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manchild is right, the spoiler slows the air creating a region of high pressure above the boot, on onto the back window, which creates downforce. However, a spoiler does tend to re-attatch the air flow at the same time resulting in reduced turbulence and less negative pressure behind the vehicle leading to a reduction in overall drag. Spoilers don't work very well on hatchbacks as described in Race car vehicle dynamics by the miliken brothers
[quote="Tp"]The function of the lip is to use the airflow, to physically push the back of the car down, for better high speed stability.quote]
That's correct Tp, that small lip protruding past the trunk captures the air spilling off the roof, and captures it as downforce. If it wasn't there, the air would come off the roof, eventually go down past the rear of the trunk area, but miss any of the bodywork. Sticking that small plate out the back captures a lot of unseen air,,and adds downforce to the rear.
I remember reading in autocar for the review of this M3 (if I'm right in what model it is), and it stated there that wee lip is worth 50kg at 150 mph (or some similar speed).
While I think that is a ridiculous number, it could be a part of the overall downforce figures, and not be the sole supplier of downforce.
The flow will not be laminar back there - it will be full of small scale turbulence, there will be 2 longitudinal vortices coming off the 3/4 pillars (back corners of the 'cabin'), these tend to push the air along the cars centreline down on to the boot, but towards the sides of the car, it gets messier.
Having the little spoiler there will promote a cleaner detachment from the car, and result in more coherent flow structures in the wake - this is favourable to a shifting seperation point, that can affect car balance, will change the wake 'plume' with time, and may result in a longer wake [resulting in more base pressure drag].
The spoiler will also induce some momentum exchange leading to downforce, it will also help entrainment of flow from outide the bottom of the rear wheels into the car's wake, again, more positive momentum exchange (downforce) and reduced wake length.
Last edited by kilcoo316 on 09 Jun 2006, 15:44, edited 1 time in total.
RaceCar Engineering did a CFD simulation on 100mm NASCAR spoilers on the rear of a sedan. While it did not make any actual downforce it reduced the rear lift of the sedan to nearly zero, so it does create a high pressure area on the back of the trunk lid. Interestingly, they also noted a decrease in drag even though there was a high pressure area, investigating this further showed that spoiler helped pull air from under the car, reducing the high pressure below the car. Because the underside of the car has a larger area than the trunk lid, it actually ended up reducing the overall drag.
It's very interesting how such a small change can have such large effects on the vehicle. Because of this, I installed a 100mm spoiler on the rear of my car, and it made a HUGE difference past 80mph. The car really feels like it hooks into the pavement at high speeds, really gives you a good sense of security, kinda hard to explain, but you definitely feel it and it makes you more comfortable going high speed.
ginsu wrote:really gives you a good sense of security, kinda hard to explain, but you definitely feel it and it makes you more comfortable going high speed.
Me!! wrote:The function of the lip is to use the airflow, to physically push the back of the car down, for better high speed stability.
Damb and blast it 
