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Truck drafting

  #11  
Old 09-22-2007, 08:12 PM
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Default Re: Truck drafting

Originally Posted by Delta Flyer View Post
A Car & Driver editor in early 2000 was challenged to get a Honda Insight over 100mpg...he was allowed to do anything, so he did it. Over 100 miles he cruised at 58mph inches behind a Ford Explorer with plywood to effectively make it like drafting behind an 18-wheeler....probably better as the plywood went lower. They were communicating by cell phone as they were inches apart - not for the faint-hearted. Undrafted, I would get about 75mpg at 58mph - he got 121mpg.

A more legit way of breaking 100mpg, would have been driving the Insight on an isolated two lane road at 40-45mph, undrafted...still not typical driving, but more legitimate than close drafting.
In previous postings elsewhere I indicated I got over 50 mpg in my '06 Prius by driving at 55 mph on the US Hwys, state and county roads.

Drafting seems just too extreme a means of gaining a couple of mpg's.

Yep. Speed kills - gas milage.
 
  #12  
Old 09-22-2007, 08:31 PM
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Default Re: Truck drafting

Legal drafting, right time, right place:


Great fun, drafting:


Darwin award:



Another approach to drafting:
 

Last edited by bwilson4web; 09-22-2007 at 09:04 PM.
  #13  
Old 09-23-2007, 04:49 AM
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Default Re: Truck drafting

Originally Posted by centrider View Post
This is your lucky day! I watched this test on the TV program, Mythbusters. Here are the results:

Mike Ryan, Hollywood stunt driver, was there to educate Grant on the ins and outs of drafting. Also on-hand was Andrew Smith, test engineer, who helped them hook up a computer to the fuel injection system to accurately measure the fuel consumption.

55mph control: 32mpg
100ft: 35.5mpg, 11% improvement
50ft: 38.5mpg, 20%
20ft: 40.5mpg, 27%
10ft: 44.5mpg, 39%
2ft: 41mpg, 29%


The fall off at 2 ft was thought to result from the difficulty of trying keep so close without running into the truck.
That was a great episode!

I only wish they took the time to measure the mpg of truck too.

All the physics that I know, says that for the mpg improvement the car experienced, the truck's mpg would decrease. Basically, you don't get anything for nothing. If the car saves a gallon of gas, the truck would have to burn (at least) an extra gallon of gas. Now, trucks get such poor mpg that the driver might not even notice the decrease. But the truck "pulling" the extra weight of a car directly behind it, has to have some affect.
 

Last edited by WaltPA; 09-23-2007 at 04:51 AM.
  #14  
Old 09-23-2007, 06:27 AM
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Default Re: Truck drafting

Originally Posted by WaltPA View Post
. . .
All the physics that I know, says that for the mpg improvement the car experienced, the truck's mpg would decrease. Basically, you don't get anything for nothing. If the car saves a gallon of gas, the truck would have to burn (at least) an extra gallon of gas. Now, trucks get such poor mpg that the driver might not even notice the decrease. But the truck "pulling" the extra weight of a car directly behind it, has to have some affect.
If the truck had a smooth after body, a tear-drop shape, the displaced air would return the kinetic energy to the truck and return to the original velocity. This would significantly improve the truck MPG but the following vehicle would not be able to draft but that is not how they are ordinarily built. But there have been some efforts in this direction:
http://www.hankstruckpictures.com/la...te_article.htm

http://www.freepatentsonline.com/4741569.html
http://www.memagazine.org/backissues...th/smooth.html

Bob Wilson
 
  #15  
Old 09-23-2007, 07:15 AM
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Wink Re: Truck drafting

Originally Posted by Delta Flyer View Post
. . .
A more legit way of breaking 100mpg, would have been driving the Insight on an isolated two lane road at 40-45mph, undrafted...still not typical driving, but more legitimate than close drafting.
Or slower to replicate the 165 MPG from last year's Insight marathon:


However, newer technology is coming, PHEV with 171 MPG:
http://hybrids-plus.com/pmwiki/index...xt.Conversion3
http://hybrids-plus.com/ht/products.html

Bob Wilson
 
  #16  
Old 09-23-2007, 07:52 AM
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Default Re: Truck drafting

Originally Posted by bwilson4web View Post
If the truck had a smooth after body, a tear-drop shape, the displaced air would return the kinetic energy to the truck and return to the original velocity. This would significantly improve the truck MPG but the following vehicle would not be able to draft but that is not how they are ordinarily built.
Actually, turbulence directly behind a vehicle is good. It "breaks" the vacuum that typically exists and that in turn reduces drag.

