Steering alignment advice

[Breeang]

I have rebuilt the front suspension on my '77- bushings, tie rod ends, ball joints ,shocks etc. When I took it for an alignment at the Porsche dealership the tech, who did the alignment -(and who also happens to be my son ) said he couldn't dial in quite enough castor angle. The car tracks and handles well except it tends to wander a little if you don't pay attention, but it used to be rock steady in a straight line. He is experienced working on the newer cars but mine is "older than he is". We are out of ideas, is there something that might need special attention, say with a large hammer, to reach the castor angle spec.?

[tobluforu]

Can you post the specs; is it just one side, both?

[Driven97]

Please post specs.

Toe tends to affect straight line more.
Castor affects more the self-centering force.

[KTL]

Did you remove all the black hardened tar crud on top of the strut mounts?

[stlrj]

Originally Posted by Breeang

...he couldn't dial in quite enough castor angle. The car tracks and handles well except it tends to wander a little if you don't pay attention, but it used to be rock steady in a straight line.

If your 911 has been lowered significantly in the front to where the chassis has an excessively nose down attitude, the castor angle would be too low and impossible to correct unless the front end were raised back to a more level or normal attitude. The adjustment at the top of the tower is of very little use for large discrepancies in castor.


Cheers,

Joe

[Breeang]

Sorry about the delay in response. I can't track down the printout, but as I recall it the castor angle was very close on both sides but just needed a tad more. The car is being driven daily but just needs a liitle attention to keep a straight line.
KTL-- where is this hardened tar crud of which you speak ? (Since I don't seem to know I obviously didn't remove any) and how does that affect castorr?
stirj-- I did lower the front a little, to euro specs, and added the bump steer spacer kit, but it doesn't look terribly nose down.

[javadog]

Most likely, you have lowered the car too much. In this case, just make the castor equal on both sides. If you want more castor, raise the car.

The "black tar" is what the factory used to waterproof the holes in the inner fender where the top strut mounts fit. This is seldom cleaned out and replaced after an alignment. Usually, what the factory put there when the car was built is still there. Once the suspension has been adjusted from the factory settings, this crap isn't doing much good.

JR

[KTL]

Castor is typically desired for steering effort and helping with the self-centering of the steering wheel, but has little effect on tracking. The self-centering is when you turn the wheel & instead of having to straighten the steering wheel, angle of the strut & forward motion of the car "unwinds" the steering wheel itself and the car goes straight.

Wandering seems like a toe issue to me. If you guys were shooting for near zero toe or a bit of toe out, the car loses some straight line stability.


If your toe checks out, you may have what's called a "radial pull" induced by a tire? Sometimes the finished alignment specs-out good but will leave you with a little bit of pull.

Instead of using cross camber to correct the pull, its better off left alone. I believe the reason being is because the cross camber can affect tire wear and shops don't want you coming back saying that a bad alignment job was done, look at my tire wear? So they choose the lesser of two evils- a bit of a pull vs. tire wear they have to defend. In your case since you know the guy, maybe he'll give your money back if you just go away?

Yeah its a good idea to find something to replace the tar goop to keep water out of the trunk. Rope caulk (like non-hardening clay) used for window sealing works but its typically white & looks sorta crappy on non-white cars.

[Breeang]

"Self-centering" is defininitely what my car now lacks. I shall follow all the excellent tips here and remove the crud and raise the nose back up a little. Thanks for all the help guys.

[johnsjmc]

I also believe not enough toe. It's very easy to add a little toe in and re test drive the car. I would do this before changing anything else. I would even do it by feel ,ie not bother with the alignment rack , just crank in a little toe on one side and test drive. (Done to one side will shift the steering wheel centre but will quickly tell you if it,s the problem.

[Driven97]

Originally Posted by Breeang

"Self-centering" is defininitely what my car now lacks. I shall follow all the excellent tips here and remove the crud and raise the nose back up a little. Thanks for all the help guys.

So when you're exiting say, a neighborhood from a stop turning right, you have to physically unwind the wheel straight? That's insufficent castor.

