The controversy of what I now know regarding suspension……….
05-22-2004, 12:41 AM
#1
Burning Brakes
Thread Starter
The controversy of what I now know regarding suspension……….
The controversy of what I now know regarding suspension……….
I am one of the many people that have lowered their car. I lowered mine for several reasons. The first was to rid the car of the low frequency pogo-ing of the outside front corner in a turn….ugh! Shame on you Porsche. Second was to bring the center of gravity lower to the ground and third was for bling, all in that order of priority. Upon researching the many methods and products used to accomplish this, I considered the following:
I finally settled on the European Bilstein PSS-9 coilover suspension for reasons of cost, availability and perceived quality of manufacturer.
I had the PSS-9’s installed and decided to do what I thought was the right thing and set the ride height and alignment to a known specification. I chose the GT-2 specification for both ride height and alignment geometry. I had them installed and the alignment set.
Specifications (S) vs. Actual Measurements (A)
Front Ride Height
S=108 to 118mm, A=113mm (Self Verified)
Rear Ride Height
S=133 to 143mm, A=134mm (Self Verified)
Front Toe Unpressed (total)
S=+5’ +/-5’, A=+9’
Front Camber
S= -1D +/- 10’, A= -1D5’
Rear Toe/wheel
S= +10’+/- 5’, A=10’
Rear Camber
S= -1D50’ +/-10’, A= -1D42’
As you can see, it couldn’t be more dead nuts perfect. In all seriousness, thank you Chris at Eurotire!
The alignment was followed by corner balancing. I set the dampers at 3 front and 2 rear to try and compensate for the inherent understeer in the car. The thinking was to make the rear slightly stiffer to induce some oversteer and hence reduce the understeer, thereby approaching a neutral handling car. Everything was set, or so I thought.
I started driving the car and realized several things.
Handling all of these effects began to get tiresome. The joy of driving was beginning to diminish. So, I decided to have the car’s alignment checked again. This was now about 9 months after the suspension installation. We found the rear tires to be toed out!! Very bad!! Cause? We think spring settling. For those that don’t know, toe out at high speeds equates to an unstable car….especially in the rear of a 911. So, with that, the high speed stability in a straight line was improved, however, the “wiggle” in a high speed turn was still there and still very unnerving. I fiddled with the damper settings across the nine setting range and found the higher numbers to be unbearable. I settled on setting No. 1 all around. This improved the car’s handling to a degree but it is very stiff, as in a racecar.
I then went on a mission. The mission statement?
“Get to the bottom of these handling glitches.”
I started by asking my immediate friends with similarly set up cars. I asked Sean (RUF PSS-9’s) and TonyNJ (H&R springs only). Sean commented that his car was rock solid. Tony shared my assessment of the “wiggle” and high speed instability. Most recently, I spoke with Vipertestarossa (Ron), who just completed his PSS-9 install. He shared the high speed instability assessment as well. On our way to Pocono Raceway recently, Jeremy at PES was behind me through a high speed left hander on the highway and said what he saw scared him. He saw my car’s rear end wiggling from side to side and he thought I was on the verge of spinning out of control. Us NJ folk tend to press our cars when we get together and I started to get concerned that the masses have not pushed their cars to the degree we have (150-180mph on deserted highways in the wee hours of the morning). Could this be? Impossible, however, why wasn’t anyone saying anything about handling glitches?
I proceeded to talk to Cary Eisenlohr of ERP Parts (CA) and Steve Weiner of Rennsport (OR) and most recently, Gary Bohrman of Exclusive Motorcars (NJ). Each of these gentlemen is wonderful. They have each contributed valuable information relating to the nuances of the 996 suspension and offered help, guidance and possible solutions. My conversations with Cary and Steve centered around products to alleviate bump steer and hard parts to remove flex in the suspension. I will discuss the topics of these discussions more thoroughly later.
