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Old 28th June 2011, 00:23   #1
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Default Power/Weight ratio : relevance for driving

Mods: I searched for this several times, using both the search function and Google - found nothing so posting this thread. Move it to a relevant thread (if one exists)

The question has been bugging me for some time, and today with the launch of Liva (best power/weight ratio due to very frugal body engineering and decent engine power) it becomes more interesting to more people.

In some detail, the question is what is the relevance of Power/weight ratio for road cars:
  1. For city driving why is it relevant
    • peak power used in the calculation will almost never be achieved in low speed traffic in city.
    • Low rpm torque has not much relevance to peak torque many times
  2. For highway driving (read high speed driving) why is it relevant -
    • most road going cars get limited in speed due to air drag (nothing to do with vehicle weight) and not due to rolling resistance (everything to do with weight)
    • Also, the airodynamic shape of the car means air flows quickly over the car and slowly under the car - in theory there is lift, though situation is not as bad as in race cars (which are also very light) - most road cars don't come with spoilers and even the ones that do have useless (but good looking) spoilers that can not possible generate down force, except via their own weight.
I'm probably missing something somewhere. Can somebody point it out?
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Old 28th June 2011, 07:44   #2
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Default Re: Power/Weight ratio : relevance for driving

Too technical for me. But I found this wikipedia page. Maybe the section for Vehicles can give you an answer
Power-to-weight ratio - Wikipedia, the free encyclopedia
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Old 28th June 2011, 10:24   #3
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Default Re: Power/Weight ratio : relevance for driving

What I understand from the power to weight ratio is if you take two cars, identical in every respect other than the engine and compare their power to weight ratios, you may get an indication of which car can generate more power than the other.

Practical considerations could include following:
1. Outright acceleration (relevant for enthusiasts)
2. Ability to lug loads up inclines/ghats (bigger family, longer vacations)
3. Ability to power other accessories (AC, coolers/chillers, winches maybe?)
4. Overtaking abilities (in-gear acceleration)

For e.g. if you compare the figures from the Wikipedia article for Fiat Grande Punto and Toyota Prius (Petrol), you will find the Punto has a better power to weight ratio, i.e. in proportion to its weight its engine develops more power. So if you pitted the Punto against the Prius, you would be able to observe one or more of the above being in favour of the Punto.

Please note, this is a layman's perspective. So I, too, would like to know if this is correct.
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Old 28th June 2011, 10:45   #4
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Default Re: Power/Weight ratio : relevance for driving

all things being equal, power to weight will define who finishes first.
but thats hardly ever the case, no?
there's hundreds of other variables coming in too.
you can put the QE2's(figure of speech, only) engine in the omni, but that flat front slab hitting the air will still keep it slow.
put the same in any other vehicle, heavier, but with some semblance of streamlining, youve got a winner.

Similarly, grip levels offered (non aerodynamic related).
one car may have 1bhp/kg, and another just 0.75.
but put different spec tyres on both cars, alter suspension, and you can swing the win either way.

So basically you cannot judge on paper using power to weight.

what youve got to consider is what is that power being put to use in?
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Old 28th June 2011, 11:34   #5
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Default Re: Power/Weight ratio : relevance for driving

Quote:
Originally Posted by vina View Post
The question has been bugging me for some time, and today with the launch of Liva (best power/weight ratio due to very frugal body engineering and decent engine power) it becomes more interesting to more people.
The one point missing in your question Vina is mileage! This also impacts the efficiency of the car.
Lighter the car, less mass to propel ergo less effort to move at at a particular speed. Add a 150 kg to this car and you get the picture right?

The positioning of the Liva and the target audience it caters to will be more interested in mileage than pick-up.
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Old 28th June 2011, 12:05   #6
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Default Re: Power/Weight ratio : relevance for driving

Quote:
Originally Posted by viper_711 View Post
The one point missing in your question Vina is mileage! This also impacts the efficiency of the car.
Lighter the car, less mass to propel ergo less effort to move at at a particular speed. Add a 150 kg to this car and you get the picture right?

