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

[Bill Verburg]

Originally Posted by phunt

Changing from 185/7-15 tires (25.2” tall) to 205/55-16 (24.9”) would lower the car about 4mm. .3” difference in tire height. Half would be .15” since you are only dealing with the radius. Measurement a. From ground to center of the wheel. That would move your 212mm b height on US cars to about the 197 you say on Euro without changing the angle of the lower control arm or geometry.

a is determined by the static loaded height of the tire not the tire OD, they all flatten to an extent that depends on PSI and sidewall stiffness. Sidewall stiffness depends on constituent material, design and wheel width the wheel width.

The a #s I quoted are the average static loaded height for the universe in my tire dB for that nominal size tire.

no matter how You choose to measure US 911 from '76 -83 were raised compared to RoW versions

The std. street spec for all 911s except those was 108+/5 mm, b w/ oe tires was 212 +/5 mm for cars that came w/ 185/70 by 15 and 197mm for cars equipped w/ 205/55 x16

the US cars for '76 & 77 would have 185/70 x15 and b of 221+/5 mm, a-b spec was 99+/-5 mm

specials like the '84 SCRS a-b spec was 144 +/5 mm and was 162+/-5 mm
'73 RSR a -b spec was 160, b was in the neighborhood of 130mm from the factory but each team did their own thing from there.

[Bill Verburg]

Originally Posted by phunt

Proper suspension geometry is based on the difference between the measurements of a and b. The 108mm that you call measurement c. Meaning it sets the angle of the lower control arm. From its pivot point to the ball joint. To maintain that angle (which will affect bump steer) when you raise the spindle 19mm you need to set that c measurement at 127mm. The 108 plus the 19 of raising the spindle. Otherwise you would reduce the angle of the control arm from the pivot point to ball joint. Bump steer is worsened by not having the steering control arm not parallel with the lower control arm taken from the center of the ball joint to the center of the pivot point. Those two arcs need to be as close as possible so no change to the dimensional measurements. Causing a pull or push on the steering knuckle. Additionally the distance from where the strut mounts on the ball joint to where it sets at the top on the chassis will not change. Maintaining original travel of the shock. That’s stock geometry. That’s why you lower a car with raised spindles vs just changing springs or spring orientation. By maintaining these angles you may not need to raise or lower the steering rack. It would depend on where the knuckle is on the strut. Weather it was raised or not.

You can measure either way, it's just that the original a - b hides the actual height w/i the formula, These days there are so many possible front tires used that , JIMHO, b is the metric I would want to know when setting the car up, Most of us aren't changing tires when we do this chore, so we really don't care. It's something that is used to initially determine what tire to use but it's not relevant after that as it's a fixed quantity.

and, yes, roll steer is the result of the different arcs defined by the A-arms and the steering arms. The #s I quoted above are just a way to keep track of the changes from stock and how to restore stock geometry in all relevant respects.


I initially began thinking about this when trying to compare my 993 w/ my 911 despite the different suspensions, It's amazing to me how similar they are in the heights of the fender tops.

here's what a stock RoW 3.2 Carrera toe and camber curves look like at stock RoW ride height w/ 205/55 x16 tires
In both cases a vertical trace is desired, the more horizontal the more change per mm of compression, the red line shows a compression of 40mm w/ corresponding toe change of -34' That's as good as it gets w/ a stock 911 strut front. It's also what gives these cars the feeling that they are alive because it causes the wheel to move in you hands.

