Practical ways to measure lift, downforce and drag on a 996TT (?)
01-29-2017, 06:02 PM
#61
I'm not so sure about that. It also gets more complicated when you mix rear spoilers/wings and underbody rear diffusers.
Consider this:
Deploying that spoiler will give you a huge improvement in downforce, mainly because it makes the diffuser more effective, not so much because of force on the spoiler itself.
On the other hand, the deployable body panel spoiler on a standard 911 up or down is very unlikely to make a noticeable drag difference. It is sitting in a mess of air either way.
Consider this:
- 911 body shape. The rear does not have a cutoff edge to drag a pile of vortex behind the car (like a car with a vertical rear does). The rear has a slope downwards which air flows down on, creating vortexes much lower to the ground than a normal car.
- Somebody went through great length to have reasonable under-body air management and then there is a serious diffuser under the rear underbody.
- If you let the air wash over the 911 rear body it will flow right into the difffuser's work area, making the diffuser less effective.
- Now consider the 911 standard spoiler (the body panel coming up), or the 991.2 sport design rear spoiler (same thing but sharper cutoff).
Deploying that spoiler will give you a huge improvement in downforce, mainly because it makes the diffuser more effective, not so much because of force on the spoiler itself.
On the other hand, the deployable body panel spoiler on a standard 911 up or down is very unlikely to make a noticeable drag difference. It is sitting in a mess of air either way.
OK, perhaps I exagerate a little but. But squid42, your post comes rather close to one of the things I've been wondering about: duck tails. In particular, copies of the 997 sport classic made for a 996.
I'm sure this is not the lap time minimizing rear aero. If it were, GT3s would have them. But I'm more interested in reducing weight than in minimizing lift/maxing downforce. I just want to reduce the lift at the front and make a balancing change at the rear. And if I can do it with a ducktail, great, because it weighs a lot less, in all the right places.
Anyway, any views re. the impact of a duck tail instead of the Carrera's pop up spoiler (or instead of anything else for that matter) on drag and lift?
01-30-2017, 08:00 AM
#62
With all the aero expertise in this thread, why measure things myself when I can just ask you guys the answer?
OK, perhaps I exagerate a little but. But squid42, your post comes rather close to one of the things I've been wondering about: duck tails. In particular, copies of the 997 sport classic made for a 996.
I'm sure this is not the lap time minimizing rear aero. If it were, GT3s would have them. But I'm more interested in reducing weight than in minimizing lift/maxing downforce. I just want to reduce the lift at the front and make a balancing change at the rear. And if I can do it with a ducktail, great, because it weighs a lot less, in all the right places.
Anyway, any views re. the impact of a duck tail instead of the Carrera's pop up spoiler (or instead of anything else for that matter) on drag and lift?
OK, perhaps I exagerate a little but. But squid42, your post comes rather close to one of the things I've been wondering about: duck tails. In particular, copies of the 997 sport classic made for a 996.
I'm sure this is not the lap time minimizing rear aero. If it were, GT3s would have them. But I'm more interested in reducing weight than in minimizing lift/maxing downforce. I just want to reduce the lift at the front and make a balancing change at the rear. And if I can do it with a ducktail, great, because it weighs a lot less, in all the right places.
Anyway, any views re. the impact of a duck tail instead of the Carrera's pop up spoiler (or instead of anything else for that matter) on drag and lift?
Another bit about the ducktail versus a wing. A car like the 991 GT3RS has both. The huge wing distracts, but it also has one of those "ski jump" spoiler like a ducktail.
It becomes a bit more clear when you look at 991gt3rs cars with the wing omitted:
From over in the 991gt3rs thread:
https://rennlist.com/forums/991-gt3-...l#post13912052
It reinforces my view that any kind of "ski jump" on a 911's back is deemed required by those with access to a wind tunnel, even if there is a wing in addition.
The way I see it the ski jump's primary purpose is to divert air from floating into the diffuser's work area.
01-30-2017, 10:09 AM
#63
RL Community Team
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With all the aero expertise in this thread, why measure things myself when I can just ask you guys the answer?
OK, perhaps I exagerate a little but. But squid42, your post comes rather close to one of the things I've been wondering about: duck tails. In particular, copies of the 997 sport classic made for a 996.
