im not one of those people to be sure, but im still here mike. with my good eye on you...;)

Factory spec begins with factory height. If that's off and no longer at factory height, then all bets are off! Lowering the suspension with the requisite nose down attitude that looks so cool creates all kinds of alignment havoc that cannot be addressed unless the chassis is brought back to factory specs.

Unfortunately, we all expect the alignment guru to perform a miracle when we hand him an impossible situation and then blame him for being an idiot!


Cheers,

Joe

I understand there will be limitations achieving factory alignment specs especially related to camber. I'm just surprised I can't meet the caster spec.

Even if the rear end is jacked up 1/2" which it's not, that would be less than 3/10th of a degree of caster change, not 8/10ths.

To answer a previous question: the car is used for playing around on back roads, the occasional sunday afternoon drive, trackday, and autocross. I'm not trying to improve a lap time or any specific goal other than just making the handling balanced and stable at speed.

For now, I'll check the alignment at home and report back any problems I find.

For the future, I'll look at perhaps an RSR front bumper with splitter to reduce front end lift and bring the car back to more period correct bodywork instead of the 965 bumper.

David - It sounds like the changes you made have all been in the suspension, so the instability you are experiencing should be resulting from those changes. To cover a few things that have been mentioned.

Bump steer - this was the first thing I thought of too. With lower spindles, this could cause the symptoms you describe. As Mahler said, most people (even shops) don't bother to truly set bump steer. You can get pretty close though by making sure the tie rods are at least close to parallel with the A arms when the car is at its normal ride height. If bump steer is your issue, to state the obvious you should only experience it when the suspension is moving like when going over bumps. If it is way off for some reason, it can certainly make a car wander ...

Aero, tires, etc. - if you weren't experiencing this handling issue before your suspension updates, I wouldn't pursue aero to solve this. Same goes for tires, pressure, and other tweaks until you figure out why it's handling differently after your updates. You should be able to get your car to drive in stable fashion without having to make changes to other components/systems IMO.

Other suspension settings - maybe I missed it, but other suspension settings/specs could be causing the instability. Spring rates, swaybar settings, etc. Also shock valving, in case that was changed. Any of them being too soft in front or too stiff in the back could make the car feel "nervous" at speed. And are you sure your front wheel bearings are properly adjusted? Wheel bearings can take a set sometimes after some driving and require adjustment. i Any wiggle there could cause the car to wander.

Scott

David , after driving your car last week, I do not think adding caster to the front would help your situation . I think it is a multi answer task . Too stiff , ride height , ect. At 110 mph on the track
the only reason for instability was the car bouncing in the air ! . I love your car ! You have done an amazing job building it . Years ago I was asked to help with the handling with an old GT car . It had 6* of pos castor, I brought it back to 4.5* pos caster and it was a class winner . Call me and we can talk . Ian

2 quick questions since you'd done a fair amount of suspension work:

First, did you corner balance the car? Not balancing the car and getting uneven weights on the two front corners can cause an unstable feeling especially over bumps.

Second, have you tried disconnecting your sway bars? Especially the front bar. The sway bar drop links should be adjusted to eliminate any lateral preload, with the vehicle on it's tires. I always disconnect sway bars to align the car, if all preload isn't removed then the sway bar just acts like a lateral torsion bar and will cause an unsettled feel in the front end.

Thanks Ian, I will be lowering the spring rates from the current 300/450.

The car is behaving better now than it ever has during my ownership and certainly better than it has the last two years without these changes so I feel I'm heading in the right direction.

I did corner balance the car with the sway bars disconnected. I then reconnected after corner balance with the car's full on weight on it to make sure I wasn't preloading the sway bars.

Here's a relatively recent picture of the car. Notice how much front grill opening there is compared to the original '79 bumper with rubber air dam. Wondering if there's too much air passing through the grill and if an actual 965 has more restriction of air going through the two lower slots .

http://forums.pelicanparts.com/uploa...1553883856.jpg

If you were developing lift from air moving through the grills in the front, then every car with a FMOC and a duct to release hot air under the car would turn into an airplane/hovercraft.
I'm with most everyone else and think your stability concerns are alignment related and not aero.
**You can totally overcompensate with a huge splitter and smash the front into the ground at speed with negative lift...but it'll still want to wander/dart if the alignment isn't right.

I'm surprised nobody mentioned the easiest way to increase caster is to raise the front and reduce your nose down attitude. It's also the easiest way to reduce negative camber. That's how the factory does it.

However, that's the last thing anyone wants to do for some reason.


Cheers,

Joe

I finally got around to setting up strings to check the alignment and boy was I surprised! I had about 10mm toe out on the front wheels measured at the lip of the 17" wheels! I stood there the whole time the laser alignment was done in the shop and made sure everything was in spec so I'm not sure how it could be so far off.

I adjusted it to about 1mm toe out (I figured if I could drive it with 10mm I'd give 1mm a try). I took it for a quick drive up to triple digit speeds and it seemed much more stable. I still need to get out on some back roads with heavy crown, grooves, and other imperfections to really test it.

Moral of the story is make sure you know who's doing the alignment and how good their machine is. Then go home, set up strings (fishing line) and double check it.

Now that I'm comfortable with aligning at home, I'll play around with it more. Plus, I can get a good feel for how many turns on the tie rods per mm of toe so I can adjust at the track.

P.S. this explains why I've had almost no understeer for the last year and half.

I haven’t read the whole thread, but..

If you add a degree of rake, you lose a degree of caster.

If your fender lip heights are 25” front and rear, then you are about 1.5 degrees more raked than stock.

The front fender opening is larger than the rear. Stock rake will roughly give you 1/2” higher front fender lip height.

I do have more rake than I should since my rear tires are 3/4" larger OD than the fronts. I can drop the rears a little bit though which will help.

My math only shows 0.32° caster difference with a 1/2" change in front to rear ride height change over the 90" wheelbase. ATAN(.5/90) = 0.3183°

I think the vast majority of my stability problem was the toe out and not the caster though.

There have been a couple of times where a pro has messed up my alignment. But that was a long time ago, I would think the current alignment racks would be OK. But maybe not.
At any rate, I would make absolutely sure that everything on your front suspension is tight. Any looseness, in bearings or struts can cause the alignment to not be consitant. However, you say the handling is better, so maybe that was your problem all along.

Another thing to be mindful of before blaming the alignment is the tires.
A soft sidewall, or tall squirmy tread blocks can make the car feel very nervous at higher speeds, especially under braking or cross winds.

Tires are Toyo R888R so don’t think it’s a tire issue.

I found another reason for the excessive toe out. I didn’t realize changing ride height affects toe that much. Turns out when I raised my car a bit after the alignment (easy with coilovers) I should have re-aligned. Duh.

Ride height affects toe when there's bump steer. Correct the bump steer and the toe won't change.

One of these days I'll make a video on how to check and adjust bump steer. There are some specific tools and techniques needed to check but it's not complicated once you've seen it done. And just adding shims without checking is NOT the way to go. Changing the caster or camber WILL change the bump steer.