The positioning of the Liva and the target audience it caters to will be more interested in mileage than pick-up.

The question is not specific to Liva, though the launch of Liva did prompt me to ask.

Once you are doing triple digit speeds on the highway, weight matters but not so much - aerodynamics matters a lot more (drag increases very fast as speed increases).

Weight has nothing to do with this part - so at highway speeds what seems obvious to you is not exactly obvious to me.
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Old 28th June 2011, 12:20   #7
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Default Re: Power/Weight ratio : relevance for driving

Quote:
Originally Posted by vina View Post
Once you are doing triple digit speeds on the highway, weight matters but not so much - aerodynamics matters a lot more (drag increases very fast as speed increases).

Weight has nothing to do with this part - so at highway speeds what seems obvious to you is not exactly obvious to me.
A few points come to my mind:
First that if we look at a particular segment (in this case the hatchbacks in India) - they more or less would have similar drag co-efficient figures. What I mean to say is that none of them are radical enough to change the figures drastically.
When you say highway speeds - do you mean highway driving or consistent high speed (more like a test track)? My understanding is that by the time drag becomes a major factor, the car would need to be about 120kmph + On typical highway driving, a car would keep changing speed between 60 to 100 (maybe down to 20kmph on some speed bump littered highways!) At this point the power to weight equation is more important imho
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Old 28th June 2011, 12:25   #8
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Default Re: Power/Weight ratio : relevance for driving

Engine power + transmission + tyre rotation = forward force.

Weight + drag + G-forces = retarding forces. At some point the acceleration stops, which is the vehicle's top speed. Though one way to look at it that vehicle is experiencing too much drag, another way to look at it is that there is not enough engine to conquer the drag. Or the weight of the load. This is why a severely engine-limited car (eg the old, 800cc Matiz) will hit triple digit speeds on the highway with one occupant, but struggle to do so with a full load of 5. Or 4.

Note that the Liva's power/weight ratio is only one part of the story, the other is the gearing which spoiled the drive experience. So it's all dependent on each other. A vehicle with a poor power/weight is not going to be quick by any stretch of the imagination, and one with a great power-weight ratio may not be the most driveable (eg drag cars with 1,000 bhp/tonne).
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Old 28th June 2011, 12:40   #9
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Default Re: Power/Weight ratio : relevance for driving

Quote:
Originally Posted by cranky View Post
Engine power + transmission + tyre rotation = forward force.

Weight + drag + G-forces = retarding forces. At some point the acceleration stops, which is the vehicle's top speed. Though one way to look at it that vehicle is experiencing too much drag, another way to look at it is that there is not enough engine to conquer the drag. Or the weight of the load. This is why a severely engine-limited car (eg the old, 800cc Matiz) will hit triple digit speeds on the highway with one occupant, but struggle to do so with a full load of 5. Or 4.

Note that the Liva's power/weight ratio is only one part of the story, the other is the gearing which spoiled the drive experience. So it's all dependent on each other. A vehicle with a poor power/weight is not going to be quick by any stretch of the imagination, and one with a great power-weight ratio may not be the most driveable (eg drag cars with 1,000 bhp/tonne).

Let me explain my reasoning in the first post (and then please pick holes in it):



(and a little nitpicking: weight enters as rolling friction of the tyres, not as weight itself)

Above about 80kmph drag starts to dominate for most street going sedans (hatches will be hit earlier, SUVs earlier than hatches, and Vans/Buses earliest of all). This is if you are travelling in still air, in case of headwinds or tailwinds adjustments need to be made to this.

By the time you reach top speed of most hatches or sedans (almost all new ones can do 130 comfortably) the majority of retarding force is actually the drag - weight/friction forms a very small part of the overall retarding force.

And as you mentioned top speed is when driving force at wheels = retarding force

So really weight should be a (very) minor factor in determining top speed.


Coming to driving at less than top speed - say 80kmph, most cars have more than enough power left in the engine, after tackling the retarding forces at these speeds, to be able to make a quick dash when needed (and when it is safe to do).



