Cirrus Killer? Cessna just doesn't get it...

"Matt Whiting" wrote in message
...
More weight means more total friction all else being equal, but it
doesn't, to a first order, change the coefficient of friction. That is
largely a function of the materials that are in contact. The total
friction force is the coefficient of friction times the normal force
(weight in this case) clamping the two surfaces together.

Actually, the determining factor is tire pressure. Now, many trucks and
SUVs use higher tire pressures than what is normally found in passenger
cars, but some are the same or lower. The actual weight of the vehicle
isn't that important, assuming adequate brakes (which, when on snow or other
low-friction surfaces, is the case for basically every vehicle), as long as
the tire pressure is sufficiently high.

As with most generalizations, one cannot simply say "all SUVs are bad", no
more than one can say "all passenger cars are good".

As far as I'm concerned, all you on the pro-SUV of this debate just got
baited into one of the classic stupid debates. You might as well be arguing
Ford vs Chevy or high-wing vs low-wing. The people claiming there's no
valid reason for driving an SUV don't have a clue, and the pro-SUV folks are
unlikely to change that. At the same time, anyone defending ALL SUVs in ALL
situations is just as lacking in clues (not that I see much of that, but
still...)

I feel dumb even bothering to post to this topic...but too many posts have
gone by arguing that weight matters, without a single mention of what really
affects the friction between the tire and driving surface: tire pressure.

Pete

Peter Duniho wrote:

"Matt Whiting" wrote in message
...

More weight means more total friction all else being equal, but it
doesn't, to a first order, change the coefficient of friction. That is
largely a function of the materials that are in contact. The total
friction force is the coefficient of friction times the normal force
(weight in this case) clamping the two surfaces together.


Actually, the determining factor is tire pressure. Now, many trucks and
SUVs use higher tire pressures than what is normally found in passenger
cars, but some are the same or lower. The actual weight of the vehicle
isn't that important, assuming adequate brakes (which, when on snow or other
low-friction surfaces, is the case for basically every vehicle), as long as
the tire pressure is sufficiently high.

How so?


As with most generalizations, one cannot simply say "all SUVs are bad", no
more than one can say "all passenger cars are good".

As far as I'm concerned, all you on the pro-SUV of this debate just got
baited into one of the classic stupid debates. You might as well be arguing
Ford vs Chevy or high-wing vs low-wing. The people claiming there's no
valid reason for driving an SUV don't have a clue, and the pro-SUV folks are
unlikely to change that. At the same time, anyone defending ALL SUVs in ALL
situations is just as lacking in clues (not that I see much of that, but
still...)

I feel dumb even bothering to post to this topic...but too many posts have
gone by arguing that weight matters, without a single mention of what really
affects the friction between the tire and driving surface: tire pressure.

That is because you aren't correct. Tire pressure only has a
significant impact on very soft surfaces such as sand, where the extra
surface area helps with flotation. In most snow, it makes little
difference.

Matt

"Matt Whiting" wrote in message
...
[...]
That is because you aren't correct. Tire pressure only has a significant
impact on very soft surfaces such as sand, where the extra surface area
helps with flotation. In most snow, it makes little difference.

Actually, we're both incorrect.

My statement was based on a theoretical understanding of friction in which
the friction depends on the force over an area. Since tire pressure
directly determines this, I assumed it had a direct effect on friction.

I found at least one reference that says that physicists ignore the area
over which the force is distributed, for the purpose of determining
friction. It did say that's actually an incorrect assumption, but that it's
"close enough" for most purposes.

I didn't bother to look further to see just how far off this "close enough"
assumption is. The reference didn't go into much detail on that regard.

Beyond that, the same reference also had a discussion of tires on snow,
oddly enough (I wasn't even looking for that specifically). They claim that
increased tire pressure actually *reduces* friction, because packed snow has
lower friction than unpacked snow, and higher tire pressures result in
greater packing of the snow.

So, tire pressure has a very significant effect on tire friction when
driving on snow. But it's opposite what would be the case on a solid
surface. So, chalk that point up for the anti-SUV crowd.

