AWD??

[GoCougs]

More or less pressure, if the actual normal force stays the same, makes no difference for friction.  On soft ice, the pressure spikes from the tread edges may help you bite into the ice, but on hard ice (ultra thin, black ice being a prime example) or HDPE, it will make no difference whatsoever.

I'd take that bet. Speaking from personal experience knobby tires do make some difference on hard/glare/boilerplate/thin ice.

[Soup DeVille]

Friction is independent of contact area.  Friction = Normal Force x Coefficient of Friction.  The latter being a material property of the two surfaces in contact.  Area of contact plays no part in it.

This has been proven false when to comes to tires for years. The short story is that tire slippage is actually a structural failure of sorts.

The long story involves Don Garlits.

But in any case, tire contact is more complicated than straight Newtonian friction.

[MX793]

This has been proven false when to comes to tires for years. The short story is that tire slippage is actually a structural failure of sorts.

The long story involves Don Garlits.

But in any case, tire contact is more complicated than straight Newtonian friction.

Already noted in another post.  See my discussion on the three modes of traction loss.  As it pertains to the OP, material failure in the tire compound was not the cause for traction loss in the video.  Nor is it the cause on ice and snow.

[Soup DeVille]

Already noted in another post.  See my discussion on the three modes of traction loss.  As it pertains to the OP, material failure in the tire compound was not the cause for traction loss in the video.  Nor is it the cause on ice and snow.

No, on snow it's the structural failure of the snow, not the tire. And I'll bet that its still the tire on that video.

Tire pressure makes a big difference in the snow, although in most snow covered roads, what you want is higher, not lower pressure. On deep snow, if you have big enough tires, airing them down makes a big difference. ( that's why snowshoes work too)

[SVT32V]

Wider tires offer no more friction than narrower ones of the same compound.  The larger contact area results in reduced shear stresses in the rubber.  There are 3 modes of "slip" where the tire meets the road.  First is the frictional limit.  This slip is when the lateral force exceeds the frictional force and the tire and the road surface slide on each other.  Second is a shear failure in the road surface.  Rare on asphalt, but the typical source of slip when driving on loose surfaces (mud, dirt, some snow conditions).  Third is shear failure of the tire rubber.  When shear stresses in the rubber exceeds the material strength of the rubber, rubber tears from rubber, leaving a skidmark on the ground.  Wider tires for performance combats the 3rd scenario.

Yes it is the 3rd scenario, and it overalls falls under rolling without slipping.

[MX793]

No, on snow it's the structural failure of the snow, not the tire. And I'll bet that its still the tire on that video.

Tire pressure makes a big difference in the snow, although in most snow covered roads, what you want is higher, not lower pressure. On deep snow, if you have big enough tires, airing them down makes a big difference. ( that's why snowshoes work too)

On polyethylene?  No, that's not a material failure.  That's a frictional limit problem.  Cf on PE is quite low.  So low that it is used as synthetic ice for skating rinks.

And I also noted that under conditions where you are trying to stay on top of deep snowpack, you do want wide, low pressure tires.  But for the street driving that any of us here do in the snow, you do not want to air down your tires.

[Byteme]

Interesting discussion.

IIRC, the advantage of lowering pressure in snow was to allow the concours of the sidewall to bite into the snow along withthe tread that normallycontacts the snow at higher pressures.  I'd guess the value is minimal with summer or all weather tires with a smooth sidewall.

The test in the video is useful if one spends a lot of time driving on polyethylene, IMO.

[MrH]

I agree that tires don't follow basic Newtonian friction, but if there were ever a situation with tires that would be close to following a typical Newtonian friction model, this is it.  It's a very low friction surface that's solid and very flat and slipping.

[Byteme]

Ice is different from polyethylene in that a tire rolling over ice is actually rolling over a very thin layer of water, perhaps just a molecule or two thck.  And the propertities of ice change as the temperature and mineral content of the ice changes.  All that will make a difference.  How much?  I don't know but it wouldn't have been that hard to make a ramp of actual ice which would have meant much better test conditions.

