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Old 03-25-2008, 01:56 AM   #11
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Experienced this phenomenon over the weekend wile driving a 2002 Toyota Corolla (2400 lbs., cD .31) instead of my 1994 Geo Metro (1700 lbs., cD .32, but modified, probably slipperier than the Corolla) on my usual coastdown stretch over Cajon Pass. Both cars have automatics, engine on coasting in neutral.

I typically crest the pass at 65 mph. My Geo will normally slow down initially to about 60 then build momentum to somewhere between 75 and 80 mph, then eventually slows to 55 in somewhere around 5.1 miles.

The Corolla loses NO speed initially, tops out at about 92 mph, slowing to 55 in just over 6 miles over the same stretch of road.

Feather thrown into the wind vs stone thrown into the wind.
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Old 03-25-2008, 03:09 AM   #12
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lunarhighway -

I keep thinking that you should carry "ballast" on the downhill run and go back up the hill empty.

Maybe I should fill my car with jugs of water for the downhill run to work and "water the begonias" in the parking lot ?!?!?!?!?

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Old 03-25-2008, 03:58 AM   #13
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i've been thinking of weight and hills as well.

what would weight do uphill... suppose you approach a short hill at a given speed, the light car will require a little extra power to overcome this extra resistance, but on the downhill it will not gain much speed (or reqire much less power)
but what about a heavy car approaching the hill at the same speed?
i'm sure this will largely depend on how steep and long the upramp is, but aren't there two forces at work that cancle eachother out? the momentum of the heavy weight that keept the car going forward and gravity pulling at the car with a greater force?

perhaps if the car can reach the top before it looses momentum it will require less power to go uphill than a lighter version, and once there it will have the benefit of the extra weight.

of course overall a light car IS better under any condition, but i supposed if used wisely extra weight could help minimize the losses it causes elswhere.
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Old 03-25-2008, 09:30 AM   #14
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coasting

Force = Mass x Acceleration

So when coasting you are converting the force into work. The work energy overcomes the resistance such as tire RR, wind and gravity.

However, Mass x Acceleration = Force. So it takes that much energy to get that 7,000 lb lead sled to move.

Ever wonder why truckers speed down hills? They are storing kenetic energy so the the mass of the truck and load will be applied to the next uphill thus saving fuel.

In summary, anytime you can cost down a hill your FE is better. Reducing your rolling resistance and wind drag helps also. Adding weight will not help. Since it takes more energy to move that weight on level or uphill slopes.

Of course if you want to solve the perpetual motion problem (overunity), you wouldn't need to worry about fuel. Good luck.
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Old 03-25-2008, 09:44 AM   #15
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Howabout if you go with a really, really, featherweight vehicle, then when you accelerate for long enough and hit the speed of light, you have infinite mass and thus infinite momentum and nothing can stop you... (But if you have the infinite amount of energy required to get there, why worry about how far you can coast after that)
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Old 03-25-2008, 09:56 AM   #16
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LOL!

Quote:
Originally Posted by RoadWarrior View Post
Howabout if you go with a really, really, featherweight vehicle, then when you accelerate for long enough and hit the speed of light, you have infinite mass and thus infinite momentum and nothing can stop you... (But if you have the infinite amount of energy required to get there, why worry about how far you can coast after that)
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Old 03-25-2008, 10:41 AM   #17
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Quote:
Originally Posted by RningOnFumes View Post
Does downforce equal added weight and thus more mass?

Is it the same way for cars that have more downforce via wings and spoilers and such...thinking of professional racing. I always thought the opposite, that it drags you down thus you'll need power to keep going.
No and yes.

No - Creating downforce doesn't add mass to the car. Mass is actual physical material. Solid, liquid, or gas. And the material's mass is independent of whether it's moving. Yes, just maybe there'a a half-ounce of air molecules pressing down on the front spoiler but not 100 lb. of molecules creating 100 lb. of downforce.

Yes - (part of your second section) - creating downforce using aerodynamics does add drag. That is, you're using the car's forward motion through the air to create this downward force. That energy has to come from somewhere, and it comes from your engine and fuel. Unless you're coasting at the time.

Of course the added drag from the front spoiler will cost you fuel or will slow you down. The penalty is worth it for a performance car because it needs the added traction. Whether or not it's the same penalty as adding say 150 lb. of weight to the front end, I don't know. My guess is that downforce from aero is less of a penalty than added weight - becuase adding weight is easy and cheap, if that were the best way nobody would bother with aero downforce.
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Old 03-26-2008, 01:05 PM   #18
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Quote:
Originally Posted by RoadWarrior View Post
...when you accelerate for long enough and hit the speed of light, you have infinite mass and thus infinite momentum and nothing can stop you... (But if you have the infinite amount of energy required to get there, why worry about how far you can coast after that)
Not quite.
The kinetic energy in a moving object increases as the mass increases, but the act of motion does not increase the mass. Twice the mass is twice the energy. A heavy car at 30 mph has more momentum than a light car at 30 mph.
The kinetic energy increases with the square of the increase in speed. Twice as fast is four times the energy. A car (heavy or light) at 60 mph has four times the momentum of that same car at 30 mph.

Taken to an extreme: E=mc2

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Old 03-26-2008, 01:43 PM   #19
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I think you're confusing newtonian momentum with relativistic momentum. http://en.wikipedia.org/wiki/Relativ...stic_mechanics
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Old 03-26-2008, 03:29 PM   #20
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Quote:
Originally Posted by RoadWarrior View Post
Howabout if you go with a really, really, featherweight vehicle, then when you accelerate for long enough and hit the speed of light, you have infinite mass and thus infinite momentum and nothing can stop you... (But if you have the infinite amount of energy required to get there, why worry about how far you can coast after that)
[tongue_in_cheek]

Interesting choice of the vehicle. I would go with a 5000lb SUV instead. Although it requires more energy to accelerate my SUV to 99%, 99.9%, or even 99.99% of the speed of light, miraculously, it requires the same infinite energy to accelerate my SUV to the speed of light as it takes to accelerate your tiny vehicle. With the same energy expenditure, I travel at the same speed, with more style (highly debatable), and my SUV probably holds up better in a crash, too.

[/tongue_in_cheek]
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