Hi Pete et al,
Unfortunately, the local hardware store is low on pitot tubes, and has completely run out of venturi pipes! To make matters even worse, our wind tunnel is being rennovated, and will not be functional for awhile. SO - ABSOLUTELY NOTHING I WRITE CAN BE SUBSTANTIATED BY FACTS.
With that disclaimer in mind, I am happy to relate our "in the pits" experiences, and what we BELIEVE to be true. Hopefully, it will carry somewhat more value than pure conjecture.
From the first few posts, it is obvious to me that we need to speak about a specific type of car (front-engine, rear-engine, mid-engine, with spoilers, with wings, with tunnels, full-body vs. open-wheel, etc.), because relocating centers of aero pressure can have a drastically different effect on each.
Also, I believe it would help our discussion to distinguish between gradients of pressure which effect the sides of the racecar (like cross-winds, or when the car is in yaw); and those which effect the upper and lower surfaces (lift or downforce). And, yes, I know that the entire body is surrounded by moving air, but for the sake of simplification . . . . .
So, while I would rather discuss something which has a high degree of adjustability, for the sake of uniformity, I shall focus on our old SC, prepped for PCA Club racing, lowered, lightened, with a splitter and an IROC tail.
We had a weight distribution of F=40% for easy figuring.
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Originally Posted by petevb
I suspect you think I'm advocating moving the center of pressure a long way- I agree that this will result in a car that understeers terminally at high speed. I believe it's common/ normal to put the COP slightly behind the CG, however, for a number of reasons:
First, so the crowd is all on the same page- it's very bad to get the COP in front of the CG. At high speed this results in the front wheels getting loaded with a higher percentage than they do at low speed. This in turn means that if the car is neutral and has balanced brake bias at low speed it will oversteer and lock the rear wheels under braking at high speed.
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While it is traditional thinking among teams to try and locate the "center" of DOWNFORCE (in this case, reduced LIFT) very near or AT the CG, we attempted to keep it more forward, in an effort to reduce high-speed lift: nose & splitter closer to the ground, less air entering under the front. Naturally, no racer balances his suspension at low speed! We used settings (and driving technique) to compensate for the chronic oversteer, typically using the 2 fastest turns on a given track as the baseline. Our class did not allow adjustable bake bias, but again we balanced everything with different brake pad compounds front and rear.
Quote:
Originally Posted by petevb
A second thing to understand is that the COP isn't static- it moves. It especially tends to move forwards when you hit the brakes, as the nose of the car tends to dive, and any low pressure area behind the front splitter is magnified due to the reduced ground clearance (aero in the ground-effect is notoriously clearance sensitive).
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All good points. This did not apply to our 911, but being near the ground is SO sensitive that many prototypes have a problem with their splitter stalling as it touches the ground under braking, having wonderful front downforce one second, and virtually none the next!
Quote:
Originally Posted by petevb
There are multiple ways to avoid this . . . The most common method used in conjunction with the above is to put the aerodynamic COP slightly behind the CG, so that when it shifts it doesn't go forwards of the CG and create instability. This also promotes slightly increased high speed stability/ under steer, which most drivers find confidence-inspiring up to a point.
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O.K. Just having come out of the Bowl, we are on the main straight at Pocono. We have been in 5th gear for awhile, and the revs have sort of maxed out. That nice, heavy motor and transaxle are physically weighting everything down pretty well in the back (60%). But there is no longer much steering "feel", even with our 245-45 Hoosiers mounted on 8" rims, and our 350mm OMP steering wheel. Now, explain to me once again how we are going to gain "high speed stability" by shifting even more (virtual) weight TO THE REAR!?!
Unfortunately, even with our Reynard, where we do have true downforce, we lowly club racers have no accurate way to actually measure any of these things.
Ed