[phunt]

Maintaining suspense geometry is what’s important. When you raise the spindle you in essence lower the ball joint. The distance between them is greater by the amount you raised the spindle. Since measurement a can not be changed. I know it depends tire selection. My tire is 205/55-16 so I think it was 303mm on your chart. So a will always be 303. If you use your metrics of 197 for b that would give you a “c” of 106 in that 108 +/- 5. However the angle of the control arm from the horizontal will have increased changing the geometry. If the center of the control arms rotational point, centerline of the torsion bar, is 197mm high. The ball joint is, in my case, 19mm closer to the ground the stock. The amount the spindles were raised. So the angle down of the control arm is steeper. To reduce that back to what would be stock you need to lower or decrease the b measurement by 19mm. That would restore the geometry. That is why I mentioned increasing the “c” measurement to 127mm. So a is 303 reduce b to 178 (197-19) that would make “c” 125 or the 106 plus 19. I feel that the angle of the control arm down from horizontal at the pivot point is the key. Keep it the same as stock and bump steer will be easier to mitigate. My car is a 66 do you know the “stock” b measurement for it? When I was mocking up my car I ended up with b at 176mm so very close to above.

[Jonny042]

For those wondering about a real-world "C" measurement on a contemporary sports purpose 911, in the second post of this thread I mentioned that a had a ride height of 151mm (accounting for the 19mm raised spindles).

So if measured by the factory method, I would have measured 170mm.

From a geometry standpoint, it would be equivalent to 170mm-19mm = 151mm

Put another way, the car was lowered 43mm from the stock 108mm (to a measurement of 151mm) via the torsion bar adjusters, and another 19mm as a result of the raised spindles, to the final measurement of 170mm.

This is the ride height at which I measured and determined that 42mm of spacer was optimal for that setup.

[Bill Verburg]

Originally Posted by phunt

Maintaining suspense geometry is what’s important.

true

Originally Posted by phunt

When you raise the spindle you in essence lower the ball joint.

true

Originally Posted by phunt

The distance between them is greater by the amount you raised the spindle.

This confuses me, the distance between what and what

Originally Posted by phunt

Since measurement a can not be changed.

true

Originally Posted by phunt

My tire is 205/55-16 so I think it was 303mm on your chart. So a will always be 303.

Range of 205/55 x16 is 303mm to 309mm, 306 mm is avg for the tires I have records for

I’d prefer the average , not the min, unless discussing a particular tire, but ok, lets use the tires where a is 303mm

Originally Posted by phunt

If you use your metrics of 197 for b that would give you a “c” of 106 in that 108 +/- 5.

Since a - b = c or the factory spec of c being 108 +/- 5mm
Then b = a - c
b = 303 – 108 = 195mm

Originally Posted by phunt

However the angle of the control arm from the horizontal will have increased,

true, but again, this was the goal.

The above is for stock ride height per RoW spec of 108mm, I didn’t measure the a arm angle, it is what it is.

The a-arm angle changes via 2 possible mechanisms
1) Use the screw adjusters to raise or lower the chassis
2) Raise the spindle, if the spindle is raised then, to maintain stock chassis height, the screw adjusters would need to be used to raise the chassis
Obviously if you wanted to lower the car by the amount the spindles were raised then no screw adjustment is necessary. so there are 2 cases to consider

Case 1: assume raise spindle 20mm and want to keep stock chassis height, c =108mm or b= 195mm,

Here the ball joint is lowered by 20mm by raising the spindles which increases the a-arm angle and at the same time the steering arm knuckle was raised by the 20mm spindle rise. To compensate and go back to stock geometry both need to be negated.

To negate the a-arm drop, raise the adjuster screws 20mm. This also raises the inner end of the steering arm 20mm which also negates the previous change in steering arm angle

Case 2: assume a 20mm spindle rise and you want the chassis 20mm lower
As above the the ball joint has dropped, This changes the a-arm angle which raises the roll center and increases compression travel. Both are good things which do not need compensation. Once again this was the goal. To restore steering geometry to stock all that is needed is to drop steering knuckle 20mm, because the inner end dropped 20mm when the chassis was lowered due to the raised spindles.

Originally Posted by phunt

However the angle of the control arm from the horizontal will have increased changing the geometry.

true, but is that not the goal of using raised spindles on a lowered car?