Yea, a true tear drop also helps "break" the vacuum by attempting to not allow it to form in the first place.
 
  #17  
Old 09-23-2007, 09:05 AM
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Default Re: Truck drafting

Originally Posted by WaltPA View Post
Actually, turbulence directly behind a vehicle is good. It "breaks" the vacuum that typically exists and that in turn reduces drag.

Yea, a true tear drop also helps "break" the vacuum by attempting to not allow it to form in the first place.
I'd be interested in reading some sources about this. Most of my reading on aerodynamics suggests otherwise:
http://aerodyn.org/Annexes/Roadv/roadv.html

However, there is one reference that suggests that at some rear slope angles, blunting an after body can reduce drag. There appears to be a critical range of angles between 30-45 degrees:
http://aerodyn.org/Frames/1roadv.html

The curve suggests shallower angles, less than 30 degrees, would continue to reduce drag. BTW, this web page agrees with much of my past reading on aerodynamics:
http://aerodyn.org/

You might also enjoy this article by some high school, future engineers:
http://mhsengineering.com/2003MPGveh...erformance.htm

The students measure drag of three shapes: box, wedge and airfoil. Their summary tables show my expectations.

Bob Wilson
 
  #18  
Old 09-24-2007, 01:36 AM
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Default Re: Truck drafting

Originally Posted by WaltPA View Post
That was a great episode!

I only wish they took the time to measure the mpg of truck too.

All the physics that I know, says that for the mpg improvement the car experienced, the truck's mpg would decrease. Basically, you don't get anything for nothing. If the car saves a gallon of gas, the truck would have to burn (at least) an extra gallon of gas. Now, trucks get such poor mpg that the driver might not even notice the decrease. But the truck "pulling" the extra weight of a car directly behind it, has to have some affect.
I think you are confusing things. You are not talking about conservation of energy here. Cars travelling in a pack all improve. It is not that the front car is pulling the other cars - rather they are all reducing wind resistance and turbulence for each other. The front car takes the brunt of it but is still benefitted by it.

Imagine we are all travelling in a tunnel. Now imagine we all have a tail wind. Would be great right? What happens in reality is that other vehicles are creating a local tail wind (or blocking and reducing head winds). It does not "cost" extra energy to do this - in fact it saves energy if there are more cars around you going the same direction/same speed.

+1 for the nighttime. The only time I had a truck get annoyed at me was at night. As far as safety is concerned - lets say that I am in a pack of cars/trucks (normal traffic) and average following distance is 1.5 car lengths. Why not settle behind a truck? It brakes a little slower and I can't see around SUVs anyway. How many people a year get decapitated by a tire .... I mean really?

The absolute best in NC is a school bus. Governed at 45 mph. I think I probably get 100 mpg in my HCH-II.
 
  #19  
Old 09-24-2007, 09:15 AM
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Default Re: Truck drafting

Originally Posted by bwilson4web View Post
I was at 101 miles per hour last weekend getting about 22 MPG. But that doesn't mean I recommend it. In this case, a better answer is to look at:

1) radar assisted cruise control - this ensures the following distance is maintained using radar or other sensor, computer controlled operation.

2) work for 'smart car transponder systems' - to establish laws and regulations so properly equipped and maintained cars can form their own 'trains' on public roads. But this means addressing some interesting engineering issues.


My excursion to 101 miles per hour was long enough to verify it was 'stable' and then I backed off. I needed the data to understand that end of the performance scale and I have no need to repeat the experiment, ever.

If someone else needs a copy of the data, send me a PM and we'll work out the logistics.

Bob Wilson
So your 'recommendation' is for Technology which does not exist outside of the lab, or an option not available for our cars.

THANKS!
 
  #20  
Old 09-24-2007, 12:33 PM
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Wink Re: Truck drafting

Originally Posted by Neil View Post
So your 'recommendation' is for Technology which does not exist outside of the lab, or an option not available for our cars.
My recommendation is to design, develop and test the missing technology and then make it available. There is a business opportunity, take it.

Bob Wilson
 

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