The True Cause of the Understeer
01-13-2009, 12:08 PM
#1
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The True Cause of the Understeer
I've read most of the available literature on the 964 understeer issue and the way I understand it, and please correct me if I am wrong, is that during a corner the AWD system takes steps to decrease the slippage in the rear differential thus partially locking the rear wheels so when you go around a corner the inside and outside wheels can't spin independently and thus try to go in a straight line, causing the understeer.
It seems that the true "fix" to the understeer problem is to fool around with the differential slippage mechanism to reduce the amount of "lock" introduced by the AWD/traction control system. Is it possible to tweak the differential slippage mechanism?
Or, and this may expose my true misunderstanding of the entire AWD system, can you swap out a C2 differential in the C4 (but retain the AWD system)?
It just seems to me there is a more sophisticated fix than tweaking suspension and tire sizes....how can we address the heart of the problem?
Thanks!
It seems that the true "fix" to the understeer problem is to fool around with the differential slippage mechanism to reduce the amount of "lock" introduced by the AWD/traction control system. Is it possible to tweak the differential slippage mechanism?
Or, and this may expose my true misunderstanding of the entire AWD system, can you swap out a C2 differential in the C4 (but retain the AWD system)?
It just seems to me there is a more sophisticated fix than tweaking suspension and tire sizes....how can we address the heart of the problem?
Thanks!
01-13-2009, 02:28 PM
#2
Three Wheelin'
Take a look at this thread. https://rennlist.com/forums/964-foru...eer-in-c4.html There is a magazine article linked in it which will be a great help. As far as messing with the awd system, I would caution against it. My car is pretty well set up, and I am pretty happy, track and street.
01-13-2009, 03:01 PM
#3
Rennlist Member
There has been a lot of discussion regarding this. My take is that some pretty smart engineers that speak German (which to me SOUNDS completely smart...) put a lot of time into designing a system that increases the safety margin of the car in bad weather. That's why I have a C4, as it is an all-weather driver for me. It also keeps me from going off the road backwards, which would be a completely likely event with my ham-handed attempts at driving fast...The alternative is a C2... which has its advantages too.
I have done some chassis mods that dial out the negative effects, and I get to keep the safety margin. It works for me.
I have done some chassis mods that dial out the negative effects, and I get to keep the safety margin. It works for me.
01-13-2009, 04:21 PM
#4
Rennlist Member
The PDAS doesn't come into play and won't induce understeer until you get close to the extremes.So most of the time, unless your doing autocross or D.E., it won't be a factor. A C4 handles differently than a C2, there are advantages and disadvantages to both.
Read the article linked to the above thread, it covers the issue very well. Also, from what I understand, braking, or tapping the brake pedal, disengages the PDAS.
Read the article linked to the above thread, it covers the issue very well. Also, from what I understand, braking, or tapping the brake pedal, disengages the PDAS.
01-13-2009, 05:11 PM
#5
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As I understand it:
At the extremes (ie. DE and AX), the limit is partially imposed by the limits of traction of the front tires. On a AWD like the C4, the front tires are powered so are "pulling" the car at the same time the rear wheels are "pushing" the car. So the front tires need traction in a straight line.
Upon cornering, the front tires are also asked to provide traction in a lateral direction when you turn the steering wheel. There is only so much the front tires can do, if you can remember the traction circle, so the tires tend to slip and not provide as much lateral traction as you like - hence understeer as you have to turn the steering wheel more to go in the direction you'd like. At the limit, this becomes ploughing where, regardless of how much you turn the steering wheel, you are not turning any further so the only what to make the corner may be to feather the gas (not lift) so the tires are asked to less in the way of straight line traction and more lateral traction.
As has been indicated, the PDAS only interferes at the real extreme to try to prevent you going off or at least go off straight first, not *** first (front first is you fault, *** first is a design flaw according to the short-sighted lawyers).
Make sense?
Marc
At the extremes (ie. DE and AX), the limit is partially imposed by the limits of traction of the front tires. On a AWD like the C4, the front tires are powered so are "pulling" the car at the same time the rear wheels are "pushing" the car. So the front tires need traction in a straight line.
Upon cornering, the front tires are also asked to provide traction in a lateral direction when you turn the steering wheel. There is only so much the front tires can do, if you can remember the traction circle, so the tires tend to slip and not provide as much lateral traction as you like - hence understeer as you have to turn the steering wheel more to go in the direction you'd like. At the limit, this becomes ploughing where, regardless of how much you turn the steering wheel, you are not turning any further so the only what to make the corner may be to feather the gas (not lift) so the tires are asked to less in the way of straight line traction and more lateral traction.
As has been indicated, the PDAS only interferes at the real extreme to try to prevent you going off or at least go off straight first, not *** first (front first is you fault, *** first is a design flaw according to the short-sighted lawyers).
Make sense?
Marc
01-13-2009, 05:11 PM
#6
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As I understand it:
At the extremes (ie. DE and AX), the limit is partially imposed by the limits of traction of the front tires. On a AWD like the C4, the front tires are powered so are "pulling" the car at the same time the rear wheels are "pushing" the car. So the front tires need traction in a straight line.
Upon cornering, the front tires are also asked to provide traction in a lateral direction when you turn the steering wheel. There is only so much the front tires can do, if you can remember the traction circle, so the tires tend to slip and not provide as much lateral traction as you like - hence understeer as you have to turn the steering wheel more to go in the direction you'd like. At the limit, this becomes ploughing where, regardless of how much you turn the steering wheel, you are not turning any further so the only what to make the corner may be to feather the gas (not lift) so the tires are asked to less in the way of straight line traction and more lateral traction.
As has been indicated, the PDAS only interferes at the real extreme to try to prevent you going off or at least go off straight first, not *** first (front first is you fault, *** first is a design flaw according to the short-sighted lawyers).
Make sense?
Marc
At the extremes (ie. DE and AX), the limit is partially imposed by the limits of traction of the front tires. On a AWD like the C4, the front tires are powered so are "pulling" the car at the same time the rear wheels are "pushing" the car. So the front tires need traction in a straight line.
Upon cornering, the front tires are also asked to provide traction in a lateral direction when you turn the steering wheel. There is only so much the front tires can do, if you can remember the traction circle, so the tires tend to slip and not provide as much lateral traction as you like - hence understeer as you have to turn the steering wheel more to go in the direction you'd like. At the limit, this becomes ploughing where, regardless of how much you turn the steering wheel, you are not turning any further so the only what to make the corner may be to feather the gas (not lift) so the tires are asked to less in the way of straight line traction and more lateral traction.
As has been indicated, the PDAS only interferes at the real extreme to try to prevent you going off or at least go off straight first, not *** first (front first is you fault, *** first is a design flaw according to the short-sighted lawyers).
Make sense?
Marc