19mm raised front spindles - adjustable bump steer or no?

[phunt]

About doing something with your own car. I know that is not what you are saying. Sorry if I implied that. It is a different post and topic. I know you, Bill, are not doing that. I like your thinking and inputs.
I feel that the position of the ball joint reference to the horizontal plane of the t-bar has to be lower or the control arm drooping or angling down. That is what the “c” measurement is ensuring. I believe it references the ball joint and the pivot point not the axle/spindle and the pivot point. Because of the negative camber added as the ball sweeps up along the pivot arch moving the bottom of the strut out. You can also say it moves the top of the strut “in” that is what adds the negative camber. Lower beyond that point you can keep the tires contact patch square with stiff springs and ad a stiff roll bar. To me that’s a negative effect on the ride quality. I am a soft spring big bar guy.
I will admit I am NOT a Porsche guy. And I have come to find out I do not like MOST Porsche guys. My car is definitely a hot rod.
As far as my car it is NOT a 72. It is a 66. You can tell be the position of the torsion bar hole on the side of the well. Yes it has a “72” style oil tank.

[phunt]

I am no artist. Hope this shows what I am trying to say.
Ball joint below pivot sweeps up and out under compression and down and in under an unloading.
That will have good effects on the camber curve
Ball joint even with or above the pivot. It still sweeps up but now moves in. That is a bad impact on the camber curve.

[Bill Verburg]

Originally Posted by phunt

.....
I feel that the position of the ball joint reference to the horizontal plane of the t-bar has to be lower or the control arm drooping or angling down.

yes but he only way to change ball joint height is by changing axle height, a
2 ways to do that
1) change tire standing loaded radius
2) raise the spindle
here's an example of (1)
when I bought my C3 new 50yrs ago, it came w/ 205/50 x 15 tires on 7" Fuchs, for the P7s that the car came w/ a is ~281 so as delivered at the dealer a - b =108, b was 173. All the other new 911's had the same a - b spec of 108 but because the had 185/70 x15 w/ a = 320 , their b spec was 212mm. They all have the same steering and bump and RC geometry, as defined by a -b. That's what a -b does. But mine chassis height was 40mm lower w/ no other effects just because of the tires. It's obvious that a shorter tire lowers the chassis, but it's not immediately obvious what a -b is measuring, as it turns out after over 50 years of thinking about this on and off(mostly off) I cam to the conclusion of what the engineers were actually describing w/ a - b, they weren't in love w/ math(well maybe they were but that doesn't matter) they weren't trying to be obtuse(probably), it is simply a description of the geometry w/ max bump travel and min roll steer at what ever ride height b is used.

Originally Posted by phunt

That is what the “c” measurement is ensuring. I believe it references the ball joint and the pivot point not the axle/spindle and the pivot point.

by definition c = a - b, the ball joint height is nowhere in there, except that for a given a the corresponding spindle position determines where the ball joint height is. if the spindle is raised from it's stock position that is the only way to move the ball joint down w/o changing b

Originally Posted by phunt

Because of the negative camber added as the ball sweeps up along the pivot arch moving the bottom of the strut out. You can also say it moves the top of the strut “in” that is what adds the negative camber.

as the suspension compresses from stock static height the camber does go a bit more neg, but not by a lot and yes it does start to go the other way eventually. See the above camber curves, if the ball joint is lower than the t-bar yeas, this caused track width to increase, I have the curves for that as well, but if the a-arm is horizontal at static ride height then the track with is strictly decreasing w/ bump.

Originally Posted by phunt

Lower beyond that point you can keep the tires contact patch square with stiff springs and ad a stiff roll bar. To me that’s a negative effect on the ride quality. I am a soft spring big bar guy.

assuming that the static geometry is in a favorable place, then yes, the faster you go, the more force goes through the suspension and the more static camber you will want and the more you want to control geometry changes due to bump/droop w/ stiffer springs, But when the suspension is made stiffer you also lose mechanical grip. So I agree, you only stiffen it the min. that keeps the car in a comfortable place to drive it. It's another of those many compromises you implement to make your usage a better experience. The factory made their compromises to suit a broad audience of end users, An owner only has to please one person(ok, sometimes 2)

[phunt]

I’m with you. We are saying the same things. But, we disagree.
As far as Porsche making the car suit the masses. I say no way. It’s a Porsche not a Cadillac sorry Mercedes. They build it the way they want it and if you don’t like it they don’t care. It is a purposeful built car. I feel optimized. So I believe you can only lower the car so much. And that’s not much. That’s why we go with raised spindles to maximize it. I know that might not be enough for everyone.
The 911 suspension is basic and so simple. The point where the top of the strut mounts is fixed can’t change it. The t-bar is fixed as well it is where it is. The only thing that can be changed is raising or lowering the ball joint the only thing you can adjust. The distance from the ball joint to the top of the strut I will call “x” that’s what changes how for the strut can compress is fixed I know tire size influence it but a is also fixed. If you set the car up with the 108 “c” measurement and the only thing you change is the spindle placement. In my case raising them 19mm. A will stay the same x will stay the same. “C” will increase by 19mm and b will decrease by 19mm. Moving the chassis down bringing the wheel well down and closer to the top of the tire lowering the car 19mm. Inside that 108 “c” you could go to the +5mm range so make “c” 113mm that’s the max. So for me 19mm plus the extra 5mm means you can only lower a 911 24mm. An inch is a lot on my car. I think the reason Porsche sets a maximum “c” measurement of 124mm when loaded when 100lbs at the strut. Is to keep the geometry of the suspension. Again I think Porsche new what they were doing. Now sure you can absolutely lower the car more. And you can compensate for anything. I 100% say make it they way you want it. Yep you are the only one that you need to make happy.
Am I right? Maybe. Are you right? Maybe. But isn’t that the brilliance?
I appreciate all of your input. Thanks

