From: Marc Thomas [mmthomas@ix.netcom.com] Sent: Thursday, September 14, 2000 2:23 PM To: 928 Subject: [928] The rear wing thing... I stayed out of the discussion on the effectiveness of the rear wing since there was way too much "opinion" and emotion expressed... However, testing has shown that the wheels will spin on mild unloading of the rear of the 928 at ~180 mph, maybe sooner/later depending on the rake of the 928. A lot of rake is good for straight line, but not for high speed turns..... Increasing the angle of the rear wing slightly will increase that number to a little bit over 190....increasing it a little more, brings it to 200 and so forth. Our goal was to have maximum course length traction for propulsion and give up rear traction on one or two bumps which greatly unload the rear...thus providing the minimum down force (minimum lift) necessary to accomplish the speed goal and successfully handle high speed turns. Thus, optimizing the drag from the rear wing to only what is necessary to get the job done. A 928 without a wing will generate tremendous lift on the rear and may never get to a top speed of over 150 ish..I have never tested a stock early 928 for high speed traction, but considering the lift generated by a S4 at 180 ish, it is most likely correct. In conclusion, the rear wing is very effective and controllable downforce/lift is achievable with modification of angle. Those of you who have taken a very good look at the car know about the rear wing thing...... The front generates tremendous downforce....that is another topic -- Marc M. Thomas DEVEK 650-592-5287 phone 650-610-0557 fax http://www.devek.net From: Jim [jim@928intl.com] Sent: Thursday, September 14, 2000 5:59 PM To: 928 Subject: [928] RE: The rear wing thing... yes , there are many opinions about aerodynamics for a 928 - the early styling exercises were in the wind tunnel as early as 1972 . Porsche had the resources to test the 928 ,the original 911s was never wind tunnel tested but drawn by " eye " with the resultant poor aerodynamics and all the spoilers , wings , tails etc. . Quoting from the book "Porsche Excellence Was Expected" by Karl Ludvigsen ..re 928 .... " it was impressively free of lift, however ; at its maximum speed of 140+ mph the lift at either end was no more than sixty pounds " page # 847 . This was for the 78 non-winged or spoilered basic 928 The photograph of the 928 in the wind tunnel ( in the color portfolio section today's Porsches )with smoke generators in front , shows five lines of smoke passing by the body ,one starts to pass under and is diffused , one starts over the hood becomes somewhat diffused then hits the windshield then nearly disappears , the other three remain visible as they bend around the body , squish closer together as they pass over the roof then spread out somewhat over the rear hatch glass . The smoke line closest to the body as it passes over the lower edge of the rear hatch appears to be at least eight inches away from the body . There is no notation of the speed used for this particular test but I would doubt that it was extremely high speed . It does seem to illustrate that the air flowing over the roof never drops down far enough to have much contact with any factory rear spoiler . At higher speeds would the airstream drop down more or stay further from the hatch ?? perhaps the engineer/aircraft types could comment . Not having a wind tunnel at my disposal , I find it hard to isolate the effects of wings and things , I did consider mounting a strain gauge to the rear hatch so that the down force could be measured but with speeds of only 130 mph the 60 lbs. does not seem significant . Jim Bailey 928 International jim@928intl.com 79 928 5spd 80 928 5spd 68 911 5spd