wet sand ,reef,river outlets.
A fourby with skinny tyres and lockers will do better than a 4wd with fats and no lockers and vice/vercer
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Reply By: Member - Blue (VIC) - Thursday, Feb 02, 2006 at 10:38

Thursday, Feb 02, 2006 at 10:38
Quite a perplexing subject this one... Everyone seems to have either a theory or an experience which leads them one way or the other... Myself, I have only had beach experience with two vehicles, my Triton and my old Rocky... With both vehicles, I had a second set of wheels wearing 31/10.5-15's... In both instances, IMO, they both performed better on the wider tyres. Both engines ran cooler in the soft stuff on wider tyres which leads me to believe that the wider tyres were easier to push around in the sand. For the record, I was running 12psi on both vehicles and in both sets of tyres.
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Follow Up By: Member - Davoe (Widgiemooltha) - Thursday, Feb 02, 2006 at 10:53

Thursday, Feb 02, 2006 at 10:53
yea but were your 1st set lower profile? higher profile tyres will elongate at low pressure more which is the most important thing rather than width
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Follow Up By: Member - Blue (VIC) - Thursday, Feb 02, 2006 at 11:09

Thursday, Feb 02, 2006 at 11:09
Davoe,
on the rocky, the stockies were 27" & 7" wide and on the Triton the stockies are about 30"&8.5" wide... In both cases, the 31's have a higher profile but are also considerably wider(pushing a bigger wave of sand). In both cases, in soft sand, on stockies i was running just under the hot mark and on the 31's, I was running just above normal temp.
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Reply By: dags666 - Thursday, Feb 02, 2006 at 10:59

Thursday, Feb 02, 2006 at 10:59
Hi guys just my 2 bobs worth ive been going to Fraser and Double Island for 30 years, driving 4x4 for 25 up the beach when you had to make your own tracks when nothing was boarded for easier driving. in my experience it what you feel comfortable driving on I run skinny tyres for years on landcriuser Ute and even a couple of trips with tyres that had 12 ply walls that where nearly bald and have never been bogged I still have the old girl. I just bought a 100 series cruiser wagon and it has the large Bridgestone’s on it and used it to tow a camper trailer up to Double Island this year. my thoughts are that tyre pressure and engine speed is more important than size my advice is if you go sand driving try to go with someone experienced and listen to their advice you will learn more in one lesson than going a dozen times on your own .every time I go I am still learning .one thing on the 100 series take the spare off and put it in the back as it acts like a plough in the soft sand dags
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Reply By: ev700 - Thursday, Feb 02, 2006 at 11:02

Thursday, Feb 02, 2006 at 11:02
Maybe one of those monthly 4X4 magazines could arrange a test?

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Follow Up By: Member - Jack - Thursday, Feb 02, 2006 at 13:47

Thursday, Feb 02, 2006 at 13:47
: )

Just gotta work out who they want to get sweet with first .. then do the test .. then recommend them to the readership like mad .. and get another year's advertising.

Don't forget to give the testers a couple of cartons of coldies so that they have something to write about ....

.. or am I being cynical.

Jack
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Follow Up By: ev700 - Thursday, Feb 02, 2006 at 17:25

Thursday, Feb 02, 2006 at 17:25
Jack

Hiya.

All true and I was being tongue in cheek. Have they got tyres with ARB stickers yet?

They could do sand rooster tails and bow waves coming down the dunes - a big colour of which could adorn the front cover. Always impresses the public.

A cynic is an enlightened optimist : )

EV700
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Reply By: The Explorer - Thursday, Feb 02, 2006 at 11:09

Thursday, Feb 02, 2006 at 11:09
Yes a wide tyre is no better on sand that a narrower tyre (of the same diameter). Its the length of the footprint that is paramount, not the width. This is what you are doing when letting your tyres down, lengthening the footprint. So a larger diameter tyre is the go for sand - doesn’t matter much about the width (but as some have pointed out you maybe worse off the wider you go as you are creating more work)- it has been an old wives tale (sorry ladies) for generations that wide tyres are better on sand - probably because it seems to make sense but in reality it not true. The dead giveaway is that even people with “fat” tyres still need to let their tyres down on sand…so where is the advantage? Simple - there is none.

My theory….
Drive forces are front to rear so this is where you need to get the contact area happening – ie you need to create a long enough contact area so that the sand to which the drive forces are being applied does not slip on itself, thereby causing you to stop. Length is required so that the frictional forces of the sand are combined in the direction to which the force is being applied. The “threshold” contact area (which will vary for weight of car and sand conditions) required is easily reached (in most normal circumstances) by letting your tyres down, even with your standard, what some call “skinny”, tyres.