As far as low speed city driving is concerned - peak power number is almost always irrelevant because nothing even close is obtained at low rpm encountered in city. And nobody does power/weight ratio with real power numbers at the more relevant rpm (no car manufacturer in India makes the torque curves easy to find in the first place)
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Old 28th June 2011, 13:01   #10
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Default Re: Power/Weight ratio : relevance for driving

Quote:
Originally Posted by vina View Post
The question is not specific to Liva, though the launch of Liva did prompt me to ask.

Once you are doing triple digit speeds on the highway, weight matters but not so much - aerodynamics matters a lot more (drag increases very fast as speed increases).

Weight has nothing to do with this part - so at highway speeds what seems obvious to you is not exactly obvious to me.
Even at highway speeds, major retarding force is the weight of the car and not the drag. Once the vehicle gains the momentum, there is inertia force (kinetic energy) which keeps the vehicle rolling for some time, but in order to keep up the momentum, vehicle needs to overcome the frictional and gravitational forces and weight of the vehicle has major role.
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Old 28th June 2011, 13:06   #11
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Default Re: Power/Weight ratio : relevance for driving

Quote:
Originally Posted by vina View Post

Coming to driving at less than top speed - say 80kmph, most cars have more than enough power left in the engine, after tackling the retarding forces at these speeds, to be able to make a quick dash when needed (and when it is safe to do).

As far as low speed city driving is concerned - peak power number is almost always irrelevant because nothing even close is obtained at low rpm encountered in city. And nobody does power/weight ratio with real power numbers at the more relevant rpm (no car manufacturer in India makes the torque curves easy to find in the first place)
...and this is what I meant in my earlier post about mileage! Manufacturers try to keep the weight down to tackle fuel efficiency on regular city driving offerings. The Japanese seem to give this more importance historically and the Europeans seem to be overlooking this in favour of other factors. The layman usually comment on this by the way the doors weight and the sound they make when shut.
I dont think Maruti have ever claimed or marketed stonking performance on their cars but have always claimed great fuel efficiency and historically most MUL offering are lighter than the competition around.
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Old 28th June 2011, 13:25   #12
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Default Re: Power/Weight ratio : relevance for driving

Quote:
Originally Posted by Guna View Post
Even at highway speeds, major retarding force is the weight of the car and not the drag. Once the vehicle gains the momentum, there is inertia force (kinetic energy) which keeps the vehicle rolling for some time, but in order to keep up the momentum, vehicle needs to overcome the frictional and gravitational forces and weight of the vehicle has major role.

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.
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Old 28th June 2011, 13:43   #13
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Default Re: Power/Weight ratio : relevance for driving

Quote:
weight/friction forms a very small part of the overall retarding force
From what I know, this is not necessarily true. The rolling resistance at high speeds (being dependent on weight as you rightly state) is equally important and in some cases is the bigger limiting factor.
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Old 28th June 2011, 13:53   #14
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Default Re: Power/Weight ratio : relevance for driving

Quote:
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.
Basically, there are two apsects to be considered
  • Power/energy required to propel the vehicle at certain speed (work done)
  • Energy required to overcome the resistance

Velocity & air drag
According to this, @ something like 70 kmph, the Friction: Drag=50:50. The frictional resistance remain same where as drag resistance grows exponentially.

But this is just the case of resistance. 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

Last edited by Guna : 28th June 2011 at 13:59.
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Old 28th June 2011, 14:17   #15
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Default Re: Power/Weight ratio : relevance for driving

power required to over come rolling resistance is a first degree function of speed.
for air resistance it is a cubed(or is it squared?i KNOW its not first degree.) function of speed.
one is linear, the other is exponential, so initially same power may suffice, but as your speed increases, the air resistance will will , quite fast, make the frictional sound minuscule.

As for sedan and hatch debate.
I think its dependent on the shape.
a teardrop shape, in a sedan or a hatch, will decide at what speed air resistance becomes dominant.
A boxy sedan like a logan may face more resistance than something shaped like the jazz.
i feel.
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