drafting lowers the mpg on draftee? - PeachParts Mercedes-Benz Forum

t walgamuth · #1 05-01-2012, 08:21 AM

A lunch buddy yesterday was telling a story about a car drafting his winnebago and complaining about it.

I said who cares, he cannot hurt you?

He said it reduced his Mpg in his vehicle.

I called BS.

anybody have any science to support the arguement one way or the other?

If two race cars can go faster together they should also get better mpg together so this car should not affect the motor home except if to slightly increase fuel economy....in my mind.

MS Fowler · #2 05-01-2012, 08:31 AM

Tom,
No science, but scientific intuition---needing massive federal funding to turn it into science.

The Winnie pushes the air out of the way. The amount of energy required to do this is directly related to the Winnie's speed, and shape ( and prevailing wind speed and direction). I see no way that a vehicle in the draft space behind the Winnie has any effect on the Winne--unless he extends a tow bar that latches onto the rear bumper of the Winnie......

Perhaps the Winnie driver believes all in life is a zero-sum game, so if the drafting vehicle benefits, then it must necessarily be costing the Winnie.

t walgamuth · #3 05-01-2012, 08:34 AM

OTOH the geese switch off the lead position....But that might only be because all the other positions are a little less strenuous.

Honus · #4 05-01-2012, 08:45 AM

Geese trade off the lead position because drafting is easier than breaking the wind, so to speak. (That reminds me of the movie "Spinal Tap." They were getting ready to come out with an album called "Break Like the Wind," as I recall.)

As I recall from engineering school (over 30 years ago!), the equations describing the aerodynamic drag on the Winnebego are parabolic equations, which means that the only conditions you need to know are the upstream conditions. What ever is happening behind the Winnebago has no bearing on the drag it sees. As I recall, the story changes as you cross the speed of sound. Then you get into hyperbolic equations that require you to know the downstream conditions as well.

Or something like that.

Your friend raises an interesting puzzler, though. On the surface, it seems like the guy dragging off the Winnebago is getting a free lunch, which generally doesn't happen in nature.

t walgamuth · #5 05-01-2012, 09:10 AM

With the Nascar racers, two are less wind resistance than each separately and added together, so the second car makes the first car move through the air easier, and himself as well.

So the car behind the winnebago may help him along a bit.

Honus · #6 05-01-2012, 09:18 AM

That's a good point concerning the Nascar racers. My guess is that they draft so close to one another that they basically form one body moving through the air. As such, they only leave one wake and it only pulls back on the rear car. If so, then each car helps the other. The Winnebago is a completely different size and shape than the car dragging on it, but there still might be a similar effect if the rear car is right up tight on the Winnebago's bumper.

lutzTD · #7 05-01-2012, 09:57 AM

I believe there is actually a slight suction created by the low pressure area behind the Winne, this means you are dragging the car from the front. On a side note, I always shake them because the idiot will pile into the back of me if something out of the ordinary happens and he cant see anything but my rear end

retmil46 · #8 05-01-2012, 02:03 PM

Quote:

Originally Posted by lutzTD

I believe there is actually a slight suction created by the low pressure area behind the Winne, this means you are dragging the car from the front. On a side note, I always shake them because the idiot will pile into the back of me if something out of the ordinary happens and he cant see anything but my rear end

That's essentially it. In NASCAR, they draft so close that the following car essentially "fills in" that low pressure area - so in effect you have one longer vehicle with more available HP and better aerodynamics.

Similar effect in ship design, as far as length to beam ratio - of two ships with the same available HP, the one with the higher length to beam ratio (ie, long and narrow) will be able to move thru the water easier and achieve a higher top speed, than one that is short and wide.

In effect, the guy behind the Winnebago is getting a free lunch, but with no adverse effect on the Winnebago itself. For the Winnebago to see any benefit from this arrangement, the guy following would have to be close enough to fill in that low pressure area - too damned close for safety.

Botnst · #9 05-01-2012, 06:01 PM

Thermodynamics.

roflwaffle · #10 05-01-2012, 06:12 PM

Quote:

Originally Posted by t walgamuth

With the Nascar racers, two are less wind resistance than each separately and added together, so the second car makes the first car move through the air easier, and himself as well.

So the car behind the winnebago may help him along a bit.

A very small bit, but yeah, that's the idea. Having something fill in the space behind any vehicle with a sharp cut-off at the end reduces it's pressure drag.

HowStuffWorks "Drafting Strategy"

That said, maybe the Prius driver is stealing the Winnebago driver's brainwaves. I think it's time to triple-bag some tinfoil hats.

t walgamuth · #11 05-11-2012, 06:25 AM

One of my lunch buddies paid me off his dollar on this the other day.

SwampYankee · #12 05-11-2012, 07:42 AM

Mythbusters did a piece on the effects on mpg by drafting behind a semi, as seen in the crappy quality video.

They tested several distances and found a negligible decrease in consumption the closer they got. I can only guess that the drafting effect is far less at highway speeds (even behind a huge draftee) than it is at NASCAR speeds with smaller, far more aerodynamic vehicles.

Coming back from Spring Break in FL way back when, I hooked up with a trucker in SC around 10pm and rode his bumper all the way back to the NY line around 10am (I had a portable CB, asked him if he minded if I snuck in behind him and did some general b.s.'ing along the way), we stopped and refueled and bought him some coffee whenever he stopped for a break, but made good time driving through the night with the cruise set at 75-80 depending on the state. We were probably lucky he wasn't a serial killer, especially since we only gained about 1 mpg over almost 800 miles.

Kuan · #13 05-11-2012, 09:15 AM

It's the vacuum or low pressure behind the vehicle that causes suction. Put two cars together and it's like making one long car, only the one in front has no vacuum sucking it backwards. That's dealt with by the rear car. So they are sharing the aerodynamic load.

MS Fowler · #14 05-11-2012, 09:18 AM

Quote:

Originally Posted by Kuan

It's the vacuum or low pressure behind the vehicle that causes suction. Put two cars together and it's like making one long car, only the one in front has no vacuum sucking it backwards. That's dealt with by the rear car. So they are sharing the aerodynamic load.

So if its a low pressure area, does the engine have higher pumping losses that lower its efficiency?
(Yeah, over- thinking)

Honus · #15 05-11-2012, 11:41 AM

Quote:

Originally Posted by MS Fowler

So if its a low pressure area, does the engine have higher pumping losses that lower its efficiency?
(Yeah, over- thinking)

Theoretically, that might be true, but if the rear car is a normally-aspirated gas engine car, all it needs to do to overcome the reduced pressure is to open the throttle a bit more.

The real answer, I think, is that the vacuum created by the wake of the lead car is much, much milder than the suction created by the engine, so it probably doesn't make much difference.

That's my story and I'm sticking to it, at least until someone who knows what they are talking about chimes in.