In a multi-link setup, as is present on nearly all modern RWD Nissans, the control arm pivot points determine the roll center, similar to a double wishbone setup, but with some variance according to where the toe and traction rod are mounted.

Front roll center is determined by the angle of the lower control arms and the angle of the upper strut mounting points. These angles, drawn in space, determine the instant centers for the front suspension (instant centers are the points that the entire suspension moves around during body roll). Then take a line from each instant center to the middle of the tire tread on the opposite side, and where these lines intersect, there is your front roll center.

Rear roll center (for our cars) is determined much like a double wishbone setup. Use the angles of the upper and lower control arms to determine the instant centers of the suspension. Then, just like the front, take a line from each instant center to the middle of the tire tread on the opposite sides. The intersection is your rear roll center.

The center of gravity is the point at which all the weight of the car is balanced. It’s roughly the crank centerline in the front, and above the differential in the back. You can find out your exact CG by using scales, but that is beyond the scope of this article.

Roll couple is the distance between your center of gravity and roll center. This is the reason that lowered cars have more body roll. The roll center moves lower (due to the change in angle of the suspension arms), while the center of gravity is only lowered as much as the body of the car. This makes the distance between the two longer, thus the roll couple longer, which gives the body more leverage on the suspension. I’m sure we all know how much of a difference leverage can make.

You actually want your roll center just under your center of gravity.

The only ball joints that I’ve seen that actually have a longer shank are Sunline Racing, Moonface, and Nagisa auto:

Nagisa:



Beware of "Roll Center Adjusting ball joints" that just space the ball joint up. The ball joint shank needs to be longer. Nagisa has a good illustration (bad on the left, proper on the right):



The above method is only so effective. Due to the angle of the ball joint, using a long shank increases ackerman, and at about 20mm of correction, the lower control arm starts running into the brake rotor.

Dan at Parts Shop MAX made a nice diagram showing how the ackerman change happens:

There aren't a whole lot of arms that offer more than 10-15mm of correction because of brake rotor clearances. Ikeya Formula has some correction, and PSM has a 5mm fine adjustment spacer.



They've updated these with bent tension rods for more steering clearance:



PSM's limit break (longer) LCA:



And GKTech's latest chromoly FLCA's. You can order these with 20mm longer shanks.:
(Image courtesy of swapmysilvia.com)


These are some nicely engineered pieces, and they fail just as they should when running into things.

Modifying the pickup points on the knuckle (usually by creating a whole new knuckle), dropping them lower. Also, raising the hub location in the knuckle, which effectively creates a "drop knuckle"; a knuckle that lowers the car while keeping roll center similar. Some of the kits actually combine both ways, raising the hub, and lowering the pickup points.

Driftworks makes drop knuckles, 45mm in the front, and 50mm in the rear. The rears have urethane bushings for the control arms:



Stock knuckle:



Driftworks knuckle:



R-chassis front Driftworks with 20mm correction:



They just released their new Geomaster V3, which was collaborated on with Wisefab. Now with 50mm of front correction and revised steering geometry. Front knuckle has drift and grip settings and the rear now has sphericals.







Parts Shop MAX forged pro knuckle:





And their new "trailing knuckle" with increased caster trail and adjustable ackerman:





Parts Shop MAX latest forged 25mm drop knuckle with spherical bushings:



And their "toe control" brackets, which allow you to dial in different levels of bumpsteer:



Here's a comparo shot between the Driftworks and PSM:



Wisefabs front suspension kit, with a 56mm of correction:





Wisefab's rear kit, with all new geometry for better rear grip:







GKTech has their V3 front knuckle with 40mm drop (preorder available with roadcourse specific steering arm):



They also have this cool diagram of the different steering geometries of their knuckles/kits:



GKtech also offers R-chassis knuckles which can be used on the Z32.



Photo courtesy of Chris York:





And their rear 40mm rear knuckle with new kinematics for better camber and to curves and corrected anti-squat. This knuckle is also significantly lighter than stock:







And GKtechs bolt on knuckle adapter with 40mm of correction:





Track Day Performance has a billet aluminum front suspension kit with 70 or 100mm of correction.







And their rear kit (I don't have shots of the arms that come with )



Heatmaker knuckles offer 60mm of correction:

You can move the whole subframe up, correcting the suspension geometry much like the modified knuckles.

SPL’s subframe adjusters (which can be either set for stock, or raised for ride height correction). These also can be ordered in an offset pattern, so you can mount a S14/15 subframe in your S13:



PSM's slip fit risers (Installing these was cake):



NOTE: Raising the subframe also increases anti-squat.

