My sad suspension
08-29-2006, 04:05 AM
#31
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Bill, from earlier discussions with you plus the info above, it sounds like you ended up with someone else's springs on the front of your car, or maybe your own swapped front to rear. It sounds like you really enjoyed the way that it handled initially. Maybe the best thing would be to try to get it back to that state.
There is info in the WSMs about free length of the springs, how much force it should take to compress to a specific length, etc. While it's a given that the springs will not meet the same spec they did 170K ago, performing these measurements will help you determine if you're in the ballpark. Without looking up the exact numbers for your car, it seems the fronts compress to ~210mm @ ~6200N and the rears compress to ~270mm @ ~4800N. Off the top of my head, it looks like over-compressing a rear spring to 210mm might get you almost to the same force at that length as for a front spring, but not quite. I couldn't find the free length for the rears so I'm guessing.
Anyway, whether or not the problem lies there, adding a stiffer swaybar is not the correct solution to a bottoming problem where, supposedly, only the shocks were changed. Either those shocks weren't worth their weight in pigeon poop from day 1, or the springs were swapped for softer units. Let's assume for a second that it's the shocks and there was no change in the springs. in this case, the shocks were clearly not doing the job as well as the "worn-out" shocks they replaced.
Here is where it gets a little silly. If the shocks were not able to damp the suspension in order to reduce bottoming, then adding a heavier swaybar would only amplify the deficiency of the shocks. Springs hold the car up. Shocks control movement of the car on those springs. A heavier swaybar effectively increases the spring rate on the outside wheel while turning(by compressing the opposite spring, but leave that aside for the moment) *or* when hitting a bump with only one wheel. To keep it all properly under control, you would need even heavier damping for it to behave with the heavier swaybar.
Cars with a too-heavy front swaybar will tend to understeer. If the suspension is under-damped, it could set up an oscillation as it alternately pushes and grabs. It should be balanced by a proportionally heavier rear swaybar.
Now, those are mostly gross effects, and hard to measure precisely in practice. But I'll wager that you will find something of interest if you check the springs against the specs.
If you could stiffen the rear bar easily, perhaps by borrowing a set of Ott droplinks, or go back to the stock front bar, and you find that it reduces the problem, then that's a useful data point -- it would support the hypothesis that the suspension is no longer balanced, and it's easier than pulling the springs out.
Sometimes the best way to move forward is to take a couple of steps back. The springs are really at the heart of the car's handling balance. Check that first. Maybe even get a set of sport springs so you know exactly what you have. Then get some good dampers on there... Bilsteins seem to get a lot of recommendations. Get that razor-sharp handling back. Then, if it has more body roll than you care for, experiment with poly bushings, bars, etc -- but I'd say you should add stiffness to front and rear bars at the same time until you're satisfied with the degree of body roll, then fine-tune front or rear stiffness to get the over/neutral/under steering behavior that you want.
Just my $0928 worth...
There is info in the WSMs about free length of the springs, how much force it should take to compress to a specific length, etc. While it's a given that the springs will not meet the same spec they did 170K ago, performing these measurements will help you determine if you're in the ballpark. Without looking up the exact numbers for your car, it seems the fronts compress to ~210mm @ ~6200N and the rears compress to ~270mm @ ~4800N. Off the top of my head, it looks like over-compressing a rear spring to 210mm might get you almost to the same force at that length as for a front spring, but not quite. I couldn't find the free length for the rears so I'm guessing.
Anyway, whether or not the problem lies there, adding a stiffer swaybar is not the correct solution to a bottoming problem where, supposedly, only the shocks were changed. Either those shocks weren't worth their weight in pigeon poop from day 1, or the springs were swapped for softer units. Let's assume for a second that it's the shocks and there was no change in the springs. in this case, the shocks were clearly not doing the job as well as the "worn-out" shocks they replaced.
Here is where it gets a little silly. If the shocks were not able to damp the suspension in order to reduce bottoming, then adding a heavier swaybar would only amplify the deficiency of the shocks. Springs hold the car up. Shocks control movement of the car on those springs. A heavier swaybar effectively increases the spring rate on the outside wheel while turning(by compressing the opposite spring, but leave that aside for the moment) *or* when hitting a bump with only one wheel. To keep it all properly under control, you would need even heavier damping for it to behave with the heavier swaybar.