[giant_mtb]

Why didn't they just tilt up an ice rink?

[Byteme]

Why didn't they just tilt up an ice rink?

Why indeed.  Pretty easy to use the same technology they use in a rink to refigerate the ramps and make them icy.   Or even better, build a ramp in winter conditions and run the test.    Running the test in non-winter weather, i.e. warm weather, you still have the problem of tires at warm weather temps.  We know tire compounds optimized for warm weather driving are often poor for winter driving and vice versa. 

All I'm saying is the test showed a vehicle can make it up a plastic ramp being flooded with water.  That result may or may not replicate the vehicle's ability to climb icy hills in winter.  I assume Suburu is using this test for advertising purposes so I'd also assume they selected tires to optimize the vehicles performance for the conditions of the test.  It sure wouldn't be the first time a manufacturer did anything like that.

[MX793]

Ice is different from polyethylene in that a tire rolling over ice is actually rolling over a very thin layer of water, perhaps just a molecule or two thck.  And the propertities of ice change as the temperature and mineral content of the ice changes.  All that will make a difference.  How much?  I don't know but it wouldn't have been that hard to make a ramp of actual ice which would have meant much better test conditions.

They wet the PE to give it more ice-like properties.  This is also done when used as synthetic ice.

Ice properties change considerably with temperature.  The melting pressure doubles between 30F and 20F, and continues increasing as temperatures drop.  Its compressive strength also changes significantly with temperature.  I believe it roughly triples between 30F and 10F.  Depending on how cold it is, you may not really get that thin water layer.  You also may or may not be able to bite into it with just tire tread.  Hence why I've referred to hard ice and soft ice.  When temperatures are above 20F, ice is relatively soft.  Impurities like air bubbles make a big difference too.

[GoCougs]

The other issue is ambient temp - tires perform differently at different temps. Even if a true icy road surface is ginned up I'm not sure how accurate the test would be at warm temps. Also, pretty much no one drives up/down an icy ~30 degree road.

[Soup DeVille]

The other issue is ambient temp - tires perform differently at different temps. Even if a true icy road surface is ginned up I'm not sure how accurate the test would be at warm temps. Also, pretty much no one drives up/down an icy ~30 degree road.

30.5

As if they set it to find the exact incline where the Subie could run it and the other cars couldn't.

[Byteme]

They wet the PE to give it more ice-like properties.  This is also done when used as synthetic ice.

  Depending on how cold it is, you may not really get that thin water layer. 

Latest research shows that Ice  has a layer of liquid water on it's surface at about any temperature below freezingwe are likely to encounter in our daily lives.  This explains how someone not even moving on ice is able to slip without any frictional and negligable pressure melting.

From:  http://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=all

Explaining Ice: The Answers Are SlipperyPublished: February 21, 2006       But maybe it should. After all, ice is a solid, and trying to glide on thin metal blades across the surfaces of most solids — concrete, wood, glass, to name a few — results in ear-piercing sounds and ungraceful stumbles. Though the question may seem to be a simple one, physicists are still searching for a simple answer. 
     
The explanation once commonly dispensed in textbooks turns out to be wrong. And slipperiness is just one of the unanswered puzzles about ice. Besides the everyday ice that you slip on, there are about a dozen other forms, some of which experts suspect exist in the hot interior of Earth or on the surface of Pluto. Scientists expect to discover still more variations in the coming years. 
     
Ice, said Robert M. Rosenberg, an emeritus professor of chemistry at Lawrence University in Appleton, Wis., and a visiting scholar at Northwestern University, "is a very mysterious solid."
       
Dr. Rosenberg wrote an article looking at the slipperiness of ice in the December issue of Physics Today, because he kept coming across the wrong explanation for it, one that dates back more than a century.
       
This explanation takes advantage of an unusual property of water: the solid form, ice, is less dense than the liquid form. That is why ice floats on water, while a cube of frozen alcohol — which has a freezing temperature of minus 173 degrees Fahrenheit — would plummet to the bottom of a glass of liquid alcohol. The lower density of ice also means that the melting temperature of ice can be lowered below the usual 32 degrees by squeezing on it.   
     
