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Old 01-06-2008, 05:10 PM   #11 (permalink)
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Also the Prius has more frontal area than the Insight.

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Old 01-06-2008, 05:46 PM   #12 (permalink)
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Quote:
Originally Posted by Smoky View Post
Apples to apples:
You do know that force of drag also has a frontal area component? Drag coefficient is only useful to compare shapes.


CdA = Drag Coefficient * Frontal Area

Insight 5.1 square feet

Prius: 6.045 square feet

mpg difference: 16.6%
CdA difference: 18.5%

The other thing is that if a person is that concerned about fuel economy, he often learns to drive correctly. This means that more chemical energy from fuel is going to push air out of the way (and a little to deform tires, but only a little), and less to heat up brake pads (or the engine/exhaust gas with engine braking, or heat up the battery with inefficient regeneration).

If you eliminate braking, the only factor weight will play is in increasing rolling resistance, which is minimal at most speeds a car will travel in comparison to aero. And note virtually everyone here is decreasing rolling resistance by inflating their tires. 45-60psi would be usual. So aero is an even larger factor.

The "fast back crap" will reduce drag by probably 20-30%. It will add maybe 50kg, at most - or 2% of the weight of a typical truck.
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Old 01-06-2008, 05:48 PM   #13 (permalink)
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Originally Posted by basjoos View Post
Also the Prius has more frontal area than the Insight.
Damnit basjoos, stealing my thunder!
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Old 01-06-2008, 05:53 PM   #14 (permalink)
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Originally Posted by MetroMPG View Post
Newton: you've got a class of vehicle in Australia not sold in North America that's ideally suited to this approach: the car-based "ute". (Correct me if I'm wrong.)
That we do!
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Old 01-06-2008, 06:50 PM   #15 (permalink)
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I think everyone may be misinterpreting my comments. I am not trying to start any arguments I just want to assist in finding more ways to make everyones vehicle more efficient.

There are 2 different threads on this exact same subject and I have posted on both.

I agree that aero is important. I do address frontal area in my other posts. In the auto industry we are under enormous pressure to reduce frontal area, reduce drag and weight.

I think nearly everyone on this forum is on the right track with regard to aero. Because of this I have chosen to center my comments on the weight issue. I don't claim to be an expert.

For the driving most people do, 60% city 40% highway, weight plays a major role in reducing efficiency. In city driving speeds rarely get over 40mph. Most driving involves stop and go. The heavier the car the more energy it takes to get moving. Once the car is moving the role weight plays is reduced and the importance of aero increases.

If weight is not a factor then F1 cars would not be using expensive and exotic materials. They will spend millions to save 1 gram of wieght. You can go to any entry level race series and see how they make their cars more efficient. They take out the weight. Aero is important but they are doing pretty much what you all advocate here (and I agree with).

Reducing weight is one of the easiest and cheapest ways to increase efficiency.

Again, I am not trying to argue or disagree in any way and in fact I completely agree with your comments. I think everyone has something to bring to the table and everyones opinion is worth consideration.
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Old 01-06-2008, 06:56 PM   #16 (permalink)
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Another comment, not to aero bash, just to inform.

Be careful with the boat tail trick. Many times this will make the car less efficient. Many of the aero scientists I have worked with really push for a 'chopped off' rear end to to help meet the aero requirements. They comment that several cars benefit from this. Examples they use are: Nissan 300 ZX, Honda CRX, Honda Insight, and Toyota Prius.

Again, I am just passing on info and not trying to discredit anyone.
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Old 01-06-2008, 08:14 PM   #17 (permalink)
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Originally Posted by newtonsfirstlaw View Post
If you eliminate braking, the only factor weight will play is in increasing rolling resistance, which is minimal at most speeds a car will travel in comparison to aero.
Only factor???

If only the world was flat, my mileage would be so much better...
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Old 01-06-2008, 09:00 PM   #18 (permalink)
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Only factor???

If only the world was flat, my mileage would be so much better...
Increased gravitational potential energy is only wasted if it goes into brake pads, or if your car is going a lot faster on the downslope than you ordinarily would drive. Hills are like perfect batteries for storing energy.

In the ideal world, all traffic lights would be at the crests of hills.
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Old 01-06-2008, 09:06 PM   #19 (permalink)
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Quote:
Originally Posted by Smoky View Post
Be careful with the boat tail trick. Many times this will make the car less efficient. Many of the aero scientists I have worked with really push for a 'chopped off' rear end to to help meet the aero requirements. They comment that several cars benefit from this. Examples they use are: Nissan 300 ZX, Honda CRX, Honda Insight, and Toyota Prius.
You do realize that if you put a boattail on the rear end of a truck, it starts to look an awful lot like the rear half of a Prius, CRX, Insight... etc? In addition, you can enhance flow separation at the back of the boattail you have just made with a little extra overhang.
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Old 01-06-2008, 09:18 PM   #20 (permalink)
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Quote:
Originally Posted by Smoky View Post
Be careful with the boat tail trick. Many times this will make the car less efficient. Many of the aero scientists I have worked with really push for a 'chopped off' rear end to to help meet the aero requirements.
I've never heard of a boat tail making a car less efficient. It's just that the advantage is proportional the the area where the boat tail shape terminates. In other words, since the lines on the usual car result in flow separation over the entire back half, having a sloped back end so that the flow doesn't separate over the entire rear profile is an improvement. The Prius would probably be more efficient w/ more of a boat tail, but the improvements would decline asymmetrically compared to the increase in length and drop in usable area.

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