Bilstein PSS-9's Do Not Mix With GT2 Ride Height - The Write-Up
#1
01-20-2005, 10:56 PM
Join Date: Jul 2003
Location: New Jersey
Posts: 4,343
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Bilstein PSS-9’s Do Not Mix With GT2 Ride Height - The Write-Up
Bilstein PSS-9's Do Not Mix With GT2 Ride Height
There has been a lot of debate and general talk about the Bilstein PSS-9 struts for use in the Porsche 996 Twin Turbo. I have gone into much detail about the pros and cons of using this suspension here:
https://www.6speedonline.com/forums/...threadid=10493
and here:
https://www.6speedonline.com/forums/...threadid=11386
In general, the controversy has related to the purpose of the suspension and its use. The suspension's purpose is to provide a means for ride height adjustment compared to the stock, nonadjustable, suspension as well as provide a means for damping adjustments compared to the nonadjustable stock suspension. Contrary to popular belief, its purpose is not to allow random or excessive lowering of the car. There are specific limitations that Bilstein recommends, an envelope of ride heights, which should be followed.
The suspension is nicely designed and has its purpose as stated above, however, if the ride height is lowered such that it is outside of Bilstein's specifications, handling irregularities will be experienced when the suspension is pushed hard. I have experienced this first hand, as referenced in the preceding links. I had set my ride height to the Porsche GT2 specification (which by all measurements was within the Bilstein factory recommendations) and the handling irregularities surfaced. Since I required the GT2 ride height as a project constraint, I decided to change out the PSS-9's in favor of a more highly adjustable and race oriented system. Refer to the second link for additional information pertaining to this.
The crux of the problem is the shock travel, and more importantly, the loss of it during the lowering of the car. Before I continue, some definitions are in order:
Definitions
Shock Travel = the total distance from full extension of the shock piston to full compression of the shock piston in the shock body including compression of the shock bump stop.
Bump Stop = A polymer sleeve surrounding the shock piston meant to cushion the bottoming out effects of a fully compressed shock. These components can be visible on the outside of the shock piston shaft or internal within the shock body as is the case with the PSS-9's.
Droop = The amount of travel of the piston relative to the shock body from full extension to a position of rest with the car's weight statically applied to the suspension. Droop can easily be measured with a zip tie on the piston pressed against the shock body inlet with the piston fully extended (wheel off the ground) and then lowering the car to rest statically on the ground. The measurement of the movement of the zip tie is the droop.
The Problem
The PSS-9's have a shock travel of approximately 100mm. This equates to about 4 inches. This 4 inches of travel is comprised of 50mm of free travel of the piston in the shock body and another 50mm of travel that compresses the internal bump stop. Please reference the following diagram for the remainder of this commentary. This diagram shows a cross section of the PSS-9 bump stop and the Load/Strain curve (similar to a stress/strain curve). I will explain:
The Math
When the shock is compressed 50mm from fully extended, it contacts the top of the bump stop. Any more compression of the suspension compresses the bump stop through the remaining 50mm of total travel (50mm + 50mm = 100mm). The bump stop is 65mm tall. Therefore at full compression, the bump stop will be 15mm squished (engineering term
).
Now for the fun part. If you look at the load/strain curve to the right, you will notice the stiffness of the bump stop is fairly linear through its first 35mm of compression and then becomes exponentially stiffer with any more compression. At 45mm of compression it is essentially a solid object.
Now, here is the problem. Listen closely because this is the important part.
The droop on the front of the 996 Twin Turbo with the Bilstein PSS-9's installed (determined by the spring stiffness used) is approximately 50mm (I measured this myself). Hmmmmmâ€Â¦how coincidental!! Lets do the math.
100mm (total travel) - 50mm (droop) = 50mm (usable compression travel while driving)
This means, at rest with a GT2 ride height, as stated in my first link, the suspension is poised right at the top of the bump stops. So, already having pointed out that we have 35mm of linear compression of the bump stop before it turns into a solid object.
So what's left???