According to the frequently cited — if incorrect — explanation of why ice is slippery under an ice skate, the pressure exerted along the blade lowers the melting temperature of the top layer of ice, the ice melts and the blade glides on a thin layer of water that refreezes to ice as soon as the blade passes.     
   
"People will still say that when you ask them," Dr. Rosenberg said. "Textbooks are full of it."     
   
But the explanation fails, he said, because the pressure-melting effect is small. A 150-pound person standing on ice wearing a pair of ice skates exerts a pressure of only 50 pounds per square inch on the ice. (A typical blade edge, which is not razor sharp, is about one-eighth of an inch wide and about 12 inches long, yielding a surface area of 1.5 square inches each or 3 square inches for two blades.) That amount of pressure lowers the melting temperature only a small amount, from 32 degrees to 31.97 degrees. Yet ice skaters can easily slip and fall at temperatures much colder. 
     
The pressure-melting explanation also fails to explain why someone wearing flat-bottom shoes, with a much greater surface area that exerts even less pressure on the ice, can also slip on ice.   
   
Two alternative explanations have arisen to take the pressure argument's place. One, now more widely accepted, invokes friction: the rubbing of a skate blade or a shoe bottom over ice, according to this view, heats the ice and melts it, creating a slippery layer.     
 
The other, which emerged a decade ago, rests on the idea that perhaps the surface of ice is simply slippery. This argument holds that water molecules at the ice surface vibrate more, because there are no molecules above them to help hold them in place, and they thus remain an unfrozen liquid even at temperatures far below freezing.       
Scientists continue to debate whether friction or the liquid layer plays the more important role. Dr. Rosenberg, asked his opinion, chose a indecisive answer: "I say there are two major reasons."   
   
The notion that ice has an intrinsic liquid layer is not a new concept. It was first proposed by the physicist Michael Faraday in 1850 after a simple experiment: he pressed two cubes of ice against each other, and they fused together. Faraday argued that the liquid layers froze solid when they were no longer at the surface. Because the layer is so thin, however, it was hard for scientists to see. 
     
In 1996, Gabor A. Somorjai, a scientist at Lawrence Berkeley Laboratory, bombarded the surface of ice with electrons and watched how they bounced off, producing a pattern that looked at least partially liquid at temperatures down to minus 235 degrees. A couple of years later, a team of German scientists bounced helium atoms off ice and found results that corroborated the Lawrence Berkeley findings.     
 
"The water layer is absolutely intrinsic to ice," Dr. Somorjai said.
       
The findings, he said, fit with a simple observation that suggests friction cannot be the one and only explanation of slipperiness. When a person stands on ice, he added, no heat is generated through friction, and yet "ice is still slippery."   
     
But a colleague of Dr. Somorjai's at Lawrence Berkeley, Miquel Salmeron, while he does not dispute Dr. Somorjai's experiment, does dispute the importance of the intrinsic liquid layer to slipperiness. 
     
In 2002, Dr. Salmeron and colleagues performed an experiment. They dragged the tip of an atomic force microscope, resembling a tiny phonograph needle, across the surface of ice. 
     
"We found the friction of ice to be very high," Dr. Salmeron said. That is, ice is not really that slippery, after all.     
   
Dr. Salmeron said that this finding indicates that while the top layer of ice may be liquid, it is too thin to contribute much to slipperiness except near the melting temperature. In his view, friction is the primary reason ice is slippery. (The microscope tip was so small that its friction melted only a tiny bit of water, which immediately refroze and therefore did not provide the usual lubrication, he said.) 
     
Dr. Salmeron concedes, however, that he cannot definitively prove that his view is the correct one.   
     
"It's amazing," he said. "We're in 2006, and we're still talking about this thing."     
   
Ice formed by water behaves even more strangely at lower temperatures and higher pressures. 
     
