Norcal Lemons update!!!! 187whp 246torque :>)
11-04-2009, 12:35 PM
#91
Rennlist Member
Didnt I say this (the cut off teardrop shape) above in my post?
and yes, the ugly wake is still there, i agree, even if the pointier rear bumper helped. but, a cut off tail might be better. just removing the bumper might not do it, but a GT40 cut, at also an angle might work. that would be an interesting design.
mk
and yes, the ugly wake is still there, i agree, even if the pointier rear bumper helped. but, a cut off tail might be better. just removing the bumper might not do it, but a GT40 cut, at also an angle might work. that would be an interesting design.
mk
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.
Airflow sticks to the 928's round tail follows it around and makes an ugly wake behind the car.
11-04-2009, 12:40 PM
#92
Rennlist Member
Did you not read my first paragraph of post #86????
"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. "
Now, as I just mentioned, sure it would be interesting to cut off the bumper and make it flat and angled. As i mentioned, the angle might be critical on how the air fow comes off the bottom of the car. that coupled with the defuser probably completes a good aero package design. Most of the fastest race cars now, have no rear end.
edit: as a note, the 928 is far from a tear drop shape. (maybe going backward), but it is heading that way toward the rear before it is rounded out. If it was tear drop shaped the car wold be 3 feet longer in the rear. The early 928s had the little kicktail, which did reduce drag. also the bumper was flatter, but not angled like the traditional Kammback.
http://ateupwithmotor.com/component/...ticle/159.html
The real question would be, would the mod to make the rear of the 928 sharper and angled back, do anything with aero dynamics, and what would the kick tail added do as well. I dont think anyone here knows for sure, but there is one thing that we do know, the difference betweek a GT40 and the 928 is not that great as far as aero, even with its Kammtail. Getting back to the subject, we are trying to help a race car go faster. 1-2hp gain is not going to do much, but a little downforce might. I remember when one of the top Speed Touring cars had lost its trunk lid, with wing attached during a race. His lap time didnt suffer at all. Pretty funny seeing that rear hatch flapping in the breeze with the wing attached, and all the anouncers saying how "loose" the car was going to be now, and he didnt miss a step and ended up in the top 3 in the race, even with major rear end damage and loss of its rear wing assembly. something to think about. we are shaving hairs here.
What do you think the gains would be as far as aero? Again, Bill's example of the .25 from .35 Cd is a little misleading due to that being on the 1940s car and lots of other things were changed. (as he referenced). what is more important, the kick tail or the angled flat , inverted rear end? all questions that come to my mind.
mk
"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. "
Now, as I just mentioned, sure it would be interesting to cut off the bumper and make it flat and angled. As i mentioned, the angle might be critical on how the air fow comes off the bottom of the car. that coupled with the defuser probably completes a good aero package design. Most of the fastest race cars now, have no rear end.
edit: as a note, the 928 is far from a tear drop shape. (maybe going backward), but it is heading that way toward the rear before it is rounded out. If it was tear drop shaped the car wold be 3 feet longer in the rear. The early 928s had the little kicktail, which did reduce drag. also the bumper was flatter, but not angled like the traditional Kammback.
http://ateupwithmotor.com/component/...ticle/159.html
The real question would be, would the mod to make the rear of the 928 sharper and angled back, do anything with aero dynamics, and what would the kick tail added do as well. I dont think anyone here knows for sure, but there is one thing that we do know, the difference betweek a GT40 and the 928 is not that great as far as aero, even with its Kammtail. Getting back to the subject, we are trying to help a race car go faster. 1-2hp gain is not going to do much, but a little downforce might. I remember when one of the top Speed Touring cars had lost its trunk lid, with wing attached during a race. His lap time didnt suffer at all. Pretty funny seeing that rear hatch flapping in the breeze with the wing attached, and all the anouncers saying how "loose" the car was going to be now, and he didnt miss a step and ended up in the top 3 in the race, even with major rear end damage and loss of its rear wing assembly. something to think about. we are shaving hairs here.
What do you think the gains would be as far as aero? Again, Bill's example of the .25 from .35 Cd is a little misleading due to that being on the 1940s car and lots of other things were changed. (as he referenced). what is more important, the kick tail or the angled flat , inverted rear end? all questions that come to my mind.
mk
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 ....
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 ....
Last edited by mark kibort; 11-04-2009 at 01:42 PM.
11-04-2009, 01:31 PM
#93
Under the Lift
Lifetime Rennlist
Member
Lifetime Rennlist
Member
The 928 shape is exactly backwards in terms of aerodynamics. Some version of a Kamm tail will almost certainly improve rear end airflow. It cannot make it any worse than it is already.
11-04-2009, 02:00 PM
#94
Rennlist Member
Bll, the kamm tail is not that counterintuitive. you can see that most of the flow is established via the sloping roof line. aero engineers figured that out many years ago. I think just now, cars are being made to use this principle, and the prius and others are good examples. our cars aero is not that bad. .32 to .34 back in 1987 was not bad at all. what was the Ford GT 40??? Do you know? It was .39 WORSE than the 928.