In layman's terms, ackerman is the steering angle difference between the right and left front wheels when turned. This has important implications in drifting and in road course work. When turning normally, the wheel to the inside of the corner turns a smaller circle than the outside one. This makes the ideal angle between the two different. In long sweeping corners, low ackerman is advantageous, while tight hairpins require more positive ackerman. Drifting has the same requirements, except there are situations where you are turning much farther than a roadcourse car would be. Too much ackerman in high speeds corners (or at high steering angles) means that the wheel with less angle is scrubbing, which slows the car down and increases tire wear. Too little ackerman in tight corners means the opposite, while turn-in speed is reduced and more input required.

Basic image of ackerman reduction:



Now let's talk about steering axis inclination (SAI). This is also known as kingpin inclination or kpi, but since cars haven't had kingpins since the 30's, I feel like we should use the proper term. This is another thing that is very easily misunderstood, as the kinematics are quite dynamic throughout the steering range.
For the purpose of our discussion, I'm going to talk about SAI as if it includes caster. By definition SAI is the angle of the suspension from the top mount of the shock through the ball joint when viewed from the front only, but since our cars are always going to have positive caster and our goal is to understand how the suspension angle effects the geometry throughout steering lock, it's easier to condense the two.
Let's start with a car going around a track in standard racing fashion. Front camber while steering is determined by the SAI, and with positive caster, this means the outside wheel gains negative camber and the inside one positive. This is very beneficial in typical racing, as the outside wheel needs negative camber to offset body roll and tire deflection. This can obviously be adjusted so as to be optimal per setup. In drifting, quite the opposite is true. Countersteering into a corner means the lead wheel gains positive camber and the trailing negative, leaning the tires up on their edges and reducing contact patch. This effect also makes the steering want to self-center, as the car lifts in the front and the weight of the car wants to push the steering back to center. There is a balance to be struck here, and it largely depends on how the driver likes it. Professional drifting has moved towards reduced caster and therefore flatter contact patch at big steering angles. This increases controllability at speed and allows for effective braking. A large amount of grassroots drivers prefer standard or even increased caster though, this makes the car self-steer more aggressively, and can help with hucking big backwards entries. The front tires will start to slide and then smoothly return when correcting, which makes the entry look much smoother.

There is also caster trail to consider. With the need for larger front contact patches and less static caster, the steering can lack feel and lead to a driver having to steer excessively, pulling the wheels away from lock and manually returning the wheels to center. By setting the hub farther back in the knuckle (front to rear), the caster trail can be increased, meaning that the steering still wants to return to center even at low static caster angles. I borrowed this image from interia-ms.com as it illustrates it nicely:



PSM, Wisefab, and TDP, and Driftworks all have knuckles with increased caster trail available. GKtech has one in development. PSM has a good side-by-side so you can see the difference. Trailing knuckle on the left and standard on the right.



With the increased prevalence of reducing static caster angles, companies have developed offset strut tops. This allows the tension rod to be used as a fine adjustment, and keeps the wheel more centered in the well.

GKtech has two versions, one that you need to cut the top of the tower to fully utilize, and another thicker one that bolts in with no modification:





TDP's is similar to the second GKtech:



PSM has one available for their coilovers specifically:



Wisefab was pretty much the originator of the idea, first with their fixed position "non-FD legal" tops, and then with their adjustable "FD legal" variant. Fixed position is pictured above in the picture of the full kit (Part II, 3.).

There are several ways to go about getting more steering angle; like tie rod spacers, steering rack spacers, and obviously modified or new knuckles.

Classic steering angle spacer:



Big steering angle spacer:



There are an incredible amount of companies that offer modified knuckles now, including:

MA-Motorsports modded:



Billspeer 555 Knuckle:



Abercrombie:





Driftworks claims 65 degress of steering lock on their V2 Geomasters:





Parts Shop MAX has 65+ degrees of steering lock:





Wisefab also claims 65 degrees of lock:



And their ackerman adjustment spacers:



Track Day Performance claims up to 70 degrees of lock:



GKtech claims 70 degrees of lock with their V3 knuckles and LCA combo:

Another thing that's rarely mentioned when talking about all this increased angle, is the tendency of the steering rack to go "over center". This is where the angle of the tie moves farther than parallel with the steering arm of the knuckle.



This can be alleviated a couple ways:

Bump steer describes the toe change due to suspension travel.
This is caused by the angle of the tie rods and the distance between the tie rod ends changing when the suspension travels. For no bump steer it would be ideal to have the tie rods allways beeing horizontal, but because that is not possible we're trying to get as far as possible to horizontal every time.



The problem on lowerd cars is, the more the car is lowerd the more bump steer will occur.



This can be fixed through different options. As an example tie rod ends with longer spine (SPL below)



Also drop knuckles (nearly all knuckles shown above) help alot with this because the car naturaly sits lower. So the tie rods remain in a more horizontal position.