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    Re: 997 Carrera S or 997 C4S.... Dilemna?

    Quote:
    Carlos from Spain said:
    Quote:
    Silver Bullet said:
    I don't understand why a car with power flowing to the front wheels - the steering wheels - would UNDERsteer.



    Silver Bullet, the answer is because in RWD cars, the front wheel's available traction is dedicated exclusively to steering the car and making it turn, but in an AWD car, the front wheels also have to share that available traction for steering with applying torque to the ground, therefore more traction is used up and the limit of the traction of the front wheels is more likely to be reached in an AWD causing it to understeer more. The front wheels' grip are under higher stress becuase they have to steer and push, hence the tend to understeer more.



    Carlos,

    what happens with a RWD when the rear wheels loose traction (to much power used). Your front wheels have still enough traction for steering, but your real wheels don't have. In that case your rear part will go in the direction of centrifugal force direction. You will turn or you have to steer in the opposite direction. In any case the resulting force (where you intended to drive) is smaller, meaning you are driving slower.

    I think in this case (and maybe only in this case) the AWD could help you. By transfering power from the rear wheels (so they don't loose traction) to the front (not to much, so they still have traction for steering).

    Similar (not exactly) things happens with a LSD.

    If you can create a smart AWD (and not a 50/50) then you would gain some advantages. Of course if someone is capable of driving a RWD without loosing rear traction he/she don't need a AWD.

    AM

    Re: 997 Carrera S or 997 C4S.... Dilemna?

    So it holds true that the Traction Circle theory states that a wheel can mutually exclusively either accelerate 100%, brake 100% or lateral forces 100% at any one time.

    Re: 997 Carrera S or 997 C4S.... Dilemna?

    mumbasic,

    Without really getting into it lets say that in most situations, not all, when ever you loose traction around a bend you loose time. But a car that tends to loose more the your front wheels (i.e. FWD and AWD) causes more havoc on your times than a car that tends to loose your rears more. We are not talking about safety, we are taking about which is faster around the track. Neutral to oversteer setup is better for lap times than neutral to understeer, and you will see that in racecars or sportcars, and thats one of the reasons whay racecars and serious sportcars have a rear-weight bias on purpose.
    The AWD in its attempt to take away some of the traction load of the rears in a critical situation causes the car to behave less sporty around the bend, its more noble at the limit, more grip in slippery surfaces, etc. but at the expense of sportiness. AWD gives you more grip but that does not mean necesarily faster, they are two different things.
    Best example is why the RWD 911 version is always faster around the track than the AWD, or why the GT3/GT2/GT versions are all RWD. However, with cars nowadays with such crazy HP numbers for a street car as 500HP +, it becomes more important to control all that torque that is appplied to the wheels so AWD is less of a compromise for sportiness so the advanatage of RWD is less in those scenarios in terms of lap times, and becomes more of an issue now of sporty-behaviour and sensations the driver whats from the car. So 500HP sedans in the market are more likely AWD but 500HP supercars are all RWD still.

    Re: 997 Carrera S or 997 C4S.... Dilemna?

    While I know all this to be true, it's definitely not intuitive, and it remains hard to understand, for me at least . . .

    Re: 997 Carrera S or 997 C4S.... Dilemna?

    There's a good explanation of these issues (totally in line with Carlos, as usual) on this forum's Boxster section:

    http://www.rennteam.com/showflat.php?Cat...e=0&fpart=1

     
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