Cycling Mob Cycling Forums - View Single Post - "Humans 'very likely' making earth warmer" is wrong

02-09-2007, 06:04 AM

Bill Baka wrote:
> R.H. Allen wrote:
>> Deputy Dumbya Dawg wrote:
>>
>>> I may agree with you if this limo was in space but here on earth with
>>> gravity your argument does not hold water. The more weight (mass
>>> affected by the force of gravity) the more friction and the more
>>> energy to maintain the velocity.
>>
>> No. You're neglecting inertia. A moving object tends to keep moving,
>> and if it's heavy it's harder to stop than if it's light.
>
> That may be over simplifying the problem, but Newton and those other
> scientists did get it right, and that was taught way back in grade
> school.

Agreed, but it was sort of my response to the other poster's "you're
neglecting friction" comment. Would probably have been more obvious if I
had actually put it next to that comment, though.... :-)

> If this is done in neutral it will determine the effects of weight on
> the tires combined with the actual rolling resistance. Do it with the
> transmission in gear and you will really notice the difference of engine
> drag.

Sure, but if the two hypothetical vehicles are truly identical aside
from their weight, the engine drag should be the same for both. I'm just
trying to simplify the problem to show the other poster why (1) weight
results in negligible gasoline consumption when maintaining constant
speed and (2) a heavier car will travel farther in a coast, which is the
opposite of what he claimed.

>> -- which would produce a force of 620N. Over the course of a mile,
>> that would require 32,000J of extra energy compared to the lighter car
>> to maintain constant speed.

I made a bit of an error there -- the heavier car would require 480kJ of
extra energy compared to the lighter car, not 32kJ (assuming 600N and
300N are required to keep the heavy and light cars, respectively, at
constant speed). Assuming 30 mpg and 25% conversion efficiency, that's
40MJ/mile for the light car and 41.92MJ/mile for the heavy one, so the
added weight produces a 4.8% increase in fuel consumption. Considering
that we're talking about a *doubling* in weight -- essentially a fully
loaded one-ton pickup that itself only weighs a ton, or about a third of
what such vehicles usually weigh -- that's still pretty small. A more
realistic figure for such a vehicle would be more like 1.5% more
gasoline (though of course the whole analysis is so simplified that the
only real-world conclusion you can draw is that weight has a negligibly
small effect).

>> Let's say the lighter car gets 30 mpg and both cars transfer energy
>> from the gasoline to the road at 25% efficiency. There are 120 million
>> joules in a gallon of unleaded gasoline, so 40 million joules are
>> burnt each mile. The extra 32,000J the heavier car requires each mile
>> correspond to 128,000J/mile of extra gasoline. Therefore, the extra
>> weight degrades the car's mileage to 29.9 mpg. The difference of 0.1
>> mpg may as well be zero considering that it's an overestimate to begin
>> with, and that other factors such as driving habits and regular
>> vehicle maintenance (or lack thereof) make a far greater difference in
>> mileage than that.
>
> I was initially talking about a constant 65 MPH on cruise control or a
> very calculating driver who knows how to hold the speed with very little
> throttle jockeying.

I suspect something like rain-slickened roads or a difference in the
direction of the wind are enough to cause more than a 0.1 mpg difference
in mileage on repeated trials in either of those cases. The point being
that no matter how good your test conditions, I just don't think weight
is going to make a noticeable difference in gas mileage at constant speed.

>> Now if you factor in aerodynamic drag, both vehicles -- being
>> identical aside from weight -- will face the same drag force. They
>> will expend the same amount of energy overcoming it to maintain a
>> constant speed. However, if you let your foot off the gas and coast to
>> a stop, you'll find the heavier car coasts farther. I refer you back
>> to Netwon's second law to understand why.
>
> That applies very well to a car doing 65 MPH, but the place where you
> will find the answer to the amount of rolling resistance by the tires is
> around 20--30 MPH. The bleed off of speed with my big Chrysler was about
> 40 to 45 seconds with wind resistance being a lesser factor.
> Anyone who doubts the effect of the air should take a car out and try it
> for themselves.

I agree completely. Using the figures at:

http://en.wikipedia.org/wiki/Drag_coefficient

which themselves apparently come from Car & Driver magazine, I get 243N
drag for the 1999 Honda Insight at 65 mph and 36N at 25 mph. It weighs
834kg, so wind resistance will dominate rolling resistance at 65 mph,
and vice versa at 25 mph. The Hummer H2 at 25 mph faces as much drag as
the Insight at 55 mph, but because of its much greater weight you still
get more rolling resistance than wind resistance at that speed.

> BTW, the slick looking car may be less aerodynamic than the brick
> looking car. Why? Take a look at the underside and see how much junk the
> air on the bottom has to go through.

Well, that's not the *only* reason -- there's often a big difference
between what people think is slick-looking and what is actually slick
from an aerodynamic perspective. In general, a bulbous front end that
narrows toward the rear (a bit like the cross-section of an airplane
wing) will produce the best aerodynamics, but that's not a good look for
a sports car. Cars that look like they're designed to knife through the
air are often called slick, but generally have poor aerodynamics (though
it's often done on purpose to produce downforce for better traction,
which explains why Formula 1 race cars have far worse drag coefficients
than school buses and Hummers). If I'm not mistaken, the General Motors
EV1 has to this day the lowest drag coefficient ever measured on a
production vehicle.

That said, the junk on the bottom of the car makes a big enough
difference that an acquaintance of mine once considered starting a
business based on covering that stuff up to improve aerodynamics. He
abandoned the idea -- I think he concluded there wouldn't be a big
enough market in his area to make the business viable, but I don't
recall the details.

