Drafting behind trucks on the highway.

Factual Questions
41

[QUOTE=kanicbird]
IIRC the 2 second rule only applies to cars, while following trucks it was 1 second and motorcycles it was 3 seconds (if you were driving a car). The reason is that trucks take much more distance to stop, motorcycles take much less.
[/QUOTE]

Don’t believe it. For starters there is Perception time (the time it requires for you to recognize the danger) and Reaction time (the time it takes for you to get your foot to the brake pedal), on average these take about one second each, so your already in trouble if you’re only one second behind the truck. Then it’s quite possible you might be looking somewhere else, at your speedometer, at a billboard, etc., that increases the perception time.
A full two seconds is the minimum for good conditions and a dry road, add more for bad weather.

42

Is draft the same as slipstream?

43

Yeah, but I haven’t heard that term in ages.
On second thought, they aren’t exacty synonomous, slipstream is the effect behind the lead vehicle, drafting is what the following vehicle is doing, or you could say the second vehicle was slipstreaming.

44

[QUOTE=A.R. Cane]
Sure, drafting a truck can improve your mpg, but it’s dangerous and, as a retired trucker, I wouldn’t tolerate it. There’s also no free lunch, so your gain is the trucks loss.
[/QUOTE]

This is wrong. It actually helps the truck because the truck+car is a more aerodynamic shape than just the truck. The trailers on the truck end in a big square cutoff, which is an awful aerodynamic shape. Having the car there helps at least theoretically.

There are two components to drag. One is called the pressure, or form, drag, and the other is called the friction drag. The latter is exactly what it sounds like, i.e. the drag from friction between the air and the truck’s surface. This is constant regardless of what is behind the truck. Form drag is a bit more complicated. The truck is in air, which exerts a pressure on it. If the truck is not moving the pressure is equal on all surfaces, and no net force is developed.

On the other hand, when it’s in motion the pressure at the front of the truck is different than the pressure at the rear. The pressure at the front of the truck is higher than atmospheric pressure because the truck knocks the air out of the way as it travels forward. The pressure at the rear of the truck is lower because the truck creates a vacuum behind it. Force is equal to pressure*area, with the force being applied in opposite direction at the front and back of the truck. Since the area is the same, the higher pressure on the front creates a net force opposing the trucks motion. I’ve done some simplifying, but this is the basic principle at work.

If you think about what the back of a truck looks like, it’s basically a big rectangle. It displaces a lot of air, so the vacuum formed behind it is large. When a car comes closer it essentially extends the aerodynamic profile of the truck. Since the car is much more aerodynamically shape, it smooths out the flow around the truck and reduces the size of the vacuum formed.

45

[QUOTE=treis]
This is wrong. It actually helps the truck because the truck+car is a more aerodynamic shape than just the truck. The trailers on the truck end in a big square cutoff, which is an awful aerodynamic shape. Having the car there helps at least theoretically.

There are two components to drag. One is called the pressure, or form, drag, and the other is called the friction drag. The latter is exactly what it sounds like, i.e. the drag from friction between the air and the truck’s surface. This is constant regardless of what is behind the truck. Form drag is a bit more complicated. The truck is in air, which exerts a pressure on it. If the truck is not moving the pressure is equal on all surfaces, and no net force is developed.

On the other hand, when it’s in motion the pressure at the front of the truck is different than the pressure at the rear. The pressure at the front of the truck is higher than atmospheric pressure because the truck knocks the air out of the way as it travels forward. The pressure at the rear of the truck is lower because the truck creates a vacuum behind it. Force is equal to pressure*area, with the force being applied in opposite direction at the front and back of the truck. Since the area is the same, the higher pressure on the front creates a net force opposing the trucks motion. I’ve done some simplifying, but this is the basic principle at work.

If you think about what the back of a truck looks like, it’s basically a big rectangle. It displaces a lot of air, so the vacuum formed behind it is large. When a car comes closer it essentially extends the aerodynamic profile of the truck. Since the car is much more aerodynamically shape, it smooths out the flow around the truck and reduces the size of the vacuum formed.
[/QUOTE]

Check the link I posted in post #12, it seems that “experts” can disagree. Here’s an excerpt:
"Two vehicle drafting is not the same as pack drafting in NASCAR, or the pelloton in bicycle racing. In a two vehicle draft situation, the lead vehicle’s vacuum “shadow” is “filled” by the parasitic vehicle, and thus the vacuum shadow of the lead vehicle is extended by about the length of the parasitic vehicle (at normal road speeds of up to 60 mph), because the presence of the parasitic vehicle prevents the rapid collapse of the vacuum shadow by pressurized air pushed out of the way by the lead vehicle. In effect, the vacuum shadow becomes longer and more turbulent for the lead vehicle, costing it mileage (or in NASCAR situations, top speed). In the case of vehicles of very different lengths and aerodynamic cross sections, the effect will be somewhat proportional to the order in which the vehicles run, i.e. a truck followed by a car will get a marginally longer, dirtier vacuum shadow with increased parasitic losses, whereas a passenger car drafted by a 40’ long trailer truck will lose its vacuum shadow entirely in the length of the truck’s following aero-profile. So, for best mileage, what car drivers would really want is to get big semis to tailgate them very closely – "

I’m retired, so it’s a moot point for me.