What you want to keep constant is the geometric relationship between the spindle, a-arm and steering arm, changing any of these necessitates a change in the others

[nickelplated5s]

Graphs are fine but just graphs. My A:C is about 43%. Control arms as pictured above are close to horizontal with no raised spindles. Two ways to 'skin a cat' and my shocks don't bottom or top out. What I am curious about is how PHunt is getting away with 205/16 in the front of a 66.

[nickelplated5s]

Whelp, no answers. I can figure out Bill's A based on posts. Spindle based on same. C? FWIW mines pretty darn close to the 108mm above. Correct me if I'm wrong but my assumption is to get RC=CG.
I've run 205's in the rear. Even with rolled fenders I wouldn't try those in the front.

[phunt]

The point of raised spindles is so you don’t have to change other settings or angles so the car suspension is like factory. But then you are only getting 19mm of lowering. If you want more then yep you have to change other things. I have the 935 style front so I can change width a little. On a fixed control arm length when you raise or lower the ball joint in that arch of travel it will move I. And out depending on where in the arch it is. Which is good because it adds or subtracts camber. And I meant the distance between the spindle center and the ball joint. Which like you say change roll characteristics.
As far as how I am running 205/55-16 fronts an my SWB I have custom wheels with offsets that I had specified. I also am running 225/50-16 rears. Lots of room just have to know how to use it

[Bill Verburg]

Originally Posted by phunt

The point of raised spindles is so you don’t have to change other settings or angles so the car suspension is like factory.

again , not true

to keep things simple only consider the heights in 2 respects
1) wrt to the ground
2) wrt to the torsion bar center


the top of the shock is variable wrt to the ground but fixed wrt the torsion bar, i.e raise the t-bar and the top shock mount rises.

the rack height is variable wrt the ground and only variable wrt the t-bar by the thickness of what ever spacer is placed between it and it's cross bar fixture, max variance is ~10mm, i.e. a rack spacer increases the rack height wrt the t=bar and the ground

t-bar height is variable wrt the ground, via 2 mechanisms,
1) screw adjusters
2) axle height, a, variance

axle height, a, is variable wrt the ground only if the tire has a different static loaded tire height,
axle height wrt the t-bar varies if the spindle is raised (or lowered) on the lower strut, or if the t-bar screw adjusters are changed, this is a - b


so,

b is a measure of the height of the chassis above the ground

a - b is the difference between the axle height and the t-bar which is also a measure of the difference in height between the axle and steering rack. This is the thing the factory engineers where most interested in back in the '60s. It is a measure of is how much bump steer(aka rolll steer) has been added to the car by lowering the t=bar height, b, via whatever mechanism.

stock a - b is 108+/-5mm

lower the car, by whatever method, to a - b of 148mm and you have increased the difference in rack height to axle height by 40mm, this means that to compensate and correct roll steer back to stock the steering knuckle must be lowered 40mm

an additional thing that happens when a - b is increased from 108mm is the steering effort also goes up because of the increased angularity of the steering arm.

[Bill Verburg]

Originally Posted by nickelplated5s

Whelp, no answers. I can figure out Bill's A based on posts. Spindle based on same. C? FWIW mines pretty darn close to the 108mm above. Correct me if I'm wrong but my assumption is to get RC=CG.
I've run 205's in the rear. Even with rolled fenders I wouldn't try those in the front.

If you don't have Bilsteins much of this is moot

Konis ir Boges may hvve different
lengths
different strokes

the a, b, c discission is probably relevant but since you don't believe it, also moot

[Jonny042]

Originally Posted by phunt

I have the 935 style front

I'm probably going to go with a 935 front setup for my lightweight project. Do you happen to know the weight savings? The adjustability is also part of the attraction. Does the 935 front move the rear inner pivots up slightly compared to the factory crossmember?

Not my pic, but a good representation:

[nickelplated5s]

A shock is a shock Bill so not moot. My question is what A:C ratio? I've heard of spindles being raised 15mm to 40mm. Obviously A has changed over the years due to tire and rim choices. I understand early R's had the 19mm raise. I don't have the data for other changes. RC=CG?

Show us a pic of your rims P, please. I'm partial to my Fuchs though. Curious as to the motor and brakes too.

