drafting semi's - Fuel Economy, Hypermiling, EcoModding News and Forum

mattbatson · 12-15-2011, 03:51 PM

okay, first off I know it is dangerous...

I recently discovered that you can be 50 to 100 ft back and still see a benefit?

I heard mythbusters found a pretty significant increase all the way back to 100ft?
Or did i hear wrong?

I am pretty confident that i can still be safe at 50-100 ft back, and was wondering if anyone with scangauges have done any informal tests with this?

In the past I've drafted within 10-15 ft off semi's with little problem.
However, i dont drive with that kind of risk level anymore...

but 100 ft is quite safe in my opinion

thoughts?

99metro · 12-15-2011, 03:59 PM

Well, 65-70 feet or so is about 1 of their length. Too close for me. I like to be 3 of their lengths behind and no more than 55 mph. I love slow trucks!

PS Yes, I use the scanguage to confirm this - AND it depends on wind speed and direction. Side winds - forget it. Head winds - nice benefit.

mattbatson · 12-15-2011, 04:02 PM

Quote:

Originally Posted by 99metro

Well, 65-70 feet or so is about 1 of their length. Too close for me. I like to be 3 of their lengths behind and no more than 55 mph. I love slow trucks!

PS Yes, I use the scanguage to confirm this - AND it depends on wind speed and direction. Side winds - forget it. Head winds - nice benefit.

okay, so a full semi truck length back you are seeing an increase?

i understand about the side winds...been there done that

so 65-70 ft back, what kind of increase are we talking here?

99metro · 12-15-2011, 04:38 PM

I can get roughly 10-20% from drafting. Over 55 mph the benefits decrease. In my little car, I can feel the difference like night and day. I get back a little too far and I have to stay squarely behind him to stay in the draft. A little to the left or right can throw me back out of the envelope. I get way too far behind and I can feel the buffeting which tells me I am out of the envelope, plus the scangauge confirms this. After 65 mph, I give up and fall back to my grandpa driving. This is just my informal findings.

mattbatson · 12-15-2011, 07:30 PM

Quote:

Originally Posted by 99metro

I can get roughly 10-20% from drafting. Over 55 mph the benefits decrease. In my little car, I can feel the difference like night and day. I get back a little too far and I have to stay squarely behind him to stay in the draft. A little to the left or right can throw me back out of the envelope. I get way too far behind and I can feel the buffeting which tells me I am out of the envelope, plus the scangauge confirms this. After 65 mph, I give up and fall back to my grandpa driving. This is just my informal findings.

wow, well 10-20% is a lot.
Damn, I need to bite the bullet and buy a scangauge...with xmas time here, we have been spending and spending and spending, lol

I hate to sound like I'm beating a dead horse...but WHY do you think over 55 there is a decrease in benefit? If find most of the semi's in my area are doing 65 or so....
And how far, that you have found personally, is usually too far away? A full truck length? Less or more?

I will get a scangauge soon, and try and do some testing and report what I find.

I'm just so intrigued as I thought I would never draft again until I found that you can do it from so far away. To me, this is a safe enough distance...100ft that is...

Cd · 12-15-2011, 07:46 PM

So I saw this gasoline tanker and decided to give chase....

Steaming · 12-15-2011, 09:43 PM

Last time I drove down the highway in the rain, I observed the spray in the air comming off the back of a semi in front of me.

I'm guessing it must be a rough indicator of the proportions of the hole they punch through the atmosphere and may give an idea of where the sweet spot is for drafting in other conditions. (Although I suspect the weight of the water vapor would tend to show a smaller clear pocket than in dry conditions.)

JRMichler · 12-15-2011, 10:22 PM

I drafted behind a Greyhound bus on a motorcycle once. The turbulence was like getting between two guys boxing each other wearing pillows for gloves. It was on one of those California 12 lane freeways with heavy traffic.

I did not save any gas, and got beat up in the process.

FXSTi · 12-15-2011, 10:28 PM

Quote:

Originally Posted by mattbatson

wow, well 10-20% is a lot.

I hate to sound like I'm beating a dead horse...but WHY do you think over 55 there is a decrease in benefit?

Over 55 the benefit of the cleaner air is overcome by all the other factors of increased speed.

I'm a big fan of minimum safe following distances. Two seconds is good three is better. The faster the truck is going the further back you should follow. 65-75 feet at 55 is way closer than what is safe. At 65+ that distance is plain stupid. I don't like people riding my butt, why would I do it to others?

Kirk

Mustang Dave · 12-15-2011, 10:53 PM

Quote:

Over 55 the benefit of the cleaner air is overcome by all the other factors of increased speed.

I'm a big fan of minimum safe following distances. Two seconds is good three is better. The faster the truck is going the further back you should follow. 65-75 feet at 55 is way closer than what is safe. At 65+ that distance is plain stupid. I don't like people riding my butt, why would I do it to others?

I agree. Safety is more important to me than miniscule increases in fuel economy.