Kamm back speed at which it starts to work?
 

Kamm back speed at which it starts to work?
I used to have a book that gave a guide to the speed at which the airflow held its shape, but I have misplaced it.
Any references online etc.

Cheers

speed
 

The coefficient of aerodynamic drag for your car is fixed,and remains constant,once up to about 20-mph ( 33 km/h ),and the experts now say that aero drag is significant by 35-mph ( 56 km/h ).

aerohead -

Since I think most cars spend the majority of their time above 30 MPH, would it be safe to say that aero-mods "always help", but that the return, the "bang for the buck", is better above 35 MPH? I keep thinking there's an "economy curve" that would slope up and apex around 55 MPH, and then start to slope down with diminishing returns. Orrrrrrrrr, would the apex of the curve change as a function of the Cd (aka basjoos)? Kind of a BSFC for each Cd.

CarloSW2

apex
 

I think the low speeds are a"wash" for aero,and emphasize to people that I meet, that my mods are virtually useless around town.So yeah,I think they really begin to shine at the higher velocities.------------------ The thing about the "apex" is that as one continued to streamline towards the ultimate,say Cd0.11-12.that you could push that curve up,forestalling the effects of the air.---------- The whole premise of CAR and DRIVER's "Crisis-Fighter-Pinto" was,leave the 70-mph speed-limit alone,and streamline the cars so the achieve the same mpg at 70,they then achieved at 55.----------------------- With the T-100,I've got data from 45-mph(72km/h),to 80 mph(129km/h),and the speed/mpg plot is pretty linear.The best mpg is at 72 km/h (39-mpg),it falls to (36.5 mph) at 88 km/h,and by 120 km/h she's down to 32-mpg.The jump to 129km/h costs a whole mpg.It's not legal to test at higher speeds so I'm stuck.Best mpg should occur in the 35-40 mph range (56-64 km/h) but it's not safe for me to attempt steady-speed driving at those kinds of velocities to check mpg ( no scan-gauge!).These lower speeds will always remain the domain for best mpg but I can't imagine the highway motorist surrendering to such speeds.So in the meantime,the mods will help.

Speaking of... In which direction does the BSFC curve shift/stretch as a result of a decrease in Cd?

Kammback creates drag. It doesn't reduce drag.

To make an object stable while in motion, the center of gravity must be forward of the center of pressure. This is true for high speed automobiles, airplanes, rockets or javelins.

Kammbacks are used in Top Fuel Funny Cars for two reasons. 1. to keep downforce or pressure on the rear tires. 2. To increase drag aft of the CG.

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.

IMHO, dams, wings, spoilers, kammbacks are "thangs" that are used to overcome bad aero design. Anything and everything that has mass and moves in a fluid creates drag.

Bsfc
 

I think at the eco-modding forum there are some BSFC maps which have been posted in the sticky.Hucho gets into this very well,shows the shifts,and how the gearing should be optimized to get the full-monty.Without the graphics we shouldn't even touch this,and we probably should take it over to the general eco-modding page.I don't have my text.From G.W.Carr,"The Development of a Low-Drag Body Shape for a Small Saloon Car",MIRA Report #2,1965, he shows that you can lose up to 45% of the streamlining benefit without gear-matching.------------ That's all I have with me.------------ The engine see's a lower load with streamlining,and will over-rev without the taller gearing,shortening it's life if not self-destructing.

kammback
 

The Kammback citing may have been used in the context of separation-free flow up to the point where the car body was chopped off.And with Cd0.37,the K-cars were remarkably "cleaner" than their contemporaries.And yes,the K-cars did have cross-wind stability problems.The up-shot is that Dr.Morrelli solved that one back in the 1980s,so we're better protected from those challenges.

BSFC only has to do with the engine. Aerodynamics does not effect it whatsoever. It affects how much power it takes to cruise at a given speed, so it affects where on the BSFC map you fall at a given time, but it does not change the map in the slightest.

-soD

aero/BSFC
 

Hucho claims in his text that should a person streamline their car,that the road load can shift so far,as to knock the engine out of it's island of highest BSFC.The car will also overspeed,and if the gearing is not changed such that the engine "see's" the same load as before the streamlining,then MPG potential from the drag reduction can suffer up to 60%.

