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Old 28th June 2011, 14:42   #16
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by vina View Post
Are you sure about this?

In a lot of places I have read that once you cross 60-70mph (100kmph roughly) air drag dominates. And this is for sedans. For hatches with their flat backs, the speed will be lower.


I dont think its a question of air drag atall for daily driven vehicles , cars today in market have diff drag coef ,but not radically different. Some are better some not so good. But diff is marginal,even in case of different segments like hatch or sedan.

Air drag is a complex matter.
Commenting on drag just by looking at the shape of the car would not be wise. Its all in formation of layers.
On other hand
Power/weight is holy grail of automobiles.
But even on this front, cars on indian market do not differ too much. Maruti offers some of the lightest cars,but that is done for saving material cost,more than anything else.

For average joe
Power/weight and torque/weight would be more noticable,because budget cars would rarely do speeds where the car would sway due to turbulance of passing traffice and air drag be noticed.
Air drag is always there,its just that at speeds around 130+ for indian cars it becomes noticable enough(most of them have very similar drag coef.)
In theory drag would increase by square of speed,but it is not that straightforward in real world. Else manufacturers would not be using billion dollar windtunnels.
In real world air drag depends on how boundary layers created by diff surfaces intract with each other.
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Old 28th June 2011, 15:39   #17
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by dustom_99 View Post
I dont think its a question of air drag atall for daily driven vehicles , cars today in market have diff drag coef ,but not radically different. Some are better some not so good. But diff is marginal,even in case of different segments like hatch or sedan.

Air drag is a complex matter.
Commenting on drag just by looking at the shape of the car would not be wise. Its all in formation of layers.
On other hand
Power/weight is holy grail of automobiles.
But even on this front, cars on indian market do not differ too much. Maruti offers some of the lightest cars,but that is done for saving material cost,more than anything else.

For average joe
Power/weight and torque/weight would be more noticable,because budget cars would rarely do speeds where the car would sway due to turbulance of passing traffice and air drag be noticed.
Air drag is always there,its just that at speeds around 130+ for indian cars it becomes noticable enough(most of them have very similar drag coef.)
In theory drag would increase by square of speed,but it is not that straightforward in real world. Else manufacturers would not be using billion dollar windtunnels.
In real world air drag depends on how boundary layers created by diff surfaces intract with each other.
going from 70 to to hundred would give you about double the drag in a normal car.
(130+ i think is when a properly designed wing would kick in).

airtunnels for road going cars are purely post many other considerations.
they need to strike the balance between optimum shape, and optimum space inside the cabin, and looks.



Otherwise, what you get, based on just shape is a car producing 18kWor roundabout 3HP doing 120+ with a Cd of less than 0.10.(nuna, solar champ)

See, drag is related to speed to the 3rd power, so you can imagine what difference similar Cd's with even a 0.01 variation would produce.



consider,what is it that stops your engine from revving to redline as easily in 5th as compared to 1st?
put it on a rolling road, you'll easily be able to redline in all gears.

Last edited by mayankk : 28th June 2011 at 15:40.
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Old 28th June 2011, 15:45   #18
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by Guna View Post
Even in a world without any resitance, you need power/energy to displace mass and this is very significant. More mass and more velocity, you need to more energy

More mass + more velocity = more energy is correct.

But more energy = more power is not necessarily correct (and this is one myth everyone seems to have on TBHP)

Once you have attained a particular speed, you don't need more energy to move that mass, it will keep moving (Newton's first law) - you'll need more energy if you are losing energy.

In a world without any resistance, once you have reached 100kmph, even if you turn your engine off, the car will keep going at 100kmph indefinitely.

