Drafting

Roger Mellie
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Drafting

Postby Roger Mellie » Fri Feb 19, 2010 11:35 pm

I note that "drafting" (saving fuel by keeping in the slipstream of an HGV) is one way of improving mpg.

However how close do you have to be, for there to be an effect? I don't want to tailgate lorries; not just because of safety, but I find it harder to maintain a steady speed with less than a two-escond gap (which is not good for mpg or brake wear). Will a two-second gap still keep you in a lorries slipstream? How far from the rear of a lorry does the slipstreaming effect disappear?

Fallguy
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Re: Drafting

Postby Fallguy » Sat Feb 20, 2010 11:23 am

I don't know if there is a definitive answer to this question - it would depend on the lorry you are following and the aerodynamics of the car you are in.

As a subjective measure I have noticed slight improvements in fuel efficiency when following a lorry at 2 to 3 seconds distance - like you I am not keen to tailgate. I guess the closer you are the less induced drag and skin friction there will be.

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frv
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Re: Drafting

Postby frv » Mon Feb 22, 2010 12:28 pm

I've found a bit more from that Mythbusters exercise:

Drafting a Big Rig

Myth: Drafting a big rig saves fuel

They emphasized again and again how dangerous drafting a big rig is: ~3/4 of truck/car accidents are caused by person driving the car and you're driving in the blind spot of the truck. 150ft is minimum recommended following distance at 55mph, so even the 100ft test is considered dangerous.
Small-scale test

NASA let them do a small scale test to study the aerodynamics of a big rig. They captured video of smoke travelling over a minature big rig and verified that there is a low pressure area behind. They then stuck a minature car to a force gauge to study the difference with and without drafting.

* 7 car lengths: 21% drag reduction
* 10ft: 60%
* 6ft: 80%
* 2ft: 93%

Full-scale test

Freightliner lent the MythBusters one of their new Cascadia big rigs, which they are billing as the most aerodynamic big rig on the market.

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 fuel economy actually dropped at 2ft. Andrew Smith's theory was that at 2ft, Grant got nervous with the throttle as it was difficult to maintain that 2ft gap.

confirmed


So you still get an 11% advantage at 100 feet so there should still be some benefit even further back if it doesn't feel "safe" at this distance....
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Karl
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Re: Drafting

Postby Karl » Mon Feb 22, 2010 1:34 pm

That was a great Mythbusters episode!!

I think they are very right about not being too close - the smoother you can be with the throttle the better in my experience. I always keep to the 2 second rule and can definitely feel the difference when I pull out to over take.

32MPG @ 55MPH... that's America for you! I would cry if that's what my car could achieve!
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Karl
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Re: Drafting

Postby Karl » Mon Feb 22, 2010 1:34 pm

The episode for the video is actually on the videos page

http://hypermiler.co.uk/hypermiling-videos.html
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Fallguy
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Re: Drafting

Postby Fallguy » Thu Feb 25, 2010 8:37 pm

Thanks for that,
Useful info there. Glad to know I wasn't hallucinating as to the fuel efficiency improvements!

Roger Mellie
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Re: Drafting

Postby Roger Mellie » Wed Mar 17, 2010 6:54 pm

Thanks for all that info, much appreciated.

I believe the stopping distance for 50mph is 13 car lengths and 60mph 18 car lengths-- so I'd imagine 55mph is around 15 or 16 car-lengths then?

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Karl
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Re: Drafting

Postby Karl » Tue Mar 30, 2010 7:23 pm

I wouldnt take a "average" stopping distances to seriously tbh. Many of todays cars stop way faster than that. I say do what you feel comfortable with. I normally keep about 2 secs behind anything.
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Fallguy
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Re: Drafting

Postby Fallguy » Tue Mar 30, 2010 8:48 pm

On non-motorway roads I prefer to leave alot more braking distance as it offers more chance for engine braking when you hit a junction or slow moving vehicle - wonder if the benefits of doing this on the motorway would outweigh the benefits of drafting?

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Karl
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Re: Drafting

Postby Karl » Wed Apr 07, 2010 2:40 pm

wonder if the benefits of doing this on the motorway would outweigh the benefits of drafting?
If you have the balls to "safely" sit 10ft from the back of a lorry - I doubt coasting into junctions will give you anywhere near a 39% increase. Just sitting 2 secs behind a lorry combined with sitting at about 56MPH must be hugely more efficient than doing 65/70 in the fast lane.
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