Back to our F1 cars. The F1 car can only accelerate at 1.5 g's because that is all the grip the tires can achieve (assuming low speeds where aerodynamic drag and down force is not significant to screw up the numbers). Now, the tires have basically the same grip accelerating, decelerating, or cornering (that is known as the traction circle). Therefore, an F1 car can only brake at 1.5 g's before the tires say no more! No more? But wait, why can an F1 pilot brake at 4, 5, or even 6 g's? Aerodynamic drag! An F1 can NOT brake at 6 g's when he is going 20 mph--the tires simply don't have that much grip. But he can certainly brake at 6 g's (I am taking your word for it here on the 6 g part) when he is bombing down the straight at 200+ mph and looking at a concrete barrier coming up to say hello because aerodynamic drag is "helping" him slow down.
Back to nuts.
David
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Your 'aerodynamic drag' is a factor at high speed, but more important is the aerodynamic downforce added to the cars static weight which increases the grip available from the tyres when operating at higher speeds. After all 'they' say that at around 200mph most modern single seaters develop enough downforce to enable them to be driven on an upside down surface ( and I dont mean any track downunder in S.A. Aussie, or NZ.)
. To accomplish this there has to be around 2g + of generated downforce.