I notice some people state – “but I put on wider tyres and they were better” – maybe they were but it wasn’t because they were wider – keep in mind a tyre has a number of characteristics – width is only one of them….why are you assuming it’s the width that is giving the advantage on sand? Maybe the wide tyre was a slightly larger diameter, maybe it flattens more front to rear that the older tyre you had?

There are advantages and disadvantages of replacing your standard tyres with something a bit wider but don’t do it expecting it to be better in sand just because they are wider. You maybe disappointed.
Cheers
Greg
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Reply By: Member - Stephen M (NSW) - Thursday, Feb 02, 2006 at 11:11

Thursday, Feb 02, 2006 at 11:11
Have a look in last months 4x4 monthly magazine and they have done a test using ink on the tread pattern then rolling accross paper at different pressures quit interesting. Also did testing on sand (beaches etc) Regards Steve M
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Follow Up By: Member - Stephen M (NSW) - Thursday, Feb 02, 2006 at 11:18

Thursday, Feb 02, 2006 at 11:18
I also use skinnys on the hilux up at stockton beach (newcastle) and keep the all terrains for off road and general running around the cheese cutters as people call them are heaps better in the sand as they dont have such a build up of sand infront to push out the way and I have tryed both definately easier with the skinnys and I dropped them all to 16 psi to see what the comparrison was. Worth the half hour to change my wheels over before I leave home when I know Im going to the beach
The skinnys not as good for general driving thats just my opinion, Im far from an expert when it come to tyres. Regards Steve M
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Reply By: Kiwi Kia - Thursday, Feb 02, 2006 at 11:16

Thursday, Feb 02, 2006 at 11:16
Geeezzz, lots of theories arn't there ! Ok, here is mine and it works.

The ideal footprint is like a catterpilar or, if you like the tracks on a bulldozer - long and thin. Wide tyres push loose sand in front like a snowplow and create far to much drag. So the best tyres are thin but with a long footprint. Best way to get a long footprint is to use the highest tyre that you can so it's rolling footprint is as long as possible. Go have a look at any photos, videos etc. of the type of wheels that the early pioneer dessert travelers used - tall and narrow. The wheels on some of Tom Kruze's trucks appear to be over waist high and look at the conditions that he got through before he even had 4wd ! Wide tyres are great for distributing load but you do not want that big wave of loose sand in front of them sucking away your horsepower. Some trucks have so much spare horsepower that they can use wide tyres and getaway with it. If you don't have mega hp avaiable then go for the higest tyres you can.
AnswerID: 152484

Reply By: flappa - Thursday, Feb 02, 2006 at 11:22

Thursday, Feb 02, 2006 at 11:22
Its not all about covering the ground.

Skinnys work because they have a longer footprint , so the build up of sand in front of the tyre is less , therefore doesn't take as much power to push through it.

Not a huge problem for modern vehicles.

It was for older vehicles.

The old 800cc Zuk my Dad had was unstopable on the beach the skinny tyres , it wouldn't move when larger tyres were fitted.
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Reply By: Rotty - Thursday, Feb 02, 2006 at 11:47

Thursday, Feb 02, 2006 at 11:47
This discussion has produced many thoughts and opinions on what is best and reference has been made to 2358516r tyres, these would not be classed as thin, thin would be 175. The first figure I understand represents the width so 235's I do not class as thin. The other reference to elongating the footprint seems misplaced. Have a look at your tyre when it is flat, which way does the tyre get bigger on the ground, across the width (vehicle sideways) or length (front to back) by the greatest amount? My understanding is that the sidewalls are softer construction to allow flexing whilst the actual tread area is firmer so any reference to tyre footprint/area not being related to tyre width does not appear correct.

In theory a round object eg tyre sitting on a flat surface will have a certain contact area that relates to the width of the tyre so a tyre 175mm wide will have an area of 175 x 10mm = 1,750sqmm( using a fixed 10mm front to back contact length) whilst a tyre 235mm wide will have 235 x 10 = 2,350 sqmm contact.

This issue of fitting a larger diameter tyre to the vehicle is not possible in most instances. How do you fit a 17inch diameter wheel under a vehicle that is factory fitted 15inch wheel arch. You don not without some major modifications, so to increase the footprint you made the tyre wider (from 175 to 235).

As with all discussions on this forum there is always going to be a difference of opinion and to each their own, so if you beleive skinny tyres are the go so be it, I know what my vehicle can and cannot do and I will stick with what I know until there is factual data to prove me wrong.