Water — H2O — seems to be a simple molecule: two hydrogen atoms connected to a central oxygen atom in a V-shape. In everyday ice, which scientists call Ice Ih, the water molecules line up in a hexagonal pattern; this is why snowflakes all have six-sided patterns. (The "h" stands for hexagonal. A variation called Ice Ic, found in ice crystals floating high up in the atmosphere, forms cubic crystals.) 
     
The crystal structure of the ice is fairly loose — the reason that Ice Ih is less dense than liquid water — and the bonds that the hydrogen atoms form between water molecules, called hydrogen bonds, are weaker than most atomic bonds.     
   
At higher pressures, the usual hexagonal structure breaks down, and the bonds rearrange themselves in more compact, denser crystal structures, neatly labeled with Roman numerals: Ice II, Ice III, Ice IV and so on. Scientists have also discovered several forms of ice in which the water molecules are arranged randomly, as in glass. 
     
At a pressure of about 30,000 pounds per square inch, Ice Ih turns into a different type of crystalline ice, Ice II. Ice II does not occur naturally on Earth. Even at the bottom of the thickest portions of the Antarctic ice cap, the weight of three miles of ice pushes down at only one-quarter of the pressure necessary to make Ice II. But planetary scientists expect that Ice II, and possibly some other variations, like Ice VI, exist inside icier bodies in the outer solar system, like the Jupiter moons Ganymede and Callisto.     
   
With pressure high enough, the temperature need not even be cold for ice to form. Several Februaries ago, Alexandra Navrotsky, a professor of chemistry, materials science and geology at the University of California, Davis, was visiting Northwestern. She was sitting in office of Craig R. Bina, a geophysicist, and looking out over frozen Lake Michigan. "Ice might have been on our minds," she recalled. 
     
The scientists started considering what happens to tectonic plates after they are pushed back down into Earth's interior. At about 100 miles down, the temperature of these descending plates is 300 to 400 degrees — well above the boiling point of water at the surface — but cool compared with that of surrounding rocks. The pressure of 700,000 pounds per square inch at this depth, Dr. Bina and Dr. Navrotsky calculated, could be great enough to transform any water that was there into a solid phase known as Ice VII.       
No one knows whether ice can be found inside Earth, because no one has yet figured out a way to look 100 miles underground. Just as salt melts ice at the surface, other molecules mixing with the water could impede the freezing that Dr. Bina and Dr. Navrotsky have predicted.   
   
Ice also changes form with dropping temperatures. In hexagonal ice, the usual form, the oxygen atoms are fixed in position, but the hydrogen bonds between water molecules are continually breaking and reattaching, tens of thousands of times a second.
       
At temperatures cold enough — below minus 330 degrees — the hydrogen bonds freeze as well, and normal ice starts changing into Ice XI.
       
William B. McKinnon, a professor of earth and planetary sciences at Washington University in St. Louis, said that astronomers were probably already looking at Ice XI on the surface of Pluto and on the moons of Neptune and Uranus. But instruments currently are not sensitive enough to distinguish the slight differences among the ices.     
 
The most recently discovered form of ice, Ice XII, was found just a decade ago, although hints of it had been seen years earlier. John L. Finney of University College London, one of the discoverers of Ice XII, said that trying to understand all the different forms of ice was important for an understanding of how the water molecule works, and that was important in understanding how water interacts with all the biological molecules in living organisms.       

"It gives you a very stringent test for our understanding of the water molecule itself," he said. 



A more scholary article can be found at:   http://lptms.u-psud.fr/membres/trizac/Ens/L3FIP/Ice.pdf

[r0tor]

Static friction is not independent of surface area.  Sliding friction is.  A tire not spinning or sliding is in static friction and thus dependent on the surface area.

[FoMoJo]

Ice is complicated .

[Byteme]

Static friction is not independent of surface area.  Sliding friction is.  A tire not spinning or sliding is in static friction and thus dependent on the surface area.

Conclusion?

This has morphed into a very interesting discussion, BTW.

[Eye of the Tiger]

Latest research shows that Ice  has a layer of liquid water on it's surface at about any temperature below freezingwe are likely to encounter in our daily lives.  This explains how someone not even moving on ice is able to slip without any frictional and negligable pressure melting.