My conversation with Gary was different. Gary sets up race cars and high performance modifications for Porsches and he suggested starting with one thing at a time and use process of elimination. Gary suggested I start by determining my suspension “droop” since he thought I was hitting my bump stops. The droop measurement would tell me how much travel was remaining. Droop is the travel distance between the fully extended position of the suspension (wheel off, corner jacked up) and the statically load, at rest, position. He told me to tie a zip tie to the shock piston, under the boot with the wheel off the ground and then lower the car, roll back and forth to settle, and then measure how far the tie has moved. I did this and measured 54mm of droop. Remember this because it is very important as you will see.
I am one of the many people that have lowered their car. I lowered mine for several reasons. The first was to rid the car of the low frequency pogo-ing of the outside front corner in a turn….ugh! Shame on you Porsche. Second was to bring the center of gravity lower to the ground and third was for bling, all in that order of priority. Upon researching the many methods and products used to accomplish this, I considered the following:
- European Bilstein PSS-9 – available at the time of my purchase
- USA Bilstein PSS-9 – unavailable at the time of my purchase
- RUF PSS-9
- FVD PSS-9
- H&R coilovers – opted against due to comments about the system dropping the car lower than advertised.
- Cross – not available at the time of purchase
I finally settled on the European Bilstein PSS-9 coilover suspension for reasons of cost, availability and perceived quality of manufacturer.
I had the PSS-9’s installed and decided to do what I thought was the right thing and set the ride height and alignment to a known specification. I chose the GT-2 specification for both ride height and alignment geometry. I had them installed and the alignment set.
Specifications (S) vs. Actual Measurements (A)
Front Ride Height
S=108 to 118mm, A=113mm (Self Verified)
Rear Ride Height
S=133 to 143mm, A=134mm (Self Verified)
Front Toe Unpressed (total)
S=+5’ +/-5’, A=+9’
Front Camber
S= -1D +/- 10’, A= -1D5’
Rear Toe/wheel
S= +10’+/- 5’, A=10’
Rear Camber
S= -1D50’ +/-10’, A= -1D42’
As you can see, it couldn’t be more dead nuts perfect. In all seriousness, thank you Chris at Eurotire!
The alignment was followed by corner balancing. I set the dampers at 3 front and 2 rear to try and compensate for the inherent understeer in the car. The thinking was to make the rear slightly stiffer to induce some oversteer and hence reduce the understeer, thereby approaching a neutral handling car. Everything was set, or so I thought.
I started driving the car and realized several things.
- First, I found the outside front corner pogo-ing was somewhat diminished however still existed albeit at a lesser amplitude but higher frequency. I attributed this response to the stiffer springs of the PSS-9’s. Not a big deal but still bothersome after supposedly “upgrading”.
- The second effect was of a substantially “unsettled” feeling at high speeds. The feeling was best described as the car wanting to spin out of control if I did not apply 100% focus to every bump and nuance encountered by the suspension and steering at speed. I hate to use the term, but the rear of the car would tend to wiggle laterally. I am referring to speeds in the range of 140-170mph in a straight line and 100-140mph through turns.
- The third and last effect was what I thought was bump steer. The car’s steering would require constant correction through a corner, regardless of speed if it was anything but a racetrack smooth surface.
Handling all of these effects began to get tiresome. The joy of driving was beginning to diminish. So, I decided to have the car’s alignment checked again. This was now about 9 months after the suspension installation. We found the rear tires to be toed out!! Very bad!! Cause? We think spring settling. For those that don’t know, toe out at high speeds equates to an unstable car….especially in the rear of a 911. So, with that, the high speed stability in a straight line was improved, however, the “wiggle” in a high speed turn was still there and still very unnerving. I fiddled with the damper settings across the nine setting range and found the higher numbers to be unbearable. I settled on setting No. 1 all around. This improved the car’s handling to a degree but it is very stiff, as in a racecar.
I then went on a mission. The mission statement?
