Mythbusters : Drafting video
 

[IMG]ttp://www.youtube.com/watch?v=Cu0uNPFCf68[/IMG]

I missed this one. Amazing reduction in drag from drafting !
I'd like to see the full episode to see how they built that wind tunnel.

Watch it fast before it gets taken down !

EDIT : I'm seeing some 'wierdness' in the site lately. I'm not seeing what I wrote ( here it is again )


I missed this one. Amazing reduction in drag from drafting !
I'd like to see the full episode to see how they built that wind tunnel.

Watch it fast before it gets taken down !

http://www.youtube.com/watch?v=lttgT1XZVvE

Part one

http://www.youtube.com/watch?v=y18mewmFNOQ

Drafting with a motorcycle.

EDIT : Scratch that - he on a bicycle .

I hesitate to even comment but here goes:

I think Mythbusters is right and their numbers are probably valid. Once upon a time on motorcycle I got behind a horsetrailer with horses in it- the ultimate draft vehicle because horse trailers are close to the ground (unlike that semi and trailer), have a large rear x-section, and because when horses are in them the driver is going to be extra careful and avoid sudden speed and lane changes unless it's really an emergency (unlike semi drivers who might swerve or lock up their trailer brakes to shake off a tailgater).

There was very little traffic but I got in "the bubble" undetected by the driver (I'm pretty sure) and hung out in there for a looooong time (150 miles at 75 mph!). I noticed that when I pulled in real close- maybe not 2 feet but... let's just say not far from that- I had to back off the throttle some and the temp gauge suddenly shot up. Bike had no windshield and I had my visor up- amazingly calm back there, not even the expected turbulence and buffeting. I didn't stay that close for long because the temp gauge climbed up against the red, I clearly heard the fan kick on, and I was sweating bricks too, so backed off enough to feel airflow again and see temp gauge go down. Still, at that point (I dunno- 25'?) much much less air to punch through than non-drafting although it was buffety and turbulent and unpleasant besides. Got way better mpg that tank even though cruise speed for that 150m was 20 mph faster than I usually go and also that segment was only part of a tank. Sorry, even though I calculated fe at most every fill, I didn't keep fe logs on the bike back then so I won't quote an exact value but yes it seemed to be a better than 40% improvement. Yup, it definitely works.

That was years ago and that was also the last time I took chances drafting even though there was no incident whatsoever. The chance of an incident is extremely high and I'm not suicidal these days... but sometimes we do stupid things in the name of science right??? Also it seems potential draft vehicles are never going the speed I want to go anyway. Seems like they are always kicking up rocks and gravel too. I don't draft (unless following using the 2 second rule counts as drafting) and don't recommend it.

Frank, I remember we had a conversation on another post related to drafting. I asked about tailgaters, and whether or not they hurt your FE.
I forget what the verdict was on that.
I remember the point that the vehicle behind you supplies it's own power, and this offsets any parasitic 'drag' - as in drag on your own vehicle.
Still, it would seem to me that eventhough they supply their own power, they add to your overall frontal area and wake size ( if there is a large SUV tailgating you.)
Besides, their vehicle now provides less power to the vehicle in front, since it has less drag to overcome.

I still slightly confused still about that.

Frank - about the horse trailer incident...the first thing that comes to mind is horse manure...flying horse manure.

Yeah if they farted I knew it.

Well if NASCAR is any indication, tailgating helps BOTH vehicles out.

There is almost no danger driving 2' behind another vehicle, except for the cosmetics on the bumpers. Even if the lead vehicle slams on their brakes, the relative speed at impact is under 8 MPH. Pilots flying in really close formation know this. At 200 mph, a NASCAR driver can't ease over to the wall and touch it because of air pressure, but he can swerve into it from farther away.

I can sympathize with the mythbusters for overdramatizing the danger, because a long train of close tailgaters would need progressively better reflexes if the lead vehicle slowed slightly, and they are in the entertainment biz anyway, but they promoted another myth. Perhaps we will get electronically controlled brakes to allow close following.

This was tested about 5 years ago... There was a "Smart Highway" test road build and the test mules were 5 or 7 Ford Taurus cars. They all had sensors front and rear and sensors in the roadway that would let the vehicles communicate with each other while driving. The drivers basically didn't have to do a thing, the vehicles steered themselves as well as maintained speed and also, a very close following distance. This allowed the vehicles to get better than average FE and were much safer as they could react faster and smoother than the human counterpart.

I'll try to see if there is any more info on that project.

I believe that is how the "Smart" cruise controllers and lane stability systems came about in some of the higher end cars.

EDIT:
Ah! found the name of the program: Intelligent Vehicle-Highway Systems (IVHS) initiative.

ARTICLE HERE
I guess I was off a bit, this program was started in 1994 and Demo'd in 1997.