[winders]

Keep in mind the specific reason spindles are raised when lowering a 911 is to move the roll center to best possible height for handling performance. Any other benefits are serendipitous and any negatives are lived with.

[Bill Verburg]

Originally Posted by winders

Keep in mind the specific reason spindles are raised when lowering a 911 is to move the roll center to best possible height for handling performance. Any other benefits are serendipitous and any negatives are lived with.

For the hard core track guy that's probably true, for street use the travel is probably more important, it was for me.

both are important

[Bill Verburg]

Originally Posted by nickelplated5s

A shock is a shock Bill so not moot. My question is what A:C ratio? I've heard of spindles being raised 15mm to 40mm. Obviously A has changed over the years due to tire and rim choices. I understand early R's had the 19mm raise. I don't have the data for other changes. RC=CG?

Show us a pic of your rims P, please. I'm partial to my Fuchs though. Curious as to the motor and brakes too.

all you care about is the absolute relative heights, not the ratios.


follow the notionary line perpendicular to the shock top mount out to where it meets the notionary line extension of the A-arm, they meet at the 'Instant Center'
then follow another notionary line from the IC to the center of the contact patch,

the intersection of this line and the vertical line thru the Center of Mass is where the Roll center is, the RC is the axis around which roll takes place, it is not a fixed point it moves around at the behest of the change in A-arm angle

b is not fixed either, it also changes as the result of A-arm angle i.e. suspension bump/droop


all roll forces act through the CoM

The distance from the RC to the CoM is the lever arm that all roll forces act on, the longer is the more roll.

In the following diagram the car has been lowered by adjusting the the t-bar adjusters.
a is constant
b has gone down and w/ it the height of the steering rack
a -b has gone up

IC has gone down, taking RC w/ it

CoM has also gone down, but not as much as the RC


here A1 is the stock lever arm length and A2 is the arm length when the car has been lowered


here you see the stock relation between shock top, t-bar, a, b, rack and knuckle


here the relationship between them when lowered, b has gone up, no compensation to rack or knuckle height here, either the rack goes up, the knuckle goes down or some combination of both is needed to compensate and get b back to spec,108+/-5 mm

[Bill Verburg]

Originally Posted by nickelplated5s

..
Show us a pic of your rims P, please. I'm partial to my Fuchs though. Curious as to the motor and brakes too.

Fuchs are fine if you want 6 or 7 front, but the 8x16 ET23.3 Fuchs has an ET that is too small to fit the front comfortably w/o a lot of compromise

to fit an 8 in front you need ET25 to 30 w/ ET 27 as arguably the best compromise,

On mine I have 8ET25 w/ the 225/50 x17 tire with a wide squarish shoulder, i.e most difficult fit. I can run at b of ~114mm, a compromise is that I have shaved the inner fender ledge to almost nothing

in back Fuchs 7 ET23.3, 8 ET10.6, or 9 ET15 fits any SC/Carrera fender well / up to a 255 tire w/ no compromises
9.5 ET19 centers the tire perfectly in the well no compromises w/ a 255 tire but w/ a 275 the trailing arm bolts and oil lines need to be modified.

in the back of a n/b 7 ET23.3 fits as long as the tire isn't too tall, a 215/60x 15 is too tall, a 205/60 x15 is not, to fit an 8 the ET needs to go up, similar to what is needed in front, here 8ET23.3 can be squeezed in w/ a 225 tire but compromises need to be made, an 8 ET28-32 allows a 225 to fit comfortable as long as it's not too tall

[phunt]

The only way to properly lower a car is to raise the spindles. Any other way changes the geometry. You have to live with whatever comes from it. Good or bad. With upper and lower A arms you can get away with a lot more. My car only has a lower A arm so it matters what you do a little more.