“Get to the bottom of these handling glitches.”
I started by asking my immediate friends with similarly set up cars. I asked Sean (RUF PSS-9’s) and TonyNJ (H&R springs only). Sean commented that his car was rock solid. Tony shared my assessment of the “wiggle” and high speed instability. Most recently, I spoke with Vipertestarossa (Ron), who just completed his PSS-9 install. He shared the high speed instability assessment as well. On our way to Pocono Raceway recently, Jeremy at PES was behind me through a high speed left hander on the highway and said what he saw scared him. He saw my car’s rear end wiggling from side to side and he thought I was on the verge of spinning out of control. Us NJ folk tend to press our cars when we get together and I started to get concerned that the masses have not pushed their cars to the degree we have (150-180mph on deserted highways in the wee hours of the morning). Could this be? Impossible, however, why wasn’t anyone saying anything about handling glitches?
I proceeded to talk to Cary Eisenlohr of ERP Parts (CA) and Steve Weiner of Rennsport (OR) and most recently, Gary Bohrman of Exclusive Motorcars (NJ). Each of these gentlemen is wonderful. They have each contributed valuable information relating to the nuances of the 996 suspension and offered help, guidance and possible solutions. My conversations with Cary and Steve centered around products to alleviate bump steer and hard parts to remove flex in the suspension. I will discuss the topics of these discussions more thoroughly later.
My conversation with Gary was different. Gary sets up race cars and high performance modifications for Porsches and he suggested starting with one thing at a time and use process of elimination. Gary suggested I start by determining my suspension “droop” since he thought I was hitting my bump stops. The droop measurement would tell me how much travel was remaining. Droop is the travel distance between the fully extended position of the suspension (wheel off, corner jacked up) and the statically load, at rest, position. He told me to tie a zip tie to the shock piston, under the boot with the wheel off the ground and then lower the car, roll back and forth to settle, and then measure how far the tie has moved. I did this and measured 54mm of droop. Remember this because it is very important as you will see.
05-22-2004, 12:41 AM
#2
Burning Brakes
Thread Starter
This morning I decided to call Bilstein USA. I had an eye-opening (or is it eye-popping??!!) conversation with the actual technician that installed the first trial set of PSS-9’s in a sample 996TT and test drove the car. He is located in Bilstein’s west coast facility. This is where it gets really interesting. He told me the PSS-9 dampers have 100mm of travel. That is just shy of 4 inches. Great! The 100mm is comprised of 50mm of normally damped travel of the shock piston and another 50mm of travel utilizing compression of the progressive internal bump stop. PSS-9 bump stops are internal. So, let’s do the math. Oh, screw the math!!! I am at 54mm which means I am ON THE BUMP STOPS WITH THE CAR AT REST!!!!!!
I cannot think of a strong enough word for what I was feeling upon hearing this!!!
The moral of this chapter of the story? DO NOT LOWER YOUR 996TT to GT2 RIDE HEIGHT USING PSS-9’s ALONE!!!
Gary was right in his initial assessment of hitting the bump stops. Kudos Gary!
So now I know, the funny steering response is due to being on the bump stops. Jeesh!! Any appreciable compression of the suspension in a bumpy corner results in the non-linear response of the bump stops!! The reason it is tolerable right now is because the damper settings are all at No. 1. Since the suspension is so stiff, it does not have much translation and therefore cannot be affected by the profound stiffness increase of the progressive bump stops. In effect I am limiting my suspension travel to an estimated 1 inch or so. This is evident when you push down on the front end. The car does not budge. The KPV Racecar has arrived. That was a joke.
So what about the high speed wiggle? Well, its source is yet unresolved, although I do have a theory. I believe since the tires are 19’s, offering essentially 0 compliance, all bumps and road undulations must be thrown into the suspension. This is where my conversation with Cary has much merit.