46

That is just some guy on a message board that doesn’t know what he is talking about. Check out the link provided by the poster before him.

Here is another cite:

47

[QUOTE=Martin Hyde]
Being one second behind any vehicle just is not advised. Is better gas mileage worth your life? What if something goes wrong and the trucker has to make a sudden stop, looks in his mirrors and sees no one and begins to slow down? Well, hopefully you react in time, but since you’re about four times as close to the truck as you are advised to be when driving on the interstate, you may not react quickly enough.

Maybe you’re the best driver in the world though, and have nothing to fear. Personally I wouldn’t risk it, it’s just way too easy to break concentration for a split second. On the interstate, when moving at speeds of 65 mph +, I always try to avoid clumps of vehicles and have a good deal of room behind and in front of me, that way worst case scenario I can probably lock up my breaks and veer off the road if it looks like something insane is going on in front of me.

I once had a car about 100 yards in front of me spin out with no warning whatsoever and if not for a few inches we’d have collided with me going around 70 mph–I’d rather not find out what that feels like.
[/QUOTE]

I don’t know where you live, but I drive the Cleveland/Akron/Canton interstates quite frequently. Around here, if you try to leave a safe distance in front, they just claim it. They almost clip your headlights as they pass then pull in front. I mean the cars, not the trucks. Well, 99% of the cars.

48

Why has no one has mentioned the worst downside to this? Being stuck behind a truck for 3 hours? If anyone here says they would rather take a 3 hour trip stuck behind a truck the whole time than be 5 dollars poorer then I don’t believe them.

49

I cycle through town regularly, and am always amazed at the difference tucking in behind a bus makes. Even if the bus is just passing, the pull as it goes past is incredible. Of course, you have to already be moving fast enough to get the effect.

My point, though, is this: I find that on a bike drafting a bus is hard work, pedalling fast enough to keep up and get the benefits of the pull, but it does make it possible to go quicker than normal (especially into a strong headwind). I assume, therefore that despite being at least as worn out at the end, I’ve expended less energy getting there? Or is the physics of the interaction between something as small as a bike with a double-decker bus (single-decker buses don’t work as well) different to the truck-car relationship?

50

[QUOTE=Bootis]
Why has no one has mentioned the worst downside to this? Being stuck behind a truck for 3 hours? If anyone here says they would rather take a 3 hour trip stuck behind a truck the whole time than be 5 dollars poorer then I don’t believe them.
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We all waited for you. :smiley:

But you’re right; the backside of a truck makes a non-entertaining view.

A.R. Cane I get your point about the debris, But about the braking or swerving; you must be joking. My car stops in 1/3 of the distance a truck takes, reaction-time is really not an issue.

Do people in modern, normal cars that are in good repair EVER hit the backside of a truck?

51

[QUOTE=The Librarian]

A.R. Cane I get your point about the debris, But about the braking or swerving; you must be joking. My car stops in 1/3 of the distance a truck takes, reaction-time is really not an issue.

Do people in modern, normal cars that are in good repair EVER hit the backside of a truck?
[/QUOTE]

If you’ll read my post again you’ll see that I included driving alongside trucks in my statement, but don’t delude yourself, newer semi rigs are equipped w/ ABS and, when moderately loaded can stop nearly as quickly as autos. Empty, or lightly loaded, semis are the most difficult to stop as they tend to jackknife more easily.

To address your second question, I ran across this:
Center for National Truck Statistics The University of Michigan:
For the 1997 and 1998 data years, UMTRI’s Center for National Truck Statistics collected data on rear underride as part of its Trucks Involved in Fatal Accidents survey. The purpose of the survey was to evaluate the incidence of underride in these fatal crashes. Supplemental data were collected on each rear-end crash involvement. Data collected included whether the truck had a rear underride guard, whether the striking vehicle underrode the truck, and how much underride occurred. A total of 853 fatal crash involvements occurred in 1997-1998 in which a nontruck vehicle struck a truck in the rear. Underride was reported in 518 of the 853 rear-end crashes. Among 276 straight trucks struck in the rear, there was no underride in 78 involvements, some underride in 152 involvements and underride could not be determined in 46. Among 541 tractors with one or more trailers struck in the rear, no underride occurred in 124 involvements, some underride occurred in 357 in, and underride could not be determined in 60 involvements.There were 979 fatalities in these rear-end crashes; 900 of the fatalities occurred in the striking vehicle. Of the striking vehicle fatalities, 211 occurred with no underride, 565 when some underride was recorded, and 124 when underride could not be determined

52

[QUOTE=A.R. Cane]
If you’ll read my post again you’ll see that I included driving alongside trucks in my statement, but don’t delude yourself, newer semi rigs are equipped w/ ABS and, when moderately loaded can stop nearly as quickly as autos. Empty, or lightly loaded, semis are the most difficult to stop as they tend to jackknife more easily.(snip)

[/QUOTE]
(bolding/underlining mine)
Don’t worry; I’ve worked long enough around truck drivers to know a semi has some huge blind spots and I try to give the big fellas plenty of room.

