Wingless GT4
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Booth9999 (02-27-2022)
05-18-2017, 12:25 PM
#17
Rennlist Member
Let me still suggest that you consider swapping out the front splitter from the Boxster Spyder when you pull the rear wing. I'll explain why:
You're correct that at sane speeds aero forces are relatively small, and in a straight line they do work very much like different fuel loads in the car. Not many crashes occur in a straight line, however, and when cornering the dynamics are very different.
The Supertest wind tunnel shows the iron rotor GT4 has 38 lbs of front downforce at 124 mph, 68 lbs rear. Remove the rear downforce and the front will increase to around 50 lbs- think seesaw.
We don't have aero data for a Cayman without a wing, but we can bracket it. A Boxster Spyder makes 15 lbs of lift @124 with a deployable spoiler, a stepped rear deck and no front downforce, so we know it's well more than that. Meanwhile an old no spoiler 911 made nearly 200 lbs of lift at the same speed. The spoilerless Audi TT was 150 lbs; I'd guess at a bit less than either of these.
If we split the difference and go with 90 lbs due to shape, then add a bit to both the front and rear due to the seesaw effect again (load added to nose of the car will lift the rear up) to come to a guess of 60 lbs front downforce, 100 lbs of rear lift.
As you rightly point out you could easily come to similar relative values by running a bunch of heavy luggage up front with a full tank, so it doesn't sound so bad until you consider the dynamics.
The great thing about downforce is that it's extra force pressing the tires into the road without the normal penalty. The 60 lbs of downforce is going to increase front axle grip by 4%, but without an extra 4% of mass the front wheels need to push sideways when cornering (as if you just added luggage). Basically the front will get 4% more stuck, while the rear will get 6% less stuck.
A 10% difference might not sound like a lot, but think in terms of fitting 10% less sticky tires, MPSC2s up front and old all seasons in the rear, and you start to get the picture. Obstical avoidance on the freeway is probably worst case- you swerve violently to avoid something on the road, the front sticks like glue and the rear steps out.
The original Audi TT was recalled worldwide when an aero imbalance of a smaller magnitude killed a number of drivers back in 2000. The crashes occurred mainly in Germany, as below 100 mph even bad aero isn't all that bad. Thus if you're diligent and serious about keeping your foot out of it you should be ok, but you should be aware of what will happen if you ever get more enthusiastic. This effect will grow exponentially, and by 150 it will get positively lethal. The Spyder spoiler lip would be cheap insurance if you're in doubt- the numbers suggest it will nearly balance the aero out, and I'd happily drive the car that way.
Back to your original question, most common 3D printed parts will be very UV and heat sensitive and unlikely to last long even when painted. They will also need some surface finishing. There are some more durable 3D printed materials that are far less common, or some low volume resin castings made from 3D printed parts and silicon molds that might be more suitable if you can get a small group together. For one off I'd probably simply bite the bullet and machine from aluminum.
You're correct that at sane speeds aero forces are relatively small, and in a straight line they do work very much like different fuel loads in the car. Not many crashes occur in a straight line, however, and when cornering the dynamics are very different.
The Supertest wind tunnel shows the iron rotor GT4 has 38 lbs of front downforce at 124 mph, 68 lbs rear. Remove the rear downforce and the front will increase to around 50 lbs- think seesaw.
We don't have aero data for a Cayman without a wing, but we can bracket it. A Boxster Spyder makes 15 lbs of lift @124 with a deployable spoiler, a stepped rear deck and no front downforce, so we know it's well more than that. Meanwhile an old no spoiler 911 made nearly 200 lbs of lift at the same speed. The spoilerless Audi TT was 150 lbs; I'd guess at a bit less than either of these.
If we split the difference and go with 90 lbs due to shape, then add a bit to both the front and rear due to the seesaw effect again (load added to nose of the car will lift the rear up) to come to a guess of 60 lbs front downforce, 100 lbs of rear lift.
As you rightly point out you could easily come to similar relative values by running a bunch of heavy luggage up front with a full tank, so it doesn't sound so bad until you consider the dynamics.
The great thing about downforce is that it's extra force pressing the tires into the road without the normal penalty. The 60 lbs of downforce is going to increase front axle grip by 4%, but without an extra 4% of mass the front wheels need to push sideways when cornering (as if you just added luggage). Basically the front will get 4% more stuck, while the rear will get 6% less stuck.
A 10% difference might not sound like a lot, but think in terms of fitting 10% less sticky tires, MPSC2s up front and old all seasons in the rear, and you start to get the picture. Obstical avoidance on the freeway is probably worst case- you swerve violently to avoid something on the road, the front sticks like glue and the rear steps out.
The original Audi TT was recalled worldwide when an aero imbalance of a smaller magnitude killed a number of drivers back in 2000. The crashes occurred mainly in Germany, as below 100 mph even bad aero isn't all that bad. Thus if you're diligent and serious about keeping your foot out of it you should be ok, but you should be aware of what will happen if you ever get more enthusiastic. This effect will grow exponentially, and by 150 it will get positively lethal. The Spyder spoiler lip would be cheap insurance if you're in doubt- the numbers suggest it will nearly balance the aero out, and I'd happily drive the car that way.
Back to your original question, most common 3D printed parts will be very UV and heat sensitive and unlikely to last long even when painted. They will also need some surface finishing. There are some more durable 3D printed materials that are far less common, or some low volume resin castings made from 3D printed parts and silicon molds that might be more suitable if you can get a small group together. For one off I'd probably simply bite the bullet and machine from aluminum.
