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Question concerning semi drafting.
Doing much highway driving, when the opportunity comes up I am not one to pass on a good draft.
I realize that being behind a semi in a low pressure/reduced air resistance area poses large FE gains.
I have never really noticed even a small RPM drop while drafting. Should the RPM's drop in a situation like this, especially when briefly as close as 20ft behind a big rig?
Or does the engine continue running at a constant RPM while less fuel is injected due to the lessened demand for power, yielding no RPM drop?

I'm split here... Though I think it would be the latter.
_Rance

There will be less fuel injected, but the same rpms. There will then be a very large rpm drop as you rear-end the semi that just slammed on the brakes.

Don't Draft.

Question concerning semi drafting.
Doing much highway driving, when the opportunity comes up I am not one to pass on a good draft.
I realize that being behind a semi in a low pressure/reduced air resistance area poses large FE gains.
I have never really noticed even a small RPM drop while drafting. Should the RPM's drop in a situation like this, especially when briefly as close as 20ft behind a big rig?
Or does the engine continue running at a constant RPM while less fuel is injected due to the lessened demand for power, yielding no RPM drop?

I'm split here... Though I think it would be the latter.
_RanceScanGage and/or the instant mpg would reflect the difference.

Now for a question of my own? Why did you think we would promote drafting? I'm not trying to be personal, but several places we have clearly stated CleanMPG does not endorse semi drafting, yet it's as if the warnings were never there.

Much of the media has put out stories with a clear slant against hypermiling and one of the things most mentioned is semi drafting.

It's not a common practice and we don't encourage it.

Why do people think we encourage drafting anyway?

I'd really like to know.

I understand that its very dangerous, and I'm not to say that I am a proponent of the NASCAR style draft. I would always be at a safe distance when I did do it. The 20ft was an example, only witnessed in tight traffic conditions, thats not where I would sit.
I have found that 200 feet back from a semi helps FE (Mythbusters), and thats where as close as I'll get, too many factors that could prove fatal.
I know that it isn't endorsed here and I know that its discouraged, part of the reason I joined, to help spread the word on benifits of hypermiling and shy from the so-called hypermiling vex.
My question was just out of curiosity because it was something that I have noticed, thats all.

_Rance

Did not mean it to come off as an attack, but much of the media is hell-bent to say it's a common practice. Squadrons do it, birds have done it forever, and I've seen trucker convoys that seem to be drafting each other.

Close semi drafting can result in a 50% gain, but that's well in the danger zone...I refer to the Car and Driver editor taking an Insight to 121mpg in 2000 at 58mph....80-85mpg is what you would get not drafting.

To answer the question, it would lower the throttle required, but not the rpm. There's a specific ratio between engine and wheels, through the transmission. That can't change unless speed changes. So what changes it engine output, lower to handle the lower demands on it.

HyperCivic, I understand what you are saying. I think word choice is important. "Drafting" takes place at high speeds on a sanctioned racetrack. On the nations highways, anyone attempting to emulate a NASCAR draft is "tailgating," and is not a safe driver (and certainly does not have the training or skill of a professional racecar driver).

if you are following a Semi at the well known "safe" distances, then you are not faking a NASCAR draft, nor are you tailgating. You are, in fact, within the law - but realizing a legal benefit - safely. All plusses.

So, while following a semi at normal distances, the benefit will not be seen as a decrease in RPMs, since that is basically hardwired to the wheels - which are still turning at the same speed. If you have a ScanGauge, you may notice a decrease in TPS, LOD, GPH, and a corresponging increase in instantaneous MPG. I notice such a thing as trucks pass me.

My question is why are you drafting semis at all if you are interested in better FE? You can gain far more FE by slowing down and maybe doing P&G than you can by drafting at a safe distance at typical truck speeds.

I'll second Lightfoot's advice - be wise and follow it

Everybody talks about the semi slamming on their brakes and being powerless to rear end them while drafting. It seems to me like the poster is following at a safe distance. I think as long as you can maintain at least a second or two away from the semi, you should be in the clear. For my car anyway, I have 4 wheel antilock disc brakes with semi performance tires and performance oriented organic brakes and a tight suspension. I don't tailgate semis but even if I did, I know I wouldn't slam into one even if it locked up its brakes. Maybe an Insight is a different story. You tell me. An Insight is like 2000 pounds right? How much does a Semi weigh again? Thats why I get nervous when theres a porche or something in front of me because his stopping power is clearly greater than mine, therefore posting a rear ending threat.

Can we please not go through the exercise again. We just recently went though this in two threads where people could not seem to understand that 1-2 seconds is not enough time when something gets thrown back at you or the truck has to make an emergency stop. Seems like it is troll bait all the time now. If you would have done a search on drafting you would have found several topics already available to read. Here are the two I am referring to, please don't add to them they are dieing but hopefully they will answer all your questions as to why we are so adamant about not drafting.

http://www.cleanmpg.com/forums/showthread.php?t=11954
http://www.cleanmpg.com/forums/showthread.php?t=11964

Why do people think we encourage drafting anyway?
I'd really like to know.

