The controversy of what I now know regarding suspension.
#61
05-23-2004 | 12:10 PM
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Originally posted by KPV
MKW,
The front height has nothing to do with the carrier, regardless whether it is different. It is to the tension strut bolt to the frame. The frames are the same, as are the bolts.
I am unaware of a rear spacer. Can you be more specific?
The ROW and USA turbos have a 20mm ride height range, whereas the GT2 has a tighter range of 10mm.
MKW,
The front height has nothing to do with the carrier, regardless whether it is different. It is to the tension strut bolt to the frame. The frames are the same, as are the bolts.
I am unaware of a rear spacer. Can you be more specific?
The ROW and USA turbos have a 20mm ride height range, whereas the GT2 has a tighter range of 10mm.
Ken , before you do too much more, I suggest setting your front higher at 118 and rear lower at 128 and realign. Make sure they check static and dynamic rear toe -in ! I believe by setting to " GT2" rear height spec , your car has too much rake , which would unload the rear aerodynamically at these super high speeds where you notice the wiggling most.
Last edited by MKW; 05-23-2004 at 12:51 PM.
#62
05-24-2004 | 06:48 PM
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I am unfamiliar with the different reference points in the rear. I physically measured mine the other day and the rear reference point was very clear to me.
When I initially had the 9's installed, I originally had the rear 1/4" lower and decided to raise it due to it appearing to be visually squatting at rest. I couldn't live with that. Nonetheless, the performance was the same.
I am comfortable with the ride height I have chosen and I am viewing this project with that as the one and only constraint.
It is interesting that you speak of dynamic toe-n (which really should be termed as dynamic toe). I get the jist though. That was my whole argument for adding additional toe-in so that the thrust forces on the rears do not initiate a toe-out at speed. I was advised by Bob Rouleau on Rennlist that Porsche Motorsports GT2's run up to +17 minutes of toe-in at the rear. The max factory spec is +15 minutes. I am at +10 minutes. I believe the combination of a change to +15' toe-in at the rear, the rubber donut replacement and the kinematic toe link (affording more precise toe adjustment than the eccentrics) will solve the high speed instability problem.
When I initially had the 9's installed, I originally had the rear 1/4" lower and decided to raise it due to it appearing to be visually squatting at rest. I couldn't live with that. Nonetheless, the performance was the same.
I am comfortable with the ride height I have chosen and I am viewing this project with that as the one and only constraint.
It is interesting that you speak of dynamic toe-n (which really should be termed as dynamic toe). I get the jist though. That was my whole argument for adding additional toe-in so that the thrust forces on the rears do not initiate a toe-out at speed. I was advised by Bob Rouleau on Rennlist that Porsche Motorsports GT2's run up to +17 minutes of toe-in at the rear. The max factory spec is +15 minutes. I am at +10 minutes. I believe the combination of a change to +15' toe-in at the rear, the rubber donut replacement and the kinematic toe link (affording more precise toe adjustment than the eccentrics) will solve the high speed instability problem.
#65
05-25-2004 | 02:17 AM
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Originally posted by KPV
ebaker,
I believe you are correct after thinking this through a little further. I could, however, increase the stiffness of the main spring while maintaining its present length, thereby limiting almost all droop to that produced by the tender spring alone. Alternatively, I could lengthen and stiffen the main springs as previously suggested, as long as i removed the same length from the compressed height of the tender spring.
My brain hurts......
ebaker,
I believe you are correct after thinking this through a little further. I could, however, increase the stiffness of the main spring while maintaining its present length, thereby limiting almost all droop to that produced by the tender spring alone. Alternatively, I could lengthen and stiffen the main springs as previously suggested, as long as i removed the same length from the compressed height of the tender spring.
My brain hurts......
BTW...wonderful info in your thread. We all owe you a debt of gratitude for bringing these issues to light.
Last edited by john stephanus; 05-25-2004 at 03:19 AM.
#66
05-25-2004 | 11:01 PM
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Dock,
So it looks like the X73 is between the GT2 and ROW.
John,
Reducing droop is a good thing. The main reason is that the spring is still pushing up in the droop range. You ideally want your car to be functioning at or near its static ride height all the time. That was the whole idea behind the concept of active suspension. The suspension would actually apply force to counteract the shortening or lengthening effects.
Let's look at a very simple case, if you are accelerating, we all know the car will nose up in front and squat in the rear. Assuming the car does not have enough power to lift the front wheels off the ground, if you limit droop, the nose up can only go so far. This is due to the fact that the front cannot be raised any more without lifting the front wheels since the suspension is completely extended. Believe it or not, I have been told that tethers (or bands) on the springs have been used to limit droop. Basically, extend the suspension to the desired droop distance and apply a tether from one of the top spring windings to one of the bottom windings thus limiting how far the spring can extend and also limiting the lifting force it can throw into the car.
