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)?

 

Are you sure about the ROW number?

 

Here are the X73 settings, where they're different from the others:

Front camber: -45' +/- 15'
Rear camber: -1D40' +/- 15'

I'd go with X73 or GT2 settings.

 

Yes Dock.
This further supports my argument. Remember this is the toe DIFFERENCE at 20 degree steering lock. Since all components, that could potentially affect geometry, are equal in the USA and ROW TT's, except the length of the springs, the ROW car is experiencing a greater change in geometry at the limits of its range (due to the lowering). Porsche recognizes this and that is why there is greater allowance.

 

Dock,
More negative toe results in more twitchy handling. The -2D 20' of the ROW car says to me it will be more twitchy at 20D lock and is a result of the limitations of the rest of the common components between the ROW and USA cars.

 

What tires are you running? Did you increase the front wheel width or offset? What about ride height? Have you driven it with stock wheels/tires to see if that helps.
I'd bet you need to fine tune the toe in, possibly with trial and error. Maybe increase the front toe in.

 

Then why is the X73's toe difference @ 20D lock -1D50' +/- 30'? It's less than ROW's -2D20', but is lower than ROW.

 

Dock,
As engineers, sometimes we find ourselves taking printed data as gospel and then justigyin git somehow. Maybe the documentation I have is flawed? Maybe the toe diff. should be the same between the USA and ROW turbo.
Can you please post the alignment specs for the X73 in their entirety as I did above?
Thanks!

ebaker,
I am running 235/35/19 and 315/25/19 Yoko AVS Sports. Wheel width was increase 1/2 inch over stock in both front and rear. Offsets...not sure. No longer own the stockers and they wouldn't fit over the big brakes anyway. I think toe has something to do with my problem.

 

Ken
Just a little insight. I find the same problem with straight line stability when I load down my MiniVan (don't laugh). The weight is mostly in the rear and causes the rear to squat more than the front. Camber change, caused by reduction in axle height, would seem to be the cause. Toe would not come into play in my senario.
I would also be interested in varition of ride height vs recommendations, with your 19 wheels.

 

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?????????

 

From the 996 Turbo repair manual group 44-1 page 2...

X73 alignment specifications.

FRONT

Front Toe Unpressed (total) = +5' +/- 5'
Toe difference angle @ 20D lock = -1D50' +/- 30'
Camber = -45' +/- 15' with a max difference left to right of 20'
Caster = 8D +/- 30' with a max difference left to right of 40'

REAR

Rear toe per wheel = +10' +/- 5' with a max difference left to right of 10'
Camber = -1D40' +/- 15' with a max difference left to right of 15'

 

WOW


I'm definately no engineer but a little explanation from Tom at Lucent helps me understand some of this info. The knowledge you guys are providing is great. Thanks for the info, I'll keep reading.

 

Is positive (degrees) toe toe-in or toe-out?

 

Positive degrees is toe in.

 

I'd try increasing the front toe-in. If you set the toe to +9' per side that's only 1/16 inch total toe-in. Your wheels probably have more outboard offset than stock, you may need a little more toe to stabilize it.