Dec 8, 2005 12:40:55 AM
- LowPolarMoment
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Board: Porsche - 911 - 996 - Turbo
Language: English
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Dec 8, 2005 3:03:12 AM
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Turbo Al
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Re: 4WD and Understeer
Quote:
LowPolarMoment said:
I've never driven a 4WD car. Many posts seem to imply that there is an understeer problem with 4WD. I'm thinking that having the front wheels pull you in the direction you're steering would reduce understeer. Can anyone explain?
I do not believe that a 4WD car PER SE causes an "understeer problem," HOWEVER, for 2 otherwise similar cars (e.g., a 911Carrera2 versus a 911Turbo with AWD, both rear-engined and thus slightly rear weight-biased), while the 911Turbo is also slightly heavier (partly due to its turbo's, cooling system and air intake pathways) compared to the RWD Carrera2, it is also relatively more front-weight biased (or at least less rear weight-biased) compared to the C2, since the former has a heavier front transaxle to power, as well as turn, the front wheels. This relative front-weight bias, I believe, increases the front slip angle relative to the rear slip angle of the Turbo versus that of the Carrera2, since the slip angle increases with increased weight (front and rear) carried over each axle. Slip angle is the tendency of the front or rear of the car to "twist" from centrifugal force, during turns-intentional and unintentional
-and greater sprung mass over the individual axle/wheels (front-versus-rear) increases this slip angle for a given centrifugal force-of course, all else being equal-similar suspension setups, for example). In addition, a car with a given low polar moment ( 
), as well as given center-of-gravity, will influence the slip angle for a given centrifugal force. This is also why front-engined cars (most cars on the road) are so understeer-prone versus rear-(and mid-)engined cars-like the 911, even though the 911Turbo (and C4) with AWD is slightly less OVERsteer-prone than is the C2. BUT this relative decrease in oversteer versus the C2 does NOT mean the AWD 911Turbo is significantly understeer-prone.
Dec 8, 2005 4:36:06 AM
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JP66
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Re: 4WD and Understeer
I have a 996 C4S and it is my first Porsche. It is also the only Porsche I've ever driven. I do however like to push my car, esp in corners, and I do experience understeer occassionaly, more so in the wet, obviously. When I sense the car understeering in the wet or dry, I let off the gas trying not to brake, and the front wheels quickly recatch. I don't know about others, but having 4WD/AWD, I have experienced understeer 
Dec 8, 2005 11:03:19 AM
- LowPolarMoment
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Re: 4WD and Understeer
Thanks for your comments Al. My gut feeling was that having a little extra weight in front would help grip. Imagine how little the car would turn if there were zero weight on the front wheels. Also, I assumed that having the front wheels pulling you in the direction you were turning would offset any disadvantages. As far as front wheel drive cars understeering, I don't recall seeing a front wheel drive car with much bigger tires in front than in back, the reverse of the 911, to compensate for the extra weight. I think that would help cure the problem but most front wheel drive cars are not designed for ultimate handling.
Dec 8, 2005 2:43:53 PM
- Grant
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Re: 4WD and Understeer
A tire has a maximum amount of friction it can produce. It can produce this friction in a longitudinal way (acceleration or braking) or a lateral way (turning). If the tire is being asked to produce longitudinal and lateral acceration at the same time, it can produce less of each type of acceleration compared to when it is asked to produce only one type.
Since an AWD car is asking its front wheels to provide forward acceration as well as lateral (turning), it cannot turn-in in SOME situations as well as a RWD car while accelerating.
Since an AWD car is asking its front wheels to provide forward acceration as well as lateral (turning), it cannot turn-in in SOME situations as well as a RWD car while accelerating.
Dec 8, 2005 3:45:43 PM
- Turbo 911
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Re: 4WD and Understeer
Like Al Pettee says - Porsches are rear biased so 4WD should actually be helpful in cornering although slightly slower in straight lines. See www.track-challenge.com and compare the Hockenheim? times of the 996 C2 and C4 - as Hockenheim? is a small track, the C4 won due to faster cornering speed but was slower on the Nurburgring due to the straights.
Re: 4WD and Understeer
Quote:
Al Pettee said:Quote:
LowPolarMoment said:
I've never driven a 4WD car. Many posts seem to imply that there is an understeer problem with 4WD. I'm thinking that having the front wheels pull you in the direction you're steering would reduce understeer. Can anyone explain?
I do not believe that a 4WD car PER SE causes an "understeer problem," HOWEVER, for 2 otherwise similar cars (e.g., a 911Carrera2 versus a 911Turbo with AWD, both rear-engined and thus slightly rear weight-biased), while the 911Turbo is also slightly heavier (partly due to its turbo's, cooling system and air intake pathways) compared to the RWD Carrera2, it is also relatively more front-weight biased (or at least less rear weight-biased) compared to the C2, since the former has a heavier front transaxle to power, as well as turn, the front wheels. This relative front-weight bias, I believe, increases the front slip angle relative to the rear slip angle of the Turbo versus that of the Carrera2, since the slip angle increases with increased weight (front and rear) carried over each axle. Slip angle is the tendency of the front or rear of the car to "twist" from centrifugal force, during turns-intentional and unintentional
Al sums it up nicely. as an ex c4 owner and now longtime tt ownwer, i think that understeer with the latter is less of a problem than with the c4. this has also to to do with the applicable power IMO, the turbo has such thrust that you are thankful for the 4wd especially in the wet. just did some corners last week in italy on the wet sliding over all four wheels, it's just brilliant! or if you take monza for example the 4wd really helps you in the corners after the chicanes, you can bslide the car really nicely in third and even fourth. you certainly can do this with a gt2 as well, but i dont know if my skills would be up to it... i'd actually be very curious to try a gallrdo on the racetrack in the wet to understand if it inspires the same confidence as the tt..
