Suspension Basics for the 996TT by Mikelly

9 minute read

Suspension Basics for the 996TT

Compliments of Mikelly @ www.renntrack.com

I'm starting this thread in hopes that it'll help all of us collectively agree as a community on some of the more common issues and solutions for the 996TT platform!

First, let's look at what the suspension consists of and what is lacking...

According to the books I’ve read on the development of the 996TT, the original intent of the suspension on the 996TT was to create a great Grand Touring, sporty and comfortable ride. It was not intended that the stock suspension be used as a road race platform. An inspection of the bushings and mounts for the suspension and drive train will show them to be very soft and compliant. Further, any upgrades to the engine/turbos will create excessive movement or porpousing of the drive train. Excessive movement can pull on intercooler hoses, vacuum lines, and cause the exhaust system to make contact with the rear bumper cover.

The basic design of the 996TT is similar to that of the other 911s based on the 996 chassis, except for the fact that the AWD system requires a different front suspension design/geometry, and the pickup point of all four corners is altered to accommodate this 4 wheel drive solution.

The 996 Turbo has a different offset to the front suspension upright/knuckle and this plays into the issues of compatibility for front brake rotors, as described in the Brake tech DIY sticky I started elsewhere in this category.

Toe and camber are adjustable on the 996TT from the factory, although camber is limited to approximately 1.5 degrees negative front and 2.0 degrees negative rear. Caster can ONLY be altered via the strut mount at the top of the coil spring and is very limited in adjustment in that range. OEM right height is not adjustable on the 996TT (However, the GT2 does have threaded collars for coil over ride adjustment.) and the sway bars are not adjustable. The X73 suspension package offers springs that are approximately half an inch lower with a few other improvements in stiffer bushings. The struts on both the standard and the X73 are valved specifically for the grand touring intent of the chassis.

There are a number of “solutions” that people have asked about… Some want to alter the ride height without creating a less comfortable daily driver. Others want to drive at the track occasionally while using the car on weekends and for fun. Still others want the best of both worlds; GT3 handling with GT2 power… 

So if we're talking "levels" then I suppose we could try something like this:

Level 1 - OEM Stock

Level 2 - Coil overs for asthetics

Level 3 - DE beginners

Level 4 - DE Advanced

Level 5 - Club Racers (Tom Kerr on Crack!)

Let's start a discussion and I'll edit this initial thread as we go to keep everything in the first thread. I also plan to do that to the Brake DIY so people don't have to read 20 pages to find the answers they're looking for...

Here is some interesting Tech info from the Factory Service Manual:

Scaling and Corner Balancing - Maximum Load Differences for the GT2, per wheel axle should be 15 KG – Adjustments made at diagonal corners for balancing (e.g. right front to left rear)

Ride Height for the GT2 – Front - 118MM minimum up to 123MM Rear – 133MM Minimum up to 138MM. Total per axle is 5mm adjustment

There is much variance on Turbo and Turbo X73/S models

Standard 996Turbo Front - 158 +-10MM Rear – 163 +-10MM 

RoW Front - 138 +-10MM Rear – 153 +- 10MM

X73 Model Front - 128+-10MM Rear – 133 +- 10MM

Suspension geometry is complex. You need to understand the physics at play when thinking about the right components for your suspension needs.

Polar Moment of Inertia, Roll center, Bump travel, Ackerman, camber arch, compression damping, compression rebound, caster, camber, toe, cross camber, are all important in understanding what you're asking of your chassis. Slip angles, reciprocating mass, static mass, where the mass lies within the chassis, all play parts as to how a suspension will perform a given task. I hope we can get into a technical discussion about suspension dynamics and what this list of parts means in the big picture! 

But for now... 

BUSHINGS BUSHINGS BUSHINGS:

The drivetrain in these cars porpouses bad enough with a modified powerplant that you can break loose boost hoses and cause exhaust tips to come OFF! This has happened to me and others...