When I spoke at length with Cary, he kept focusing on the rear of the car. His basis for argument was that there is so much weight back there that any small deviations in the suspension throughout its range will quickly affect the ride and handling characteristics of the car. Do I agree? Hell yes!! As I understand it, the rear suspension of the 996TT is a derivation of the Weissach rear axle of the late 928. It was groundbreaking at the time and has been refined for use in the 996TT. He quickly pointed out one major shortcoming of the rear suspension though. There are others, but this is the big one. The control arms are held in place against fore/aft movement with “drag struts”. These struts extend from the chassis towards the rear of the car and attach to the control arms approximately 2/3 of the way out towards the wheel pick-up point. They are basically stabilizers. Well, these stabilizers have 2 inch rubber donut isolators connecting them to the control arms. I am told, especially with increased horsepower (1 ½ times factory hp spec in my case) these donuts will flex under load. The load can stem from acceleration, braking, or cornering. This deformation can lead to toe changes in the rear of the car. Toe changes at speed can lead to instability. He recommended replacing the rubber donuts with a spherical bearing. He also recommended replacing the upper non-adjustable kinematic toe link with an aftermarket adjustable version.
The front suspension is an entirely different matter. Three primary effects occur by lowering the car via coilovers alone:
There are arguably two ways to go about solving these front suspension issues. The right way and the compromised way.
Oh, wait! I could always raise the car back up to the factory ride height!
I cannot think of a strong enough word for what I was feeling upon hearing this!!!
The moral of this chapter of the story? DO NOT LOWER YOUR 996TT to GT2 RIDE HEIGHT USING PSS-9’s ALONE!!!
Gary was right in his initial assessment of hitting the bump stops. Kudos Gary!
So now I know, the funny steering response is due to being on the bump stops. Jeesh!! Any appreciable compression of the suspension in a bumpy corner results in the non-linear response of the bump stops!! The reason it is tolerable right now is because the damper settings are all at No. 1. Since the suspension is so stiff, it does not have much translation and therefore cannot be affected by the profound stiffness increase of the progressive bump stops. In effect I am limiting my suspension travel to an estimated 1 inch or so. This is evident when you push down on the front end. The car does not budge. The KPV Racecar has arrived. That was a joke.
So what about the high speed wiggle? Well, its source is yet unresolved, although I do have a theory. I believe since the tires are 19’s, offering essentially 0 compliance, all bumps and road undulations must be thrown into the suspension. This is where my conversation with Cary has much merit.
When I spoke at length with Cary, he kept focusing on the rear of the car. His basis for argument was that there is so much weight back there that any small deviations in the suspension throughout its range will quickly affect the ride and handling characteristics of the car. Do I agree? Hell yes!! As I understand it, the rear suspension of the 996TT is a derivation of the Weissach rear axle of the late 928. It was groundbreaking at the time and has been refined for use in the 996TT. He quickly pointed out one major shortcoming of the rear suspension though. There are others, but this is the big one. The control arms are held in place against fore/aft movement with “drag struts”. These struts extend from the chassis towards the rear of the car and attach to the control arms approximately 2/3 of the way out towards the wheel pick-up point. They are basically stabilizers. Well, these stabilizers have 2 inch rubber donut isolators connecting them to the control arms. I am told, especially with increased horsepower (1 ½ times factory hp spec in my case) these donuts will flex under load. The load can stem from acceleration, braking, or cornering. This deformation can lead to toe changes in the rear of the car. Toe changes at speed can lead to instability. He recommended replacing the rubber donuts with a spherical bearing. He also recommended replacing the upper non-adjustable kinematic toe link with an aftermarket adjustable version.
The front suspension is an entirely different matter. Three primary effects occur by lowering the car via coilovers alone:
- The steering tie rod angle goes from angled downward from the chassis to the wheel carrier to upward.
- The control arm angle goes from angled downward from the chassis to the wheel carrier to upward.
- The shock travel is diminished by the drop distance.