Cheers
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Follow Up By: tim_s - Thursday, Feb 02, 2006 at 12:25

Thursday, Feb 02, 2006 at 12:25
Rotty

It seems that you still do not understand. Your statement

"In theory a round object eg tyre sitting on a flat surface will have a certain contact area that relates to the width of the tyre so a tyre 175mm wide will have an area of 175 x 10mm = 1,750sqmm( using a fixed 10mm front to back contact length) whilst a tyre 235mm wide will have 235 x 10 = 2,350 sqmm contact"

is just plain wrong. Your error is assuming that the 10mm front to back contact length remains the same. For a given pressure this is NOT the case.

Pressure in a tyre is measured in PSI or Pounds Per Square Inch. That is the amount of load that the tyre can support for a given contact area. So at the same pressure and same load the contact area HAS to be the same. The skinny tire may have to flatten more to get there, but the contact area is NOT dependant on width. Please try and understand this
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Follow Up By: robak (QLD) - Thursday, Feb 02, 2006 at 13:48

Thursday, Feb 02, 2006 at 13:48
Hi Rotty

It's good to have these discussions to help us understand what we're all dealing with and improve our driving.

However I'm (partly) with tim_s on this.

TYRE PRESSURE is the key. I have been shown and am now convinced (see link i posted above) that no matter what width or height of the tyre, the contact with the ground will be the SAME for the SAME pressure. So it wrong to say that wider tyres have a bigger footprint.

20 PSI is EXACTLY that. 20 pound per square inch (of tyre).

If your car weighs 4000 pounds then at 20 PSI you have 200 square inches of tyre contact with the road/sand. Thats 50 square inches per tyre.

Now if your tyre is 10 inches wide, the lenght of the contact patch is 5 inches. If your tyre is only 5 inches wide, the lenght of contact is 10 inches. (assuming side wall strength to be negligible)

I think that (and it's only my theory) one benfit of wider tyres is that they can run on lower pressures.

Keep those brians churning.

R.

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Follow Up By: Equinox - Thursday, Feb 02, 2006 at 15:00

Thursday, Feb 02, 2006 at 15:00
Quote: "20 PSI is EXACTLY that. 20 pound per square inch (of tyre)."

Rotty - I thought it would be 20 pounds per square inch of air in the tyre.

Eq.
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Follow Up By: Equinox - Thursday, Feb 02, 2006 at 15:02

Thursday, Feb 02, 2006 at 15:02
That previous was for robak:)
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Follow Up By: robak (QLD) - Thursday, Feb 02, 2006 at 15:27

Thursday, Feb 02, 2006 at 15:27
EQ

I don't think that's right. Pressure is measured in as the force per area. Not as a volume.

(Just thinking aloud here, so I'm quite happy to be wrong)

20 PSI means that the air inside exerts 20 pounds of pressure on every square inch of tyre (in all directions).

If you applied greater pressrue the tyre would squash, therefore reducing it's volume and increaseing pressure until the equilibrium was established. Simillarly if the pressure on the outside was less the tyre would expand and increase in volume and decrease the pressure inside until the equilibrium.

Hope that makes sense 'cause I'm staring to get confused.

Another example:

Think of a large rubber band. At first it's easy to pull but it gets harder when you stretch it until the force you exert on it equalls the force it has to pull itself back together. The pressures are equal.
Now use that same idea in a balloon. At first it's easy to blow up but get harder and harder until the balloon exerts a pressure on the air inside that is equal to what your lungs can push out .

Ok. I've come to the conclusion that I can't really exaplain it.
Where's an engineer when you need one?

R.
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Follow Up By: tim_s - Thursday, Feb 02, 2006 at 15:44

Thursday, Feb 02, 2006 at 15:44
Robak

Your in luck. The pieces of paper on may wall proudly state that I am a qualified engineer. How about that!!

I think you examples of a balloon and rubber band simply show that a material gets progressively more difficuly to extend/make bigger, the bigger you make it. Not sure if this relates to tyres.

The pressure inside a tyre is the force it exerts on the wall of the tyre and so is no volume related as EQ suggested. For a tyre suspended in mid air, the shape is the same all around. Put a tyre in a bucket of water and the pressure all around will indeed squash to the tyre a little until the pressure increase can withstand the water pressure. However, once in contact with the ground, the tyre deforms until the contact area can support the load (Load = force (or weight) divided by area). The only part that deforms is the bottom. As you suggested this may result in a slight decrease in volume and therefore increase in pressure, however the remaining portion of the tyre will probably just expand a little due to the increased pressure, resulting in no overall change.
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Follow Up By: Member - Captain (WA) - Thursday, Feb 02, 2006 at 16:50

Thursday, Feb 02, 2006 at 16:50
Hi tim_s,

I think you may have confused the internal pressure of the tyre with the load weight, area (contact patch) and constrained volumes.