The 928 shape isnt "exactly" backwards, but it is better backwards, because it has two sloping shapes together. the rear end is not that bad, and if it had some mods, maybe like the Kamm tail, it could be better. How much, we dont know. YOU dont know. go plug the dimensions in a simulator and tell us what you get. in my estimation, the gains, if any might ony be a couple of points on the Cd scale.
Now, you want to talk race cars? toss all the aero fuel efficiency stuff out the window. you want big downforce front and rear, and that comes as a cost. drag of a f1 car is well over our cars Cd. almost 1! not .3, not .5, but near 1. downforce comes with a cost of drag. Most wings have a lifting force to drag ratio of near 10:1, so you do the math. Now, go look at our frontal area, the type of day you are running in, and the drag coefficient and you can get an actual drag number. At 100mph, divide it by about 4.5 and thats the actual torque loss, gain, or cost to our cars at the engine.
mk
GT40 specs
http://www.gt40spf.com/gt40-specifications.aspx
The 928 shape isnt "exactly" backwards, but it is better backwards, because it has two sloping shapes together. the rear end is not that bad, and if it had some mods, maybe like the Kamm tail, it could be better. How much, we dont know. YOU dont know. go plug the dimensions in a simulator and tell us what you get. in my estimation, the gains, if any might ony be a couple of points on the Cd scale.
Now, you want to talk race cars? toss all the aero fuel efficiency stuff out the window. you want big downforce front and rear, and that comes as a cost. drag of a f1 car is well over our cars Cd. almost 1! not .3, not .5, but near 1. downforce comes with a cost of drag. Most wings have a lifting force to drag ratio of near 10:1, so you do the math. Now, go look at our frontal area, the type of day you are running in, and the drag coefficient and you can get an actual drag number. At 100mph, divide it by about 4.5 and thats the actual torque loss, gain, or cost to our cars at the engine.
mk
GT40 specs
http://www.gt40spf.com/gt40-specifications.aspx
That is right and the Kamm tail provides the airflow of the extended tail in a counterintuitive way. It works. MANY, MANY cars use it to improve fuel efficiency, including the Toyota Prius. IT WORKS or they wouldn't have done it. I know it was tested and proven on this car.
The 928 shape is exactly backwards in terms of aerodynamics. Some version of a Kamm tail will almost certainly improve rear end airflow. It cannot make it any worse than it is already.
The 928 shape is exactly backwards in terms of aerodynamics. Some version of a Kamm tail will almost certainly improve rear end airflow. It cannot make it any worse than it is already.
11-04-2009, 02:59 PM
#96
928 Collector
Rennlist Member
Rennlist Member
No no Bill, Mark may have a point ... just look at this very lucid report he wrote: http://apps.pdos.lcs.mit.edu/scicach...rk+Kibort.html
11-04-2009, 03:21 PM
#97
Rennlist Member
Bill, here are your comments again below. I dont understand your issue with my responses. We all are fully aware of the general aerodynamic shape that is apart of most cars today, as well as race cars. My entire response was urging brian to put downt the rear hatch, as that long roofline, and subsequent "blunt" back, does not make a Kammback. (ala, firebird example you posted)
We can see the GT40 aero is WORSE than the 928, as far as Cd. Why is that? We all know that the concept of the Kammback, was a decending roof line toward the rear, with a sharp cut backward at the end, thus fooling the air, for the most part, into thinking the entire shape was a full tear-drop, when it was only missing the rear portion. The kicktail and sharp cut back is just noise in the overall drag/performance. (Unless you can prove to me with actual numbers that it isnt). You then quote the classic, exagerative, and misapplied, misleading gains of the Kammback design on the BMW, as it lowered the Cd from .35 to .25. DID I MENTION THAT BMW was from the 1940s?????
I never discredited Kamm and his design. I only said that raisng a roofline does not make a Kammback. cutting the rear end off our cars and slanting it back, will make little difference on the race track. However, it might on the open road as far as fuel efficiency, where small changes in drag, make big differences in gas mileage.
Thats it. Im sorry, if it is arrogance you think I have with bringing up valid questions and or points, in response to GROSS generalizations and misapplied, and weighted theories, then yes, Im arrogant. Im not going to follow the herd on this one, there are too many loose ends, and you know it!
<snip> "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"
__________________
(ala, firebird example you posted) We can see the GT40 aero is WORSE than the 928, as far as Cd. Why is that? We all know that the concept of the Kammback, was a decending roof line toward the rear, with a sharp cut backward at the end, thus fooling the air, for the most part, into thinking the entire shape was a full tear-drop, when it was only missing the rear portion. The kicktail and sharp cut back is just noise in the overall drag/performance. (Unless you can prove to me with actual numbers that it isnt). You then quote the classic, exagerative, and misapplied, misleading gains of the Kammback design on the BMW, as it lowered the Cd from .35 to .25. DID I MENTION THAT BMW was from the 1940s?????
