Date:         Thu, 18 Aug 2005 08:39:04 -0700
Reply-To:     John Bange <jbange@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         John Bange <jbange@GMAIL.COM>
Subject:      Re: How much fuel efficiency gained by shedding 300 lbs?
In-Reply-To:  <25340-43040D0C-1356@storefull-3178.bay.webtv.net>
Content-Type: text/plain; charset=ISO-8859-1

> You have to be able to maintain the mass movement.
> And this is where the gas mileage gets eaten up, being able to maintain
> movement.
> 
> On hill or dale, or on the flat.

Hill or dale, yes, you will eat more fuel pulling that weight; but on
flat ground, not really. Isaac Newton's First Law of Motion (Law of
Inertia):
"An object at rest tends to stay at rest and an object in motion tends
to stay in motion with the same speed and in the same direction"
There are only two forces acting on the inertia of an already rolling
vehicle: rolling friction and wind resistance.

> 
> <<but on flat ground at steady speed the van could be 6 tons and you'd
> get the same MPG.>>
> 
> Nope.
> 
> Lay 12,000 lbs into your Van and see how wonderful the fuel economy is,
> on a window pane flat pavement.
> 
> I'll run mine empty, at the same exact speed as your going, all the
> time.
> 
> It takes work to maintain the movement that your Vans going to require
> to move and maintain a specific speed that 12,000 lbs. mass will
> require.

You're correct, but only for insignificant quantities of "correct".
Rolling friction with an impossible 6 ton load will account for an
approximate loss of 20hp. This is less than the horsepower loss
between 60mph and 75mph. This may seem like a lot, but the point is
that it's still only a fraction of the wind resistance loss, even when
it's EIGHT TIMES the practical load limit of the vehicle.
  
> <<At steady speed you are not accelerating the mass, you are only
> pushing the wind out of the way, so mass doesn't matter.>>
> 
> Nope.
>

YES. See The Law of Inertia!
 
> And how do you get to this magic steady speed ?
> 

We're NOT TALKING ABOUT ACCELERATION. This is about MPG while
maintaining a steady highway speed. We've already allowed that extra
weight takes more fuel to bring up to speed or pull up a hill. Flat
ground, constant speed.

> Forget the wind, it's only part of the problem you drew.
> 

What? Wind resistance is MOST of the physics problem. Getting back to
the point of the original discussion, 300 pounds will make a
difference of 1hp on the rolling resistance side. On the wind
resistance side, 1hp is LESS than the difference in horsepower
requirement  between 60mph and 61mph.

> I think this is a real bad example of what your trying to say.
> 

I think he was exagerrating to make a point.