Cars with a too-heavy front swaybar will tend to understeer. If the suspension is under-damped, it could set up an oscillation as it alternately pushes and grabs. It should be balanced by a proportionally heavier rear swaybar.
Now, those are mostly gross effects, and hard to measure precisely in practice. But I'll wager that you will find something of interest if you check the springs against the specs.
If you could stiffen the rear bar easily, perhaps by borrowing a set of Ott droplinks, or go back to the stock front bar, and you find that it reduces the problem, then that's a useful data point -- it would support the hypothesis that the suspension is no longer balanced, and it's easier than pulling the springs out.
Sometimes the best way to move forward is to take a couple of steps back. The springs are really at the heart of the car's handling balance. Check that first. Maybe even get a set of sport springs so you know exactly what you have. Then get some good dampers on there... Bilsteins seem to get a lot of recommendations. Get that razor-sharp handling back. Then, if it has more body roll than you care for, experiment with poly bushings, bars, etc -- but I'd say you should add stiffness to front and rear bars at the same time until you're satisfied with the degree of body roll, then fine-tune front or rear stiffness to get the over/neutral/under steering behavior that you want.
Just my $0928 worth...
08-29-2006, 06:01 AM
#32
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Is it possible that at high speeds the ride height (front ?) is changing due to lift ?
As far as I understand this problem, it's only at high speed that problem manifests itself.
What ride heights is George running?
As far as I understand this problem, it's only at high speed that problem manifests itself.
What ride heights is George running?
08-29-2006, 09:32 AM
#34
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Bill,
It sounds like hypercoil springs are in order. Go stiffer for what you are doing so you have more control at high speeds. Old springs do not act the same as new ones, once they are tired your going to notice it especially at high speeds.
Also, what did Koni do for your shocks, rebuild or replace? What is your confidence level on the rebuild? I don't know if they meet the same stringent qualities for a warranty rebuild as they do when they make them new.
I have done tokico 5 way shocks, new stiffer springs and swaybar and the car was incredible. My current 928 seems okay so I havent done anything yet.
It sounds like hypercoil springs are in order. Go stiffer for what you are doing so you have more control at high speeds. Old springs do not act the same as new ones, once they are tired your going to notice it especially at high speeds.
Also, what did Koni do for your shocks, rebuild or replace? What is your confidence level on the rebuild? I don't know if they meet the same stringent qualities for a warranty rebuild as they do when they make them new.
I have done tokico 5 way shocks, new stiffer springs and swaybar and the car was incredible. My current 928 seems okay so I havent done anything yet.
08-29-2006, 01:22 PM
#35
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Originally Posted by GlenL
Ok, "bad swaybar" one more time. Sure sounds like it.
Was the stiffness of the Koni's checked for eveness before installation?
But I swear the first set was downright awful out of the box. These new ones seem far better by the bumper test. At this point, I am not blaming the Konis and my stock sport springs.
How about low or uneven tire pressures? Steering rack bushings sloppy?
Alignment guys are usually really good at spotting bad suspension parts. I'd still jack it up, pull a wheel and use a crowbar to move things.
FWIW: I'm running the Koni's and while the street ride is harsh the track use is good. Set 2/3rds hard. Very smooth and predictable in hard corners and sweepers. My guess is something (or things) are loose or broken.
Last edited by Bill Ball; 08-30-2006 at 01:55 PM.
08-29-2006, 01:39 PM
#36
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Originally Posted by dr bob
Bill--
How much adjuster is used up compared to when you bought the the car? How lond is the spring installed height with the car on the ground? It's short division so even I can usually do the calc in my head.
How much adjuster is used up compared to when you bought the the car? How lond is the spring installed height with the car on the ground? It's short division so even I can usually do the calc in my head.
As springs get tired and the rate decreases, there's more opportunity for punishmnet of the shocks. The shocks really don't care how much the springs are carrying or howe heavy the car is, so long as they aren't bottomed out. If they are bottomed out hard, there's a good chance they are damaged internally. If/when you change to the Bilstiens, maybe a set of springs are in order.
The lower spring rates from worn springs, with a lowered car, makes you a potential poster child for bump steer issues.