Well, that's easy, 35mm of compression travel. What is that in English you ask?
1 3/8" of Compression Travel For Use While Driving
The Moral of the Story
If you are going to use PSS-9's, be very, very cognizant of the amount of lowering you dial in. It is easy to see that, if running at GT2 ride height, once you compress the suspension 1 3/8", you simply run out of suspension travel. When pressed hard, this can relate to horrendous handling irregularities since there is a tremendous amount of inertia of the moving car that suddenly stops its suspension translation. Smoothness becomes abrupt character changes in the handling.
There has been a lot of debate and general talk about the Bilstein PSS-9 struts for use in the Porsche 996 Twin Turbo. I have gone into much detail about the pros and cons of using this suspension here:
https://www.6speedonline.com/forums/...threadid=10493
and here:
https://www.6speedonline.com/forums/...threadid=11386
In general, the controversy has related to the purpose of the suspension and its use. The suspension's purpose is to provide a means for ride height adjustment compared to the stock, nonadjustable, suspension as well as provide a means for damping adjustments compared to the nonadjustable stock suspension. Contrary to popular belief, its purpose is not to allow random or excessive lowering of the car. There are specific limitations that Bilstein recommends, an envelope of ride heights, which should be followed.
The suspension is nicely designed and has its purpose as stated above, however, if the ride height is lowered such that it is outside of Bilstein's specifications, handling irregularities will be experienced when the suspension is pushed hard. I have experienced this first hand, as referenced in the preceding links. I had set my ride height to the Porsche GT2 specification (which by all measurements was within the Bilstein factory recommendations) and the handling irregularities surfaced. Since I required the GT2 ride height as a project constraint, I decided to change out the PSS-9's in favor of a more highly adjustable and race oriented system. Refer to the second link for additional information pertaining to this.
The crux of the problem is the shock travel, and more importantly, the loss of it during the lowering of the car. Before I continue, some definitions are in order:
Definitions
Shock Travel = the total distance from full extension of the shock piston to full compression of the shock piston in the shock body including compression of the shock bump stop.
Bump Stop = A polymer sleeve surrounding the shock piston meant to cushion the bottoming out effects of a fully compressed shock. These components can be visible on the outside of the shock piston shaft or internal within the shock body as is the case with the PSS-9's.
Droop = The amount of travel of the piston relative to the shock body from full extension to a position of rest with the car's weight statically applied to the suspension. Droop can easily be measured with a zip tie on the piston pressed against the shock body inlet with the piston fully extended (wheel off the ground) and then lowering the car to rest statically on the ground. The measurement of the movement of the zip tie is the droop.
The Problem
The PSS-9's have a shock travel of approximately 100mm. This equates to about 4 inches. This 4 inches of travel is comprised of 50mm of free travel of the piston in the shock body and another 50mm of travel that compresses the internal bump stop. Please reference the following diagram for the remainder of this commentary. This diagram shows a cross section of the PSS-9 bump stop and the Load/Strain curve (similar to a stress/strain curve). I will explain:
The Math
When the shock is compressed 50mm from fully extended, it contacts the top of the bump stop. Any more compression of the suspension compresses the bump stop through the remaining 50mm of total travel (50mm + 50mm = 100mm). The bump stop is 65mm tall. Therefore at full compression, the bump stop will be 15mm squished (engineering term
).Now for the fun part. If you look at the load/strain curve to the right, you will notice the stiffness of the bump stop is fairly linear through its first 35mm of compression and then becomes exponentially stiffer with any more compression. At 45mm of compression it is essentially a solid object.
Now, here is the problem. Listen closely because this is the important part.
The droop on the front of the 996 Twin Turbo with the Bilstein PSS-9's installed (determined by the spring stiffness used) is approximately 50mm (I measured this myself). Hmmmmmâ€Â¦how coincidental!! Lets do the math.
100mm (total travel) - 50mm (droop) = 50mm (usable compression travel while driving)
This means, at rest with a GT2 ride height, as stated in my first link, the suspension is poised right at the top of the bump stops. So, already having pointed out that we have 35mm of linear compression of the bump stop before it turns into a solid object.