When a tyre on a vehicle has minimal air in it, it is "flat" as the internal air pressure cannot support the weight on the "normal" contact area, hence the contact area increases as per P=F/A. As you increase the internal air pressure (P), the tyre pumps up and the contact area (A) decreases as vehicle weight (F) is constant. This part of your explanation is correct.

However, once you get above a certain pressure (different depending on tyre type, but in the order of 30 psi), as the tyre internal pressure continues to rise, the tyre shape is constrained by design and the contact area remains roughly the same.

Think about it, pump a tyre up to 100 psi and it still has roughly the same contact area as say 30 psi. But once you get below 30 psi, the contact area starts to change rapidly as you let more air out. I can get into more detailed physics if you like, but hopefully you get the idea.

Cheers

Captain
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Follow Up By: tim_s - Thursday, Feb 02, 2006 at 17:26

Thursday, Feb 02, 2006 at 17:26
Captain

I can not get my head around your explaination. How can the contact area not continue to decrease with increasing pressure.

I agree that with a "flat" tyre, the internal air pressure can not withstand the weight of the car and therefore it collapases. However, this load is being applied to the rim of the tyre (that's what holds the car up, which is inturn transfered to the ground). Since the rim can not deform, there is a critical pressure required to hold up the car. For your P=F/A, F is the weight of the car, A is the projected area of the rim. Both are constant. Therefore once the internal pressure is below F/A collapse happens. Lets assume this critical pressure is 5 psi (guessing, would need to measure a rim), then the additional pressure applied to the rim causes compressive stresses in the rim, which are easily withstood because the rim is "rigid".

For a tyre than can deform, we agree to the point of 30psi. Consider a car with 30 psi in each tyre and a load of 300 pounds per wheel (makes the math easier). Then the internal pressure within the tyre has to push the car up with 300 pounds of force. This acts on the rims. There must be an equal and opposite force equal to 300 pounds into the ground through the tread. Since the pressure is 30 psi, then the contact area is 10 sq inches.

Take your case where we have 100 psi in there tyres. The load on each tyre remains the same and therefore the tyre holds up the car with 300 pounds of force. Thus the balancing force downwards is 300 pounds. If the area is still 10 sq inches as you suggest, then the contact pressure (force/area) must be 30 psi. What happens to the remaining 70 PSI? Stress in the sidewall? Given that the tyre can still deform (or should that be "undeform"), I can not see how this can be sustained. The constraint from the tyre shape may be having a small effect, but I doubt it is significant.

Please get into more detailed physics to prove me wrong.

Cheers

Tim

PS I haved been proved wrong more times that right!
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Follow Up By: Member - Captain (WA) - Thursday, Feb 02, 2006 at 18:21

Thursday, Feb 02, 2006 at 18:21
Hi Tim,

OK, there are two pressures here, one is the tyre pressure, internal to the tyre and the other is the contact pressure (ground force ) excerted by the tyre on the ground. I think you are confusing internal tyre pressure with contact pressure.

The contact pressure (ground force ) is purely a function of area and weight of vehicle. The weight is fixed so the only way to vary ground force is to increase area. So we let the tyre down (forget about internal tyre pressure for now) and we increase the contact area hence lower the ground force and we can now drive on soft sand. Same principle as wearing snow shoes on snow (increase contact area with same weight to decrease ground force). Similairly, decrease contact area and you increase ground force. Hopefully this makes sense!

Now, the internal tyre pressure is a function of tyre volume (volume relatively fixed but with ability to change contact area on ground).
As we increase the tyre pressure from flat, the tyre shape changes from "flat" to round. Now the tyre roundness is purely a function of the tyre type, vehicle weight and pressure.

If a tyre behaved perfectly linear as per your theory, it would have infinite area at zero pressure, however it clearly doesn't as the tyre type constrains this. Similairly, as we increase tyre internal pressure, we see a linear response over typically 5-30 psi, then tyre type constrains the contact area.

Don't confuse tyre pressure and ground pressure, they are two totally different forces.