I'm sure this is not the lap time minimizing rear aero. If it were, GT3s would have them. But I'm more interested in reducing weight than in minimizing lift/maxing downforce. I just want to reduce the lift at the front and make a balancing change at the rear. And if I can do it with a ducktail, great, because it weighs a lot less, in all the right places.
Anyway, any views re. the impact of a duck tail instead of the Carrera's pop up spoiler (or instead of anything else for that matter) on drag and lift?
OK, perhaps I exagerate a little but. But squid42, your post comes rather close to one of the things I've been wondering about: duck tails. In particular, copies of the 997 sport classic made for a 996.
I'm sure this is not the lap time minimizing rear aero. If it were, GT3s would have them. But I'm more interested in reducing weight than in minimizing lift/maxing downforce. I just want to reduce the lift at the front and make a balancing change at the rear. And if I can do it with a ducktail, great, because it weighs a lot less, in all the right places.
Anyway, any views re. the impact of a duck tail instead of the Carrera's pop up spoiler (or instead of anything else for that matter) on drag and lift?
What is it that the car is doing now that you want to change?
01-30-2017, 03:29 PM
#64
Three Wheelin'
01-30-2017, 03:51 PM
#65
Three Wheelin'
The discussion goes side-tracked, my core point is that I would love for somebody to actually measure. The most straightforward way I can see is measure suspension height at a reproducible speed run.
Another bit about the ducktail versus a wing. A car like the 991 GT3RS has both. The huge wing distracts, but it also has one of those "ski jump" spoiler like a ducktail.
It becomes a bit more clear when you look at 991gt3rs cars with the wing omitted:
From over in the 991gt3rs thread:
https://rennlist.com/forums/991-gt3-...l#post13912052
It reinforces my view that any kind of "ski jump" on a 911's back is deemed required by those with access to a wind tunnel, even if there is a wing in addition.
The way I see it the ski jump's primary purpose is to divert air from floating into the diffuser's work area.
Another bit about the ducktail versus a wing. A car like the 991 GT3RS has both. The huge wing distracts, but it also has one of those "ski jump" spoiler like a ducktail.
It becomes a bit more clear when you look at 991gt3rs cars with the wing omitted:
From over in the 991gt3rs thread:
https://rennlist.com/forums/991-gt3-...l#post13912052
It reinforces my view that any kind of "ski jump" on a 911's back is deemed required by those with access to a wind tunnel, even if there is a wing in addition.
The way I see it the ski jump's primary purpose is to divert air from floating into the diffuser's work area.
The shaped decklid theoretically helps pull air from the engine bay from the vent under it. It redirects air and changes how the vacuum effect of air filling in the gap behind the car works, and now the vacuum can pull hot air out of the engine compartment. Maybe helps slightly with drag reduction since it is being filled with air instead of pulling the car back slightly? Not sure. Also it helps direct and clean up airflow/increase velocity of air moving between the decklid and the airfoil (wing). This allows wing to be more effective. There is probably a fringe benefit of helping a diffuser, but I don't think it is primary.
01-30-2017, 07:41 PM
#66
The duck tail or ski jump on the 991 GT3 and GT3RS, and also I believe on the 997 GT3s, is probably not actually there to help a diffuser, as no 991 or 997 GT3s have one.
I'm also wondering if it's as big as the 991 duck tail that's available from Porsche as part of an aero kit. The GT3 version looks a fair amount smaller. (Though this is from memory, I don't have photos in front of me.) I am inclined to agree that even if Porsche put it there with the wing, it probably also deliveres benefits without the wing. But if the aero kit ducktail is larger and/or steeper, then I'm not sure what to think of that.
Indeed, I'd very much like to know if the 991 duck tail has aero advantages (or disadvantages), so I asked my local Porsche Zentrum, but they seem unable to find any info. Not that I'm sure they're trying very hard ... .
I'm also wondering if it's as big as the 991 duck tail that's available from Porsche as part of an aero kit. The GT3 version looks a fair amount smaller. (Though this is from memory, I don't have photos in front of me.) I am inclined to agree that even if Porsche put it there with the wing, it probably also deliveres benefits without the wing. But if the aero kit ducktail is larger and/or steeper, then I'm not sure what to think of that.
Indeed, I'd very much like to know if the 991 duck tail has aero advantages (or disadvantages), so I asked my local Porsche Zentrum, but they seem unable to find any info. Not that I'm sure they're trying very hard ... .
