you put 500 pounds on 1sq inch versus 500 pounds on 100sq inches ... it's the same "force"

500lbs on 1sq inch = 500 lbs per sq inch

500lbs on 100sq in = 5 lbs per sq inch.

BIG DIFFERENCE
[/quote]

still the SAME force ... yes the "per sq inch" is 1/100th but you also have 100 more ... it's still a normal force of 500lbs

Here, simply go read
http://en.wikipedia.org/wiki/Traction_%28engineering%29

Note, the "common" approximation formula for friction has NOTHING todo with pounds per sq. inch ... it simply is the normal force and the coefficient of friction ...

BUT ...

It is important due to broad application to point out the specific case of multi-wheeled vehicles or vehicles with multiple contact patches between the tire and the road surface. The constant coefficient of friction approximation is not adequate to describe real world maximum traction situations. If the normal force is increased, per given area of contact patch, the coefficient of friction decreases and as the normal force decreases, the coefficient of friction increases. If this were not true, then increasing tire width, lowering tire air pressure or increasing tire diameter (all of which increase the area of the contact patch) would have little effect.

Read this part AGAIN

If the normal force is increased, per given area of contact patch, the coefficient of friction decreases and as the normal force decreases, the coefficient of friction increases. If this were not true, then increasing tire width, lowering tire air pressure or increasing tire diameter (all of which increase the area of the contact patch) would have little effect.

This could also be stated to say ...
"If the contact patch is increased, per given normal force, the frictional force increase, and as the contact patch decrease, the frictional force decreases."

To spell it out ...

... If the normal force is increased, per given area of contact patch, the coefficient of friction decreases ...

SO ... 500 pounds on one square inch will have a LOWER coefficient of friction. call this U1
if I have 1 of these I have a normal force of 500 lbs

... and as the normal force decreases, the coefficient of friction increases.

SO ... 5 pounds in one square inch will have a HIGHER coefficient of friction. call this U2
if I have 100 of these I have a normal force of 500 lbs

SO ...

U2 * 500 lbs > U1 * 500 lbs

Note, the "common" approximation formula for friction has NOTHING todo with pounds per sq. inch ... it simply is the normal force and the coefficient of friction ...

Here is a quote from the wikipedia article you pointed me to:

Traction between two surfaces usually depends on several factors including

Material properties of each surface.
Macroscopic and microscopic shape or "roughness".
Force of contact.
Area of contact.
Contaminants at the material boundary including lubricants and adhesives

Notice the third factor there where it says force of contact, that is PSI

To spell it out ...

Quote
... If the normal force is increased, per given area of contact patch, the coefficient of friction decreases ...
SO ... 500 pounds on one square inch will have a LOWER coefficient of friction. call this U1
if I have 1 of these I have a normal force of 500 lbs


Quote
... and as the normal force decreases, the coefficient of friction increases.

SO ... 5 pounds in one square inch will have a HIGHER coefficient of friction. call this U2
if I have 100 of these I have a normal force of 500 lbs

SO ...

U2 * 500 lbs > U1 * 500 lbs

WHAT?

Look it is very simple, 4,000lb vehicle 1,000lbs on each tire. If the contact area is 10 square inches then the force is 100lbs per square inch. If the contact are is 20 square inches then the force is 50lbs per square inch.

Now back to ice. when a vehicle is driving on ice many of the components of traction are compromised, One aspect of traction which is not affected is (force of contact) see above. The greater the force between the tire and the ice the more traction you are going to have. Period, end of story.

Look what is your point with all this, If you are saying that a tire with a smaller contact patch on ice does not provide more traction than a larger patch when all other factors, vehicle weight, tread, rubber compound, etc are equal then you are just plain wrong.

A 32 inch tire (what 10.50, 11.50?) is a huge tire to run on a vehicle as light as a Samurai in the winter. You don't specify what surface you are talking about when you say "I lower the psi to 5psi or less and that thing will go anywhere" It has already been discussed that the off road trucks in Greenland run big wide flotation tires at very low tire pressures. This is of course because they want the trucks to stay on top of the snow, and the way to do that is to increase the size of the contact area between the tire and snow, which decreases the PSI between the surfaces. If this is what you are doing then you are right. However as I stated above a tire at 5 psi is not going to be safe at hwy speeds, and we are talking about snow now not ice.

The tires are 32"x10.50 TSL radials. And I am talking snow. Last year on the first snow fall we got about 4" of fresh snow before I had a chance to lower the air pressure from 15psi in the tires and my Sammy would not get out of it's own way on the way to work. The snow was starting to melt and get slushy so it was not packed slick snow. After work I lowered the pressure to 5psi and had no problems, and the snow conditions were the same.

