Yaw axis instability in straight line at 80

Guys,

Yesterday I noticed some serious straight line yaw axis instability at 80 on the free way in my 73 T coupe with a 2.7 MFI engine.

When I got home I jacked up the front end and did the 12/6 o'clock push pull on the front wheels. At 12/6 I got noticable free play but at 3/9 o'clock things were tight. The car was aligned 2 months ago and seemed alright when I got it back.

I have 205 15 Yokohamas on 15x6 fuchs. There was a bit of wind but the instability seemed to be the same no matter what direction I was travelling. Changing lanes and interstates did not make a noticable difference

Other than worn ball joints what other things cause high speed straight line instability? Do worn ball joints sound like the culprit here?

Thanks
John, proud member # 322
73 S with sporto and 73 2.7 coupe

Quote:

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Originally Posted by caproader

.... what other things cause high speed straight line instability?

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Insufficient castor.

How much castor did your alignment report show when you had it aligned?

John:

General instability is usually caused by unwanted toe changes either up front or at the rear.

Assuming for the moment that you have dealt with any bump steer issues,....this can be due to bad or improperly adjusted wheel bearings, wrong toe settings, tie-rod assemblies, ball joints, loose hubs (rotor mounting bolts), worn or seized bushings, and a bad tire or two.

Caster settings rarely causes this unless its at or below 5 degrees.

Time to do some serious checking (and if you haven't measured bump steer, its time to do that first).

Hope this helps,

Steve,
Thanks for the tips. Here is a quick summary:
Bump steer kit installed

Bearings adjusted (No free play when wheels push/pulled at 9/3 o'clock)

Alignment done by string at Wright Tuning, Nov 2005. Therefore, no print out of figures. However as John manages a number of racing teams, I trust his work. I do not have the figures he uses here in the office.

Turbo Tie rod kit installed prior to Jan 2006 alignment. As I remember, I also installed new ball joints at that time.

"A" arm bushings may be a factor. When I rebuilt the suspension, I reused the orig A arm bushings to preserve ride quality, but rotated them 180 degrees. Car has 99k miles

When I push/pull the right front wheel at 12 and 6 o'clock (car on jack stands), I get a definate 'clunk' as opposed to a mushy bushing sort of squish.

My plan is to:
Corral neighbor kid to push pull/wheel while I crawl underneath and inspect for movement.
Replace 205 tires with a 195 set from the other car, then test drive
Check results with John Wright
Replace ball joints due to 12/6 clunk mentioned above, then test drive
Based on movement inspection, replace upper stut mount at top of fender well.
If all else above fails, replace A arm bushings with aftermarket.

thanks again for the direction.

John

Hi John:

FWIW,......One checks wheel bearing freeplay by gripping the wheel/tire at the 12 & 6 position. If it "clunks" more than a little bit, its time to pull the bearings/races and carefully clean and inspect them. Further, worn spindles will do the same things and that requires new struts.

Ball joints cannot be "felt" by the above method; one must use a big prybar and feel any vertical freeplay when the ball joint is unweighted by VERY carefully prying upward between the bottom of the strut & A-arm.

A-arm bushings really cannot be rotated for effect. If they are sagging or otherwise worn, they should be replaced. As you know, Porsche doesn't sell these separately; these come bonded onto new A-arms and those are not cheap. We've had the best luck with the Elephant Racing Poly-Bronze greasable bushings as their ride quality is FAR superior to any kind of plastic ones.

Hope this helps focus your efforts toward resolution. Make sure that you have about 1/16" total toe-in at the front end (un-pressed).

Steve,

Thanks again for the thoughts.

Now I'm embarrassed to have to quote Emily Latella from SNL:

Never Mind.

When I went home last night, I re-checked the bearing adjustment. Sure enough, they were loose. After I tightened them and took the car out, a large part of the problem went away. So now how do I figure out if the spindle is bad?

The struts are used Bilstein 3.5" that I used to install aluminum calipers. When I put them on last November, I inspected the spindle for scoring, cleaned and greased the bearings using the same races, and then adjusted the bearings.

John

Hi John:

Ahh,..I'm glad you found that,....:)

Determining spindle wear can be tricky since thats a judgement call and takes experience to inspect & measure to see if that contributes to the problem.

When the spindles are worn, one cannot adjust the wheel bearings sufficiently to control the unwanted toe changes that cause the instability that you were feeling (how that for a run-on sentence?). :) :)

When in doubt, take a micrometer to a new one and measure the diameter & taper in 3 or 4 places to compare it against the one in question.

Steve,

Once again, thanks for the great thoughts. For now, I think that having adjusted the bearings, I'll run the car for a hundred miles or so and then see where things are.

Thanks again. I'll check back in in a hyndred miles.

John