01-30-2017, 07:50 PM
#67
I would like to have a more tied down front end at high speeds. But just reducing front lift without changing the rear would leave the car unbalenced. (And succeptible to unrequested oversteer at high speeds.) So I need a similar reduction in lift at the rear. For instance, with between 6 and 13 kg of front lift at 124 mph (like a TT or GT3), I'd aim for 0 to 6 kg of downforce at the rear.
01-30-2017, 07:52 PM
#68
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02-03-2017, 04:58 PM
#69
Race Car
I don't think it would be practical to measure differences in down-force by trying to measure suspension height/travel while trying to drive the same road at the same speed. A stretch of road will have a near infinite number of surface variations causing the suspension to move almost all the time, so you would have to take an average of all the suspension height difference measurements. Also, the change in down-force will most probably produce a minuscule difference in suspension height, making accurate height measurements a top priority. Just trying to drive in precisely the same line on a long enough stretch would be next to impossible. Then there would be temperature variations that would probably skew the measurements. There is probably no accurate way to do this other than a wind tunnel where many variables are held constant.
Dan
Dan
02-03-2017, 05:25 PM
#70
I'm a bit more optimistic, for two reasons. First, I would not be the first to do this.
Second, while your concerns make perfect sense, I don't see any of them as deal killers. The faster one goes, the bigger the height differences are. And while I would indeed never take the same line again with milimeter precision, that's the point of averages. Finally, there are systems that calculate the averages for you. And if they turn out to be too expensive, when the height is displayed as a graph, it isn't too hard to see difference in averages.
Second, while your concerns make perfect sense, I don't see any of them as deal killers. The faster one goes, the bigger the height differences are. And while I would indeed never take the same line again with milimeter precision, that's the point of averages. Finally, there are systems that calculate the averages for you. And if they turn out to be too expensive, when the height is displayed as a graph, it isn't too hard to see difference in averages.
02-03-2017, 05:39 PM
#71
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I would like to have a more tied down front end at high speeds. But just reducing front lift without changing the rear would leave the car unbalenced. (And succeptible to unrequested oversteer at high speeds.) So I need a similar reduction in lift at the rear. For instance, with between 6 and 13 kg of front lift at 124 mph (like a TT or GT3), I'd aim for 0 to 6 kg of downforce at the rear.
02-05-2017, 07:33 PM
#72
02-06-2017, 10:50 AM
#73
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Have you fitted your car with adjustable sway bars and/or coilovers? Using them to tune the car's handling is much more effective and consistent than aero tweets.
Info here:
https://rennlist.com/forums/996-turb...sway-bars.html
and here:
https://rennlist.com/forums/996-turb...uspension.html
Info here:
https://rennlist.com/forums/996-turb...sway-bars.html
and here:
https://rennlist.com/forums/996-turb...uspension.html
Last edited by Carlo_Carrera; 02-06-2017 at 02:44 PM.
02-08-2017, 07:20 PM
#74
Have you fitted your car with adjustable sway bars and/or coilovers? Using them to tune the car's handling is much more effective and consistent than aero tweets.
Info here:
https://rennlist.com/forums/996-turb...sway-bars.html
and here:
https://rennlist.com/forums/996-turb...uspension.html
Info here:
https://rennlist.com/forums/996-turb...sway-bars.html
and here:
https://rennlist.com/forums/996-turb...uspension.html
I gather that my aero concerns puzzle you. Perhaps because my situation is a bit different from yours. I have a Carrera, not a Turbo, which is much more tied down in front. And I have autobahns handy, so I get to experience just about as much lift as the car can generate.
02-08-2017, 07:41 PM
#75
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Coilovers yes, sway bars no, maybe some day. But my concern with the aero is not exactly the same. The handling is fine at low to medium speeds. At high speeds, the front can feel light and lacking in directional stability. Even if suspension tuning could fix that at high speed, it would have the wrong effect at other speeds.
I gather that my aero concerns puzzle you. Perhaps because my situation is a bit different from yours. I have a Carrera, not a Turbo, which is much more tied down in front. And I have autobahns handy, so I get to experience just about as much lift as the car can generate.
I gather that my aero concerns puzzle you. Perhaps because my situation is a bit different from yours. I have a Carrera, not a Turbo, which is much more tied down in front. And I have autobahns handy, so I get to experience just about as much lift as the car can generate.
This is the Turbo version.