By the way, in a world without any resistance (air drag or friction) no street car will ever attain any speed at all - friction is required for a car to start moving.
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Old 28th June 2011, 15:47   #19
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Default Re: Power/Weight ratio : relevance for driving

In terms of relevance, it's the torque to weight that matters, along with other factors like:

- At what rpm max torque is produced
- Gearing
- Turbolag

At the end of the day, numbers only mean so much. The proof of the pudding is in the eating. The Santro 1.1 has lesser torque than the older i10 1.2, but man, what a torquey engine! Phenomenal driveability. On the other hand, the Linea MJD makes a lot more torque than the Logan. But one drive in the city and you'll love the Logan as much as you hate the MJD's turbolag.
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Old 28th June 2011, 15:48   #20
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by vina View Post
By the way, in a world without any resistance (air drag or friction) no street car will ever attain any speed at all - friction is required for a car to start moving.
This had me smiling!
I'm glad some speed-junkie never found a genie and wished for a world with no resistance so that he could go realllllllly fast!!
surprise, surprise!!!
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Old 28th June 2011, 16:00   #21
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by vina View Post
... By the way, in a world without any resistance (air drag or friction) no street car will ever attain any speed at all - friction is required for a car to start moving.
Incorrect! The exhaust will push the car to some speed in some time! Please account for all the parameters / variables properly, instead of throwing some out. There is at least 1 member on TBHP whose car can be propelled by the power output of his sub-woofers alone - he just has to drive it with the hatch open!

You must be sub-consciously assuming there is no air also? Like in space - no friction, no air? Pray, how will the engine operate under such circumstances?

Last edited by DerAlte : 28th June 2011 at 16:02.
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Old 28th June 2011, 16:06   #22
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by GTO View Post
In terms of relevance, it's the torque to weight that matters, along with other factors like:

- At what rpm max torque is produced
- Gearing
- Turbolag

At the end of the day, numbers only mean so much. The proof of the pudding is in the eating. The Santro 1.1 has lesser torque than the older i10 1.2, but man, what a torquey engine! Phenomenal driveability. On the other hand, the Linea MJD makes a lot more torque than the Logan. But one drive in the city and you'll love the Logan as much as you hate the MJD's turbolag.

You have nailed the essence of what I had mentioned in the bold letters above - but at the same time you are running away

For example even torque/weight ratio doesn't mean much if the peak torque is reached at a high rpm - as in case of turbo lag. What matters usually is how much power the vehicle can muster when you need it. Low turbo lag diesels for example are most likely to be first off a signal thanks to the massive torque available to low rpm.


My question actually goes pure technical - from a practical driving standpoint a car with lower power/weight ratio (e.g. Manza) may not achieve any better acceleration than another with better ratio (e.g. Indica Vista 90) on a test track, but on a real road, the Manza driver can easily compensate by giving more throttle - pickup will usually be limited by what is ahead of your vehicle and what the guys on your side are doing.

That done, at cruising, even with similar engines Manza in the above example may achieve faster speeds than Vista90, simply because the boot allows better rear aerodynamics (even if not perfect) than Vista's hatch door.


Quote:
Originally Posted by mayankk View Post
going from 70 to to hundred would give you about double the drag in a normal car.
(130+ i think is when a properly designed wing would kick in).

airtunnels for road going cars are purely post many other considerations.
they need to strike the balance between optimum shape, and optimum space inside the cabin, and looks.
Just to add - several microlight planes have their stall speeds less than 70kmph, in fact even commercial small planes have stall speeds less than 100kmph. Anyone who thinks airdrag is not a problem can do a simple experiment - pillion ride a mobike with a pole in hand, and ask to be ridden at 40+ - now try to keep the pole vertical.


At a speed of about 80-90kmph a well designed sedan has airdrag vs. rolling resistance in a ratio of about 2:1. At 120kmph, airdrag almost completely takes over (because drag force increases as square of speed, while rolling resistance force doesn't increase much with speed)
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Old 28th June 2011, 16:08   #23
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by vina View Post
Once you have attained a particular speed, you don't need more energy to move that mass, it will keep moving (Newton's first law) - you'll need more energy if you are losing energy.