If you're on the freeway and have to constantly put small inputs in to stay in the lane, that's insufficient toe.

[johnsjmc]

Remember it appears the car used to track straight before the rebuild of the front end. Unless you have bent something (crashed) the caster angles were probably a little low before and the car still tracked straight. It,s likely toe related and easy to fix.
The tar mentioned is sealer on the upper strut mounting bolts. I expect your son would have seen that and removed it if needed to move the struts. Euro height isn,t low enough to reduce caster significantly below spec.

[Jesset100]

So, if he needed (wanted) to dial in more caster beyond the factory max how would it be accomplished?
Just asking

[mar2mar]

Same thing happened to me after aligning my car with a complete front end rebuilt. Turns out the front wheel bearings was a bit loose on both sides.

[Driven97]

Originally Posted by Jesset100

So, if he needed (wanted) to dial in more caster beyond the factory max how would it be accomplished?
Just asking

Some aftermarket camber plates have a slightly smaller spherical bearing than the stock rubber donut. Or he could enlarge the hole. Or use an offset balljoint to move the lower pivot forward. Or use some custom A-arms to do the same. Or all of the above. Some are better ideas than others.

Caster is moving the pivot point forward in relation to the contact patch. Not unlike the wheels on your office chair of the same name, the difference in points creates a lever arm for forces to act on the wheel and make it want to "follow."

Another neat benefit is that since the axis is tilted back, you get kinematic camber gain with wheel angle. Think of a chopper motorcycle with extended forks - crazy caster. Turn the wheel, and it leans way over. The 911 has a super simple MacPherson strut front suspension, which has very little camber gain. A little bit of caster gives you camber when you need it. Cool, right?

This Mercedes has a lot of caster showing what I mean:



Also a bit of cool classic car trivia - the legendary E30 M3 was heavily revised in the front suspension to have a lot more caster than the rest of the E30 line.

On the flip side, too much caster increases steering effort as you have to "lift" the wheels, especially with wide tires. Also too much makes the steering feel vague, as the axis becomes too separated from the contact patch.

tl;dr the 911 doesn't have that big of an adjustment range. It shouldn't make that much of a difference where you set it.

[javadog]

[QUOTE=Breeang;7776873 I shall follow all the excellent tips here... and raise the nose back up a little. [/QUOTE]

Before you do that, you might want to show us a picture of the car as it sits now. A look at the post-alignment numbers would also be helpful.

JR

[stlrj]

Originally Posted by Breeang

"Self-centering" is defininitely what my car now lacks. I shall follow all the excellent tips here and remove the crud and raise the nose back up a little. Thanks for all the help guys.

Raising the nose is the easiest most effective way to increase castor. It has other benefits too, like reducing steering effort and understeer.

http://www.youtube.com/watch?v=lMBwto6QCF0


Cheers,

Joe

[javadog]

Raising the nose without regard to the height of the rear, is what I was cautioning against. Then there's the issue of corner balancing, which may have not been considered.

JR

[johnsjmc]

Originally Posted by stlrj

Raising the nose is the easiest most effective way to increase castor.

Unless the nose is down like a jacked up "Starskey and Hutch" hotrod that is unlikely the problem. Look at simple first and add some toe in.
If I recall the spec for the old chassis(torsion bar) is something like 6 deg pos castor. Anything above about 4 deg should be sufficient to drive straight.
I,ve seen 3/4 deg difference from one alignment rack to another on the same car and same day.

[johnsjmc]

Originally Posted by javadog

Raising the nose without regard to the height of the rear, is what I was cautioning against. Then there's the issue of corner balancing, which may have not been considered.

JR

Corner balance. HaHa . Last time I was in our local P car dealer (newly opened last year). I asked if they could do corner balance, and/ or kinematic toe on my 993.
I am a retired tech and have access to an alignment rack and own corner scales but don,t have a kinematic toe guage.
Dealer service manager and head tech didn,t even know what I was talking about.
In the case being discussed I haven,t heard any indication of gross corner balance issues like early brake lock up on one side or pulling while driving.
Again keep it simple . Add toe in first .