Re: 4WD and Understeer
I have to say that my 997 C4S -20mm hardly manifests any understeer. The 997 C4 understeers less then the 996 C4 and the 997 C4 - 20mm understeers even less then the 997 C4...
I must admit that I only have 1700km on it, lots on the highway so I still need some more testing.....
I must admit that I only have 1700km on it, lots on the highway so I still need some more testing.....
Re: 4WD and Understeer
997 is a whole different car in terms of under/over steering biasing. Porsche took care of the complaints on this issue...On the 996 this issue exists, it's real, the car was designed this way but it can be addressed easily. This understeer biasing if even more noticeable (and annoying) on the track. Meanwhile for those of you who don't want to spend money on their cars just use 33front and 39rear psi of tyre pressure and drive with full tank and this will dial out 50%of the understeer depending on the tyre brand...
Re: 4WD and Understeer
Quote:
Gnil said:
I have to say that my 997 C4S -20mm hardly manifests any understeer. The 997 C4 understeers less then the 996 C4 and the 997 C4 - 20mm understeers even less then the 997 C4...
I must admit that I only have 1700km on it, lots on the highway so I still need some more testing.....
They completely re-did the front suspension and used thinner anti-roll bars, etc.
Dec 13, 2005 8:56:54 PM
- Paul S in TX
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Re: 4WD and Understeer
Quote:
Grant said:
A tire has a maximum amount of friction it can produce. It can produce this friction in a longitudinal way (acceleration or braking) or a lateral way (turning). If the tire is being asked to produce longitudinal and lateral acceration at the same time, it can produce less of each type of acceleration compared to when it is asked to produce only one type.
Since an AWD car is asking its front wheels to provide forward acceration as well as lateral (turning), it cannot turn-in in SOME situations as well as a RWD car while accelerating.
Grant's explanation is how it's always been explained to me, also. And having owned all of the following: 996 C2; 996 C4; 996 Turbo; and 997 S--no question they all tend to understeer and the AWD tend to understeer more when trying to turn in under power at the track and such. But when you're not placing demands on the tires, that tax the available friction, it isn't an issue.
Jan 10, 2006 7:29:24 PM
- TT Surgeon
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Re: 4WD and Understeer
I dunno guys, I have a C4S which I push very hard at some of the fastest tracks around here, before I did the X73 I experienced some understeer, since I've done it understeer is almost nonexhistant (with sport cups). Maybe a TT is more prone bc of more power and a little more weight?
Jan 11, 2006 10:03:03 PM
- Dock (Atlanta)
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Re: 4WD and Understeer
Quote:
C4S Surgeon said:
I dunno guys, I have a C4S which I push very hard at some of the fastest tracks around here, before I did the X73 I experienced some understeer, since I've done it understeer is almost nonexhistant (with sport cups). Maybe a TT is more prone bc of more power and a little more weight?
My Turbo understeered a bit before X73, but the suspension upgrade basically eliminated it. The suspension and alignment settings play a big role in the amount of understeer present.
Concerning the front wheels being driven in an AWD Porsche...at turn-in under throttle the front tires shouldn't normally be set instantaneously to maximum lateral grip, so "powering" the wheels and asking for lateral grip at the same time should not be a performance issue on turn entry.
Jan 11, 2006 11:16:40 PM
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Turbo Al
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Re: 4WD and Understeer
Quote:
Dock (Atlanta) said:Quote:
C4S Surgeon said:
I dunno guys, I have a C4S which I push very hard at some of the fastest tracks around here, before I did the X73 I experienced some understeer, since I've done it understeer is almost nonexhistant (with sport cups). Maybe a TT is more prone bc of more power and a little more weight?
My Turbo understeered a bit before X73, but the suspension upgrade basically eliminated it. The suspension and alignment settings play a big role in the amount of understeer present.
Concerning the front wheels being driven in an AWD Porsche...at turn-in under throttle the front tires shouldn't normally be set instantaneously to maximum lateral grip, so "powering" the wheels and asking for lateral grip at the same time should not be a performance issue on turn entry.
Yes-very imnportant point: When discussing understeer and comparing RWD to AWD cars, more than just the drivetrain determines the amount of understeer, and suspension changes can significantly increase or decrease this for RWD and AWD cars. And some AWD cars can have less understeer than some RWD cars (even when both are rear-engined like the 911C4 and 911C2) based on suspension geometry alone.
Also on the 911C4 and 911TT, the AWD (and specifically FWD contribution) is variable of course, according to rear-wheel slip, even though weight bias is unchanged.
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