So what's left???
Well, that's easy, 35mm of compression travel. What is that in English you ask?
1 3/8" of Compression Travel For Use While Driving
The Moral of the Story
If you are going to use PSS-9's, be very, very cognizant of the amount of lowering you dial in. It is easy to see that, if running at GT2 ride height, once you compress the suspension 1 3/8", you simply run out of suspension travel. When pressed hard, this can relate to horrendous handling irregularities since there is a tremendous amount of inertia of the moving car that suddenly stops its suspension translation. Smoothness becomes abrupt character changes in the handling.
Last edited by KPV; 07-16-2006 at 10:19 PM.
#2
01-20-2005, 11:06 PM
love your posts man. so informative, detailed, and practical. exactly what we want to hear. and it's refreshing to see such an enthusiastic member who really tests his car. really are helping out lots of members. my friend is contemplating what suspension for his Turbo, and i told him not to get the PSS-9's because of your post (he wanted his car low)!
#3
01-21-2005, 01:47 AM
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Join Date: Jul 2003
Location: Los Angeles, CA
Age: 45
Posts: 15,576
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ken's post is always very informative & practical. however, there are few side notes i'd love to add. since our selection is narrow down to the following:
JIC
Bilstein PSS-9 (including Techart, Ruf)
Moton
JRZ
H&R (spring & coilover)
Eibach
KW
X73
Euro M030
based on the above 9+ options, their price difference is from anywhere south of 400 to north of 6,000. therefore, which suspension is best for the car, the answer will be which one fits the driver's need the most.
from my experience, 7 out of 10 customers will not know the difference between the above 9 options. so the key factor is really depends on who's selling it & who's installing it. here are the common problems we've experienced (not in particular order):
1. Customers drive in with a car that's never been aligned after suspension work.
2. Customers drive in with a car that's been poorly aligned & already cause potential damage to both suspension & car.
3. Customers want corner balancing without knowing what is it & were only told to do so.
4. Customers want to spend the top dollar for something they'll ever need.
5. Customers want to spend the least money but don't care about the end result.
6. Customers want to drop the car beyond certain point which exceed the limit of the particular suspension.
7. all of above plus 8, 9 ,10...
our goal, as many as other shops out there, is to insure customers spend the right money to get the best result of their need. we need to guide them to the right way otherwise they will never mod the car again for countless hassle. at the end it's not just about buying one suspension & making one choice. it's about telling the positive feedback of that particular set up was recommended.
up to date, 90% of the customers report back 100% positive feedback of the PSS9s & other suspensions we've installed. the rest 10% have been guided to the GT direction including myself because i found trying to make apple to orange is simply pointless. all being said, majority of people only wants to improve the handling while achieve the certain lowered look, then u can't argue with PSS9.
hope this help for someone is making decision on suspension because not everyone is expert & no expert knows everything.
JIC
Bilstein PSS-9 (including Techart, Ruf)
Moton
JRZ
H&R (spring & coilover)
Eibach
KW
X73
Euro M030
based on the above 9+ options, their price difference is from anywhere south of 400 to north of 6,000. therefore, which suspension is best for the car, the answer will be which one fits the driver's need the most.
from my experience, 7 out of 10 customers will not know the difference between the above 9 options. so the key factor is really depends on who's selling it & who's installing it. here are the common problems we've experienced (not in particular order):
1. Customers drive in with a car that's never been aligned after suspension work.
2. Customers drive in with a car that's been poorly aligned & already cause potential damage to both suspension & car.
3. Customers want corner balancing without knowing what is it & were only told to do so.
4. Customers want to spend the top dollar for something they'll ever need.
5. Customers want to spend the least money but don't care about the end result.
6. Customers want to drop the car beyond certain point which exceed the limit of the particular suspension.
7. all of above plus 8, 9 ,10...
our goal, as many as other shops out there, is to insure customers spend the right money to get the best result of their need. we need to guide them to the right way otherwise they will never mod the car again for countless hassle. at the end it's not just about buying one suspension & making one choice. it's about telling the positive feedback of that particular set up was recommended.
up to date, 90% of the customers report back 100% positive feedback of the PSS9s & other suspensions we've installed. the rest 10% have been guided to the GT direction including myself because i found trying to make apple to orange is simply pointless. all being said, majority of people only wants to improve the handling while achieve the certain lowered look, then u can't argue with PSS9.
hope this help for someone is making decision on suspension because not everyone is expert & no expert knows everything.