Cheers

Captain
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FollowupID: 406308

Follow Up By: D-Jack - Thursday, Feb 02, 2006 at 20:57

Thursday, Feb 02, 2006 at 20:57
I've got a really scientific observation. If you follow a car on the sand with skinny tyres, the with of the footprint is skinnier than that of a fatter tyred car. If you let a skinny tyre down to a pressure which gives a 300mm (long) footprint in a 175mm width, why can't you let a fatter tyre down to give a 300mm length footprint in a 265mm width. May take different tyre pressures to get the 300mm length, but then the fatter tyre even though it is pushing more sand in front will not sink as far as the skinnier one (particulary when stationary), because the kgs of pressure relative to the contact area will be less for the fatter tyre??? I can't keep up with all the scientific guff, but I can't see how you could get any other deduction from my example than fatter tyres will float you better over sand.

Also, if skinnier is better, then theorietically? you could use really skinny tyres (100mm) and it should be better, I don't think they would work real well, so I think there is probably some sort of parabola factor here.

D-Jack
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FollowupID: 406340

Follow Up By: The Explorer - Friday, Feb 03, 2006 at 00:40

Friday, Feb 03, 2006 at 00:40
D-Jack - While the original question did ask if "skinny" tyres are "better" I think the general consensus is (and in fact) they are no worse (i.e. no advantage to be gained by putting on tyres just because they are wider). There are a number of variables that must be considered when assessing a tyres capabilities on sand and width is only one of them - most people ignore all others factors except width when carrying out their "backyard" assessments.

Theoretically, going skinnier (and not increasing diameter and therefore front/rear footprint) will eventually go passed the threshold required for sufficient drive on sand (ie don’t try running bike tyres on the Tojo..but a 100mm wide 10 metre diameter tyre may be OK), but I suspect (put money on it) even the skinniest available 4WD tyre exceeds the threshold width and will still be capable on sand if reduced to a suitable pressure.

Bottom line is whatever tyre you’re using, just reduce pressures (to increase footprint length) when driving on sand…it is honestly that simple. If you get stuck it is very likely you haven’t let your tyres down enough (however narrow or wide they maybe), so let them down some more.

Cheers
Greg
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Follow Up By: robak (QLD) - Friday, Feb 03, 2006 at 09:59

Friday, Feb 03, 2006 at 09:59
D-Jack

I reckon you've hit the nail on the head. Skinny tyres give NO advantage when at the same pressure as fat tyres, and therefore the same contact area.

BUT fat tyres can give you greater contact area because they can be deflated more.

If you blow up a pushbike tyre tube and a car tyre tube to 30 PSI, lay it flat on the ground and then stand on each one. Which one will hold you up off the ground. The skinny tyre need a higher PSI to keep you up.

So , there must be a perfect size tyre which, on one hand, doesn't build up too much sand in front , and on the other lets you drop your psi low without you riding on your rims.

According to the People in the Dakkar Ralley that size is a 235.

Captain,

So what your saying is that above 30 psi the pressure builds up without changing the contact area? A bit like having a solid but hollow tyre right?

Cheers guys, it was a good post and some good responses. Something to think about next time where in the sand.

R.

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Reply By: signman - Thursday, Feb 02, 2006 at 11:57

Thursday, Feb 02, 2006 at 11:57
Watching the recent Dakar Rally, I noticed the Mitsubishis & Touregs were running comparatively narrow tyres (compared to say a 31x10.5). Now some of the sand situations they were in was VERY soft- even heaps softer to the drifts on Stockton and Fraser.
I think the techos associated with those teams would have a fair idea!!
AnswerID: 152491

Follow Up By: signman - Thursday, Feb 02, 2006 at 12:03

Thursday, Feb 02, 2006 at 12:03
A follow up to my own follow up.
The Dakar Pajeros were running BFG 235/85x16 (oáll width 234mm)
A typical 31x10.5/15 o'all width 280mm.
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FollowupID: 406215

Reply By: ImEasy - Thursday, Feb 02, 2006 at 12:04

Thursday, Feb 02, 2006 at 12:04
Smaller is way better, just ask my wife, she alway's tell's me, "It's not the size that count's, it's the way you use it"...
AnswerID: 152492

Follow Up By: ImEasy - Thursday, Feb 02, 2006 at 12:05

Thursday, Feb 02, 2006 at 12:05
Sorry, wrong forum.......
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FollowupID: 406216

Reply By: Member - MrBitchi (QLD) - Thursday, Feb 02, 2006 at 12:23

Thursday, Feb 02, 2006 at 12:23
Here's a link which explains how tyre pressures work.