From:  http://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=all

Explaining Ice: The Answers Are SlipperyPublished: February 21, 2006       But maybe it should. After all, ice is a solid, and trying to glide on thin metal blades across the surfaces of most solids — concrete, wood, glass, to name a few — results in ear-piercing sounds and ungraceful stumbles. Though the question may seem to be a simple one, physicists are still searching for a simple answer. 
     
The explanation once commonly dispensed in textbooks turns out to be wrong. And slipperiness is just one of the unanswered puzzles about ice. Besides the everyday ice that you slip on, there are about a dozen other forms, some of which experts suspect exist in the hot interior of Earth or on the surface of Pluto. Scientists expect to discover still more variations in the coming years. 
     
Ice, said Robert M. Rosenberg, an emeritus professor of chemistry at Lawrence University in Appleton, Wis., and a visiting scholar at Northwestern University, "is a very mysterious solid."
       
Dr. Rosenberg wrote an article looking at the slipperiness of ice in the December issue of Physics Today, because he kept coming across the wrong explanation for it, one that dates back more than a century.
       
This explanation takes advantage of an unusual property of water: the solid form, ice, is less dense than the liquid form. That is why ice floats on water, while a cube of frozen alcohol — which has a freezing temperature of minus 173 degrees Fahrenheit — would plummet to the bottom of a glass of liquid alcohol. The lower density of ice also means that the melting temperature of ice can be lowered below the usual 32 degrees by squeezing on it.   
     
According to the frequently cited — if incorrect — explanation of why ice is slippery under an ice skate, the pressure exerted along the blade lowers the melting temperature of the top layer of ice, the ice melts and the blade glides on a thin layer of water that refreezes to ice as soon as the blade passes.     
   
"People will still say that when you ask them," Dr. Rosenberg said. "Textbooks are full of it."     
   
But the explanation fails, he said, because the pressure-melting effect is small. A 150-pound person standing on ice wearing a pair of ice skates exerts a pressure of only 50 pounds per square inch on the ice. (A typical blade edge, which is not razor sharp, is about one-eighth of an inch wide and about 12 inches long, yielding a surface area of 1.5 square inches each or 3 square inches for two blades.) That amount of pressure lowers the melting temperature only a small amount, from 32 degrees to 31.97 degrees. Yet ice skaters can easily slip and fall at temperatures much colder. 
     
The pressure-melting explanation also fails to explain why someone wearing flat-bottom shoes, with a much greater surface area that exerts even less pressure on the ice, can also slip on ice.   
   
Two alternative explanations have arisen to take the pressure argument's place. One, now more widely accepted, invokes friction: the rubbing of a skate blade or a shoe bottom over ice, according to this view, heats the ice and melts it, creating a slippery layer.     
 
The other, which emerged a decade ago, rests on the idea that perhaps the surface of ice is simply slippery. This argument holds that water molecules at the ice surface vibrate more, because there are no molecules above them to help hold them in place, and they thus remain an unfrozen liquid even at temperatures far below freezing.       
Scientists continue to debate whether friction or the liquid layer plays the more important role. Dr. Rosenberg, asked his opinion, chose a indecisive answer: "I say there are two major reasons."   
   
The notion that ice has an intrinsic liquid layer is not a new concept. It was first proposed by the physicist Michael Faraday in 1850 after a simple experiment: he pressed two cubes of ice against each other, and they fused together. Faraday argued that the liquid layers froze solid when they were no longer at the surface. Because the layer is so thin, however, it was hard for scientists to see. 
     
In 1996, Gabor A. Somorjai, a scientist at Lawrence Berkeley Laboratory, bombarded the surface of ice with electrons and watched how they bounced off, producing a pattern that looked at least partially liquid at temperatures down to minus 235 degrees. A couple of years later, a team of German scientists bounced helium atoms off ice and found results that corroborated the Lawrence Berkeley findings.     
 
"The water layer is absolutely intrinsic to ice," Dr. Somorjai said.
       
