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    Questions on sport chassis (20mm)

    I just had a chat with a friend of mine who sold his turbo and ordered a 997S cab. We were discussing about the sport chassis (-20mm option). He told me he wouldn't consider it strictly due to comfort issues, he's an oldish dad and anyway, they don't offer it for the cab version.
    But from a technical standpoint, he told me it's a great option but only if you drive in a very sporty manner, implying mostly on the track.
    He told me the PSM ruined his rear brakes on the Turbo every time he tracked it because as soon as a wheel has less traction than the other one, the PSM brakes the other wheel individually. Here's the point of LSD, no PSM breaking action (until you reach the threshold of the LSD), you keep your brakes longer.
    Is that correct guys?
    What would be the other advantages of the LSD? Better traction at the exit of a curve?
    RC, what did you mean when you wrote that, with LSD, your car almost feels like a Carrera 4?
    In fact, if I understand well, the LSD is just like the viscous coupling system of a Carrera 4 but instead of transmitting traction from the rear axle to the front axle when needed, it transmits traction from a wheel of the rear axle to the other wheel of that same axle? Correct?
    Can you have LSD on an AWD Carrera? Theoretically, you'd have then a set up that would provide ultimate traction to each wheel individually right?
    I guess that would be a feature of the next Turbo, but what is the difference if it's managed mechanically or electronically?
    Thanks.

    Re: Questions on sport chassis (20mm)

    with LSD you must push the gas pedal after ,if you want to oversteer the car .
    if you push the gas pedal early ,the car run straight ....like a Carrera 4 .LSD push the car!
    but when you are in the middle of the curve ,with LSD, the car turns more ...because the interior wheel don't loose aderence ,and the sport crono make the difference ...(psm effect must be late)
    hope you understand my orrible english .

    Re: Questions on sport chassis (20mm)

    Quote:
    Fanch said:
    I just had a chat with a friend of mine who sold his turbo and ordered a 997S cab. We were discussing about the sport chassis (-20mm option). He told me he wouldn't consider it strictly due to comfort issues, he's an oldish dad and anyway, they don't offer it for the cab version.
    But from a technical standpoint, he told me it's a great option but only if you drive in a very sporty manner, implying mostly on the track.
    He told me the PSM ruined his rear brakes on the Turbo every time he tracked it because as soon as a wheel has less traction than the other one, the PSM brakes the other wheel individually. Here's the point of LSD, no PSM breaking action (until you reach the threshold of the LSD), you keep your brakes longer.
    Is that correct guys?




    Francois,

    The function of the PSM you are talking about in the Turbo example is called the ABD (Automatic Brake Diferential) which is part of the PSM. It acts as an LSD but works "electronicallY" by braking the slipping wheel, while the LSd does this "mechanically" by transfering torque to the opposite wheel (instead of braking the slipping wheel). The difference is that the ABD by applying brake instead of managing the torque is less sporty and is more intrusive, hence it limits the car more in at the limit track driving, and with the LSD allows you to be on the brakes longer into the curve and "playing" with the rear more.


    Quote:
    What would be the other advantages of the LSD? Better traction at the exit of a curve?



    Yes, thats another advantage, because the LSD not only works in managing the torque from left to right in deceleration but also in acceleration. While the ABD doesn't. However the ABD is not the same as another electronic differential typen refered to as "active differentials" such as in the Porsche 959 (Porsche was the inventor of this type of systems and the first one to use it, it was in the 959) or Mitsu EVO. These do work like mechanical LSD only suposedly better becasue they not only act like a LSD when slippage occurs but also "before" it occurs if the car predicts slippage may happen like if driver applies full throttle suddently.

    Quote:
    RC, what did you mean when you wrote that, with LSD, your car almost feels like a Carrera 4?



    I think what he meant was that with the rear LSD the car has so much rear traction the its not necesary to transfer torque to the front like the 996C4 does. In the 996C4 the rear does not have as much traction therefore its more dependant on recruiting the front wheels for traction more.

    Quote:
    In fact, if I understand well, the LSD is just like the viscous coupling system of a Carrera 4 but instead of transmitting traction from the rear axle to the front axle when needed, it transmits traction from a wheel of the rear axle to the other wheel of that same axle? Correct?
    Can you have LSD on an AWD Carrera? Theoretically, you'd have then a set up that would provide ultimate traction to each wheel individually right?



    Thats exactly right, they are all diferentials, what we refer to the LSD in the -20mm 997S is a REAR differential. If that same diferential system is placed in the center and is used for managing torque forn rear to front instead of right to left then you have an AWD car, and its called a CENTRE LSD.