What does the T-bars have to do with anything? The only thing they do , besides act as a spring, is give the lower A arm a pivot point. It is fixed in the car and can not be changed. Just like the upper perch where the top of the strut mounts. Unalterable besides the small adjustments for caster and camber it is fixed. That is why the location of the ball joint matters. Even to the amount of travel you get on the shocks. Which is what I am worried about. If you lower the car by raising the position of the ball joint in reference to the pivot point/t-bor you will have adverse effects on geometry. That is why the 108 “c” measure is so important. In a strut system like on my car when the ball joint is below the pivot as the suspension compresses under load, as in a turn, the the A arm will swing up relative the pivot. So as the ball joint swings up it will also move out away from the pivot or center of the car. As it does this it will move the bottom of the strut out away from center line adding negative camber. As the inside tire of the turn will become less loaded the A arm will swing down pulling the bottom of the strut in towards the pivot adding positive camber. So additional negative camber on the outside wheel and positive camber on the inside well. Yep that’s a good thing. When you lower the car by raising the ball compared to the pivot like straight out or worse slight angled up. When the wheel compresses in a turn it will pull the bottom of the strut in adding positive camber. Yep that’s a bad thing. That is why that lower control when video the front of the car has the droop below the pivot point. As shown in one of those pics above showing the rack and tie rod orientation. Both of those shown would suffer bump steer. So yep I am sure you can change that “c” measurement to what ever you want and lower your car accordingly. But does it change the geometry? Yep. Is it for the better? Nope. I guess like what they say about these cars, and the way they drive, shift, run, or whatever you just have to get use to it.
We are lucky in these cars that where the tie rods pivot and where the A arms pivot are very close in line. So the more parallel you can get them the less bump steer to deal with

[phunt]

I have been given a bunch of BS reasons as to why things are and what is right or wrong to do to my own car. I have been ripped off by well known and supposedly top people in this Porsche community. “The Porsche way” is not necessarily the way for me. But as for my engine. It is custom built by me. It is a 3484cc. You do the math. It has Mahle pistons built to my needs Pauter rods also to my spec. I chose a specific rod/stroke ratio. The rods are longer than stock. Twin plug 3.2 heads with different sized valves. RSR high butterfly but SFI. Cam is bespoke grind lift duration timing and LSA all to my list. Very high VE.
Here are my wheels. Rear 225/50-15. Rear fender height at the lip 25 1/4” the fronts are 205/55-16. No fenders in the picture but it is set at 127 “c” plenty low and the 25 1/4 rear makes for a nice look I think perfect rake

[nickelplated5s]

Thanks for the pics P. I'll have to parse the rest for later. For some reason I thought you were talking about a 66 but the pics show 72, hence the tire width. Curious about the calipers though. Love to sit copilot in it.

Short version or search my user name; Stock spindles, custom short throw front shocks, very low i.e. track car. I can break the rear if I try but have never lost the front. At one point I did put a smaller sway in the rear though.

The motor goes in next week for static dyno tuning. It will be interesting as the rear bushings were done with the motor out. Decided against the GT3 front when I did that and Boxster brakes.

[Bill Verburg]

Originally Posted by phunt

The only way to properly lower a car is to raise the spindles.

that depends on how much lower the car is going to be, there is no point where below you have to do one thing and above another

if you want to go down 10mm then likely the screw adjusters will be adequate and certainly a 10mm rack spacer to compensate for the drop in rack height while lowering.

certainly, if going down 20mm raising the spindles is the way to go, here a rack spacer of 11mm w/ a 9mm drop in knuckle height compensates

Originally Posted by phunt

Any other way changes the geometry. You have to live with whatever comes from it. Good or bad.

any change in ride height b, changes both roll center geometry and steering geometry., you do want to live w/ RC geometry change as that is good, you do not have to live w/ the bad roll steer geometry, unless you want to. To correct it just change the steering knuckle height.