Want to try explaining why I had more traction after lowering the PSI with your theories?

As for the surface, it be anything. Better traction in the woods around here, won't sink in the sand, made getting to work easy and put the jeep boy's to shame when it broke trail in snow run's.


And remember, these things are light. My sammy is way light and tire PSI is a different thing. Take my "huge" tires for example. Even though it is a radial, the sidewalls do not even start to show a bulge till the PSI is under 10psi. At 5psi they look like a standard radial with 32psi. I have taken them down to 2psi and they still do not look any where near flat.

So a 5000lb rig with the same tires and psi is going to act different than a rig that weighs 2500lb's, or less.

Want to try explaining why I had more traction after lowering the PSI with your theories?

They are not my Theories, but common knowledge, in Michigan were I learned these techniques and they were confirmed by independent research done by spyder0069

Also, I was agreeing with you in the post you replied to. That whole bit about the trucks in Greenland was confirming what you said was true.

I did not pay close enough attention to what spyder0069 said earlier about deep snow. I was assuming he was talking about road travel. Sure in deep fresh snow lower pressures are better. It would be a very rare situation however where lowering tire PSI would be useful for road travel.

I run 20psi on my sami all day long maybe 25psi on the highway if I'm going on a long trip  I run 15psi on snow and ice because it does better for ME then 20psi and thats with a 31x11.50.

But slush  that's a different story

I'm not trying to start an argument, I'm just stating what works best for ME


Ryan

It would be a very rare situation however where lowering tire PSI would be useful for road travel.

As I have posted, my Samurai will not drive down the road safely with a tire pressure above 10psi in the winter weather. It is all over the place because it wants to wander because it has no traction or grip. I lower the psi to around 5psi and I have control.


You state you are in your 50's. What did your grandfather tell you about driving in the snow?

My grandfather told me thirty years ago that for snow you need narrow tires to help dig down to the road surface in loose snow and wide tires to help in the slick stuff like ice and packed snow. Then again he was an old time truck driver.

But think about it. Take a block of ice and see how easy a razor blade will go from one end to the other, yes you can use the back side of the blade if you want. Now take a 1/2" piece of an old wiper blade and see how easy it is to go from one end to the other, and yes you can use the wiping edge.



By the way, wet melting ice is a whole other subject.   Keep you ice near freezing.

Never mind sidekicksrock, I see you are a flat lander. Things are a bit different when you have to go up and down hills in this stuff.

You only have about a 1300' change in elevation across your whole state. I get close too that driving the 7 miles to work.

I'll tell you something

The snow tire's on my beater I have no idea on their psi, they are not flat and they get me back and forth to work.

But it is stock and when you start modding things you need to change the way you think about things.

Quote

Note, the "common" approximation formula for friction has NOTHING todo with pounds per sq. inch ... it simply is the normal force and the coefficient of friction ...

Here is a quote from the wikipedia article you pointed me to:

Traction between two surfaces usually depends on several factors including

Material properties of each surface.
Macroscopic and microscopic shape or "roughness".
Force of contact.
Area of contact.
Contaminants at the material boundary including lubricants and adhesives

Notice the third factor there where it says force of contact, that is PSI

no, that is the force normal. force is *NOT* pounds per square inch (see below)

The other 4 factors impact the coefficient of friction. It is the fourth factor that raises the coefficient as the area of contact increases. Obviously the other impacts on the coefficient can come into play as well (e.g. hydroplaning is a form of the lubricant and it's effect will "lower" the coefficient as the area increases).

Quote

To spell it out ...

Quote
... If the normal force is increased, per given area of contact patch, the coefficient of friction decreases ...
SO ... 500 pounds on one square inch will have a LOWER coefficient of friction. call this U1
if I have 1 of these I have a normal force of 500 lbs


Quote
... and as the normal force decreases, the coefficient of friction increases.

SO ... 5 pounds in one square inch will have a HIGHER coefficient of friction. call this U2
if I have 100 of these I have a normal force of 500 lbs

SO ...

U2 * 500 lbs > U1 * 500 lbs

WHAT?

Look it is very simple, 4,000lb vehicle 1,000lbs on each tire. If the contact area is 10 square inches then the force is 100lbs per square inch. If the contact are is 20 square inches then the force is 50lbs per square inch.