Tyre pressures

AnswerID: 152494

Reply By: Rotty - Thursday, Feb 02, 2006 at 12:51

Thursday, Feb 02, 2006 at 12:51
Tim
Maybe you do not understand
You are entitled to your opinion regarding skinny tyre perference over wide on sand but if you look at the link to the landrover site then you may be able to picture where the tyre width comes into the picture. Picture this, two tubes, same diameter but one is 100mm long and one is 50mm long, which one makes a larger impression when you push them both onto a soft surface, the 50mm long one or the 100mmm long one? This is the footprint/area. If you say the 50mm long one then you keep driving the skinny 175 tyres in soft sand with no solid base and see how deep the tyre sinks into the sand, in mud you want to get deeper to reach the firm base and to get traction.

Your "facts" have not proved a thing to support the arguement for skinny tyres at the same pressure as opposed to wide tyres at the same pressure on sand.

We will appear to remain with a difference of opinion regardless of the arguements. Cheers to you and enjoy the experience.
AnswerID: 152499

Follow Up By: tim_s - Thursday, Feb 02, 2006 at 13:44

Thursday, Feb 02, 2006 at 13:44
Rotty

I agree that we are all entitled to our own opinion. However, when somebody is giving advise based on incorrect facts, there can be harm done. No where in my posts have I said I had a preference, I simply stated that I "thought" skinny tyres were better.

You again provided an example where there would be no deflection of the item. Without deformation, what you have said is quite correct. However, this is not realistic with respect to tyres. How many times does it have to be said that the width of the tyre has no effect on the footprint, only its shape. Nowhere on the landrover site does it say that the wider the tyre the bigger the footprint for a given pressure. The reason that it does not say that is that it is simply not true.

I give up

Happy driving
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FollowupID: 406235

Reply By: Philip A - Thursday, Feb 02, 2006 at 13:15

Thursday, Feb 02, 2006 at 13:15
Good post on the age old question.
Having lived in Saudi for 2 years and seen many combinations i would like to add 2 cents worth.
1 Dedicated sand tyres are sold in Saudi and elsewhere. They are like an aircraft tyre. They have no tread except for a number of longitudenal lines, and they are V shaped, so that when fully inflated only about one inch contacts the road.
The theory is that the shape minimizes the frontal area, and the longitudinal (lateral? circumfrential?) lines stop the sand from moving out towards the sides.
Friends report these are the ducks guts on sand but diabolical on road.
They are quite high in profoile maybe over 100% , and are balloon tyres which are quite wide at their widest point say 10 ins on a narrow rim. One of friends got them on a Patrol but we had rain one day in Riyadh and they were totally undrivable, so he swapped them..
2 The next best were Michelin Saharas. Obvious usage from the name. These had a round profile with rounded tread blocks, like an AT but everything is round. They are still sold mainly in truck sizes I believe. They are still pretty bad on road.
They are usally in narrow sizes like 205R16.
Personally I used 205R16 Continental HT tyres on 6 inch riims on my Range Rover in the Sahara with good results.
I think there are several elements that make a good sand tyre ..
1 Fine tread with deep water grooves
2 Round shoulders
3 Flexible sidewalls. the 15 inch widies usually have very much thinner sidewalls Than LT 16 inch tyres, and in the US and elsewhere are called amazingly enough "Floatation tyres"
4 also the tyre width vs the rim width is important. Generally I noticed a tyre on the narrowist sensible rim was best as it allowed the sidewalls to bulge better.

So there is no one answer but a good compromise is a HT with round shoulders and deep rain grooves. Narrow is probably better but its more the relationships above that matter.
Wide 15 inch tyres may be good DESPITE their width not because of it, because of other factors such as rounded shoulders,flexible sidewalls that are intrinsic in the floataion design and a wide tyre vs the rim size..
NOW I will get flames from those with muddies that glide over sand.
BUT I have been there and done that in the Sahara, which makes Big Red look like a Molehill. What a disappointment it was!
regards Philip A
AnswerID: 152506

Reply By: hopscotch - Thursday, Feb 02, 2006 at 13:36

Thursday, Feb 02, 2006 at 13:36
Our 'entertainment' on Fraser is to sit on the patio of an afternoon and watch the vehicles get 'bogged' as they come up from the beach into Happy Valley. It appears to be of little consequence what size, make or model or what tyre pressures or types of tyres less than 20% get through without some sort of problems. Even some of the Kingfiser 4 x 4 tour buses with their BIG oversize singles have two or three goes when it has been very dry. My mate with his Hilux on skinnys (and years of experience) and me in my Pajero with wide tyres (265x70x16) have no trouble at all and we are using that track up to six times a day. I use 4wd at all times once I leave Rainbow Beach but the Hilux does it frequently in 2wd. (He doesn't want to wear out the front diff!!!)
What we have noticed is that some drivers will race in at speed but in top gear and then have to 'grab' for a downstick when only partway through. That loss of momentum brings on the grief. Much better to go in fairly hard in 2nd and have the constant power up the sleeve. I grew up on the Darling Downs so black soil and mud are more my scene but watching the locals can be beneficial. When in Rome!!!

