heinrich

The fuel tank gains very little protection from that plastic tail ....

mark kibort

The hp savings of what you are talking about is in the lower single digits.
top speeds at thunderhill are 100mph to 130mph most of the fastest and slowest cars, and its only for a few seconds a lap in total. most of time is spent around the 80-85mph range. the pointier rear bumper of the S4 created more efficient aero, and the wing did as well. making a blunt rear end is not going to help aero. a few other things could have caused the better aero of the Lemans Ford. The kick tail is one, not the blunt rear end. The angle of the rear end, maybe.

There is plenty of protection for the tank with the surrounding structure, with the bumper cover over it. that bumper cover acts like a giant cushion spreading the force over a wide area, for most impacts. did you ever see anderson NAILED by that vet in the WC Las vegas race in 2000? (or mine by a miata at low speed). no frame damage, no tank damage and thats with no main bumper. Its a colapsable plastic tank, like what was used in the early 996 cup cars. even if they are hit, they deform before they crack and break. very safe in my opinion. fuel cells generally are installed with a majority exposed below a bumper anyway. they fail with enough impact and are more exposed than the stock tank, even without a bumper insert. I think a rear- side impact and roll over is what i would be worried about as the stock tank would be opened at the filler neck and leak out. fuel cells are safer from that perspective.

mk

vwdmc16

yeah im worried about teh filler neck breaking, and even if the kamm tail does nothing around t hill, it would look neat. anyone that can photoshop wanta help??

Bill Ball

Mark! I hear Pete Brock is still alive. I'll try to look him up so you can tell him his idea was ****. I suppose because you didn't think of it first, it must be ****. I've read his accounts of the mods made to the car and the Kammback was absolutely key. They did raise the roofline as well and that helped too. You make such ridiculous assertions based on nothing sometimes and this is one of them. Look up the Kamm engineering papers first before you discredit it so casually. Read about the Ford GT 40's kammback too. Read about BMW research that showed the drag coefficient from .35 to .25 using the Kammback. MANY, MANY cars still use this design, maybe in a form you don't recognize, including most hybrids and wind tunnel tests confirm the benefit. Sometimes your outrageous arrogance gets under my skin, but then I realize it is such a big part of what drives you.

Bill Ball

Not true at all. Wake up!



Actually, I'm kidding. This version, based on a 1978 Tras Am turned out to create too much vacuum and drag. The right Kammback or tail is not to be a big slab on the rear, but involves taper to a smaller slab.

mark kibort

Bill, First of all, Welcome BACK!!

Now, I was not coming close to saying Pete Brock's design was "shxx". What I was saying was I didnt think that the changes to the 928 suggested would or could resemble those done by Pete on the Ford Daytona. we have seen many cars that have the rear end approach a tear drop, that reduce aero dynamic drag. this is just basic aero 101. many of the things that have been tried were contrary to intuitive design for aero that worked,only to later be explained.

The Kammback design is not just a blunt rear end , or a high roof line, but the shape of the rear, and bottom how they join at the rear.
Do you have the data for the aero dynamic drag of the Ford daytona with and without the "kick tail". were the 150mph to 185mph changes do only to a rear end change of a single component? keep in mind, most of the fast cars at lemans, need downforce. most of the drag on a race car comes from downforce, just as the primary cause of drag on an airplane is due to lift. the most streamline shape is a tear drop. you dont see the back of jets, or other subsonic flow vehicles with blunt rear ends, unless they are supersonic.
Now, if you have to live with a bunt rear end , then there are things that have been done to help with drag. Im sure many of the aspects of the Kammback design help in this area. Im also sure, its a combination of shapes that helped with the aero efficiency of the design.

What gets under my skin, are ideas that come from something that has been refined and tested , where parts are slapped on someothing with expected results to be the same. THATS all Im saying. Are you saying that raising a roof line and blunting a rear end will help the 928 aero to a noticable gain in aero? If so, you need to re-think the many other factors that will be in effect with the aero of our car's shape.


I dont think just removing a 928 bumper, making a flat angled surface like the kammback will do the exact same thing it did on the Ford Daytona.

Mark

heinrich

Yet another area Mark Kibort qualifies himself as an expert .... fluid dynamics, aerodynamics, aircraft design, gear ratios and mathematical equations, auto racing, mechanics, the Kamm tail ... the man's talents are endless Bill, we can all learn from such a man ... sit at the feet of the master, so to speak

mark kibort

Bill, what also gets under my skin, is overstating or misapplying simple facts.
Yes the Kammback design was an improvement in aero above the current designs of the day, but you have to understand what the previous shapes looked like. quoting that there was a .35 vs .25 Cd change due to the Kammback tail is a LOT misleading! That was a 1940's BMW 328, not a new one.
Here is what it looked like and here is the article that talks about the Kammback design.
http://homepage.mac.com/christopher....backStory.html

Ideally, the tear drop shape is most efficient, but is usually impractical. the slanted rear ends , off boxey looking cas helped with aero. the main benefits of the Ford Daytona were due to its over all shape, and that kick tail would add downforce as it does in nascar, probalby hurting aero drag, but thats a common trade off of most race car designs. F1 cars are over .45 Cd, due to downforce aides.

mark kibort

Quite the contrary, I try to bring common sense to the topics.