Look it is very simple, 4,000lb vehicle 1,000lbs on each tire. If the contact area is 10 square inches then the force is 100lbs per square inch. If the contact are is 20 square inches then the force is 50lbs per square inch.

Now back to ice. when a vehicle is driving on ice many of the components of traction are compromised, One aspect of traction which is not affected is (force of contact) see above. The greater the force between the tire and the ice the more traction you are going to have. Period, end of story.

You still don't get it ... the force is the SAME ... it's 1000 lbs.
Force is NOT measured in "pounds per square inch" it's measured in pounds (or newtons). Pressure is force divided by area and is measured in puonds per square inch (or newtons per square meater, or pascals).

Look what is your point with all this, If you are saying that a tire with a smaller contact patch on ice does not provide more traction than a larger patch when all other factors, vehicle weight, tread, rubber compound, etc are equal then you are just plain wrong.

 

PSI = Pounds per square inch = Force divided by the area

Units:  Pounds
Area:  Inchs

If you've ever had a trailer, when you go to disconect it from your truck, what is the first thing you do?  You put a board under it.  If you dont it will sink into the ground.

The Force is the same, but if you change the area of that force and spread it over a larger area, the PSI is lowered and the board will no sink into the ground.

A skinny tire and fully inflated tire will concentrate that force to a smaller area and will give you more digging power.  A wide flat tire will spread that force out and not dig so much and alow you to "float" more.

Now, given the weight of your vehicle and the size of your tire, mabe airing down will give you the performace that you are looking for.  Or mabe airing up is what you need.  Alot also depends on what medium you are driving on.

Now, given the weight of your vehicle and the size of your tire, mabe airing down will give you the performace that you are looking for.  Or mabe airing up is what you need.  Alot also depends on what medium you are driving on.

I'll go with that, very well said.

This does it for me I'm done. I am not going to argue about this anymore.

Yeah, I don't think there is a perfect answer here.  As I hit the slush and so forth probably would want more air, then on the same trip when I hit the fresh drifts at lower speed would probably want less pressure.  :^)  I think the better answer may be I need more weight in the rear.  I have the two 70lb bags of sand.  Anyone put more than that in?  Also anyone put extra weight in the rear in the summer to gain traction on the trails?  Just seems so light that the rear tires spin pretty easy.  The fronts grab and pull but its like the rears are outrunning the fronts.  :^)

Yeah, I don't think there is a perfect answer here. *snip* I think the better answer may be I need more weight in the rear. 

There may not be a perfect answer, but we all have a resposibility to help each other out, and just because 2 people get better traction with 42psi in their Tracker's tires doesn't mean it should be suggested to everyone else.  You have to remember, not everyone has been driving these for a long time, doing something like almost doubling tire pressure could get a new driver into trouble. Driving a Tracker in the winter for the first time can be scary, especially if you switched from a fwd car, or heavy pickup.
Now: having said that, I did do a quick google search and found this writeup on tirerack.com. It does prove your theory, but not for the same reasons, and not as extreme. They recommend 3-5psi for temperature offsets, softer winter tire makeup(winter tires are made of softer rubber). It doesn't say that it will help you cut through the snow better, or put more weight on the surface. I looked in my manual, and Suzuki does not recommend any pressure changes for winter, I'm guessing they assume there won't be a "summer" tire installed

http://www.tirerack.com/winter/tech/techpage.jsp?techid=168

It reminds me of a Dr Suess story where one creature is going west the other east and niether would step to the side to let the other past. The moral some psis work well for some  psis for others. Spyder0069 experiment safley and find what works best for you

This is my first winter (heck first month) with the sidekick.  Yesterday I put on 75 miles with the tires (235's) at 25 psi and it felt much more locked in.  Granted alot of the snow has melted.  But even before when it was dry it felt much more loose and of course rode hard.  Now I also repaired my tube that connects the front of the A arms that was rotted out too. I think that also played a big factor as the new tube didn't just slide through and I had to put a jack to it and pry it to fit through the other half.  That tells me that there is enough force to cause twisting /tweeking of the suspension if that is damaged and I bet could also have caused the loose feeling.  The new tube I ended up with was 1 5/8" outer diameter by 1/4" thick wall by 3 feet long.  Probably the toughest part on the suzuki now.   Installed a CB yesterday.  Picked up a small cobra from walmart for $35  and it fit perfectly where the ashtray was (had to pull out the metal ashtray bracket. Picked up a mirror mount antenna from our truck stop and side mounted it on the pillar behind the driver.  Ran the antenna wire through the zipper down the pillar, under the carpet, through the center console. Looks decent.