I believe people think this will be an easy/painless way to increase mpg. Which IMHO it isn't. I hate getting near any other vehicles especially Semi's.

Plus one of the first things visitors read when they come here is
http://www.cleanmpg.com/forums/showthread.php?t=1510 and the first actual photograph is drafting. Most people dont read they just skim to the pics.

I believe people think this will be an easy/painless way to increase mpg. Which IMHO it isn't. I hate getting near any other vehicles especially Semi's.

Plus one of the first things visitors read when they come here is
http://www.cleanmpg.com/forums/showthread.php?t=1510 and the first actual photograph is drafting. Most people dont read they just skim to the pics.

I think you have an excellent point there. Maybe Wayne needs to rethink the presentation of that particular part of the article. Like 2 inch read font that says, Don't Do This!

It's interesting that the media seems to want to paint hypermilers in a bad light because of the drafting issue. It's been my experience that it is the non-hypermilers that are aggressive, that speed, and that tailgate. I can be following a semi at 3 seconds, and car after car will pull in between me and the semi. Why not make a big issue about these idiots "drafting"? An honest reporter would point out that because most drivers *are* idiots, a hypermiler drafting close in to a semi wouldn't be doing anything more dangerous than what the typical non-hypermiling commuter does already. How many times have you seen some jackass speeding, weaving in and out of traffic, tailgating, honking their horn, blink their lights at people? Why do those idiots get a pass, but hypermilers are crucified for occasionally drafting a semi at 3 seconds?

It intresting we say that driving 1 to 2 seconds behind is drafting. I find the majority of people behind me are 0.5 to 1.5 seconds behind me, and their gole clearly isn't to improve fuel economy. The original rule of thumb was distance was 2 seconds, and today's state handbooks suggest 2, 3, or 4 seconds (depending on state), with most suggesting 4, but when you look at the laws, they are often vague (must maintain safe distance) or distance specific (x feet, or x feet at speed y). Very rarely do the laws match the handbook sugestions, although following the handbook pretty much keeps you legal.

Strange how outsiders can bad mouth hypermilers who tailgate while often people the same people who would be tailgating us as far closer distances.

As far as techniques go, forget it. Even with semi's, which produce relatively massive areas which you could benefit from, you can't get close enough with any safety to take advantage. The only way we will ever be able to draft safely is when we have the adaptive cruise control where computers monitor distance and keep us at a safe distance. They can react almost instanty...at least to the point we actually could ride half a car length with confidence. However if I had a car with that tech today, and it was legal to use it in that manor, I still wouldn't unless everyone else had it. Image sitting 10 feet behind a semi at 70mph with some guy doing similar to you. The semi brakes hard, your car automaticly stops with it, but the guy behind you without the tech can't react in time and push you and your car under the semi. Kiss your everthing good bye.

One last thing, the new aerodynamic rigs (or more importantly trailers) don't produce much more draft area than a car. As the wind comes around the back, it closes up fairly smoothly. So as more of those become common, it will be even more foolish...hey I like that...."Don't be fuelish, but more importantly don't be foolish".

Hi Shirobi:

___You are right as 200’ is well outside a close-in. I mention 1.5 + is a good number but as most others said, far right lane and let those “speeders” provide us with a free ride in the traffic side. As congestion builds but traffic is still moving, a surf works great as well. Jsut no close-ins if you can help it.

___Good Luck

___Wayne

Now for a question of my own? Why did you think we would promote drafting? I'm not trying to be personal, but several places we have clearly stated CleanMPG does not endorse semi drafting, yet it's as if the warnings were never there.I'm not trying to be personal either, and to be honest I don't see a single thing unsafe about following another vehicle -- ANY VEHICLE -- at a prudent distance. In fact it is unavoidable on any but the most lightly travelled roads.

But from my POV the "don't draft" warnings appear to be a reaction rather than a primary tenet. For an example of what I mean, just do a search of the glossary for the word 'draft'; you get:

Close-in draft
Distant draft
drafting
Surf draft
Traffic-side draft

That's a lot of hits for a supposedly dirty word. ;) Might I suggest replacing the word 'draft' with some other term? 'Wake surf' or somesuch...

Nothing beats drafting a semi going 50. Nothing.
I've noticed there's more of them these days.
If you guys want to leave me that slot, I'll take it.
I ain't gonna hurt that truck with my plastic car.
And I ain't gonna let him hurt me.
Thanks for your concern.
Now bug off.

Hi Ptero, please let us know how many rock chips and windshield cracks your propensity for tailgating nets you. I prefer to abide with the law on this issue, and be a safe, considerate driver.

Hi Ptero,

On a dry calm day in the mid 80's, in a 2nd Gen Prius, at 53 mph, and super highway mode one can do 65 (mid 70's with a following wind) mpg. That beats drafting a semi at 1 second, usually, since they are up and down in speed so much in metropolitan traffic. In the right lane a little slower than traffic, the engine stays at a near constant speed, and so does the car. And this is very advantageous to fuel economy.

If you can find a DWL solution in your Smart, a similar situation may also occur.