mpg and weight

[GeekGuyAndy]

Mentalic, what about parking in N and using the hand brake? Does that count as not using the brake? Maybe just putting blocks around the wheels too. I can picture someone jumping out a 2mph and running infront of the car and pushing it to a stop to not use the brake. Or how about a good foot drag, that will slow it down too!

[skewbe]

All this brake stuff is based on the premis that if you so much as touch the brakes, or look at them wrong, then there can be no benefit to having a little extra weight ever or under any circumstances. It is a silly thing.

Back to the real question, weight v/s mpg:
Sailplanes may add weight so they can win cross country and endurance competitions, an example of extreme pulse and glide. Discuss.

[Bill in Houston]

Originally Posted by skewbe

Back to the real question, weight v/s mpg:
Sailplanes may add weight so they can win cross country and endurance competitions, an example of extreme pulse and glide. Discuss.

We kind of beat this to death while you were gone, just a week or two before you came back. It was my assertion that theoretically, done right, on the right terrain, extra weight could give you a better P&G and better mileage. In most people's daily driving, extra weight will lower mileage, though.

[GeekGuyAndy]

Sailplanes (gliders) are a bad example I think. Aren't they just towed to altitude and let go? If I was towed to the top of a hill and let go of course I would want more weight, but since I have to drive up there first I think less weight is always better. Unless I could always drive downhill

[lca13]

I think Bill is right. I also think there are no real world examples of the theoretical case... but we did beat it to death. The whole glide twice as long argument is invalid because it takes twice the energy to get to speed.... ie, half weight cars may not glide as far but you can get two accel/glides for every one of the heavier (minus waste).

Conservation fo energy is the defining argument here. As long as there is waste, more energy expended is worse, even if you think you are getting more of it back.... the most fuel efficient vehicle will weight zero lbs, and while there may be a sweet spot where waste energy is so high that the extra energy used in accelerating the heavier car can acutally do better than the slightly lighter car (but all operating conditions might have to be right), this sweet spot weight is likely very low WRT typical weights... like maybe the weight of the engine only moving itself at 20% efficiency.... add a few lbs and pulse and glide just right and you migh see an advantage.

[Sludgy]

Weight causes several subtle changes in a car's mileage.

On the plus side:

Rolling resistance goes down.
Less fuel is expended accelerating the car mass a given speed.

On the negative side:

The suspension and tires are more lightly loaded, raising the car and increasing its effective frontal area.
On gas cars, the lower load on the engine means a more closed throttle, and higher pumping loss.

There are lots of reasons that mileage isn't proportional to vehicle mass.

[cfg83]

Bill -

Originally Posted by Bill in Houston

We kind of beat this to death while you were gone, just a week or two before you came back. It was my assertion that theoretically, done right, on the right terrain, extra weight could give you a better P&G and better mileage. In most people's daily driving, extra weight will lower mileage, though.

I think you are referring to this thread :

Weight reduction - not always good?
http://www.gassavers.org/showthread.php?t=4050

I think there is a "sweet spot" for every car, but it's dependent on alot of other external conditions being just right (traffic, commute topography, blah blah blah ...).

CarloSW2