[Vwdiesel] mileage

[decker at toledotel.com]

Hi Eric;
    You and Val can argue about physics but in a real world of stopping a
big diesel tractor and a long semi-trailer or a set of doubles I can tell
you about how they brake since I have done it for 50 years. If you are empty
and in a panic stop you put on the air and the tires will almost invariably
smoke (skid) and then bounce so you have to back off on the air till they
start rolling again and have some adhesion to the pavement. Between air time
and skid time you do very little braking. If you are loaded to say 100,000
pounds you have so much mass moving that when you put on the air and the
wheels lock the tires skid and you aren't stopping well. If you are lightly
loaded you have a pretty good balance between the mass you are trying to
stop and the adhesion your tires have on the pavement so you stop rather
quickly. Read about ABS brakes.  A big share of you new cars have them
because the computer can sense when the tires would skid and can modulate
brake pressure to prevent you from skidding them no matter how had you press
on the brake pedal.
    One of the tough things about driving a big rig in traffic is when you
leave enough following distance to be safe most likely some stupid car
driver will pull in front of you and behind the rigs in front of him.
Apparently they don't comprehend that they have just driven their mobile
coffin into emanate danger till you can back off enough to have a safe
following distance. Sometimes it feels like you are driving your truck
backwards. When you are drafting you don't bother me since what is behind me
has to look out for themselves. Last year we had a driver with a set of
doubles just barely get stopped because some fool in a motor home just
stopped in the road in front of him for no apparent reason. It would not
have been a problem except that the new  Ford pickup that was following to
close got eaten up when he hit the back trailer. It probably was inattention
since he should have been able to stop faster than our doubles driver. He
wasn't hurt it totaled his pickup.
Brian Decker
----- Original Message ----- 
From: "Erik Lane" <eriklane at gmail.com>
To: "Val Christian" <val at valchristian.com>
Cc: <vwdiesel at vwfans.com>
Sent: Saturday, November 28, 2009 6:44 PM
Subject: Re: [Vwdiesel] mileage


On Sat, Nov 28, 2009 at 5:26 PM, Val Christian <val at mongo.mongobird.com>
wrote:
> Apparently the physics classes didn't work.
>
> Assume that you have a human sized sheet of sandpaper. Put a feather on
> the sandpaper sheet and try to pull it. No problem, just a little
> resistance, as you scuff the finish on the wood floor.
>
> Now on that human sized sheet of sandpaper, put the biggest, meanest
> and heaviest person you can think of. Now pull it. Can't move it?
>
> That's the physics part.
>
>
> F(f) = uN, where N is the normal force. Weight and N work together,
> and the big guy on the sandpaper increases the N. Therefore the
> frictional force increases linearly with an increase on the weight
> (hence the normal force).
>
> Seriously, you took university physics in college, and engineering
> courses? Sorry I'm being hard, but it's pretty elementary.


You're right, it's elementary. You completely forgot about inertia.
The frictional force increase is directly in proportion to the
increase in weight, but the inertia is also a direct proportion. The
feather takes almost nothing to stop, while the heavy person takes a
*lot* to stop. The equations for the distance that something will
travel don't have weight in them, because it cancels out. (Well,
actually mass, but since we're doing all of this on Earth, that's not
an important distinction.)

If you want to play this game some more I can go find equations to back it
up.

Erik
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