Originally Posted by phunt

With upper and lower A arms you can get away with a lot more. My car only has a lower A arm so it matters what you do a little more.

arguably double A arm has better bump and drop curves. Either needs similar corrections to complement any given change

Originally Posted by phunt

What does the T-bars have to do with anything? The only thing they do , besides act as a spring, is give the lower A arm a pivot point. It is fixed in the car and can not be changed. Just like the upper perch where the top of the strut mounts. Unalterable besides the small adjustments for caster and camber it is fixed.

correct, both are the only fixed reference points to describe any given geometry that is implemented. You need these as reference points for the rack, spindle and knuckle heights. You don't change reference points they just give perspective and define consequences.

Originally Posted by phunt

That is why the location of the ball joint matters.

the only way the ball joint matters is that it clears the wheel. Other than that it's only significance is as the outer end of the A-arm where is determines the A-arm angle and hence determines the line to the instant center. You do want it to be low enough that the IC line goes as high as possible. The only way to change the ball joint height is by changing the spindle or axle height.

Originally Posted by phunt

Even to the amount of travel you get on the shocks. Which is what I am worried about. If you lower the car by raising the position of the ball joint in reference to the pivot point/t-bor you will have adverse effects on geometry.

Most of us want to lower our cars but a Safari car has the opposite goal, they would raise the spindle height which raises the ball joint which changes the A-arm angle which changes the IC etc. on a safari car the steering compensation would be to raise the knuckle

Bilsteins at stock height only have ~3.5" of travel, lowering the car via t-bar adjusters removes 1 " of travel per 1" of lowering

which is why raising the spindles is such a good idea, raise the spindle 1" and the compression travel remains the same. AND you improve RC height as a freebie

The issue is that when the spindle is raised 1" the steering knuckle is also raised 1", This causes the roll steer geometry to get far worse, which is why you also need to lower the knuckle and or raise the rack to correct it.

Originally Posted by phunt

That is why the 108 “c” measure is so important. In a strut system like on my car when the ball joint is below the pivot as the suspension compresses under load, as in a turn, the the A arm will swing up relative the pivot. So as the ball joint swings up it will also move out away from the pivot or center of the car. As it does this it will move the bottom of the strut out away from center line adding negative camber. As the inside tire of the turn will become less loaded the A arm will swing down pulling the bottom of the strut in towards the pivot adding positive camber. So additional negative camber on the outside wheel and positive camber on the inside well. Yep that’s a good thing. When you lower the car by raising the ball compared to the pivot like straight out or worse slight angled up. When the wheel compresses in a turn it will pull the bottom of the strut in adding positive camber. Yep that’s a bad thing. That is why that lower control when video the front of the car has the droop below the pivot point. As shown in one of those pics above showing the rack and tie rod orientation. Both of those shown would suffer bump steer. So yep I am sure you can change that “c” measurement to what ever you want and lower your car accordingly. But does it change the geometry? Yep. Is it for the better? Nope. I guess like what they say about these cars, and the way they drive, shift, run, or whatever you just have to get use to it.
We are lucky in these cars that where the tie rods pivot and where the A arms pivot are very close in line. So the more parallel you can get them the less bump steer to deal with

the importance to car of c = 108 is that it describes the geometric relationship between the axle the t-bar and and ground, This geometry can exist at any ride height b because it is independent of the absolute t-bar height b It only depends on the distance between the axle and t-bar heights.. It merely defines the relation between axle height and t-bar height This in turn defines the roll steer character of the suspension, when it becomes bigger by whichever method(remember there are 2 ways this can happen) the roll steer gets worse, At best roll steer is mediocre in these cars, see the above roll steer curves, and no it can't be eliminated, You can just move one way or the other along the curves.


No one is saying that 108 is perfect, it is just what the factory engineers recommended. It just places the suspension action in a linear portion of the curve that has the steepest possible slope. i.e a great compromise.

Most lowered car are fine w/o compensation because the curve is so linear and the cars aren't lowered too much. The problems arise at the extremes,

remember there are 2 related issues here
1) available bump travel
2) roll steer

both are changeable, for a better and for worse

lastly no one is telling anyone what to do w/ their cars, All that is described here is the Physics of it the suspension and the consequences of some specific changes.

You do you