BTW, how do you measure downforce on the car accurately?
05-18-2017, 02:46 PM
#18
I am pretty sure the 911r has different splitter, it looks smaller, like the spyder.
I think the Spyder also has larger blocks in front of the front wheels than the gt4 came with. I am guessing the 911r has something similar as it would let porsche use the same under tray.
I think the Spyder also has larger blocks in front of the front wheels than the gt4 came with. I am guessing the 911r has something similar as it would let porsche use the same under tray.
05-18-2017, 04:41 PM
#19
Measuring downforce is difficult without a wind tunnel. Race cars are wired with ride height sensors or strain gauges that let the data acquisition system compute the loads accurately, but that's a very significant project that's beyond most. Typically good drivers at the club level can give feedback by feel, wheel lockup, etc that lets a race engineer balance the car without knowing absolute numbers.
05-20-2017, 04:30 AM
#20
Rennlist Member
Let me still suggest that you consider swapping out the front splitter from the Boxster Spyder when you pull the rear wing. I'll explain why:
As you rightly point out you could easily come to similar relative values by running a bunch of heavy luggage up front with a full tank, so it doesn't sound so bad until you consider the dynamics.
As you rightly point out you could easily come to similar relative values by running a bunch of heavy luggage up front with a full tank, so it doesn't sound so bad until you consider the dynamics.
Only comment is regarding his comment about adding heavy luggage whether up front or back, which you covered nicely, but to emphasize your point further, the weight added is not really a substitute to down force generated by aero. This dead weight, he thinks about adding, can shift dynamicly from front to back or vise verses, or side to side, depending on car attitude, accelerating or braking or cornering right or left and is much more variable & less uniform than say the aero loading which is less variable & more uniform during various car maneuvers at high speed, and can't be really compared or be a substitute for aero loading. Which is why as you pointed out, it is imperative to use the Spyder front lip if he is planning to remove the rear wing from his GT4. Again great wrtie up. Mark
06-09-2018, 02:35 AM
#21
Finally!
We have blanking plates! After starting the process with three different fabricators who never got the job done, I came across a master fabricator with the skills to make very high quality custom blanking plates. They look great, and work even better. I have attached pictures to show how the car looks without the wing and to show the plates up close.
06-09-2018, 07:40 AM
#22
Hey, a GT4 touring! 
if you don’t mind a critique of the finished product, I think it looks pretty darn good from the side, but curiously, looks “unfinished” (like something is missing) from the quarter rear view.
if you don’t mind a critique of the finished product, I think it looks pretty darn good from the side, but curiously, looks “unfinished” (like something is missing) from the quarter rear view.
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rafamrqs (07-15-2021)
06-09-2018, 10:40 PM
#25
Rennlist Member
Plates look really nice. +1 for body color, which may help them (and the lack of the wing) recede visually.
If only someone would make a Ferrari F355-style or Ruf/Stola 987 hatch for the 981. I liked the idea of going wingless on the GT4, but I began to understand why the factory put the lettering on the panel (which I never liked) above the spoiler on regular Caymans vs. below it—it wants something on it. Not sure what, but something.
If only someone would make a Ferrari F355-style or Ruf/Stola 987 hatch for the 981. I liked the idea of going wingless on the GT4, but I began to understand why the factory put the lettering on the panel (which I never liked) above the spoiler on regular Caymans vs. below it—it wants something on it. Not sure what, but something.
06-10-2018, 08:57 AM
#26
Rennlist Member
Plates look really nice. +1 for body color, which may help them (and the lack of the wing) recede visually.
If only someone would make a Ferrari F355-style or Ruf/Stola 987 hatch for the 981. I liked the idea of going wingless on the GT4, but I began to understand why the factory put the lettering on the panel (which I never liked) above the spoiler on regular Caymans vs. below it—it wants something on it. Not sure what, but something.
If only someone would make a Ferrari F355-style or Ruf/Stola 987 hatch for the 981. I liked the idea of going wingless on the GT4, but I began to understand why the factory put the lettering on the panel (which I never liked) above the spoiler on regular Caymans vs. below it—it wants something on it. Not sure what, but something.
For adding lightness the current hatch could be done completely in CF, including the hatch window, paint the window area black and use a video feed center rear view mirror.
Agreed with the lettering, but that's more for walking around it and looking down, when on the road looking from a lower vantage point not so much an issue.
06-10-2018, 02:33 PM
#27
Rennlist Member
I'd like something like that hatch, but not one that blocks the passenger side rear quarter view.
For adding lightness the current hatch could be done completely in CF, including the hatch window, paint the window area black and use a video feed center rear view mirror.
Agreed with the lettering, but that's more for walking around it and looking down, when on the road looking from a lower vantage point not so much an issue.
For adding lightness the current hatch could be done completely in CF, including the hatch window, paint the window area black and use a video feed center rear view mirror.
Agreed with the lettering, but that's more for walking around it and looking down, when on the road looking from a lower vantage point not so much an issue.
The Stola/Ruf solution is flawed for exactly the reason you point out: 3/4 visibility.
If we are just dreaming, the 355 had the solution. Integrating a similar rear window on a hatch that opens and closes might be a bit tricky, but I suspect it could be done—especially if it wasn't so large an aperture. I'm amazed that one of the companies that makes the 986/987 Boxster > Cayman conversion bodywork kits—or bodywork for race cars—has not done this already. The question, of course, is whether there's a market for it. If it looked right (factory) and saved real weight, there might be.
