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Old 12-23-2009, 09:08 PM   #2
PoorMans180SX
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This is the roll center, suspension pickup, and steering angle modification thread.

Part III: Steering Angle Modification

Let's talk about steering geometry for a bit. There's a lot to consider when you're picking out parts for steering angle. We now have tons of options. When I first made this thread, it was pretty much cut n' shut knuckles or Driftworks.

Ackerman in the steering:

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 road course car would be. Too much ackerman in high speed 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 number 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.

Caster trail:

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.



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 have been plenty of companies that have come and gone over the years, making their own version of modified or new knuckles for the s-chassis:


Naoki Nakamura's now infamous B-knuckle:






MA-Motorsports modded (no longer available):



Billspeer 555 Knuckle (no longer available):



Abercrombie (no longer available):





Driftworks claims 65 degrees 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 (No longer available):



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



Overcentering:

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 Image borrowed from interia-ms.com



This can be alleviated a couple ways:

1. Moving the steering rack forward:







2. Spacing the tie rods farther forward, using spacers like these Driftworks ones:

Version 2:



Or the Parts Shop MAX version:



and GKTech:



These do the same thing as the eccentric bushings, but have more correction. And there have been talks of solid steering rack bushings warping and becoming loose.

NOTE: These could put stress on your rack that it was not designed for.


3. Using Parts Shop MAX steering rack eccentric bushings:







Dan gathered some interesting information about how moving the rack forward effects ackerman angle:




4. Once you're using high-clearance tension rods/control arms, you'll need to worry about your trailing wheel hitting your sway bar. There are two solutions for this:

The GKTech high clearance sway bar:







And the Bink Industries high clearance bar, available in street and Pro versions, and residing on quite a few Pro and Pro-am cars. These come with endlinks, D-bushes, and brackets.

Street:







Pro:







Parts Shop Max has a high-angle sway bar as well. Comes with brackets, bushings, and locks.





Depending on your knuckle/LCA/tie rod end combo, you're probably going to need longer tie rods. Here's a nice list of lengths:


Inner:
M12x1.25:
11.10" - '89-'94 240sx (S13)
M14x1.5:
11.37" - '95-'98 240sx (S14)
11.42" - '94-'99 Maxima (A32)
11.42" - '96-'99 I30 (A32)
12.00" - '97-'01 Q45 (FY33)
12.00" - '01-'06 Q45 (F50)
12.91" - '99-'03 Maxima (A33)
12.91" - '00-'04 I30 (A33)
13.30" - '03-'08 Maxima (A34)
9.25" - PSM Short
11.73" - PSM Medium
13.93" - PSM Long
14.50" - GKTech

Outer:
M12x1.25:
3.88" - '89-'94 240sx (S13)
M14x1.5:
3.63" - '95-'98 240sx (S14)
5.00" - '88-'94 Maxima (J30)
x.xx" - '94-'99 Maxima (A32)


Let's discuss.
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Last edited by PoorMans180SX; 05-27-2021 at 07:27 AM..
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