KPV

I have been struggling with a handling glitch recently and I figured I would post some information in an effort to collect some ideas.

The Problem
When I am in a fast corner (60-80mph) OR at very high speeds (130mph+) in a straight line, I find a need for constant steering correction.

In a corner, I find myself continually correcting for any slight undulations in the road surface. Constant attention is required. I tested this the other day and found, in a constant radius turn, the lift-throttle oversteer occurs as predicted and the effect is diminished when cornering while accelerating (Read unweighting front suspension while corning)

In a high speed straight line, I find the car to be very twitchy as if the car was caught in the truck tire ruts that commonly occur on highways. The steering requires constant attention.

My Theory and Some Background

I believe the effect I am experiencing is bump steer. For those that don’t know, bump steer is the steering effect that results from changes to the suspension geometry as it goes through its range of motion (generally compression). It becomes more and more prevalent in lowered cars. The reason for this is that the geometry of the lower control arms is altered by lowering. As an example (actual values are different) if the factory ride height USA TT has control arms that angle down 10 degrees towards the wheel when the car is at rest, their range, through compression of the suspension, may bring them to a horizontal position. Conversely, a lowered ride height USA TT may start with the control arms horizontal and at 10 degrees above horizontal at full compression. The tip of the control arm at the wheel will either be moving away from or towards the chassis as a result of suspension movement as it follows its arc of movement. Since the control arms are located behind the steering rack tie rod ends the following should theoretically occur (please keep in mind these are generalizations):

Factory downward sloping control arm
Undergoes a positive toe change through suspension compression

Lowered (assumed) level control arm
Undergoes a negative toe change through suspension compression

In my old 280Z set up for the track, we actually shimmed beneath the steering rack to preserve the control arm angle geometry.

When I lowered my TT with the Euro version PSS-9’s, I told the shop to set it up per the GT2 ride height and alignment spec. Before I get flamed, my choice and direction were simply because I figured same chassis, same ride height, same geometry rather than arbitrary values for each. At the time, I was unaware that the control arms were different lengths as well as other aspects of the suspension geometry. I confirmed all of the final settings met spec.

Remedial Plan

I have an appointment with the alignment shop for next Tuesday, April 27, to check and change the geometry. I am thinking of going to strict TT spec. (USA or ROW not sure)

The alignment geometry for the three cars are as follows:

Front Axle

Toe unpressed (total)
USA TT: +5’ +/-5’
ROW TT: +5’ +/-5’
GT2: +5’ +/-5’

Toe difference angle at 20 degree lock
USA TT: -1D20’ +/-30’
ROW TT: -2D20’ +/-30’
GT2: -1D50’ +/-30’

Camber (with wheels in straight ahead position)
USA TT: 0D +/-15’
ROW TT: -30’ +/-15’
GT2: -1D +/-10’

Max. Camber difference, left to right
USA TT: 20’
ROW TT: 20’
GT2: 15’

Caster
USA TT: 8D +/-30’
ROW TT: 8D +/-30’
GT2: 8D +/-30’

Max. Caster difference, left to right
USA TT: 40’
ROW TT: 40’
GT2: 40’

Rear Axle

Toe per wheel
USA TT: +10’ +/-5’
ROW TT: +10’ +/-5’
GT2: +10’ +/-5’

Max. Toe difference, left to right
USA TT: 10’
ROW TT: 10’
GT2: 10’

Camber
USA TT: -1D25’ +/-15’
ROW TT: -1D25’ +/-15’
GT2: -1D50’ +/-10’

Max. Camber difference, left to right
USA TT: 15’
ROW TT: 15’
GT2: 15’

Vehicle Height

Front Axle height with 18” wheels (from road surface to lower edge of hexagon-head bolt(a/f 18) of the tension-strut screw connection to the body)
USA TT: 158mm +/-10mm
ROW TT: 138mm +/-10mm
GT2: 118mm +/-10mm

Rear Axle height with 18” wheels (from road surface to locating bore in the rear axle side section (between toe and camber eccentrics))
USA TT: 158mm +/-10mm
ROW TT: 148mm +/-10mm
GT2: 133mm +/-10mm

Request from you guys

So, with all of that, my questions are:

1. Where is Cary? I really need to talk to you on the phone!!
2. Does anyone have any advice or insight?
3. Should I follow the USA TT spec or ROW TT alignment spec since they are based on same parts only lower springs (ROW specs seem more appropriate in my opinion)?

SPR

sounds like your front toe is off for sure, check the individual toe and total toe

KPV

SPR,
I looked at my specific alignment output.

FRONT:
Left Camber = -1D 0'
Right Camber = -1D 0'
Cross Camber = 0D
Left Toe = +0D 3' 0"
Right Toe = +0D 2' 24"
Total Toe = +0D 5' 24"

REAR:
Left Camber = -1D 54'
Right Camber = -1D 48'
Left Toe = +0D 7' 48"
Right Toe = +0D 7' 12"
Total Toe = 0D 15'

All of these values meet the spec for the GT2 setup. As a matter of fact, I could not be more pleased at how close they got everything to what I told them. I am very happy with Eurotire.

So, now what?????????

SPR

Interesting then bump steer in mid corner might then just be your issue. I have also read about the shimming of the rack and pinion. It would be interesting to see what angle the tie rods come out at in relation to the horizontal on a stock turbo and then measure yours. And or have someone jack up an end up and down to see the range and if it is creating enough resistance. You should speak with trg, or Stever Werner, I have found him extremely knowledgable

http://www.rennsportsystems.com/2.html

www.theracersgroup.com

Steve Weiner-Rennsport Systems

Ken:

From your description, I'd say that you are getting some bump steer.

When you go back to the alignment shop, measure the bump steer (toe change) from 1.5" in droop to 1.5" in bump on the machine and record the results.

You might need the ERP special tie rods that permit correction at each tie rod end; the steering rack cannot be moved on these cars.

E-mail me directly and I'll send you a picture of them as we do these all the time.

viperbob

Ken,

What is your vehicle height at the moment? I see you alignment settings, but knowing how much your a car has been lowered would be helpful...

KPV

SPR,
I am beginning to think I may have two problems. The high speed twitchiness is probably due to the front wheels being thrusted beyond 0 toe to a slightly negative value. The mid-corner steering input neecessity is probably due to bump steer.

Steve,
All of the mid-corner corrective steering seems to point to bump steer. I will specifically ask them to measure the toe change as you indicated. I assume this is achieved simply by jacking one corner's suspension?
I have sent you n email requesting the picture of the ERP special tie rods. Please send it as soon as possible.

Viperbob,
The vehicle height is GT2 spec. See first post in this thread.

boqueron

Ken, How did you finally solve your problem ? Did you change to TT spec ? If so, did you keep the TT heighth and, if so, which stiffness settings on your Pss9' s ?

Doug H

He went with Motons I believe. They are slick and he seems ery jappy with them. Check out the 6speedonline.com board for a write up. They sound like the ****, but they are apparently very pricery for a suspension set up. Look for "My last major mod" thread.

KPV

For those interested in the final resolution of my suspension issues, please reference te following:

Suspension Resolution Writeup


Track Testing of New Suspension