I still think it's a silly argument.

Pete

Peter Duniho wrote:

"Matt Whiting" wrote in message
...

[...]
That is because you aren't correct. Tire pressure only has a significant
impact on very soft surfaces such as sand, where the extra surface area
helps with flotation. In most snow, it makes little difference.


Actually, we're both incorrect.

My statement was based on a theoretical understanding of friction in which
the friction depends on the force over an area. Since tire pressure
directly determines this, I assumed it had a direct effect on friction.

I don't recall area being a part of the thoeretical equation. My
Physics book says that F=uN, where F is the total force due to friction,
u (mu) is the coefficient of static or dynamic friction as the case may
be, and N is the normal force holding the two surfaces together. Area
isn't part of the equation. Now there are materials reasons that area
does have an impact, that that isn't in the basic theory.


I found at least one reference that says that physicists ignore the area
over which the force is distributed, for the purpose of determining
friction. It did say that's actually an incorrect assumption, but that it's
"close enough" for most purposes.

Actually, every reference I've ever seen ignores area, because it is
only a factor in special circumstances and then it is related to the
materials failing, not to the underlying theory of friction.


I didn't bother to look further to see just how far off this "close enough"
assumption is. The reference didn't go into much detail on that regard.

That is because you are wrong and didn't want to further show that.


Beyond that, the same reference also had a discussion of tires on snow,
oddly enough (I wasn't even looking for that specifically). They claim that
increased tire pressure actually *reduces* friction, because packed snow has
lower friction than unpacked snow, and higher tire pressures result in
greater packing of the snow.

That is a somewhat specious description, but in any event tire pressure
is at best a second or third order effect, it isn't a first order affect.


So, tire pressure has a very significant effect on tire friction when
driving on snow. But it's opposite what would be the case on a solid
surface. So, chalk that point up for the anti-SUV crowd.

Still wrong. Tire pressure is little affect.


I still think it's a silly argument.

Yes, when I'm shown to be wrong, I usually think it was a silly argument
at that point as well. :-)

Matt

"Matt Whiting" wrote in message
...
[...]
Actually, every reference I've ever seen ignores area, because it is only
a factor in special circumstances and then it is related to the materials
failing, not to the underlying theory of friction.

Whatever. It should be painfully obvious that area DOES have an effect,
since you cannot decrease the area arbitrarily without affecting friction.
Make the contact point small enough, and friction WILL increase.

[...]
That is because you are wrong and didn't want to further show that.

Now you're just being stupid. I found a reference that showed I was wrong,
commented on that here, and you're accusing me of not want to FURTHER show I
was wrong? Just how much "wrong" do you need someone to own up to before
you are finished insulting them?

[...]
That is a somewhat specious description, but in any event tire pressure is
at best a second or third order effect, it isn't a first order affect.

No one said anything about whether it's "first order", "second order", or
"millionth order". You claimed it "makes little difference", when in fact
it does make more than a "little difference".

[...]
I still think it's a silly argument.

Yes, when I'm shown to be wrong, I usually think it was a silly argument
at that point as well. :-)

I was talking about the argument about whether SUVs are a valid choice for
any driver, genius.

Pete

"Matt Whiting" wrote in message \

That is because you aren't correct. Tire pressure only has a significant
impact on very soft surfaces such as sand, where the extra surface area
helps with flotation. In most snow, it makes little difference.

Makes a big difference on packed snow and ice. Ice racers use inner tubes
and run the tire pressures very low. Pump them up and they don't stick.

moo

Happy Dog wrote:

"Matt Whiting" wrote in message \


That is because you aren't correct. Tire pressure only has a significant
impact on very soft surfaces such as sand, where the extra surface area
helps with flotation. In most snow, it makes little difference.


Makes a big difference on packed snow and ice. Ice racers use inner tubes
and run the tire pressures very low. Pump them up and they don't stick.

Most also use studs or spikes. :-)


Matt