#4
01-21-2005, 02:20 AM
Ken,
As always, you are the PH.D to our turbo's.......... At least for the Florida Crew this information regarding suspension is useless because as you are always stating we dont have curve's or turns, just straight aways...........
J/K just messing with you Ken
As always, you are the PH.D to our turbo's.......... At least for the Florida Crew this information regarding suspension is useless because as you are always stating we dont have curve's or turns, just straight aways...........
J/K just messing with you Ken
#5
01-21-2005, 07:31 AM
Ouch my head hurts. Any recommendation for setting the ride height for us dummies. Mine (PSS9s) are set to the GT2 specs. Being the chicken I am I haven't noticed any instability problems but also don't reach the speeds you have. Too many deer and old farmers in pickup trucks around here.
#6
01-21-2005, 08:11 AM
Damn Ken, nice work.
Arling, I'm a little confused on the intent of your post. While I can appreciate the points you make about many different types of customers, you lead off with a "however". I think Ken's post is quite accurate and explains the problems of slamming your tt using pss9's. It does induce serious suspension/handling issues. The less you lower the car, the less travel you are giving up. He is not saying the PSS9's are bad, he is saying lowering your car too much will hurt your handling if you use PSS9's. If that is your caution you offer your customers, OK. Otherwise, I think there may be a different issue.
Finally, if you don't drive your car at real high speeds, aggressively, or on bad roads, you may not notice much of an effect.
Just to add on Ken’s post. What is different about the Moton’s, JRZ’s etc, is that you can adjust the position of the shock so that you don’t lose any travel when you lower the car. This is what is not possible on the PSS9’s.
Arling, I'm a little confused on the intent of your post. While I can appreciate the points you make about many different types of customers, you lead off with a "however". I think Ken's post is quite accurate and explains the problems of slamming your tt using pss9's. It does induce serious suspension/handling issues. The less you lower the car, the less travel you are giving up. He is not saying the PSS9's are bad, he is saying lowering your car too much will hurt your handling if you use PSS9's. If that is your caution you offer your customers, OK. Otherwise, I think there may be a different issue.
Finally, if you don't drive your car at real high speeds, aggressively, or on bad roads, you may not notice much of an effect. Just to add on Ken’s post. What is different about the Moton’s, JRZ’s etc, is that you can adjust the position of the shock so that you don’t lose any travel when you lower the car. This is what is not possible on the PSS9’s.
Last edited by ColorChange; 01-21-2005 at 08:33 AM.
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#8
01-21-2005, 08:18 AM
Great, so how much suspension travel did you gain back over the 1 3/8"(PSS9's) by switching to Moton's is the real question, based on your GT2 ride height prerequisite?
Last edited by pieball; 01-21-2005 at 08:22 AM.
#10
01-21-2005, 08:24 AM
Tim, I’m staying out of this one. Another one for the mixer is the KW race suspension that we are currently using on our race cars. It is being developed by PSI and the results are promising. We used to use Moton.
#11
01-21-2005, 08:31 AM
Join Date: Jul 2003
Location: New Jersey
Posts: 4,343
Rep Power: 198
Addendum
Tim hit the nail right on the head with
"It does induce serious suspension/handling issues. The less you lower the car, the less travel you are giving up. He is not saying the PSS9's are bad, he is saying lowering your car to much will hurt your handling if you use PSS9's.
and
What is different about the Moton’s, JRZ’s etc, is that you can adjust the position of the shock so that you don’t lose any travel when you lower the car. This is what is not possible on the PSS9’s.