In a world without any resistance, once you have reached 100kmph, even if you turn your engine off, the car will keep going at 100kmph indefinitely.

By the way, in a world without any resistance (air drag or friction) no street car will ever attain any speed at all - friction is required for a car to start moving.
BTW, there is also gravitational force which pulls the vehicle down and opposes the horizontal force. But this force is probably factored as part of the frictional resistance.
Yes you need traction (which requires friction) to move the car
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Old 28th June 2011, 16:10   #24
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by DerAlte View Post
Incorrect! The exhaust will push the car to some speed in some time! Please account for all the parameters / variables properly, instead of throwing some out. There is at least 1 member on TBHP whose car can be propelled by the power output of his sub-woofers alone - he just has to drive it with the hatch open!

You must be sub-consciously assuming there is no air also? Like in space - no friction, no air? Pray, how will the engine operate under such circumstances?

I knew you or Sutripta sir will correct me on this.

In fact if you neglect air drag, and still account for exhaust, the speed of the car will keep increasing as time passes by.

The fastest recorded ground speed in the world was done in a car with a turbojet engine - even that doesn't need wheel traction to go forward.



Anyway, you got the idea of what I was commenting on. Let's get back to the topic now.
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Old 28th June 2011, 16:16   #25
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Default Re: Power/Weight ratio : relevance for driving

What about the safety aspect? I for one would feel safer in a "heavier" car than a light one!!
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Old 28th June 2011, 16:20   #26
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Default Re: Power/Weight ratio : relevance for driving

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... Let's get back to the topic now.
What topic? Where to get back? You yourself digressed!
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Old 28th June 2011, 16:29   #27
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Default Re: Power/Weight ratio : relevance for driving

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What about the safety aspect? I for one would feel safer in a "heavier" car than a light one!!

Let's keep safety for a different thread for now. Otherwise people will start commenting how a lighter car can be safer - and we will lose the track immediately.
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Old 28th June 2011, 16:37   #28
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Default Re: Power/Weight ratio : relevance for driving

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Let's keep safety for a different thread for now. Otherwise people will start commenting how a lighter car can be safer - and we will lose the track immediately.
yes please.
and also the diesel and torque bits also.
THAT discussion heads on a tangent in a hurry!
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Old 28th June 2011, 17:22   #29
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Default Re: Power/Weight ratio : relevance for driving

To me,

Everything else remaining same,
1. Car with more power/weight than another translates to a higher max speed.

2. Car with more torque/weight than another means faster 0 to 60/100 numbers.

But the key thing is everything else remaining same like air drag, wheel friction.



That is why if you lightweight your existing car, it will be a better drag racer.

Last edited by dot : 28th June 2011 at 17:25.
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Old 28th June 2011, 19:30   #30
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Default Re: Power/Weight ratio : relevance for driving

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Originally Posted by vina View Post
what is the relevance of Power/weight ratio.
What power to weight ratio does is just give a peek into the vehicles performance, and is NO WAY near the actual performance. IMO, where the power to weight ratio gains relevance is when comparing cars with similar engines and drivetrains. In this case, the lighter car has a clear advantage. Like the 1.8 tsi and the 1.3mjd. Otherwise, the P/W ratio points little towards vehicle performance.

On the other hand, the useful'est values that a manufacturer can provide to predict the vehicles performance is the power/torque per rpm graph.

Quote:
Originally Posted by vina View Post
For city driving why is it relevant
For city driving a meaty low end value for torque will gain relevance. Torque/weight @ rpm is the value that one can use to predict the closest realworld performance.
Quote:
Originally Posted by vina View Post
For highway driving (read high speed driving) why is it relevant
For optimum performance in higway driving, you need a near flat torque curve that extends till the end rpm's and and a wide power band.




IMO, the area under the graph decides the vehicles real world performance - the closest. The more, the better. Where as P/W ratio may be a spike in the graph, which is practically useless, unless you drive in that narrow rpm range always.
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