The findings, he said, fit with a simple observation that suggests friction cannot be the one and only explanation of slipperiness. When a person stands on ice, he added, no heat is generated through friction, and yet "ice is still slippery."   
     
But a colleague of Dr. Somorjai's at Lawrence Berkeley, Miquel Salmeron, while he does not dispute Dr. Somorjai's experiment, does dispute the importance of the intrinsic liquid layer to slipperiness. 
     
In 2002, Dr. Salmeron and colleagues performed an experiment. They dragged the tip of an atomic force microscope, resembling a tiny phonograph needle, across the surface of ice. 
     
"We found the friction of ice to be very high," Dr. Salmeron said. That is, ice is not really that slippery, after all.     
   
Dr. Salmeron said that this finding indicates that while the top layer of ice may be liquid, it is too thin to contribute much to slipperiness except near the melting temperature. In his view, friction is the primary reason ice is slippery. (The microscope tip was so small that its friction melted only a tiny bit of water, which immediately refroze and therefore did not provide the usual lubrication, he said.) 
     
Dr. Salmeron concedes, however, that he cannot definitively prove that his view is the correct one.   
     
"It's amazing," he said. "We're in 2006, and we're still talking about this thing."     
   
Ice formed by water behaves even more strangely at lower temperatures and higher pressures. 
     
Water — H2O — seems to be a simple molecule: two hydrogen atoms connected to a central oxygen atom in a V-shape. In everyday ice, which scientists call Ice Ih, the water molecules line up in a hexagonal pattern; this is why snowflakes all have six-sided patterns. (The "h" stands for hexagonal. A variation called Ice Ic, found in ice crystals floating high up in the atmosphere, forms cubic crystals.) 
     
The crystal structure of the ice is fairly loose — the reason that Ice Ih is less dense than liquid water — and the bonds that the hydrogen atoms form between water molecules, called hydrogen bonds, are weaker than most atomic bonds.     
   
At higher pressures, the usual hexagonal structure breaks down, and the bonds rearrange themselves in more compact, denser crystal structures, neatly labeled with Roman numerals: Ice II, Ice III, Ice IV and so on. Scientists have also discovered several forms of ice in which the water molecules are arranged randomly, as in glass. 
     
At a pressure of about 30,000 pounds per square inch, Ice Ih turns into a different type of crystalline ice, Ice II. Ice II does not occur naturally on Earth. Even at the bottom of the thickest portions of the Antarctic ice cap, the weight of three miles of ice pushes down at only one-quarter of the pressure necessary to make Ice II. But planetary scientists expect that Ice II, and possibly some other variations, like Ice VI, exist inside icier bodies in the outer solar system, like the Jupiter moons Ganymede and Callisto.     
   
With pressure high enough, the temperature need not even be cold for ice to form. Several Februaries ago, Alexandra Navrotsky, a professor of chemistry, materials science and geology at the University of California, Davis, was visiting Northwestern. She was sitting in office of Craig R. Bina, a geophysicist, and looking out over frozen Lake Michigan. "Ice might have been on our minds," she recalled. 
     
The scientists started considering what happens to tectonic plates after they are pushed back down into Earth's interior. At about 100 miles down, the temperature of these descending plates is 300 to 400 degrees — well above the boiling point of water at the surface — but cool compared with that of surrounding rocks. The pressure of 700,000 pounds per square inch at this depth, Dr. Bina and Dr. Navrotsky calculated, could be great enough to transform any water that was there into a solid phase known as Ice VII.       
No one knows whether ice can be found inside Earth, because no one has yet figured out a way to look 100 miles underground. Just as salt melts ice at the surface, other molecules mixing with the water could impede the freezing that Dr. Bina and Dr. Navrotsky have predicted.   
   
Ice also changes form with dropping temperatures. In hexagonal ice, the usual form, the oxygen atoms are fixed in position, but the hydrogen bonds between water molecules are continually breaking and reattaching, tens of thousands of times a second.
       
At temperatures cold enough — below minus 330 degrees — the hydrogen bonds freeze as well, and normal ice starts changing into Ice XI.
       