But now for the hypothetical situation where I am driving very close behind a combination (matching it’s speed) Do you honestly think the truck can brake hard enough to suprise me?

53

[QUOTE=The Librarian]

Do you honestly think the truck can brake hard enough to suprise me?
[/QUOTE]

It’s been a while since I’ve driven a truck on the road. In the past, airbrakes were slow to activate as when the brake pedal was pressed, the air lines had to be pressurised, this takes time and is quite disconcerting until you adjust to it. This gave following vehicles an advance warning as the brake lights come on, then half a second or so later, the brakes start working. Another issue is that as the line pressurises, the brakes start operating from front to back and there can be a time when the front axle has some braking, but the last axle has none.

Newer trucks now have disc brakes, ABS, EBD and “brake by wire”. In this, the brake lines are fully pressurised with a valve and sensor at each wheel. When the brake pedal is operated, the valve opens and the brakes are operated immediately, the valve shutting off when the sensor reports the brake pressure matches the required pressure. This means the delay in braking is much less than in the past and the pressure to each wheel depends on the wheels load so there is less chance of lockup, which is taken care of anyway with the ABS. ( ETA And each wheel is operated at the same time (no front to rear delay) )

Can a truck stop as fast as a car? I can’t say :smiley: , but when the newer braking systems were on test trucks that magazines drove, the reviewers raved on about them matching car brakes in performance.

54

[QUOTE=A.R. Cane]

Center for National Truck Statistics The University of Michigan:
For the 1997 and 1998 data years, UMTRI’s Center for National Truck Statistics collected data on rear underride as part of its Trucks Involved in Fatal Accidents survey. The purpose of the survey was to evaluate the incidence of underride in these fatal crashes. Supplemental data were collected on each rear-end crash involvement. Data collected included whether the truck had a rear underride guard, whether the striking vehicle underrode the truck, and how much underride occurred. A total of 853 fatal crash involvements occurred in 1997-1998 in which a nontruck vehicle struck a truck in the rear. Underride was reported in 518 of the 853 rear-end crashes. Among 276 straight trucks struck in the rear, there was no underride in 78 involvements, some underride in 152 involvements and underride could not be determined in 46. Among 541 tractors with one or more trailers struck in the rear, no underride occurred in 124 involvements, some underride occurred in 357 in, and underride could not be determined in 60 involvements.There were 979 fatalities in these rear-end crashes; 900 of the fatalities occurred in the striking vehicle. Of the striking vehicle fatalities, 211 occurred with no underride, 565 when some underride was recorded, and 124 when underride could not be determined
[/QUOTE]

It’s hard to read much into this paragraph. How many of these accidents were the result of inattentive drivers hitting a fully stopped semi at full speed? How many were the result of an auto driver following too closely behind a semi which had to stop suddenly at highway speed? All we know from this is that a number of cars somehow hit a number of trucks, with a resulting number of underrides and deaths.

55

This is why I just roll down my window, slip a tow rope onto the truck’s bumper, and cruise a nice safe distance away from the truck. I get 100% fuel savings for the cost of a disposable rope.

56

I think the “drafting trucks” issue has been sufficiently explained, I’m gonna’ leave it to the Darwin effect from here.

57

I’ve watched Nascar on occasion and from my understanding drafting is benificial to the leader and follower.

Also, the Mythbuster’s experiment seemed to be pretty thorough and it reduced wind resistance by a lot at 6 car lengths behind (completely safe distance) so, you don’t have to be right on the truck’s bumper to get a decent effect.

58

[QUOTE=Psycho Originals]
… you don’t have to be right on the truck’s bumper to get a decent effect.
[/QUOTE]

My own (utterly non-scientific) experience is that this is correct. When I am driving on some of the narrow bush two-lane highways west of Sydney, and I pass an oncoming semi at a closing speed of about 200kmh (wassat? 120mph?) there is a delay of about a second between the time I clear the rear of the semi and the time I feel the wheel shudder and the car get buffeted by a wall of wind. Simultaneously as this happens, I see leaves and such being kicked up in a vortex. So whatever one entire second at 200kmh represents in distance, it seems like a reasonable length of road, so a car could still be in the truck’s draft a way back of its tail.