The differnce is that the Motons, JRZ's and Ohlins use a smooth shock body shaft that can be positioned anywhere (within reason) in the wheel carrier. Therefore, if you lower the car, you are lowerig the ENTIRE shock/spring assembly in the carrier, rather than lowering the shock shaft in the shock body.
This is a very important distinction, as it allows the Motons, JRZ's and Ohlins to preserve all of their intended travel!!! The JIC's allow some degree of lowering of the entire assembly although not as much as the Motons/JRZ's/Ohlins.
Back to the PSS-9's.....
Remembering that the GT2 ride height is 118mm as compared to the USA turbo's 158mm in front, it is very easy to see that if you maintain the factory ride height of 158mm, you regain much of the travel that Bilstein intended.
158mm - 118mm = 40mm = 1.57 inches
Add this to the 1.375 inches mentioned above and we miraculously have 3 inches of total compression travel from statically loaded!!!!!!!
Tim hit the nail right on the head with
"It does induce serious suspension/handling issues. The less you lower the car, the less travel you are giving up. He is not saying the PSS9's are bad, he is saying lowering your car to much will hurt your handling if you use PSS9's.
and
What is different about the Moton’s, JRZ’s etc, is that you can adjust the position of the shock so that you don’t lose any travel when you lower the car. This is what is not possible on the PSS9’s.
The differnce is that the Motons, JRZ's and Ohlins use a smooth shock body shaft that can be positioned anywhere (within reason) in the wheel carrier. Therefore, if you lower the car, you are lowerig the ENTIRE shock/spring assembly in the carrier, rather than lowering the shock shaft in the shock body.
This is a very important distinction, as it allows the Motons, JRZ's and Ohlins to preserve all of their intended travel!!! The JIC's allow some degree of lowering of the entire assembly although not as much as the Motons/JRZ's/Ohlins.
Back to the PSS-9's.....
Remembering that the GT2 ride height is 118mm as compared to the USA turbo's 158mm in front, it is very easy to see that if you maintain the factory ride height of 158mm, you regain much of the travel that Bilstein intended.
158mm - 118mm = 40mm = 1.57 inches
Add this to the 1.375 inches mentioned above and we miraculously have 3 inches of total compression travel from statically loaded!!!!!!!
#12
01-21-2005, 08:46 AM
Join Date: Jul 2003
Location: New Jersey
Posts: 4,343
Rep Power: 198
Addendum #2
The interesting, yet nearly impossible, solution for the travel reduction issues with the PSS-9's used at low ride heights is to remove the sway bar drop link attachment bracket and fin. The sway bar attachment bracket is comprised of two "L-shaped" brackets welded to the outside of the shock body. The sway bar drop link top bolt connects through this bracket.
If these brackets were removed (DIFFICULT OR IMPOSSIBLE WITHOUT DAMAGE TO THE SHOCK BODY AND INTERNALS), the shock body could slide down in the wheel carrier and allow full suspension travel much like the Motons, JRZ's and Ohlins. The sway bar drop link top bolt would then connect at and double as the wheel carrier cinch bolt just like the Motons, JRZ's and Ohlins.
The interesting, yet nearly impossible, solution for the travel reduction issues with the PSS-9's used at low ride heights is to remove the sway bar drop link attachment bracket and fin. The sway bar attachment bracket is comprised of two "L-shaped" brackets welded to the outside of the shock body. The sway bar drop link top bolt connects through this bracket.
If these brackets were removed (DIFFICULT OR IMPOSSIBLE WITHOUT DAMAGE TO THE SHOCK BODY AND INTERNALS), the shock body could slide down in the wheel carrier and allow full suspension travel much like the Motons, JRZ's and Ohlins. The sway bar drop link top bolt would then connect at and double as the wheel carrier cinch bolt just like the Motons, JRZ's and Ohlins.
#15
01-21-2005, 10:14 AM
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great write up. however can't some of the bumpstop be cut off to extend travel. and does the PSS9 allow you to add preload to reduce sag/droop.