Kamm back speed at which it starts to work?

[PaleMelanesian]

Of course, you can overcome that with Pulse & Glide....

[mobilerik]

So are you both saying the same thing...? That the BSFC curve doesn't actually shift, since that specifically has to do with the engine, but the entire driving system will bias itself downward on the map towards less efficient power? Or is the "load" on the BSFC curve based on the entire car (engine, gears, wheels, and all) so that the map will change since "the car" has changed?

Sorry this has gotten so far away from Kammbacks!

CobraBall: It sounds as if you're talking specifically about a "spoiler" of the kind that is placed at an angle to the airflow to create drag and force in a particular direction. I got the impression that Geebee was asking about a "Kammback" as a design that deliberately follows the ideal curve for some distance in order to avoid creating extra drag.

[PaleMelanesian]

'zactly. If you're running steady-state speed, you have lower engine load, which puts you into a less-efficient BSFC point.

However, for p&g, the pulse will not be much different than before. Maybe slightly shorter, due to the lower resistance, but not much. The big bonus is that the glide will go much longer. This biases the total more toward the glide-level consumption, raising the average. Glide-level being idle if you leave it running in N, or zero if you EOC.

You would move from the red zone (best) straight down, based on how much less resistance you have.

[aerohead]

Originally Posted by akashic

So are you both saying the same thing...? That the BSFC curve doesn't actually shift, since that specifically has to do with the engine, but the entire driving system will bias itself downward on the map towards less efficient power? Or is the "load" on the BSFC curve based on the entire car (engine, gears, wheels, and all) so that the map will change since "the car" has changed?

Sorry this has gotten so far away from Kammbacks!

CobraBall: It sounds as if you're talking specifically about a "spoiler" of the kind that is placed at an angle to the airflow to create drag and force in a particular direction. I got the impression that Geebee was asking about a "Kammback" as a design that deliberately follows the ideal curve for some distance in order to avoid creating extra drag.

It is a "shift".I work with this so infrequently that I'd like to have the text in front of me so I don't mis-represent it.All of the BSFC map architecture remains constant,however the road-load horsepower curve drops away and I really need to have it in front of me (Hucho's work ) to make the best go of it.I'll shoot for tomorrow as the weather is bad an I can't work.If any lurkers have the text and can post the maps please do!

[mobilerik]

Originally Posted by PaleMelanesian

Of course, you can overcome that with Pulse & Glide....

*IF* you can Pulse & Glide. In most freeway conditions other than going downhill, my truck is too big to P&G for any gains. It's too heavy to accelerate efficiently, and it slows down way too fast. I do P&G in town a lot, but acceleration is still a killer for me that frequently backfires, running into blind stops and pileups, etc, that cut my little glide short. I calculated the tradeoffs, and the tolerances for when it'll work and when it won't are pretty tight. As I continue to aeromod (perhaps with a Kammback ), Pulse & Glide is becoming a better bet. But I don't expect to ever be able to efficiently P&G on a flat highway at 50mph. And around town it'll always be a crapshoot.

[cfg83]

akashic -

Originally Posted by akashic

So are you both saying the same thing...? That the BSFC curve doesn't actually shift, since that specifically has to do with the engine, but the entire driving system will bias itself downward on the map towards less efficient power? Or is the "load" on the BSFC curve based on the entire car (engine, gears, wheels, and all) so that the map will change since "the car" has changed?

Sorry this has gotten so far away from Kammbacks!

...

Me too. Sorry to muddy the waters. I was trying to relate the concept of a 3-Dimensonal BSFC chart for a given engine to a 2-Dimensional MPG/MPH chart for a given Cd.

But I do think the segway is pretty interesting.

CarloSW2

[basslover911]

Originally Posted by CobraBall

In the movie World's Fastest Indian Burt Munro did a stability demo with a pencil, the pencil became stable when the CG was moved forward of the CP. Burt wanted to add weight to the front end of his Indian MC to shift the CG forward of the center of pressure, thus make it more stable.

GREAT movie, it just shows how much (even though a little "hollywood") backyard "engineers" can achieve and what streamlined bodies and regeared transmissions can do- in the movie they use it for speed, but the same holds true for economy...

In other words...

I want a taller 5th gear! I can downshift, thats fine im not lazy... heck its an automatic it will downshift by itself common!!!

[MetroMPG]

Originally Posted by akashic

CobraBall: It sounds as if you're talking specifically about a "spoiler" of the kind that is placed at an angle to the airflow to create drag and force in a particular direction. I got the impression that Geebee was asking about a "Kammback" as a design that deliberately follows the ideal curve for some distance in order to avoid creating extra drag.

I was confused by CobraBall's response too. Think it's a terminology mix-up.

[MetroMPG]

To answer the original question, have a look at the aero/rolling resistance / fuel consumption tool I posted:

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

The default values you see when you load the page are for a 95+ Metro hatch, but you can enter your vehicle's values if you want.

You can see that aero & rolling drag are pretty much equal at 30 mph/50 km/h.

But even at just 20 mph, aero drag still represents 1/3 of the total.

Remember the comments on this subject from the developers of the GM Volt:

One of the big surprises to emerge from the Volt's aero development has been identifying the extent to which wind drag affects the relatively low speed city driving cycle. In an environment where economy is measured in fractions of a mile per gallon, GM is finding major city mileage gains with its dynamic aero refinements on the Volt. - source

(That said, GM should be embarrassed to admit being "surprised" to learn this. Maybe their marketing people were "surprised", but the engineers weren't.)

[Christ]

MetroMPG - Unless their engineers are anything like their management staff...

Person w/ the Toy PreRunner - If you have stake pockets in your rig:

1. Put tight fitting boards in them, at least as tall as the top of your cab.
2. Get a chalk line, and learn to calculate the angle of a scalene triangle based on it's right angle size. (Or get a protractor)
3. Tap a nail into the board at the cab-line on the front,
4. Tap a nail into the rear board at a level that creates a 13* angle from the front to the rear
5. Snap your chalk line between those two points, and cut them off on that angle
6. Same on other side
7. Frame off the 4 or 6 boards in your stake pockets so they're all attached to each other, and won't move around
8. Get a canvas tarp, or something waterproof that doesn't look too "redneck" (if you care about looks)
9. Attach it to the frame, starting at the top and cleanly folding it into itself to create the taper down to the tailgate, stapling/brad nailing it in place as you go
10. Test
11. Record results

You should see some kind of drag improvement by doing this, as it effectively creates a boat-tail of your bed, one of the easiest things to do to a truck.

You will, however, notice less winter (snow and ice) traction at speed, due to the lack of downforce caused by the (add term here) effect, which creates an oblong air-swirl in the bed of the truck that helps to keep the rear planted at speed (see: Mythbusters episode on "tailgate down mileage savings")

I'm not sure if that's a misconception though (the traction thing). Although at first glance, it seems plausible, since it's a high pressure area.

Anyway, give that a shot, you might like what you see. If it works out for you, it's easily removable and replaceable, should you need your entire truck bed for something, and you could easily make an aluminum button frame and cut/sew a canvas or other material for it.