...what are you asking for? a "Smart Highway" from our government...sounds like a "motoring oxymoron" to me!

A

You have made a VERY dangerous statement. There is a BIG difference in Nascar drivers and the public drafting. Nascar drivers are trained drivers and all the drivers use the same method.

Drafting a semi as close as 2' in a car is VERY dangerous. Vehicle mass and stopping distances are two factors very different from NASCAR.


Drafting is a dangerous practice that is not recommended. There are many other more safe ways to achieve great mileage.

I'd just like to point out that stopping distances for cars are shorter than semis. So that just leaves reaction time as a factor. The reason I don't do it is flying road gators.

Ratio of Speed to Stopping Distance
Oregon Trucking Associations: Stopping Distances For Cars vs. Trucks

I think the best compromise is to draft when an opportunity arises.

Example:

On I64 yesterday the traffic was heavy. Many vehicles including big rigs were tailgating at 1 car length at 60 MPH.

I usually stay in the right lane and look for a 3 stripe cushion behind a large vehicle, preferably one with the undercarriage close to the ground.

It also sends a message to aggressive drivers that I am not interested in the left lane antics that are very dangerous. Most sane drivers will not try to pass on the right using my 150 foot cushion of distance, even when it is at least 5 times the average distance between vehicles.

Staying in the right lane with a 125 to 150 foot separation distance gives me the chance to stop completely in that cushion distance or swerve right to the shoulder if a large object is in the road.

It does require a greater level of concentration that driving on a deserted highway, but nothing compared to the level of concentration the aggressive tailgaters need to apply to their stupidity.

I would much prefer no drafting and a relaxed drive, but in some circumstances here it is just not a possibility, unless you want to feel like a doormat as people blast by you with 3 feet of clearance to pass, on the right, someone in the left lane. It's highly illegal but seldom enforced here.

regards
Mech

On the other hand, driving on typical Northern California freeways (and I suppose elsewhere), you can't possibly avoid drafting. Maybe not at 2 feet, but if you leave much more than a car length following distance from the vehicle ahead, someone is sure to cut in. So you might as well find a semi to follow...

Some days everyone was drafting except me, the only hypermiler and ecomodder doing it slowly on my own.

So I decided to learn to draft properly and with more safety. Which was good as I was driving a slow diesel van with the aero's of a brick, and drafting really helps high CDa vehicles.

Side drafting is easy and safe. Sometimes in the middle of three lanes you get cars on both sides helping. Quite a technical skill as cornering on the extreme inside line(to reduce distances traveled), cross winds, leading vehicle shapes, beside road hillsides and windblocks, have to be constantly calculated.

Rear drafting at a good distance is perfectly safe and is better than no drafting at all.

Drafting at the right time begs consideration, especially in underpowered vehicles like Japanese diesel vans. If I was climbing a freeway hill and its going to be a strong headwind near the top, I would look for a lead vehicle prior to the accent which would allow the climb without needing to change down or reduce speed. Was a very courteous thing to do for other motorists who would have been inconvenienced by my slow van.

When boating I would always look for a wake to surf, or an island to get leeward of.

Doing a night drive Wellington to Auckland when gas was expensive, around 3am I saw 15 or so big rigs all drafting each other at around 65mph. Probably saved a stack of money. Really dangerous.

Dolphins draft up front, so the lead does benefit too

Mythbusters
 

I think that the 1/24-scale windtunnel models were operating below critical Reynolds Number,so those numbers could be a little squirrely.--------- In full-scale,and without the "throttle feathering" they might have seen a full 50% improvement,which would agree with Kelly and Holcombe's research,constituting a 100% drag reduction.--------- And I'm with Bicycle Bob on the safety aspect at 2-foot clearance.Should the 18-wheeler jam the brake pedal,inertial effects would still allow for a soft kiss since they're starting at identical velocity.If they were touching it would be the safest.( test course driving,not the real world ).

...yeah, it's only those pesky "sudden" stops that get you!

Still, I was not interested in "kissing" that trailer on a bike!

Good plan. When bike racers are drafting closely, and wheels touch, the back rider goes down. There's no bumper, and they fall over if steered by a tow vehicle, or other contact.

With cars and trucks, as drafting distance decreases, the chance of collision keeps going up. However, when very close, the severity of the collision begins decreasing to zero, if the bumpers match.

The lead vehicle benefits from being drafted.
"In car drafting, the lead car is also getting a benefit. Trailing cars fill in the lead car's low-pressure wake, thereby cutting down pressure drag."
The Daytona 500: Flying Without Leaving the Ground | LiveScience

The reason close drafting is dangerous by bicycle is because if you bump, the front wheel is the first thing to touch, and with no steering damper, the wheel makes a sudden turn you weren't expecting. Same for a motorcycle, but since a car has a bumper, that's really not a fair comparison.