Personally, I would like less droop than what I have (2.125").
So it looks like the X73 is between the GT2 and ROW.
John,
Reducing droop is a good thing. The main reason is that the spring is still pushing up in the droop range. You ideally want your car to be functioning at or near its static ride height all the time. That was the whole idea behind the concept of active suspension. The suspension would actually apply force to counteract the shortening or lengthening effects.
Let's look at a very simple case, if you are accelerating, we all know the car will nose up in front and squat in the rear. Assuming the car does not have enough power to lift the front wheels off the ground, if you limit droop, the nose up can only go so far. This is due to the fact that the front cannot be raised any more without lifting the front wheels since the suspension is completely extended. Believe it or not, I have been told that tethers (or bands) on the springs have been used to limit droop. Basically, extend the suspension to the desired droop distance and apply a tether from one of the top spring windings to one of the bottom windings thus limiting how far the spring can extend and also limiting the lifting force it can throw into the car.
Personally, I would like less droop than what I have (2.125").
Last edited by KPV; 05-25-2004 at 11:03 PM.
#67
05-25-2004 | 11:28 PM
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Ken,
Not trying to get into a debate here, but one of the times I have seen tethers used is in rally cars that are airborne much of the time and then it is to protect the suspension from damage due to slamming to a fully extended position. They really do not use the tethers to reduce droop significantly here, just to stop it near the end of it's natural entension travel. I have also seen other droop limiting devices in race cars that will be lifted with built-in air jack systems and they limit the droop so that with the limited travel of the built-in jack system they can get all four wheels off the ground during pit stops. The reduced droop in this latter case is not a problem due to the smoothness of most race tracks and therefore more droop travel is not needed to follow a rougher road surface. I still think that if you reduce droop significantly on your road car, you will impair the rough or undulating road handling as you will not have enough rebound travel for the wheels to extend and stay in contact with the road surface over these undulations. As I mentioned before, the first suspension I had on my 996tt was a GT-3 cup suspension that I am told had droop limiters built into the struts/shocks and I can tell you from personal experience that it was not a good thing when really pushing it on rough roads.
Just my $.02 worth...
Not trying to get into a debate here, but one of the times I have seen tethers used is in rally cars that are airborne much of the time and then it is to protect the suspension from damage due to slamming to a fully extended position. They really do not use the tethers to reduce droop significantly here, just to stop it near the end of it's natural entension travel. I have also seen other droop limiting devices in race cars that will be lifted with built-in air jack systems and they limit the droop so that with the limited travel of the built-in jack system they can get all four wheels off the ground during pit stops. The reduced droop in this latter case is not a problem due to the smoothness of most race tracks and therefore more droop travel is not needed to follow a rougher road surface. I still think that if you reduce droop significantly on your road car, you will impair the rough or undulating road handling as you will not have enough rebound travel for the wheels to extend and stay in contact with the road surface over these undulations. As I mentioned before, the first suspension I had on my 996tt was a GT-3 cup suspension that I am told had droop limiters built into the struts/shocks and I can tell you from personal experience that it was not a good thing when really pushing it on rough roads.
Just my $.02 worth...
Last edited by john stephanus; 05-26-2004 at 01:29 AM.
#68
05-25-2004 | 11:52 PM
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John,
I do not have any personal experience with the tethering of a suspension for droop limitation. I do not know its applicability either. I was told about this and thought I would share it with you guys. I am not a suspension guru by any means, but I do have a good, and improving, understanding of it. I also like to learn all I can, technical and otherwise, about the things that interest me.
I did use the phrase "I have been told..."
In any case, for a car used on the street and partially on the track, I agree with your comments regarding the need to have a reasonable amount of droop. It would most certainly depend on where the car is used and under what circumstances of course.
I do not have any personal experience with the tethering of a suspension for droop limitation. I do not know its applicability either. I was told about this and thought I would share it with you guys. I am not a suspension guru by any means, but I do have a good, and improving, understanding of it. I also like to learn all I can, technical and otherwise, about the things that interest me.
I did use the phrase "I have been told..."
In any case, for a car used on the street and partially on the track, I agree with your comments regarding the need to have a reasonable amount of droop. It would most certainly depend on where the car is used and under what circumstances of course.
#69
05-27-2004 | 01:52 PM
Hi Ken, great talking to you tonight. Sorry I was inbetween calls and just came back in when you called (the dog barking didn't help either). Anyways, I've thoroughly enjoyed reading this thread and am glad that someone is trying to get to the bottom of this. My car is set up exactly the same as yours as per GT2 spec and I've obviously experienced some of the same issues you have.
Tomorrow the JICs go on and I'm hoping for marked improvements (that includes camber plate chatter lol)
Tomorrow the JICs go on and I'm hoping for marked improvements (that includes camber plate chatter lol)
#74
01-19-2005 | 02:30 PM
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