    So RWD cars can have a rear diferential (of any type LSD, viscous coupling, electronic, etc). And AWD have another differential in the centre that manages the torque between front and rear axl (also can be LSD , viscous, torsen, electronic, etc). So you can have a rear diff on a RWD, a centre diff on an AWD car, and even better a rear AND centre diff on an AWD car.
    And you can even have a rear, centre AND front diff on AWD car. Some AWD cars have another diferential for the front wheels specifically, called FRONT diferential, but is not that needed as the others for sport cars since the front wheels don't get as much traction normally in sportcars anyway and is used for AWD designed for slippery surface grip rather than sporty driving gripr like in 50-50 permanent torque split AWD like torsen of AUDI or in rally cars for traction in dirt, ect. The exception is in electronic diferentials which control all wheels already so also have differential capabilities in the fron axl as whell as on centre (AWD) and in rear axl.

    Quote:
    I guess that would be a feature of the next Turbo, but what is the difference if it's managed mechanically or electronically?
    Thanks.



    Yes, active electronic differential system will seem to be the case of the 997TT as rumors imply. Like I said above, the difference with these types of electronic differential (not PSM's ABD of the 996) will be that it can vary torque split front and rear, and left and right and on both axls not ONLY when slip happens as in mechanical types (LSD, viscous-coupling, torsen, etc) but also BEFORE it happens if the software predicts it may happen so it acts even quicker. For example if you floor it form a standing start, it will detect you inention and will send torque to the rear wheels in order to provide more traction before the slipping happens.

    Hope that answered your questions, its hard to put into words

    Re: Questions on sport chassis (20mm)

    Sergini,
    Your english is fine man thank you! I think I understood what you meant.
    Carlos,
    Thanks a lot for the lonf reply, you're right, not easy to explain, but I think I got it.
    If that rumor about the 997 Turbo is true though then, that means that this car will be a monster of efficiency, I mean, almost to the point of bein "too" perfect. A car that will predict when the drift will come and correct individual traction to each wheel to prevent it.
    In a way, this is amazing, the ultimate performance weapon, but on the other hand, it'll be more and more of a point and shoot car, involving less the driver, am I right Carlito?
    Btw, off topic, but did you get the PCCB on your car?

    Re: Questions on sport chassis (20mm)

    Well, it should be at least more efficient than the current viscous-coupling center diferential AWD systmem of the 996, though this one is no slouch already. For refference, the 959 had this system and the Mitsu EVO also, so you can get an idea, both highly rated. It will be perfect for making the most of the turbo power on the streets. One thing is to have good specs on paper, that easy, but another is translating them into performance and efficacy in the real world and Porsche is an expert on that

    Of course, not all active diferentials are great, depends who makes them, the "Haldex system" is an active diferential system but its not as good to, put it lightly, compared to the others. The Haldex is used in the Golf platform based VW's "4-Motion" and A3 platform (same platform as VW Golf BTW) based Audi's "Quattro". The Passat 4-motion and A4 and up Quattro use the Torsen AWD system.

    But since Porsche invented the active diff, they should do a good job so a 997TT with the a "PSK" AWD, "PDK" double-clutch, 480HP, and Porsche's know-how of how to make a car handle... I think it will be "perfect"


    As to the PCCB's, I didn't get them because I personally didn't see a benefit over steel rotors worth the money they are charging.

    Re: Questions on sport chassis (20mm)

    Quote:
    Fanch said:
    But from a technical standpoint, he told me it's a great option but only if you drive in a very sporty manner, implying mostly on the track.
    He told me the PSM ruined his rear brakes on the Turbo every time he tracked it because as soon as a wheel has less traction than the other one, the PSM brakes the other wheel individually. Here's the point of LSD, no PSM breaking action (until you reach the threshold of the LSD), you keep your brakes longer.
    Is that correct guys?
    What would be the other advantages of the LSD? Better traction at the exit of a curve?



    Fanch the whole thing about LSD is its more sport driving than anything. My friend has a 2 way LSD on his car(drifter) and his turning radius is affected because unlike the open diff. it doesnt let the wheels spin at different speeds.

    Quote:
    RC, what did you mean when you wrote that, with LSD, your car almost feels like a Carrera 4?
    In fact, if I understand well, the LSD is just like the viscous coupling system of a Carrera 4 but instead of transmitting traction from the rear axle to the front axle when needed, it transmits traction from a wheel of the rear axle to the other wheel of that same axle? Correct?



    Viscous is commonly used to link the back wheels to the front wheels so that when one set of wheels starts to slip, torque will be transferred to the other set. Not between axles The best way to understand the behavior of a viscous coupling differential is to go get an egg and do an experiment. Place it on a flat surface and see that both the shell and yolk are stationary. If you suddenly spin the egg, the shell will be moving at a faster speed than the yolk for a second, but the yolk will quickly catch up.To prove that the yolk is spinning, once you have the egg spinning quickly stop it and then let go -- the egg will start to spin again. It has everything to do with friction and what not

    Quote:
    Can you have LSD on an AWD Carrera? Theoretically, you'd have then a set up that would provide ultimate traction to each wheel individually right?



    Yes- LSD is usually for the rear differential; Viscous for the center; and Helical or torsen for the front diff.

     
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