I'd start with WEVO or GT3 Cupcar motor mounts and trans mounts. I'd also check the condition of the front diff mount and replace it if you have any evidence of damage to it.

I'd consider the following point to upgrade: 

Adjsutable thrust bushings - Added negative camber causes the front corners to gain way to much positive caster... You need the ability to dial that BACK. Adjustable "clocking" thrust bushings will give you this ability.

Pillow mounts - These mounts will help your suspension stay true in its adjustment. Without them, as you progress in your ability, you will force the upper mounts to move under severe load.

Lock plates for your TOE links - Again, as you progress, you can cause the OEM adjusters to lose their adjustment settings. These lock plates will do away with the OEM adjusters all together. 

Rear Sub-frame solid bushings... These will also stave off deflection in the drivetrain. As you progress in your driving you will notice more and more out things come "out of alignment" slightly...These mounts help lock down the rear subframe...

Now for the "fun" stuff:

GT3 Lower control arms... I'd do these FRONT AND REAR. They're pricey, but they work. Be sure to buy plenty of shims... And the shim for the PSM sensor. You can also get the (Tarred or other) monoball mount for the inner pivot, which removes the last of the rubber on that component...

Upper rear control arms AKA Dog bones - Make sure if you get these that you get STEEL insert for your heim joints. I've deformed and loosened the aluminum inserts that came with mine.

Adjustable Toe links - These will seriously improve your car's consistancy in cornering. The OEM toe links will deflect on you as you improve beyond novice/intermediate.

Adjustable Sway bars - You can go the GT3 route, or the H/R route, or whoever... But your bars are only as good as the droplink attached to them. Make sure you get a GOOD adjustable droplink that has a proven track record on the track and for sustained period of performance (years).

Lowering Springs - H&R, Bilstein, Porsche, and a host of others make standard lowering springs in a variety of types and heights... Progressive rate springs start at one rate but "progressively" get stiffer as they are compressed. Straight rate springs are a static rate through the compression range. Street cars tend to like progressive springs over straight rate springs. 

Coilovers come in many forms... Do you want looks, or looks and function? Do you have to have the very best? Here is a rundown on the coilovers out there:

H&R - They've been around for a long time and their system is a copy of the Bilstein from all I've read. I was told by one shop that Bilstein actually supplies their cores, but don't know how true that is. H&R is considered the "entry level" in coil-overs, but I'm sure they're probably on par with out of the box PSS9s. 

Bilstein - Bilstein makes a wide range of coilover solutions. They are the manufacture of many OEM components world wide and their PSS9 and PSS10s are well used. Some advantages of the Bilstein are excellent customer service and complete custom builds. Downfall is the fact that they do not come with pillow mounts. Some will also argue that the PSS9 should not be lowered to GT2 ride heights. However Bilstein and FVD both have solutions to overcome this myth. My own car has shown that indeed you can run at BELOW GT2 heights without problems of bottoming out on the bumpstops. There are a number of options for custom PSS9s, so educate yourself before you spend your money. All Bilsteins are not created equal.

CROSS JICs - These are considered a step up, if for no other reason than the fact that they come with their own camber plate/pillowmount setup. There has been much debate over their customer service, but it appears that the company has resolved it's issues and the product is being used by many here with reasonable success. Not unlike the PSS9, the JICs do have their limitations. To much spring can cause problems with tuning the compression and rebound settings.

KW - KW has been building suspensions for some time and have come into their own recently. Pricing is comperable to the JICs and they are highly adjustable in compression (14 settings) and rebound (infinate adjustment).

Moton & JRZ are more sophisticated in their design. Both employ external reservoirs and require more time to setup. It's not unheard of to have a full Moton or JRZ purchase/install run $10,000 with proper testing/corner balancing/alignment!

On the far end of the spectrum is the Ohlins, Penski and other custom shock makers!

Regardless the component, they're all only as good as the person installing and tuning the system. Remember, each part works as part of a complete package and having a suspension engineer involved in the installation and setup is key to having a well sorted suspension package!

Mike