Kevin J
AnswerID: 152509

Reply By: Member - Troll 81 (QLD) - Thursday, Feb 02, 2006 at 14:58

Thursday, Feb 02, 2006 at 14:58
Thanks for all the input guys.....was a good read
AnswerID: 152523

Reply By: spak - Thursday, Feb 02, 2006 at 15:03

Thursday, Feb 02, 2006 at 15:03
Greetings Earthlings, (dags666 your spot on)
Just to add more fuel to the fire, the father in law has been going to Fraser for over 30 yrs in an old landrover 2A, on skinny nearly bald tyre and has never been bogged once. He actually used to drive around the guys who were bogged and have a good chuckle to himself. We are about to inherit this old girl and we wont change the tyres.
It's not all about tyre/tyre pressures, although that is a big part. Common sense and willingness to listen to those who have been there and done it will also go a long way. :)

Spak
AnswerID: 152524

Reply By: Kiwi Kia - Thursday, Feb 02, 2006 at 15:03

Thursday, Feb 02, 2006 at 15:03
All you people that have ideas about a tyre getting wider when you reduce pressure had better get out to your vehicle and take a ruler and some chalk.

Put the side of the ruller up against the tread of the tyre where it touches the ground and chalk a line out on the pavement at rt angles to the vehicle. Make a chalk mark in the front and at the rear of the tyre. Measure the distance between the chalk marks. Note also that any bulging of the side wall does NOT touch the ground - it is a bulge only !!

Now reduce the tyre pressure and do it all again.
1. The fore and aft 'footprint will be a lot longer
2. The side wall bulge will be bigger but it will still NOT TOUCH THE GROUND !

Next, make a chalk on the side wall down to the ground and mark the ground as well. Roll the truck forward one revolution of the wheel and mark the ground where the chalk mark on the side wall almost touches the ground.
Measure and record the distance between the chalk marks on the ground.

Now do it all again at a different tyre pressure and report back here.

Untill you have done this or already know that the answer to the original question is NARROW tyres are best in sand you need not make any more comments as you are only repeating what a friend of a friend has told you. Wide tyres only work well when you have mega amounts of spare horsepower available to overcome the snowplowing effect of wide tyres.

Sand is not always the worst problem either. Gravel beaches consisting of almost round stones about 5 mm dia are far worse then fine sand beaches. Even the slightest wheel spin will have you sitting on the chassis rails in seconds. Very low speed without throwing any material out from behind the wheels is the only way to make progress.
AnswerID: 152525

Reply By: RupertDog - Thursday, Feb 02, 2006 at 15:52

Thursday, Feb 02, 2006 at 15:52
Get read - so many theories, so little time !

I don't think anyone has mentioned the most important fact. Big 4wds look "better" with wider tyres.

They just look odd with skinny pizza cutters !!

Even my RAV looks the part with wider tyres (and goes pretty good in the sand with wider tyres as well)

RD
AnswerID: 152534

Follow Up By: ro-dah-o (WA) - Thursday, Feb 02, 2006 at 19:35

Thursday, Feb 02, 2006 at 19:35
I reckon that is the most sensible post of the thread!!!

Its all about how you look driving to the beach, not practical use and benefits LOL
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FollowupID: 406324

Reply By: age - Thursday, Feb 02, 2006 at 16:25

Thursday, Feb 02, 2006 at 16:25
Some key principles being missed here. The air in a tyre at no matter what pressure is designed to keep the general shape of the tyre and comfort of the ride. The pressure (PSI) is a measurement of the exertion of pressure against a known area CONTAINED within that tyre and is irrelevant to mass of vehicle through tyre onto land surface - take your tyre off and measure PSI then put it back on the vehicle and measure - very minor change only caused from part of the tyre being lessened (compressed) in size by the mass of the vehicle. Pressure in tyre does not equate to pressure on road - A tyre with 50 psi in it exerts the same pressure/weight downwards as the same tyre with 15 psi it except for the weight/volume of air between the two pressure differences - which is minimal.

Now - tyres dont need air in them (be a bad ride but...) - but a solid tyre with load bearing area on a road of a set width/length exerts so much force to the ground (depending on mass it is supporting) per given area and wider longer bearing area (like when tyre deflated) exerts less for the same given area and mass. This is why whether fat or skinny tyres when you let them down you are increasing the load bearing area with the same fixed vehicle mass - end result better floatation as less force downward per given area.

