will a heavier car coast further?

[lunarhighway]

i know it takes more energy to accelerate a heavier car to the same speed as a light car, but what if they're both up to speed , wich will coast furter?

assuming they both have more or less the same tires and frontal area and Cd (in the example i'm thinking of the bigger car actually has a better Cd)
the weight difference is about 300 kg - 660lbs.

will weight effect rolling resistance, or won't this be a factor with the right tire pressure?

[Biffmeistro]

It'll effect rolling resistance, yes, but the heavier car will most definitely coast further, all other factors being equal. My old t-bird coasted a mile on level ground, from 60, and it was still going 30mph. My current car can barely make it a half mile. There's a lot more differences, but I think the big one is my tbird was 1000 pounds heavier.

[8307c4]

From an FE standpoint the heavy car loses out.
First in acceleration the coast gains don't make up for it, then maintaining a cruising speed consumes more, even with the same size engine heavier isn't the answer for better mpg.

But to answer the question, yes, the heavier car coasts further.

[lovemysan]

I would answer no. Nothing I've driven coast as good as the saturn. My brothers altima at 400lbs heavier with equal tire pressure comes well short. None of the pickups, vans, etc are any different. The only time a heavier car will coast better will be on long steep hills. This is more evident when heavily loaded because the weight vs frictional losses ratio is higher than on a lighter car. But its still more efficient to use the lighter car in every other circumstance. Of course this is all just my opinion.

[1993CivicVX]

My VX doesn't coast worth an expletive. But when its laden with people or stuff... it coasts better down hills or on flats. I believe and please correct me if I'm wrong, if you have a car like my VX with a Cd of .31 that weighs 2100 pounds, and you put 300 pounds inside the car, the car's Cd has suddenly dropped from .31 to some smaller number. But just because its Cd has dropped, doesn't mean the car is now getting better gas mileage! Of course it isn't, since the engine has to haul around more mass. Conversely, you tear out the passenger and back seats and now the car weighs less. It's cD is going to go up and it's going to coast down hills even worse. But because the engine is hauling around less mass, it's going to get better gas mileage, not worse, despite the worse cD. Well, that's how I understand it anyway. Maybe someone can add to it.

[samandw]

Originally Posted by 1993CivicVX

My VX doesn't coast worth an expletive. But when its laden with people or stuff... it coasts better down hills or on flats. I believe and please correct me if I'm wrong, if you have a car like my VX with a Cd of .31 that weighs 2100 pounds, and you put 300 pounds inside the car, the car's Cd has suddenly dropped from .31 to some smaller number. But just because its Cd has dropped, doesn't mean the car is now getting better gas mileage! Of course it isn't, since the engine has to haul around more mass. Conversely, you tear out the passenger and back seats and now the car weighs less. It's cD is going to go up and it's going to coast down hills even worse. But because the engine is hauling around less mass, it's going to get better gas mileage, not worse, despite the worse cD. Well, that's how I understand it anyway. Maybe someone can add to it.

Nope, added weight doesn't change Cd. However, it does change the ratio of gravity force to aerodynamic resistance, meaning you have a slight increase in rolling resistance, no increase in aerodynamic drag, but an increase in the gravity vector with a weight increase. This will make it coast better downhill. On the flat you have again ONLY a slight increase in rolling resistance but a significant increase in momentum. . .i.e. the heavy car will decelerate at a slower rate thus coasting further. . .

[DarbyWalters]

Originally Posted by lovemysan

I would answer no. Nothing I've driven coast as good as the saturn. My brothers altima at 400lbs heavier with equal tire pressure comes well short. None of the pickups, vans, etc are any different. The only time a heavier car will coast better will be on long steep hills. This is more evident when heavily loaded because the weight vs frictional losses ratio is higher than on a lighter car. But its still more efficient to use the lighter car in every other circumstance. Of course this is all just my opinion.

Try coasting by yourself and then again with 3 passengers ~450# total~you will coast further.

[lunarhighway]

that's what i'd expected, my main consern was that rolling resistance might be higher, although that would mainly have to be a tire thing.

my next vehicle is likely going to weight a bit heavyer (what i have now was build by 80's standards... so it's lot like i'm shopping for an suv)

but the extra weight will take extra energy so i'd like to figure out the best strategy to minimise the loss.

[thecheese429]

the heavier car has more inertia to carry it further, so it will go the farthest. a nice aerodynamic [looking] heavy can I can think of is the olds aurora. that would coast pretty far, but still, the losses are when you're accelerating. if you're on the highway/country road and not stopping, you could do more effective P&G with a heavy one.

[RningOnFumes]

Does downforce equal addend 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.

[mustngr]

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.