I never discredited Kamm and his design. I only said that raisng a roofline does not make a Kammback. cutting the rear end off our cars and slanting it back, will make little difference on the race track. However, it might on the open road as far as fuel efficiency, where small changes in drag, make big differences in gas mileage.
Thats it. Im sorry, if it is arrogance you think I have with bringing up valid questions and or points, in response to GROSS generalizations and misapplied, and weighted theories, then yes, Im arrogant. Im not going to follow the herd on this one, there are too many loose ends, and you know it!
<snip> "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"
__________________
11-04-2009, 04:00 PM
#99
Rennlist Member
Here is a pretty good story of Pete's use of the Kammback concept and the pic of the end result. notice how the bottom, sides and top are pointing to a tear-drop, convergence? then, it is cut at an angle. everything leading up to the "cut" is important.
Then, there was another version built on a cobra body as well, but did have the inverse of the traditional Kammback design. it has a slightly raised roof and the reverse angle on the rear chop. Still the form is tapped overall toward the rear before the mods. This odd design is pictured below and was built by Pete Brock when he redesigned his 289 Daytona Coupe on the new 427 Cobra chassis. This Shelby "Super Coupe" was the result. Built to achieve 215 mph at LeMans, it never got the chance
mk
http://images.google.com/imgres?imgu...a%3DN%26um%3D1
Then, there was another version built on a cobra body as well, but did have the inverse of the traditional Kammback design. it has a slightly raised roof and the reverse angle on the rear chop. Still the form is tapped overall toward the rear before the mods. This odd design is pictured below and was built by Pete Brock when he redesigned his 289 Daytona Coupe on the new 427 Cobra chassis. This Shelby "Super Coupe" was the result. Built to achieve 215 mph at LeMans, it never got the chance
mk
http://images.google.com/imgres?imgu...a%3DN%26um%3D1
11-04-2009, 04:20 PM
#100
Race Director
Thread Starter
GUYS........may I suggest a bit less of an aerodynamic debate and come back on topic to our LEMONS effort.....which BTW was offiically accepted TODAY!!!! So we are 100% IN for the Thunderhill race!!!!
Testing is TOMORROW and Friday at Thunderhill...so it will be interesting to see how the "estate" performs..especially if its raining on friday?
Testing is TOMORROW and Friday at Thunderhill...so it will be interesting to see how the "estate" performs..especially if its raining on friday?
11-04-2009, 06:01 PM
#101
Brian congrats on getting accepted, you're gonna have a BLAST!
And btw this is a LEMONS car, who cares about aero? We had a giant wooden rabbit with floppy ears hanging out the back of ours! Wasn't exactly aero-efficient, but we were pretty damn fast anyway.
And btw this is a LEMONS car, who cares about aero? We had a giant wooden rabbit with floppy ears hanging out the back of ours! Wasn't exactly aero-efficient, but we were pretty damn fast anyway.
11-04-2009, 07:20 PM
#103
Under the Lift
Lifetime Rennlist
Member
Lifetime Rennlist
Member
As I said, I'm done debating. Carry on.
Hope the car continues to work well, Brian.
Good find H'man. Here's a nice sentence:
"Here, we disconfirm that the foremost flexible algorithm for the deployment of telephony by Ole-Johan Dahl et al. is optimal."
Hope the car continues to work well, Brian.
Good find H'man. Here's a nice sentence:
"Here, we disconfirm that the foremost flexible algorithm for the deployment of telephony by Ole-Johan Dahl et al. is optimal."
11-04-2009, 07:36 PM
#104
Rennlist Member
Yeah Brian, we are all counting on you!!
Bill, there is no debate, only misunderstanding as usual for list communication.
Look, i agree with the design having its merits, but one must look a the entire system, not just one component. aero is like that. many interactive components, as you showed with the firebird picture. (high, longer roofline, blunt end, but look at the result. BAD AERO)
So, I agree and have agreed with the benefits of the design, even though I didnt invent or think of it. I also think that i corporating it to a 928 is more complicated than raising the rear hatch and pegging it in suspension, or slicing off the rear end. remember, the GT40 and MK2 and MK1 had worse aero than our 928, but had the tail. The system has to be looked at and I dont think you or anyone can debate that, or can you? 
mk
Bill, there is no debate, only misunderstanding as usual for list communication.
Look, i agree with the design having its merits, but one must look a the entire system, not just one component. aero is like that. many interactive components, as you showed with the firebird picture. (high, longer roofline, blunt end, but look at the result. BAD AERO)
So, I agree and have agreed with the benefits of the design, even though I didnt invent or think of it.
I also think that i corporating it to a 928 is more complicated than raising the rear hatch and pegging it in suspension, or slicing off the rear end. remember, the GT40 and MK2 and MK1 had worse aero than our 928, but had the tail. The system has to be looked at and I dont think you or anyone can debate that, or can you? mk