There are arguably two ways to go about solving these front suspension issues. The right way and the compromised way.
- The Right Way
This method involves the replacement of the wheel carrier casting. A new casting, with lower control arm (CA), steering tie rod (STR) and shock body (SB) pick-up points relative to the axle, is required. If you can imagine disconnecting the CA, STR and SB from the carrier, dropping the entire body with those parts, and then reconnecting them at new lower locations, then you can understand the purist approach to resolving the effects of lowering. All suspension components remain in their factory prescribed relative position to one another. I am in the process of researching if the GT2 wheel carrier or other Porsche Motorsport wheel carrier is available. If anyone knows, please advise. - The Compromised Way
The compromised way consists of replacing the STR with a revised aftermarket bump steer kit that includes a new billet steering tie rod, spherical end bearing and tapered spacer to artificially lower the angle of the STR. The shock body and control arm are left in their compromised positions. This potentially creates issues with scribed arc compatibility of the STR and CA which, in a perfect world, want to scribe very similar arcs.
Oh, wait! I could always raise the car back up to the factory ride height!
05-22-2004, 01:16 AM
#3
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Ken, Like many others, I have been following your mods and recently your pursuit of knowledge with regard to your suspension. I don't have enough technical knowledge to answer any of your questions but, I appreciate your endeavors and sharing your knowledge and results with all of us.
While I do not have an engineering background, I do enjoy my 996tt especially on the track. I have been looking into modifying my suspension and have a box of PSS9's waiting to be installed. If the PSS9's have 100mm of travel, is there a point where one could lower the car a little but not as low as the GT2 and strike a balance keeping some or all of the inherant travel? Such as to ROW specs which appears to be approx half way between GT2 and Turbo (USA).
Thanks for sharing all of your expertise and experience with us.
Rob
While I do not have an engineering background, I do enjoy my 996tt especially on the track. I have been looking into modifying my suspension and have a box of PSS9's waiting to be installed. If the PSS9's have 100mm of travel, is there a point where one could lower the car a little but not as low as the GT2 and strike a balance keeping some or all of the inherant travel? Such as to ROW specs which appears to be approx half way between GT2 and Turbo (USA).
Thanks for sharing all of your expertise and experience with us.
Rob
05-22-2004, 01:22 AM
#4
Burning Brakes
Thread Starter
Rob,
Absolutely! My post is not meant as a PSS-9 bashing session. On the contrary, the product is inherently very good. The users must understand its limitations though. I happen to be at the extreme lower limit of its usability. In essence, too low for this product. If you use PSS-9's and don't change anything else, my personal opinion is to keep them set at ROW ride height or above. This will allow them to function properly. If in doubt, defer to the ride height recommendations in the instructions.
Absolutely! My post is not meant as a PSS-9 bashing session. On the contrary, the product is inherently very good. The users must understand its limitations though. I happen to be at the extreme lower limit of its usability. In essence, too low for this product. If you use PSS-9's and don't change anything else, my personal opinion is to keep them set at ROW ride height or above. This will allow them to function properly. If in doubt, defer to the ride height recommendations in the instructions.
05-22-2004, 01:39 AM
#5
Burning Brakes
Thread Starter
Rob,
Incidentally, the ROW ride heights are:
Front 128 to 148mm
Rear 138 to 158mm
I believe your droop would be the same as what I measured as it is strictly a function of total shock travel (constant with all PSS-9's) and vehicle load distribution on the front wheels (constant for all 996TT's).
Incidentally, the ROW ride heights are:
Front 128 to 148mm
Rear 138 to 158mm
I believe your droop would be the same as what I measured as it is strictly a function of total shock travel (constant with all PSS-9's) and vehicle load distribution on the front wheels (constant for all 996TT's).
05-22-2004, 10:01 AM
#7
Still plays with cars.