On the control arm bushings: There's no way you can manually put enough force on those bushings to move them, certainly not enough movement to notice with the eye as you grunt on the end of the suspension arms. Maybe if the old bushing was 80% missing...
With the four-post lift you have, you are all ready to test the bushings under lateral load. Dial indicator on the arm, base on the inner fender well. Strap the tire to the lift tray and use a come-along or similar to pull the car sideways, simulating the side loading you experienced at speed. Dial indicator shows bushing deflection. Make yourself some room by jacking the car up and extending the suspension some before pulling. You may also try using an old bare rim (no tire) on the testing side front and have plenty of room. Carpet pad under the bare rim might help.
Just a thought...
Last edited by Bill Ball; 08-30-2006 at 01:56 PM.
08-29-2006, 01:44 PM
#38
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Originally Posted by greg928GTS
Bill, from looking at your avatar picture, the front end of your car certainly does look higher than it should be.
I had my mechanic check the front A-arm bushings and I watched him while he took a bar and moved the front suspension in such a way that there was movement. It's on my long list.
I had my mechanic check the front A-arm bushings and I watched him while he took a bar and moved the front suspension in such a way that there was movement. It's on my long list.
08-29-2006, 01:47 PM
#39
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Originally Posted by Andrew Olson
Just a random thought (ignore if it's not even worth considering), but could the shop that originally put the Koni's in have mixed up the springs by putting front on rear and rear on front? Not even sure if there is a difference, but tought I'd throw it out there.
08-29-2006, 01:53 PM
#40
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Originally Posted by Vlocity
Follow up question......if you have adjustable shocks where are they set. It also sounds like "tramling" from having improper toe or a steering rack that is moving. MK suggested a dry marker test on your driveway...but I use a Sears Laser lined up with the rear wheels and then can break the beam to get measurements at the rear and front of the front wheel. Not perfect, but accurate enough to confirm toe in or out. I actually built a trammel bar to get accurate measurements.
Are you running any steroid bars on the rear?
Lastly what about rear toe. Squating under load will move you quickly to a toe out situation which will also cause the problem you describe. Perhaps you need to add a little rear toe in to the setup.
08-29-2006, 02:00 PM
#41
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Bill...........I believe in the other thread you had mentioned owning a set of Bilsteins. Would it not be easier to eliminate the shocks and springs by replacing both? Your springs must be tierd after all this time and miles and perhaps the 'new' shocks only have shown how bad the springs really are.
If all's well after the spring/shock replacement your done.............if not at least you have piece of mind knowing the shocks and springs are new.
Bon chance.
If all's well after the spring/shock replacement your done.............if not at least you have piece of mind knowing the shocks and springs are new.
Bon chance.
08-29-2006, 02:06 PM
#42
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Originally Posted by SharkSkin
...adding a stiffer swaybar is not the correct solution to a bottoming problem where, supposedly, only the shocks were changed. Either those shocks weren't worth their weight in pigeon poop from day 1, or the springs were swapped for softer units. Let's assume for a second that it's the shocks and there was no change in the springs. in this case, the shocks were clearly not doing the job as well as the "worn-out" shocks they replaced.
Here is where it gets a little silly. If the shocks were not able to damp the suspension in order to reduce bottoming, then adding a heavier swaybar would only amplify the deficiency of the shocks. Springs hold the car up. Shocks control movement of the car on those springs. A heavier swaybar effectively increases the spring rate on the outside wheel while turning(by compressing the opposite spring, but leave that aside for the moment) *or* when hitting a bump with only one wheel. To keep it all properly under control, you would need even heavier damping for it to behave with the heavier swaybar.
Cars with a too-heavy front swaybar will tend to understeer. If the suspension is under-damped, it could set up an oscillation as it alternately pushes and grabs. It should be balanced by a proportionally heavier rear swaybar.
If you could stiffen the rear bar easily, perhaps by borrowing a set of Ott droplinks, or go back to the stock front bar, and you find that it reduces the problem, then that's a useful data point -- it would support the hypothesis that the suspension is no longer balanced, and it's easier than pulling the springs out.