William B. McKinnon, a professor of earth and planetary sciences at Washington University in St. Louis, said that astronomers were probably already looking at Ice XI on the surface of Pluto and on the moons of Neptune and Uranus. But instruments currently are not sensitive enough to distinguish the slight differences among the ices.     
 
The most recently discovered form of ice, Ice XII, was found just a decade ago, although hints of it had been seen years earlier. John L. Finney of University College London, one of the discoverers of Ice XII, said that trying to understand all the different forms of ice was important for an understanding of how the water molecule works, and that was important in understanding how water interacts with all the biological molecules in living organisms.       

"It gives you a very stringent test for our understanding of the water molecule itself," he said. 



A more scholary article can be found at:   http://lptms.u-psud.fr/membres/trizac/Ens/L3FIP/Ice.pdf

I knew that.

[r0tor]

Conclusion?

This has morphed into a very interesting discussion, BTW.

To me it looks like they didn't stop the Subaru half way up the ramp and definitely had the traction control engaged as you could see the wheel spin being controlled - resulting in more traction.  My guess is in the Mazda which most impressively spun all 4 wheels together, had the traction control turned off and the driver didn't give a crap about spinning the wheels.

[MX793]

Static friction is not independent of surface area.  Sliding friction is.  A tire not spinning or sliding is in static friction and thus dependent on the surface area.

Static friction is indeed independent of surface area.  That's physics 101.

[FoMoJo]

To me it looks like they didn't stop the Subaru half way up the ramp and definitely had the traction control engaged as you could see the wheel spin being controlled - resulting in more traction.  My guess is in the Mazda which most impressively spun all 4 wheels together, had the traction control turned off and the driver didn't give a crap about spinning the wheels.

I guess that's the key to making it up the ramp...leave the traction control on.  The RAV 4 had some weird dynamics going on with wheel spin.

[r0tor]

Static friction is indeed independent of surface area.  That's physics 101.

Try physics 201 then.  Static friction is not = u * Fn

If I have 1 piece of duct tape I can hang a 10lb weight from the ceiling.  If I have 100 pieces of duct tape, I can hang a small child from the ceiling.

[r0tor]

I guess that's the key to making it up the ramp...leave the traction control on.  The RAV 4 had some weird dynamics going on with wheel spin.

I have seen enough Rav 4s in the snow to know their AWD system is a horrendous clusterfutz

[GoCougs]

Try physics 201 then.  Static friction is not = u * Fn

If I have 1 piece of duct tape I can hang a 10lb weight from the ceiling.  If I have 100 pieces of duct tape, I can hang a small child from the ceiling.

What???

[FoMoJo]

I have seen enough Rav 4s in the snow to know their AWD system is a horrendous clusterfutz

Amazing that they don't use the Subaru system then.

[SJ_GTI]

My brother and his wife have a Rav4. He said it does well, better than his GTI (which itself is pretty good).

[r0tor]

What???

Am I really the only one that paid attention in physics...

[MX793]

Try physics 201 then.  Static friction is not = u * Fn

If I have 1 piece of duct tape I can hang a 10lb weight from the ceiling.  If I have 100 pieces of duct tape, I can hang a small child from the ceiling.

Adhesion != Friction.

Put a brick on a plank of wood with it's largest face in contact.  Elevate one end of the plank until the brick slides.  Now place one of the smaller sides on the plank and repeat.  I can damn near guarantee that the angle at which it slides is the same in both cases.

[r0tor]

Adhesion != Friction.

Put a brick on a plank of wood with it's largest face in contact.  Elevate one end of the plank until the brick slides.  Now place one of the smaller sides on the plank and repeat.  I can damn near guarantee that the angle at which it slides is the same in both cases.

Try it...

For the rest of us, a nail nailed into a board is held into wood by static friction.  What is harder to pry out of the board - a small finishing nail or a huge ass nail.  Of course its the huge ass nail... Yeesh

And by the way, if I am took your brick and super glued it without you knowing it, you would describe it as having a hell of a lot of friction.