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KPV - The rear toe spec from Motorsports for the GT2 on track is 17' per wheel, or 34' total. This makes the car very stable at high speeds. As you discovered, the rear tend to toe out under load which is why we want static toe-in. More rear toe in adds stability at the expense of tire wear. In racing when tires are not limited, it is common to use a ton of rear to-in for qualifying.
Rgds,
Rgds,
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05-22-2004, 12:49 PM
#8
Burning Brakes
Thread Starter
Bob,
Thank you for that information. The factory spec has limits of +5' to +15'. So at the very least, I should run the +15' or if I don't care about my $1K rear tire set, go with greater. Partly serious, partly joking there.
Thank you for that information. The factory spec has limits of +5' to +15'. So at the very least, I should run the +15' or if I don't care about my $1K rear tire set, go with greater. Partly serious, partly joking there.
05-22-2004, 01:38 PM
#10
Three Wheelin'
Ken, changing out the rubber to solid mono-***** should eliminate most your wiggle if all your setting are correct, at the expense of ride quality. I almost went to solids, but didn't want to loose the ride. I have the H&R coil overs set pretty low and it's maitained a very compliant ride. There was alot of hype when the PSS9 came out, but I feel the H&R coil overs are hard to beat for overall street and track performance especially if you prefer GT2 ride heights.
)or do you have direct experience with someone that has done it??
05-22-2004, 01:47 PM
#12
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That is a very interesting and informative post. I just lowered my car with H&R. I have not taken much about 130 with the new suspension. Didn't you show me the photo of low your front end was. What I am getting at is a concern that I may also have the problems you describe at high speed, but my car did not seem nearly as low as yours and I have not had up to a speed where I felt any instability since the H&R kit was added.
How much can you drop it before you encounter these issues.
Are you better off leaving it at factory settings for high speed stability issues.
I would hate to be playing around one day and get the wiggle you describe, but have a bad outcome.
How much can you drop it before you encounter these issues.
Are you better off leaving it at factory settings for high speed stability issues.
I would hate to be playing around one day and get the wiggle you describe, but have a bad outcome.
05-22-2004, 01:52 PM
#13
Three Wheelin'
Ken, I know of few turbo guys that have done the solids and have had good results. What you are feeling is most likely the rubber deflecting under loads on undulating high speed corners, I have found that a softer/compliant suspension set up at the GT2 or slightly lower height works to eliminate the wiggle. Rear sways set at loose-med loose and front full-med loose helps with the H&R coil overs. Not sure about the Pss9.
05-22-2004, 02:05 PM
#14
Burning Brakes
Thread Starter
Doug,
My car is precisely set and verified to the GT2 ride height spec. The more I research, the more I learn. At this time, I am convinced that the high speed instability is due to the effect that Bob mentioned above, namely rear toe change. I will be changing out the rubber donuts for hard parts as well as the kinematic toe link.
Please understand, there are several things going on here. The first is high speed instability. Rear toe in + hard parts should solve that. THe second is shock travel reduction. Limiting drop should combat that if you are unwilling to change out other suspension parts as well.
My belief, and I emphasize "belief", is that keeping your ride height at ROW or above has merit.
My car is precisely set and verified to the GT2 ride height spec. The more I research, the more I learn. At this time, I am convinced that the high speed instability is due to the effect that Bob mentioned above, namely rear toe change. I will be changing out the rubber donuts for hard parts as well as the kinematic toe link.
Please understand, there are several things going on here. The first is high speed instability. Rear toe in + hard parts should solve that. THe second is shock travel reduction. Limiting drop should combat that if you are unwilling to change out other suspension parts as well.
My belief, and I emphasize "belief", is that keeping your ride height at ROW or above has merit.
05-22-2004, 02:06 PM
#15
Burning Brakes
Thread Starter
Thanks again Oak. The hard parts are a definite for me. I am awaiting price and availability feedback. I have the stock sways and will change those out as well as a matter of course.