02-09-2007, 06:04 AM	#522 (permalink)
R.H. Allen Posts: n/a	Re: Why are SUVs and Christianity similar? Bill Baka wrote: > R.H. Allen wrote: >> Deputy Dumbya Dawg wrote: >> >>> I may agree with you if this limo was in space but here on earth with >>> gravity your argument does not hold water. The more weight (mass >>> affected by the force of gravity) the more friction and the more >>> energy to maintain the velocity. >> >> No. You're neglecting inertia. A moving object tends to keep moving, >> and if it's heavy it's harder to stop than if it's light. > > That may be over simplifying the problem, but Newton and those other > scientists did get it right, and that was taught way back in grade > school. Agreed, but it was sort of my response to the other poster's "you're neglecting friction" comment. Would probably have been more obvious if I had actually put it next to that comment, though.... :-) > If this is done in neutral it will determine the effects of weight on > the tires combined with the actual rolling resistance. Do it with the > transmission in gear and you will really notice the difference of engine > drag. Sure, but if the two hypothetical vehicles are truly identical aside from their weight, the engine drag should be the same for both. I'm just trying to simplify the problem to show the other poster why (1) weight results in negligible gasoline consumption when maintaining constant speed and (2) a heavier car will travel farther in a coast, which is the opposite of what he claimed. >> -- which would produce a force of 620N. Over the course of a mile, >> that would require 32,000J of extra energy compared to the lighter car >> to maintain constant speed. I made a bit of an error there -- the heavier car would require 480kJ of extra energy compared to the lighter car, not 32kJ (assuming 600N and 300N are required to keep the heavy and light cars, respectively, at constant speed). Assuming 30 mpg and 25% conversion efficiency, that's 40MJ/mile for the light car and 41.92MJ/mile for the heavy one, so the added weight produces a 4.8% increase in fuel consumption. Considering that we're talking about a doubling in weight -- essentially a fully loaded one-ton pickup that itself only weighs a ton, or about a third of what such vehicles usually weigh -- that's still pretty small. A more realistic figure for such a vehicle would be more like 1.5% more gasoline (though of course the whole analysis is so simplified that the only real-world conclusion you can draw is that weight has a negligibly small effect). >> Let's say the lighter car gets 30 mpg and both cars transfer energy >> from the gasoline to the road at 25% efficiency. There are 120 million >> joules in a gallon of unleaded gasoline, so 40 million joules are >> burnt each mile. The extra 32,000J the heavier car requires each mile >> correspond to 128,000J/mile of extra gasoline. Therefore, the extra >> weight degrades the car's mileage to 29.9 mpg. The difference of 0.1 >> mpg may as well be zero considering that it's an overestimate to begin >> with, and that other factors such as driving habits and regular >> vehicle maintenance (or lack thereof) make a far greater difference in >> mileage than that. > > I was initially talking about a constant 65 MPH on cruise control or a > very calculating driver who knows how to hold the speed with very little > throttle jockeying. I suspect something like rain-slickened roads or a difference in the direction of the wind are enough to cause more than a 0.1 mpg difference in mileage on repeated trials in either of those cases. The point being that no matter how good your test conditions, I just don't think weight is going to make a noticeable difference in gas mileage at constant speed. >> Now if you factor in aerodynamic drag, both vehicles -- being >> identical aside from weight -- will face the same drag force. They >> will expend the same amount of energy overcoming it to maintain a >> constant speed. However, if you let your foot off the gas and coast to >> a stop, you'll find the heavier car coasts farther. I refer you back >> to Netwon's second law to understand why. > > That applies very well to a car doing 65 MPH, but the place where you > will find the answer to the amount of rolling resistance by the tires is > around 20--30 MPH. The bleed off of speed with my big Chrysler was about > 40 to 45 seconds with wind resistance being a lesser factor. > Anyone who doubts the effect of the air should take a car out and try it > for themselves. I agree completely. Using the figures at: http://en.wikipedia.org/wiki/Drag_coefficient which themselves apparently come from Car & Driver magazine, I get 243N drag for the 1999 Honda Insight at 65 mph and 36N at 25 mph. It weighs 834kg, so wind resistance will dominate rolling resistance at 65 mph, and vice versa at 25 mph. The Hummer H2 at 25 mph faces as much drag as the Insight at 55 mph, but because of its much greater weight you still get more rolling resistance than wind resistance at that speed. > BTW, the slick looking car may be less aerodynamic than the brick > looking car. Why? Take a look at the underside and see how much junk the > air on the bottom has to go through. Well, that's not the only reason -- there's often a big difference between what people think is slick-looking and what is actually slick from an aerodynamic perspective. In general, a bulbous front end that narrows toward the rear (a bit like the cross-section of an airplane wing) will produce the best aerodynamics, but that's not a good look for a sports car. Cars that look like they're designed to knife through the air are often called slick, but generally have poor aerodynamics (though it's often done on purpose to produce downforce for better traction, which explains why Formula 1 race cars have far worse drag coefficients than school buses and Hummers). If I'm not mistaken, the General Motors EV1 has to this day the lowest drag coefficient ever measured on a production vehicle. That said, the junk on the bottom of the car makes a big enough difference that an acquaintance of mine once considered starting a business based on covering that stuff up to improve aerodynamics. He abandoned the idea -- I think he concluded there wouldn't be a big enough market in his area to make the business viable, but I don't recall the details.