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

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.

'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.
http://us1.webpublications.com.au/st...110216_9mg.jpg

shift
 

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!

*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.

akashic -

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

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!!!

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

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:

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

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.

OP - sorry about that. I have no idea when a Kamm is most effective, but I know the theory and practice of it's application, based on information attained from this site and other sources. None of which are personal experience.

General population - Braingle: 'Tailgate Flap' Brain Teaser

I suppose you mean me? ;) You can call me "Rik", btw. It sounds like you're describing either a very high sloping tonneau cover or the back of an aeroshell. No I don't have stake pockets. You can see my solution at the end of my build thread: http://ecomodder.com/forum/showthrea...nner-6342.html I'll be building a sturdy version tonight.

Good thing I live in San Diego, eh? ;)

In my build thread, you can see some "teardrop analysis" and discussion of my options re: a Kamm-style chopped boat tail to work for my shortbed.

K "Rik", yeah, I meant a shell :P Good work on your truck though.. your avatar image doesn't show it, so I didn't know about it. :turtle:

EDIT - I guess it does... mibad, I just saw a plain ole' Toy in that image.. I guess the mind really does see what it wants. :rolleyes:

If the K-car you are talking about is the 1978 Pontiac Firebird Wagon (see attachment), IMHO it is a "truncated hatchback" which is different from a "Kamm Back". Examples of a "truncated hatchback" would be AMC Gremlin & Lancia Y10. In the 60's and 70's it was common station wagon to have a roll-down back window built into the back wagon door. A station wagon is nothing more than a "truncated 4 door sedan." With the rear window cracked open carbon monoxide poisoning from the exhaust was a threat due to the low pressure at the rear.

Drag Racer Don Schumacher's Vega Station Wagon called the "Wonder Wagon" did not last very long. The truncated 2-door had a bad reputation of poor handling near the finish line. Don kept the name and switched to a coupe.

0.37CD is the same as the Volkswagen Tiguan (SUV). Always thought SUV, aero & CD in the same sentence was an oxymoron.:)

A true Kamm Back has flowing tapered lines and then sharply truncate the end. The best racing example of a Kamm Back may be the Shelby Cobra Daytona Coupe (below). Others examples are the 1970-86 Citroen GS, 1984-91 Honda CRX, 1992-98 Mazda MX-3, Insight, & Prius.

Professor Wunibald Kamm, head of the Automotive Research Institute at the Stuttgart Technical College developed his Kamm Back theory to shorten the length of a tear-drop-rear-end design and still have a good aero/CD.

The Fastest Indian was a little Hollywood but Burt took his Indian Streamliner Motorcycle to Bonneville 12 years in a row. The movie had to condense the 12 years into 90 minutes. Burt is one of the true LEGENDS/ICONS of high speed racing.:thumbup:

IMHO if the Kamm Back was the answer to an aero rear end design, then high speed bicycle and gravity racers would use a Kamm back in lieu of a tear-drop design.

This is Kamm's K car:

http://ecomodder.com/forum/member-ae...-kamm-back.jpg

That may be, but the Tiguan is a modern, detail-optimized shape. Compare Kamm's work to contemporary vehicles and his accomplishment is more evident. Give him a Tiguan, and he'd improve its rear design and lower its Cd too.

Nobody has suggested that a complete tear drop shape isn't more efficient.

The Kamm back is the answer to good aero design in production vehicles where (a) it's not practical to extend the vehicle's lines to the ultimate teardrop end, and/or (b) where consumers would likely reject a full or near-full tear drop as too radical. HPV's, gravity racers and solar cars don't face either of those constraints.

MetroMPG,

Thanks for the picture of Dr. K's K-car. My mistake.

After the 1934 Chrysler Airflow was introduced, the Auto Design Institute retired the "THE BUTT-UGLIEST " trophy :p to permanently to the Chrysler Design Studio.

First time I heard/read K-car: I thought of the Plymouth Relient. Not exactly an aero wonder, if yaknowuddimeen.