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:

1. European Bilstein PSS-9 – available at the time of my purchase
2. USA Bilstein PSS-9 – unavailable at the time of my purchase
3. RUF PSS-9
4. FVD PSS-9
5. H&R coilovers – opted against due to comments about the system dropping the car lower than advertised.
6. 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.

1. 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”.
2. 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.
3. 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 using 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.

 

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:

1. The steering tie rod angle goes from angled downward from the chassis to the wheel carrier to upward.
2. The control arm angle goes from angled downward from the chassis to the wheel carrier to upward.
3. The shock travel is diminished by the drop distance.

Every one of those effects severely changes the geometry and operating ranges of the individual components. This contributes to toe changes mid corner, camber loss mid corner and shock travel deficiencies. All bad. So what is the solution?

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.

So, here I am, mentally drained, a little wiser and not any happier since there isn’t any clear cut solution to resolve everything…..at least not yet.

Oh, wait! I could always raise the car back up to the factory ride height!

 

I just had a revelation! My droop is 2.125 inches. This is a function of the weight on each corner and the physical travel limit of the shock unloaded.
Although I have not measured them (totally estimating based on memory), the tender springs are on the order of 1 1/2 inches in height when the suspension is unloaded. This means the total droop is made up of 1 1/2 inches of compression of the tender springs and 1/2" of compression of the main springs. Remember the tender springs are only there to keep the main springs preloaded when the car is unweighted (as in over a drop-off). We don't want the main springs coming dislodged off their perches! As long as the tender springs apply pressure to the bottom of the main springs, they are doing their duty. So, if I replaced the main springs with 1" longer and stiffer versions, I can decrease my droop to approximately 1 inch. If I can decrease my droop 1 inch, I increase my travel 1 inch!!!
Does this hold water?

 

I think that will increase the travel by raising the ride height. Same as raising the car by adjusting the spring perches.

 

is this why people always seem to end up selling their modded tt's for gt2's and keeping them relatively stock???

 

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......

 

It sounds like you need to raise the ride height to where you have at least 10mm before the jounce bump stop starts to make contact.
Are you positive the bump stops are inside the shock? The height and stiffness of the jounce bumpers can be tuned (changed) if you can get to them. Bilstein's have the floating piston in the bottom that separates the fluid from the high pressure gas. I can't visualize how they would put a bumper in the bottom of the shock with the high pressure gas chamber there.

 

Ken,

To reclaim some suspension travel at your lowered height, you need a damper with a shorter body. Basically, your ride height is out of spec for the damper length. You can also cut the bump stops, though I don't know if that'd be possible with internal bump stops as you've indicated.

Not sure how the PSS9s are, but if they are setup like the JIC's, another option you'd have is to thread the body (as opposed to the perch) to reclaim travel. Visualize that the perch stays in it's location, and the shock body is threaded downwards, gaining damper travel.

Incidentally, I drove a Turbo with PSS9 today... they feel much more like a touring setup than anything I'd run at the track. For me, they're actually not stiff enough for what I'd prefer for the street. My JICs are at the limits (9kg/12kg) of what I can stand for the street, though I'd imagine they'd be a killer at the track. (I will find out as soon as I can hit Gingerman with this setup) Wish somebody would come up with something in between the PSS9 and JIC.

 

Great Post KPV!!

 

I'm actually having a set put on my car this morning.....

 

Ken, the only thing that comes to mind it changing the mounting points of the coil-over, but from what I have seen I don't think that it is possible. Although I may be wrong. I sure hope that you can solve you problem. Good Luck.

 

Ken:

Outstanding work! Please keep us informed of your progress as I have my PSS9's sitting and waiting for installation. You mentioned camber plates as a way to lengthen the shock stroke and regain some travel. Is this sound?

 

Ken
Thank you!! - Not only for your efforts in resolving the problems but for posting such a detailed and intelligent explaination for all of us. I am amazed at the lack of engineering that evidently went into the design of the PSS-9 as well as Porsche's blind acceptance of them (they are available and RECOMMENDED by my local dealer).
It would appear that the PSS-9s are not the proper suspention for our car (what a statement!). The proper suspension would allow for greater compression of the tender springs before the main springs compress. Can Bilstein supply stiffer tender springs or 'recalibrate' stock PSS-9s to resolve the problem?

 

Ken......WOW. You may steal the title of "Crazy lab guy" from Eric formerly known as "the plug guy." What an important thread. This is research thesis stuff.

The absence of a Bilstein engineering definitive comment is the key to this issue. There are some pretty important assumptions going on regarding the internal bumpstop, what are the properties and if it can be modified. My experience with engineering concepts is that it is easy to make logical deductions and come up with a sound result on paper when in fact there are often missing clues and design features that are unkown to the person creating the flow of logic. In other words, the (current PSS-9) engineers need to chime in at each step in the dissection of the PSS-9s. The dialogue is the true solution not a monologue. Having said this, it is highly commendable what has been reasoned and checked to this point and serious CONGRATS to Cary Gary and Steve but most of all to KEN!

There are a lot of us out here with your GT-2 setup on PSS-9s and would like to see resolution/solution to this issue. Even if it means changing to Moton, Proton or the like, the cure to this inherent instability issue is paramount.

I personally think the solution is finding out the precise function / performance of the internal (PROGRESSIVE) damper. This may be the fix without changing a lot (using just the bump steer kit?) of the CA STR geometry. Also, if this were combined with stiffer main springs, it may give just the cure of a setup between PSS-9's and JIC that Stephen was talking about. But this is just a guess. What do you think guys?

 

Great thread KPV!! One of the problems it seems, after reading your posts, is that the car is set-up way too low. I rode in a RUF Rturbo that had their version of the pss9 and it was very stable with no hint of any of the problems that plagued your car. I wonder if RUF changed some things in addition of adding their pss9. I highly doubt that so maybe their alignment set-up/ride height adjustment is a key to why this problem did not affect their cars.