[phunt]

The reason I was that I was reading this post was because I am concerned about the amount of travel I have left in my struts. Not much. Definitely not enough to use my dual spring progressive rate set up. I am afraid I will have to raise my car 😂

[Bill Verburg]

Originally Posted by phunt

The reason I was that I was reading this post was because I am concerned about the amount of travel I have left in my struts. Not much. Definitely not enough to use my dual spring progressive rate set up. I am afraid I will have to raise my car 😂

2 things will increase bump travel
1) raise struts to the max allowed by the wheels
2) shorten strut

of course the knuckle should be raised to correct roll steer

I just went through this on my car in this thread

alternatively, you could buy a shorter shock like MCS

also there is an option to install double A-arm

[Jonny042]

Originally Posted by phunt

The reason I was that I was reading this post was because I am concerned about the amount of travel I have left in my struts. Not much. Definitely not enough to use my dual spring progressive rate set up. I am afraid I will have to raise my car ��

Is this at ride height? I'd say that's a heckuva lot more than many people have. And that you may find it's fine, assuming from the pic you're still in the build stage.

[nickelplated5s]

Saw the oil door and made a wrong assumption, sorry. I am starting to get my head around this though. Mine is lower than most would want, i.e. RC & CG are pretty close. C is about spec. Arms about horizontal as pictured. The drawback is the travel. You don't get much with stiff TB's and the short throw shocks anyway. In a nutshell the spindle change allows for more travel and better handling with stock shock travel. I wouldn't want to drive cross country with mine but, boy is it fun when you push it.

Duck club past my fence is at it this am P.

[ccallana]

This has been a great thread from a technical perspective....

What I still am struggling with is what I would expect to feel behind the wheel if I've lowered my car by tweaking the torsion bar screw, but don't do anything else...

My 'b' is something like 20-30mm lower than the 197mm Bill called out earlier.. but not having any reference while driving, car *seems* fine, but maybe I'm fighting it more than I know...

its not cheap to have the spindles moved (is there anything in the Porsche world that is cheap??) - so trying to see if it is worth doing, or just keep driving with what I have.....

[Bill Verburg]

Originally Posted by phunt

I am no artist. Hope this shows what I am trying to say.
Ball joint below pivot sweeps up and out under compression and down and in under an unloading.
That will have good effects on the camber curve
Ball joint even with or above the pivot. It still sweeps up but now moves in. That is a bad impact on the camber curve.

yes w. a normal down angle geometry on the A-arm track width varies

That and inherent roll steer are why as the suspension goes through its travel as you go down the road the wheel path looks like this



and yes the camber curve also varies a bit as shown here, in the top right diagram, sturtz is camber variation, up along the positive Y axis is bump, the camber curve in bump w/ stock geometry is almost vertical, meaning little variation w/ bump


Porsche built the car for the wider Porsche market, yes it's more focused and sporting than the wider market but it is still populated mainly by affluent Doctors, Lawyers and Indian Chiefs so many of the less sporting compromises were chosen, fewer in the specials like RS or CS or GT3.

That is why when a hardcore enthusiast gets one they immediately modify them to suit their idiosyncratic tastes

[nickelplated5s]

My advice is stay close to stock CC. Driven PeteK's car, watched HarryD's & PK's at AX. Either would be fun for day trips. Mine would shave seconds off theirs but a different sort of fun for day trips. Phunt's will come in about <2200 lbs depending on a few things. If that was my build I'd put a fuel cell in it. I can tell the difference based on fuel load.

Phunt's thoughts on lighter T bars and heavier sways is good. More travel, smoother. Mine came the other way circa 83 and I lightened the sways. Set up by a race tech in the late 80's and other than bushings, shocks and brakes has stayed the same. I've got about 1/2 mile of newly paved road out my window. Small camber S curves. Speed limit is 55, crops are cut so full sight lines. It will be fun when the motor is back in and I have a dry day.

[phunt]

Yep picture is at ride height. My problem is with the dual rate springs. The black one is 200lbs the red one is 250lb. Together that makes them a 111lb spring for every 1” of suspension travel rasping only moves 1/2”. A little more for the light one a little less for the heavier one but you get the idea. So to transition to the 250lb rate the 200lb. black one. Has to bind or transition. That happens when the springs travel is used up. That black spring goes into bind at 2.7” of spring travel or compression. So that would mean the suspension would have to travel 5.4”. No way there is 5” of travel in my set up

[nickelplated5s]

I doubt I use 3" and 5 would be the undercarriage. Measured the old Bil rears for Pete. 4 1/4 if I remember and the Von's same. Trade off for softer T bars.