View Single Post
Old 12-23-2009, 10:07 PM   #1
PoorMans180SX
Zilvia.net Advertiser
 
PoorMans180SX's Avatar
 
Join Date: Nov 2005
Location: Michigan
Age: 32
Posts: 5,003
Trader Rating: (16)
PoorMans180SX is close to perfectionPoorMans180SX is close to perfectionPoorMans180SX is close to perfectionPoorMans180SX is close to perfectionPoorMans180SX is close to perfectionPoorMans180SX is close to perfectionPoorMans180SX is close to perfectionPoorMans180SX is close to perfectionPoorMans180SX is close to perfectionPoorMans180SX is close to perfectionPoorMans180SX is close to perfection
Send a message via AIM to PoorMans180SX
This is the roll center, suspension pickup, and steering angle modification thread.

Disclaimer: Modifiying a cars suspension in the ways discussed in this thread will always result in some compromise. Increased steering angle makes things rub, raising the subframe increases anti-squat, etc. Please read through and understand the concepts before you go hacking up your car in pursuit of performance. All of the suspension components are affected when lowering the car, especially when lowered to the extreme. Correctly modifying the suspension pick up points or knuckles to correct for these changes can reap large rewards, including better camber and toe curves, less body roll for a given spring rate/anti-roll bar, and more traction/grip.

This thread is focused mainly on the S-chassis, with some Z and R chassis thrown in for good measure. There are many more brands than mentioned in this thread, but I've tried to include the most original and functional options. Feel free to contribute more.


Part I: The Basics.

The Front Suspension:

Tension rods: These are used to adjust caster, and together with the lower control arm pivot points, determine anti-dive and anti-lift in the front suspension. Positive caster like the S and Z chassis have gives positive steering feel, and helps the steering return to center automatically from lock. The angle of the tension rod and lca (horizontal plane, front to back) determines anti-dive under braking situations and anti-lift on acceleration.

Tip: If you’re using your tension rods to adjust caster more than one or two degree(s) past factory settings, there will be bind in the lower control arm's traditional rubber or urethane bushing. It’s recommended that you change to an aftermarket control arm with a heim joint/spherical bearing.

Front lower control arms: Determine your roll center (basically), track width, and camber curve of the front suspension. The arc these move in is the camber curve. Together with the tension rod they form an A-arm.

Tie rods are used to adjust toe in the front, and obviously to steer your car. Aftermarket tie rod ends can be used to adjust bumpsteer by spacing the tie rod and changing it's relationship with the control arm.

The Rear Suspension:
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.

Rear upper control arms: Used to adjust static camber. Together with the lower control arm form the curve which the knuckle travels in vertically, gaining increasing negative camber with suspension compression.

Toe rods:These control static toe and toe change during suspension compression. When lowered aggressively, our cars toe and camber curves are significantly changed, becoming “steeper” (more change with less movement).

Traction rods: Determine caster (or thrust angle) on the rear suspension. For most of us, that doesn't mean much. What matters is that they have a large effect on the toe curve, or bumpsteer of the rear suspension. A good rule of thumb is to lengthen these just as much as you lengthen your RUCA to minimize bumpsteer. This also slightly reduces anti-squat, which can be beneficial or detrimental depending on your chassis and setup.

Rear lower control arms: Help determine roll center, anti-squat, and camber gain. Can be used to adjust static camber as well as the camber curve over the suspension stroke.

Anti-squat: Anti-squat is probably the single most misunderstood aspect of the rear suspension. Anti-squat by definition is the resistance to squat the rear suspension posses by way of diverting force through the suspension links. The higher the percentage the more resistance to suspension compression. Most people incorrectly assume that squat equals weight transfer to the rear, and thus increased traction. The truth is, multi-link rear suspension is vastly different than a solid axle, and squat actually reduces rear weight transfer, and takes up time compressing the suspension, where it must be damped by the shock. It is true that in traction limited scenarios, such as gravel or snow, we do want very little anti-squat, but on dry roads or the racetrack, this is not the case.
To determine anti-squat, we must first draw a line from the overall center of gravity of the vehicle through the centerline of the rear wheel. This is called the neutral line. Then we must draw lines from the rear control arm pivot points forward. Where these lines intersect, and whether they are above or below the neutral line, determines the percent of anti-squat. If they are below the neutral line, there is a negative percent of anti-squat (or pro-squat), and if they are above there is a positive percentage. Around 120% is ideal. Here's a nice GIF and a couple images Motary put together.:



Here's S14 anti-squat:



And with GKtech rear knuckle:



Unfortunately for 90's Nissans, we are stuck between two extremes. The S13 (and Z32/R32 for that matter), have a huge amount of anti-squat (somewhere around 180%), which means engine torque is very directly transferred to the tires, often shocking them into breaking traction earlier than ideal. The S14 and S15 (and R33/34), have a very small amount of anti-squat at standard height, but this quickly goes pro-squat when the car is lowered. This means the car squats excessively, changing the suspension geometry, reducing traction, and delaying the cars reaction to inputs to the throttle. Fortunately, the overwhelming aftermarket support for these vehicles have come up with some very nice solutions, which I will present below.

Roll Center:

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.



Center of Gravity:

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:

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.

Part II: Roll Center Adjustment

Here are the things you can use to adjust roll center:

1. Use a ball joint with a shank that is longer, thereby spacing the pivot point at the knuckle farther down.

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:



2. Purchasing aftermarket lower control arms that have roll center adjusting heim joints and shanks.

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.



3. 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:






6. 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. SPL provides a very good link on how pushing the Subframe affects anti-squat and roll center:

http://splparts.com/instructions/SPL_SSB_S13C.pdf
__________________
Whalen's Speed Research & Development
Racemind: racing setup and alignments

Build: http://zilvia.net/f/showthread.php?t=643065

Hit me up for BorgWarner turbos/AN PS lines/Power Steering kits/Brake line kits/Link ECUs!

Friends don't let friends buy knock-offs.

Last edited by PoorMans180SX; 11-19-2017 at 11:08 AM..
PoorMans180SX is online now   Reply With Quote
Sponsored Links