Re comment above about sidewalls not touching - have a look out the window next time in soft sand - the sidewalls do touch the sand surface, giving load bearing capability and greater surface area in touch with road/sand (as well as more friction - hence motor power being required to overcome that portion of overall resistance), but in most cases the "overall" gain is bigger than the extra resistance, and why most vehicles perform better with tyres deflated.
AnswerID: 152539

Follow Up By: Axle - Thursday, Feb 02, 2006 at 16:45

Thursday, Feb 02, 2006 at 16:45
My 10c worth. What about small tractors (or large) used by councils to maintain parks ovals etc. most have wide balloon tyres fitted. why?,so as they dont damage the suface with ruts etc. Golf course, very sensitive areas same thing, wide tyre distributes weight less ground pressure per wheel. got to be an advantage.

Cheers Axle
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FollowupID: 406280

Follow Up By: Bonz (Vic) - Thursday, Feb 02, 2006 at 18:24

Thursday, Feb 02, 2006 at 18:24
Ax I think you'll find that when the tyre diameter is limited as in your examples then wider is better. When we're talking about 16" rims and such thinner is better
.
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Follow Up By: Charlie - Thursday, Feb 02, 2006 at 18:27

Thursday, Feb 02, 2006 at 18:27
"This is why whether fat or skinny tyres when you let them down you are increasing the load bearing area with the same fixed vehicle mass - end result better floatation as less force downward per given area. "

One thing I'd add to this is many skinny tyres are designed to carry a lot of load with the load rating measured at a high inflation value often double that at which they use for a wide fifteen inch "floatation type" tyre. Deflate both tyres to twenty pounds an inch and your effectively deflating the skinny tyre more than the wide one.
The magazine test was nonsense, how can you compare a square shouldered high load index sixteen inch tyre to a fifteen inch floatation tyre, it's just a not a valid comparison
Regards Charlie
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FollowupID: 406312

Reply By: Rotty - Thursday, Feb 02, 2006 at 17:26

Thursday, Feb 02, 2006 at 17:26
Now it has become obvious why your tag is Troll, you have created a post for today that has had the most views, responses and posts and yet there is still no definitive answer just a lot of conjecture from all sorts.

End result, know your vehicle, its limitations and enjoy the off road driving whether it be toyota, ford, nissan, mitsu or lady Niva.
AnswerID: 152551

Follow Up By: gramps - Thursday, Feb 02, 2006 at 17:34

Thursday, Feb 02, 2006 at 17:34
Yep. Wide, skinny, steel, alloy, Engel, Waeco, diesel, petrol, electronic, paper, cheap, expensive. Whatever works for you is best :))))
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FollowupID: 406301

Reply By: Member - Norm C (QLD) - Thursday, Feb 02, 2006 at 18:13

Thursday, Feb 02, 2006 at 18:13
Ah! A great debate. Love it.
Just 2 points.
1. I am happy that skinny tyres (say 7inch) are better than 10 or 11 inch tyres in sand.

2. But most of us have on set of tyres, which are a compromise to meet our various driving needs. My tyres are 265mm (about 10.5 inches ) wide. I don't and won't change them when I go to Fraser. So I have to adjust my tyre pressure and driving style to get the best out of them.

So while it is a good theoretical debate. In practice, few of us do a lot of sand driving (compared to the amount of black top driving). We have the tyres we have. so the variables available are in other things, rather than tyre size.

Good discussion all the same.
AnswerID: 152556

Reply By: D-Jack - Thursday, Feb 02, 2006 at 21:12

Thursday, Feb 02, 2006 at 21:12
I've been doing a few formula calculations and came to this conclusion. If you've got the background then it may make sense.

tp=h1/2DIVIDED BY 100XacPisquared-circum>235,159*AB+iced coffee

The conclusion - WHO GIVES!!!!!!

P.s. I like beer

D-Jack

AnswerID: 152595

Follow Up By: tim_s - Friday, Feb 03, 2006 at 10:06

Friday, Feb 03, 2006 at 10:06
D-Jack

You missed some important facts in your equation

The correct on is:

Contact Area = P*(Engel current draw/Waeco current draw)*(number of Nissan 3l failures/number of people who drive Toyo's)*(BFG tyre wear/number of people being screwed on Cooper's warranty)*G'ma's age/optimum beer temp.

All this theory gave me a headache!

Tim
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FollowupID: 406412

Reply By: J.T. - Friday, Feb 03, 2006 at 00:04

Friday, Feb 03, 2006 at 00:04
D-JACK,You should have posted that on Friday(phunny).
AnswerID: 152640

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