I always back up my comments with facts or actual experience with emperical testing or SIMPLE equations.

To think that just raising a hatch, and making a flat rear end qualifies as a "Kammback" is a little silly, dont you think.

Look at the car that was changed from .35 to .25 Cd that Bill mentioned. we are talking about two different cars in the 1940s!!

Now, you or anyone can walk into this with eyes closed. Or you can use what we have as simple resources and see if this proposed mod or any aero mod will help and to what extent. Also, you have to look at the proposed changes as how they will effect a race car. the most aero dynamic race cars are hardly ones that will win races. what if this mod changes the aero balance of the car to the worse? what if it reduces downforce in the rear? Has anyone thought of that? The best race cars are not the best aero forms. they usually have HUGE drag coefficients, expecially for tracks where conering is more important than the little you might be able to be saved in aero, due to the scant time spent at those moderate high speeds. (i.e. at 80-120mph).

now, back to your dreamfest.

Below are a couple of Kammback designs. I think the 928 has already incorporated many of the features of the original design,as does many cars these days. you can see that the "Firebird" in Bills post of just raising the roofline, created the OPPOOSITE effect. So, the design is a little more involved.

mk

heinrich

Mark, the article is FULL of inaccuracies ... and it is on some dude's Macintosh Homepage .... a great authoritative source, right alongside Area 51 and alien probe pages ... say, were you abducted? That would explain a lot

That guy calls every car with a flat rear, every station wagon, a Kamm tail. Why don't we all make a home page with lots of pictures, so you can quote whatever we say there?

mark kibort

I am no expert on Kammback design, but what i do know at the high level is that it was revolutionary. basically, it tricks the air flow to thinking the shape is a teardrop, but its missing the last part, hence, you get a flat rear end. BUT, the main gains were not due to the flat rear end, but the initial slope of the roof line, as we have with the 928. the GT40, has this as well, the rear angle is important as it acts to guide the air from the rear bottom of the car, to meet the air comng off the top. all this can be seen on a wind tunnel, or even with computer modeling tools.

again, to just raise the hatch of a 928 will more approximate the firebird that DIDNT work, back in the day. remember, porsche 928s were changed and tested in the wind tunnel. Not changing the basic shape, the pointier rear bumper helped to reduce drag. I have no idea if the chopping off of our bumper, and the anglulation of the rear section would help, and either do you. But, it could work, but we have no way of testing it. im sure that, or defuser modifications, opening up the bottom of our bumper areas would help A LOT! but, for what gain? thats the question.

who is the guy, "makintosh"? I know the internet has a lot of misinformation and misleading information. thats a given.
But, there are some actual articles that talk about Kamm's designs and their function. You have to admit, changing the 928 by just raising the hatch might not do anything positive and certainly is not a Kammback design, following your fellow, Mackintosh, talking about anything with a flat rear end, being a Kammback. same thing, right?

Mk

PorKen

That's the point. What Kamm et. al. found was that you can cut off the tail of the teardrop and still get the same airflow benefits. For this to happen, however, the cutoff must be sharp so that there is airflow separation.

Airflow sticks to the 928's round tail follows it around and makes an ugly wake behind the car.

heinrich

For the education of Mr Kibort and his Mac webpage friend:

A Kamm tail is basically a teardrop (or aircraft wing) with its butt forward and the tail trailing. That tail is then cut off (not for aerodynamic purposes, but to save the massive space the long tail would take up). Kamm found that cutting the tail off at a reasonable point AFTER THE ROOFLINE AND HIPS HAD ALREADY TAPERED SIGNIFICANTLY, did not cause much change in the aerodynamic efficiencies we all know exitst in that nice long tail.

So, in essence, you do not need the whole tail.

Imagine the 928 with its roofline continuing backward at the same angle it has today ... you would see a teardrop with a long tail. Now, cut that tailk off exactly where Porsche did .. BUT, make the tail flat, and not rounded/bulged. This was the mistake that Porsche made with the tail ... they bulged it. If you flatten the 928 rear and add a duck tail to it ... you get the Daytona Kamm Tail. That bulge is horrible for turbulence. We need to lose it.

So the Kamm principle is a long sleek tail cut off somewhere along the way to the tip. NOT A STATION WAGON and NOT A PACER, which never had a tail or taper to begin with. It is all about the cross-section.

But the Kamm priniple alone will not get the 928 where it needs to be .. we need to lose that bulge, and we need a duck tail (again ... see the Daytona). THat duck tail exists in a sunken rear, or extended trailing edges. Porsche found with YEARS of extensive testing on the 911, that the duck tail was the superior design for downforce and also aerodynamics .... Looking at Kibort's red 928 pic ... you can see that the 928 IS a teardrop ... an aircraft wing ... they just missed the flat, duck-tailed part ....

Tom. M

Volvo ran their 850 station wagon in the mid 90's BTCC (British Touring Car Championship) series with great success. They found that their wagon version of the 850 had better aerodynamics than the sedan since the series didn't allow big wings off their tails..

heinrich

EXACTLY