Considering 1) there is a minimal speed differential in drafting (and its is differential between vehicles, not absolute speed, that determines impact force), 2) a semi has about 4 times the stopping distance of a car, 3)they tend to drive more steady than your average car driver, and 4) Mythbusters showed there is a benefit as far back as 100ft;
it really doesn't make sense to make a blanket statement that "drafting is dangerous".

Just like with some people claiming it is dangerous to drive slow on the highway (when in fact the opposite is true), danger isn't just about the risk of an accident, but the product of the risk of accident AND how severe that accident would be if it occurred.

However, I never draft anymore. All the semi's around here speed, all of the time. They are supposedly limited to 10mph under the limit for everyone else, but speed limits are kind of a joke.

I like you.

Thanks!

That and...

I've thought that for a long time. The drafting vehicle is providing a kind of rolling boat tail for the lead vehicle.

I wonder why truckers object so violently to being drafted. I doubt it's because of an altruistic concern for car drivers' safety. Is it because they begrudge the economy realized by the cars, or is it just the ignorant belief that that such economy is at the truckers' expense?

I've never experienced a trucker objecting to being drafted.

They do it with each other all the time out on the open road.
They flash the lights to let the passing truck know the moment its safe to pull in front of them, presumably so the drafting affect is at a maximum.
I took to doing that when I drove my RV across the country, and they would pull in a few feet ahead of me, then flash the running lights a couple times as a way of saying "thanks" (I know that's what it meant, because I asked about it over the CB).

Maybe some people in small cars drive recklessly, (like in Fast and Furious, where the sport cars drive underneath the semis!) and they prefer they just stay away from them all together?

I had one go bananas on me... same trip that I drafted that horse trailer on. Eh hole went on the shoulder and locked up his trailer brakes, throwing lots of gravel. Gotta hand it to him, that was pretty good. But I stayed back there and eventually he came to a complete stop right in the middle of the road! LMAO! I stopped too but eventually took off.

I have been sprayed with stones here in NZ too. I suppose I might be inclined to do the same. Little blowfly buzzing around my butt.

Its like someone standing too close to you. Its about standardised distances. If you go to another country or sometimes even another town everyone drives different.

On my scooter I nip in and out, travel past cars on the inside etc etc and people totally freak out!

In asian cities I would be called Mr Careful. Here I'm Mr Dangerous.

Usually 30 years plus males in large cars are the most offended. :rolleyes:

Around here they let you be most of the time. I just make sure I can always keep eye contact with one of the truck side mirrors.

When traveling in the NE states, I've noticed a bigger propensity for truckers not liking being drafted, but that might not be representative.

this makes since. my father and I drive semi trucks. and i remember being told that the bull haulers have a small section in the front of the trailer blocked off because the truck part would push the air out of the way, and inside the very front of the trailer the turbulence would create a vacuum bubble behind the sleeper and a few feet in the trailer at highway speeds. and the animals would pass out from lack of oxygen! that may be a bs trucker story i have yet to find facts on that, but I do remember trash cans are easily sucked from the side of the road if you get the rear of the trailer just right :D

I think the main reason truckers don't like people back there is because you cannot see them for like 200 feet behind the trailer. you have to manage the space around the truck because if an emergency comes up you will need to know the best way out without killing someone. also a light rear end car to car will result in some broken plastic and hurt feelings, that same accident with the rear of the trailer there is heaver damage to the car and greater chance of injury. and insurance company's almost always find a way to blame the truck driver.

I have seen a dozen big rigs sharing drafting duty (less than 75 feet separation) on I95 going south of DC for 70 miles. They didn't like me being between them (costing them money) but following up in the rear at a stopping distance gap was good for 68 MPG in my stock 94 VX, averaging 65 MPH (speed limit), when gas was $4 a gallon last summer.
Another advantage is the rest of the nuts going 80 MPH (reckless driving conviction here), would not think of using the distance in front of me for passing with a few rare exceptions.

I generally like 3 stripes between me and the host (43 feet from point to point between stripes). Since I am a slow (relative to others) driver, the right lane is where I am legally supposed to be. It also gives me an always available escape route to the right if I spot a large chuck of debris, and the time to see the debris.

regards
Mech

While drafting works, I don't do it.

The safety hazard is the least likely thing to hurt you.

The most likely thing is you will miss an exit.

The primary reason I don't even want to appear drafting is the PR in hypermiling articles....I've lost count of the misinformation this is a common practice...Even after CleanMPG.com made a press release, articles STILL said: "hypermilers draft semis". Sadly, hypermiling in the media and public is less a discussion of fact and more a smear campaign.

How far back are you when you can see the semis' mirrors ?
Any closer, and I am not comfortable.
Even at that distance, I was reminded of why drafting is dangerous - there was a large object in the road, and I almost smashed right into it.

Stay safe folks.