Here is where it gets a little silly. If the shocks were not able to damp the suspension in order to reduce bottoming, then adding a heavier swaybar would only amplify the deficiency of the shocks. Springs hold the car up. Shocks control movement of the car on those springs. A heavier swaybar effectively increases the spring rate on the outside wheel while turning(by compressing the opposite spring, but leave that aside for the moment) *or* when hitting a bump with only one wheel. To keep it all properly under control, you would need even heavier damping for it to behave with the heavier swaybar.
Cars with a too-heavy front swaybar will tend to understeer. If the suspension is under-damped, it could set up an oscillation as it alternately pushes and grabs. It should be balanced by a proportionally heavier rear swaybar.
If you could stiffen the rear bar easily, perhaps by borrowing a set of Ott droplinks, or go back to the stock front bar, and you find that it reduces the problem, then that's a useful data point -- it would support the hypothesis that the suspension is no longer balanced, and it's easier than pulling the springs out.
Sometimes the best way to move forward is to take a couple of steps back. The springs are really at the heart of the car's handling balance. Check that first. Maybe even get a set of sport springs so you know exactly what you have. Then get some good dampers on there... Bilsteins seem to get a lot of recommendations. Get that razor-sharp handling back. Then, if it has more body roll than you care for, experiment with poly bushings, bars, etc -- but I'd say you should add stiffness to front and rear bars at the same time until you're satisfied with the degree of body roll, then fine-tune front or rear stiffness to get the over/neutral/under steering behavior that you want.
08-29-2006, 02:07 PM
#43
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Originally Posted by the flyin' scotsman
Bill...........I believe in the other thread you had mentioned owning a set of Bilsteins. Would it not be easier to eliminate the shocks and springs by replacing both? Your springs must be tierd after all this time and miles and perhaps the 'new' shocks only have shown how bad the springs really are.
If all's well after the spring/shock replacement your done.............if not at least you have piece of mind knowing the shocks and springs are new.
Bon chance.
If all's well after the spring/shock replacement your done.............if not at least you have piece of mind knowing the shocks and springs are new.
Bon chance.
08-29-2006, 02:21 PM
#44
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The thing with suspension is that it's the spring that (mostly) provides the resistance and rebound to the vertical forces that are encountered while driving from bumps and force vectoring when turning. The strut's primary function is to limit the oscillation of the spring. We've all seen a car driving down the road bouncing up and down. In this case the springs are fine, but the strut has lost its ability to dampen the oscilation.
Unfortunately Bill, as you pointed this out, it only happens at high speeds, so it's hard to test for. I think you either have a a bad spring(s) or a poorly sized strut (need something with more damping). Now, the other components that come into play are:
Bushings - you suspect these
Sway bar - I doubt this as this transmits the vertical forces to both wheels, not just one
sway bar links -
Are you running stock links? The rubber inserts can wear out!
Ball joints - Assume this is good.
tie rods/joints - Also assume this is good
hub & bearing - Again, assume this is good.
Tires - I was surprised to hear you were running 16's. The forces you're creating are pretty substantial for a car with that tall of a sidewall - could have been flexing?
control arms - Assume these are good
Eccentrics - I have to assume these are good, but worth checking.
the rigidity of the chassis - Again assume this is good
Did I miss anything?
Unfortunately Bill, as you pointed this out, it only happens at high speeds, so it's hard to test for. I think you either have a a bad spring(s) or a poorly sized strut (need something with more damping). Now, the other components that come into play are:
Bushings - you suspect these
Sway bar - I doubt this as this transmits the vertical forces to both wheels, not just one
sway bar links -
Are you running stock links? The rubber inserts can wear out!Ball joints - Assume this is good.
tie rods/joints - Also assume this is good
hub & bearing - Again, assume this is good.
Tires - I was surprised to hear you were running 16's. The forces you're creating are pretty substantial for a car with that tall of a sidewall - could have been flexing?
control arms - Assume these are good
Eccentrics - I have to assume these are good, but worth checking.
the rigidity of the chassis - Again assume this is good
Did I miss anything?
08-29-2006, 02:45 PM
#45
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Originally Posted by John Speake
Is it possible that at high speeds the ride height (front ?) is changing due to lift ?
As far as I understand this problem, it's only at high speed that problem manifests itself.
What ride heights is George running?
As far as I understand this problem, it's only at high speed that problem manifests itself.
What ride heights is George running?