TB993tt

Guys, since I found the formulas on the site below I have enjoyed doinf some simple excel graphs for the lift/downforce of various Porsche models. Using data from Porsche via Paul Frere (RIP) of front lift coefficient 0.01 and rear 0.03 for the 993tt I created the chart below......those better at this stuff please feel free to correct me if it looks wrong !
http://www.reverie.ltd.uk/calculate.php

Jean

Toby

These look like reasonable numbers for sure and the progression of lift with speed as well. One needs to keep in mind that lift numbers are very volatile and subject to a number of parameters that can easily alter the readings.

The tests for coefficient of lift are usually done in a wind tunnel therefore do not take into consideration mass tranfer under acceleration, which has an impact on the balance between front and rear lift. Rake difference between two identical cars will give different readings, suspension height settings, Center of gravity height, engine power, etc..

pstoppani

So, am I reading this correctly, the tail has more lift that the front?

OT - I like Reverie... can you tell Most of the black bits on my Lotus, including much of the interior is from them. That rear wing works like a champ!

No HTwo O

To me it says the rear tail / wing / spoiler has more DOWNFORCE than the front spoiler. Not lift as you state.

I am surprised it's so little. For example at 100 MPH, the rear has about 7.5 kg of downforce, or about 16.5 pounds. That does not sound like too much. At about 150 MPH it's about 38 pounds. Still not much.

What am I missing?

malcolmd

I discovered how little downforce my car with standard aerodynamics has at Easter (see earlier post on Mt. Panorama). When cresting a rise on the main straight at 250kph there as a scary amount of lateral movement during the brief period of unweighting. An improved splitter would have been desirable. The 997tt that I followed in much of the session reported no such problem. Aerodynamics (if not looks) have got better in 15 years it appears.
Toby, I recall you looked closely at the aerodynamics of your blue beast - but I think this was for lower drag on the 300kph runs, rather than for stability.

pstoppani

Thanks, I was confused by the negative numbers for "lift"...

So, does the 'S' front splitter increase downforce appreciably? Would it make a good match for the "standard" rear wing?

Jean

This coefficient is positive therefore these are lift (as in lighter) and not downforce numbers. In fact these are quite respectable for the period especially on a drag/lift ratio basis and compared to most other sports cars of the era. These numbers are for a ROW TT..Front lift numbers on our 993s make them quite vulnerable to side winds, the front turbo S chin helped in this respect.

pstoppani

Cool, thanks. Always looking for good reasons to spend money on the car

hiflexaust

My Turbo S suffered exactly the same lateral movement as yours at exactly the same place Malcolm. I also had a very interesting experience at the crest as well, I came up behind a GT3 RS with the big wing and the wash from the wing at 250km/h + was amazing! I swear it moved me 2-3ft further across the track than I had allowed for when pulling out behind him to overtake! It felt like I had been blown sideways by a huge wind and it was very scary for the blink of an eye as I had no control over the lateral shift. Strangely it settled quite dramatically when I swapped to soft compound tyres.

Jean, the Turbo S lip may help a little but I feel it would be so slight as to be insignificant. When talking to other 993 guys at Bathurst, with or without the S lip, the cars all moved around A LOT!

malcolmd

Grant, I'm interested/surprised to hear that soft tyres made a difference!

For your interest, here is a plot of the Lat G's at the crest. Bear in mind this is mid way down a straight where the only deviation is in height.

SimonK

What you came up with is about right. 911’s do not have good downforce management. The problem is rear engine and lack of rear end diffuser under the car. Big downforce is achieved with undercar diffusers not as much with wings, that’s why 911’s have huge rear wings when racing to compensate. The reason why we don’t have rear undercar diffusers is engine and exhausts. No room for them although some have modified. 996 in 997 turbos are good for about 40kg of downforce at max speed 997 couple of kg more than 996. This combined with the lift element at speed = zero lift. So our cars are managed on a 0 lift not on downforce. Ferrari with huge diffuser manages 200kg+. Porsche 944 turbo had 140kg of downforce at 160mph because of its rear diffuser. Here is a good photo of what is responsible for serious downforce...



944 turbo; rear spoiler = 40kg downforce. Rear undertray diffuser = 100+kg.



911 solution for bigger df.

TB993tt

That seems a lot, way too much in fact. The new 997GT3RS with its huge rear wing gets 18kg front and 80kg rear at ~160mph...... can you show where this information came from ? TIA

TB993tt

Bill Verburg gave me some data from the 993 RS bi-plane rear spoiler which is probably similar to the 993GT2 bi-plane - here are the numbers, certainly makes a big difference !

SimonK

I think I've explained it quite clearly in my post; it's not about the rear top wing but the undertray diffuser or better the combo of the both. My info is from the 944 turbo cup racing book I have at home. If I have time will scan the relevant pages and post it on here.

malcolmd

Wow, that's a big difference say at 150mph. Standard rear turbo spoiler 17kg of lift, whereas the biplane RSR spoiler 58kg of downforce = 75kg improvement!
I assume we are talking of the wing at left of picture?