[cfg83]

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 ?!?!?!?!?

CarloSW2

[lunarhighway]

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.

[maxxgraphix]

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.

[RoadWarrior]

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)

[maxxgraphix]

LOL!

Originally Posted by RoadWarrior

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)

[brucepick]

Originally Posted by RningOnFumes

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.

[Lug_Nut]

Originally Posted by RoadWarrior

...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

186,000 miles per second: It's not just a good idea, it's the law!

[RoadWarrior]

I think you're confusing newtonian momentum with relativistic momentum. http://en.wikipedia.org/wiki/Relativ...ic_mec hanics

[spacepilot]

Originally Posted by RoadWarrior

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]

[Hateful]

I'd rather have a smaller car even though I risk being killed by those with style.

[Lug_Nut]

Originally Posted by RoadWarrior

I think you're confusing newtonian momentum with relativistic momentum. http://en.wikipedia.org/wiki/Relativ...ic_mec hanics

At 0.0000001% the speed of light, about 67 mph, I don't think I am. That is close enough to zero, relative to 100%, that relativistic issues are negligible. At 0.00000005% of C, or about 33 1/2 mph, even less so.

Al's elegantly simple mathematical ....

Oh, Sorry, he let his close friends call him Al.

Herr Einstein's elegantly simple mathematical equasion was developed from imagining what might happen if the energy increase seen and measured and calculated at these lowly speeds were extrapolated out to their logical, albeit theoretical, conclusion. The E=MV2 equasion was existant long before he decided to theorize taking velocity to its limit.

[RoadWarrior]

Hmmm but you appeared to be using a newtonian example to illlustrate that mass, energy and momentum do not become infinite at the speed of light.

[Lug_Nut]

They don't.
Mass does not change with speed, whether 0.0000001% nor 99.999999% the speed of light.
Energy in the moving object due to momentum does approach the infinite, but does not become infinite, as the object approaches the speed of light.
In theoretical terms the energy would become infinite if the object were ever able to move AT the speed of light, but the mass of the object would not change.

[8307c4]

Originally Posted by lunarhighway

but the extra weight will take extra energy so i'd like to figure out the best strategy to minimise the loss.

accelerate real nice and easy, really there's not much beyond the already useful hypermiling techniques but I think in a heavier car a heavy foot has a much worse effect than on a lighter car... That is in percents I think it's all the same, but at the pump and in dollars you feel it more with the heavier car, so a very light foot on the throttle and lots of hanging back and DWB.

You know it's all the same loss, whether you go from getting 40mpg to 30 or whether you go from 20 to 15 it's all the same loss here... But get to that pump and the difference is like ok on the first you have to spend 30 instead of 20 (+$10) but on the second example you have to spend 60 instead of 40 (+$20) so it'll drain your pocket book quicker no matter how you look at it, all I'm saying.

Take it nice and easy, best I can tell you.

[RoadWarrior]

Originally Posted by Lug_Nut

They don't.
Mass does not change with speed, whether 0.0000001% nor 99.999999% the speed of light.
Energy in the moving object due to momentum does approach the infinite, but does not become infinite, as the object approaches the speed of light.
In theoretical terms the energy would become infinite if the object were ever able to move AT the speed of light, but the mass of the object would not change.

Ah okay, that argument. I'll just agree that rest mass is always rest mass.

[RightontheMarc]

Someone please tell me why my car coasts worse in the winter than in the summer. My tires are not that much less psi. Is it something to do with the colder grease in the bearings? I can't figure out what the hell is wrong. I'm still getting a crappy 31mpg. I used to be able to coast all the way down this one gradual slope of road without losing speed and going below the speedlimit. In fact I used to accelerate a little. Now my speed drops about 5mph and I risk the cars behind me getting mad.

[Erik]

The transmission fluid is cold and thick in the winter- this really increases drag even in neutral

[dddon]

aahhh yes .. when i was a lad working at a "socony mobil " station ,,, we added ATF to the tranny and rear axel to make the cars more efficent ,,they were ALL stick shifts back then..
and YES! the grease in the wheel bearings adds a lot of drag to a car or truck!! during that era there was the "mobil economy run" .. the teams did all the things that you find on this site AND many other things to make their car get the best mpg they could ... the penalty for an infraction was a full stop !!! .. example: the drivers would grind the heel of their right shoe rounded so it would be smoother on the gas pedal.
so look to the proper alignment of the wheels, and synthetic lube in engine and wheel bearings, as the over the road trucks have done for many years... dddon

[RoadWarrior]

Also cold air is denser, so you'll have higher aero resistance. For example my van takes about 20.5HP in aero to cruise at 60mph in -5C and only 18.5HP at 25C