1970 911 RS clone Aerodynamics question

[minoclan]

I just finished (as done as I can be this year) the suspension on my 70. I went with the Elephant racing coilovers in the front (350 lbs) and 31mm rear torsion bars. I have the Tarrett 25mm rsr sways front and rear. Not to bore you with the details almost every suspension component has been upgraded.

I took the car to the track and was really pleased with how neutral the car was. For me it was perfect. I want to focus on downforce now.

My question is if I put a splitter on the front, what do I do to the ducktail to keep the downforce equal? I learned from my 1987 that if you do the front, you have to offset it in the rear.

I am the blue car in the picture.

[GrantG]

I have a front splitter (fairly mild one) and regular ducktail and it works well. I think with a low/firm suspension, the balance is not upset as much.

[Todd Simpson]

I don't think you'll need to do anything...at least, nothing a small sway bar adjustment can't fix depending on the track.

I've run my car with a few different splitters, and the car handled pretty much the same.

Removing the duck...THAT made a difference!

[minoclan]

Originally Posted by Todd Simpson

I don't think you'll need to do anything...at least, nothing a small sway bar adjustment can't fix depending on the track.

I've run my car with a few different splitters, and the car handled pretty much the same.

Removing the duck...THAT made a difference!

Do you have the duck tail on or off now? What is the difference in handling?

[Todd Simpson]

I did one event with no duck, the small "Lightning" circuit at NJMP. The difference was instability under braking-particularly at initial application. I figured a little over 100 pounds of lift at the end of the front straight.

Everywhere else, including the ~80MPH 180 degree turn onto the front straight the car was the same as with the duck.

The duck is back on these days!

[kiwiokie]

Paul Frere's 911 book has a good analysis of the various aero options front/rear and the effect on lift. Love that metallic blue.

[Flieger]

From Fere:

I believe "Ducktail" curves are measured with front spoiler and Bürzel installed together.










1970 C-series (no spoilers)

Cd=0.408 ClF=0.190 ClR=0.264

1972 E-series (front "S" spoiler/airdam, standard engine lid)

Cd=0.408 ClF=0.107 ClR=0.261

At 143mph, front lift reduced 40% from 83 to 46.5 kg. Rear lift was still 114 kg @ 143 mph.

Carrera RS with

Carrera RS with

standard front air dam (with blocked-off oil cooler)

standard front air dam (with blocked-off oil cooler)

and rear Bürzel

and flat engine lid

Cd= 0.397

0.409

Clf= 0.070

0.064

ClR= 0.084

0.290

front lift @ 152 mph = 35 kg

32 kg

rear lift @ 152 mph = 42 kg

145 kg

Rear-end grip increased by 20% with both front air dam and Bürzel
When a deeper front air dam was used, some front downforce was made but it caused a rocking moment which increased rear lift. The deeper air dam was only used with larger RSR whale tails.

[minoclan]

So, should I put the splitter on like Todd has?

[GrantG]

Originally Posted by minoclan

So, should I put the splitter on like Todd has?

Sure - mine is not too different and works great with ducktail...