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

[Jonny042]

Originally Posted by r lane

Silver car

Interesting. It does look like the front is too soft in roll, and "diving" in.

I wouldn't go chasing a particular angle on the a-arm - too many variables really. Basically you want to raise the spindles as much as you can. The factory had to allow for 14" wheels, don't forget. This is why we're in this pickle in the first place.

Depending on what size wheels you're running, you may only be able to raise the spindles what seems like a small amount, (19mm MAX for 15" wheels, 30mm for 16, etc.) but it would make a significant difference in the roll center. Due to the geometries involved a small change in a-arm angle can make a much larger difference in roll height.

It will also be advantageous for the camber curve which may actually be headed towards positive on that outside front wheel.

[Jonny042]

Originally Posted by r lane

My suspension guy says shock issues are about transition, and as I was already into the turn, it was a spring/roll bar issue.

He's right - this is why higher end suspensions, as the next step up from a 2-way adjustable, will add a third adjustment for low speed compression adjustment so you can adjust that very thing separately from the normal compression/rebound adjustment.

[stownsen914]

r lane - what size wheels/tires are you running? Early 911s do have a tendency to lift an inside front tire. Obviously not desirable. It would seem like you should just fix this by making adjustments to your suspension like softening the front swaybar or stiffening the rear, but it's not so simple. You may just wind up with an oversteering car. Personally for a track car I'd much prefer lifting a wheel to oversteer.

Really fixing the problem could involve altering roll centers. Or larger wheels/tires at the rear to increase grip, which would allow you to soften the front bar or stiffen the rear bar without resulting in oversteer.

[Jonny042]

Originally Posted by stownsen914

r lane - what size wheels/tires are you running? Early 911s do have a tendency to lift an inside front tire. Obviously not desirable. It would seem like you should just fix this by making adjustments to your suspension like softening the front swaybar or stiffening the rear, but it's not so simple. You may just wind up with an oversteering car. Personally for a track car I'd much prefer lifting a wheel to oversteer.

Really fixing the problem could involve altering roll centers. Or larger wheels/tires at the rear to increase grip, which would allow you to soften the front bar or stiffen the rear bar without resulting in oversteer.

It sounds like you (unless I'm the one off-track, which is totally possible) are misinterpreting his problem. His car is the silver one in the photos, and appears to be lifting the rear and diving in the front but you are making suggestions that would exacerbate the problem?

[stownsen914]

OK, I think I did misinterpret. But on rereading r lane's post, it looks like he's not asking about lifting a tire but rather why the rear is rising under braking? I don't see a lifting tire in that pic. The car is also leaning a fair bit.

So that's two different things going on. Butt in the air could be not enough anti-dive in the front and/or not enough anti-squat geometry in the rear (which can impact lifting under hard braking). Porsche addressed both on the early RSR and on the 930 by revising the suspension geometry.
Lateral leaning of course can be solved with more swaybar and/or heavier springs / torsion bars.

[r lane]

Non adjustable Von shocks, phase 5 on the front and 4 at the rear. Sanders bars 24 front and 31 rear. Hoozier slicks 205x16 front and 245x16 rear. Wish they made a 215 x 16. I have an event this weekend and will stiffen up the rear bar a bit for now. 1'' sway bar on both ends. I have a set of RSR front and rear that I may try at a later date. I don't have bound and rebound values on the Vons but am thinking they may replicate the RSR set, so need to determine that. Also dawned on me that RSR shock values are for coil over, not torsion bars. Wheel rates will be different for those 2 setups, so shock values will also need to be different. These shocks are referred to as digressive, which I think means the more acceleration in the bound or rebound motion, the shock progressivly stiffens. I think. I appreciate all of your input. Bob

[DennisMenace]

I am about to install a KW V3 with 19 mm raised spindles. Just ordered the Tarret Bump Steer Kit. Any advice how I can identify the „ideal“ bump steer correction for my car? I assume I cannot just take the 42 mm and apply it to my car but rather need to adjust it according to my ride height. Thanks a lot for any advice!

[Bill Verburg]

Originally Posted by DennisMenace

I am about to install a KW V3 with 19 mm raised spindles. Just ordered the Tarret Bump Steer Kit. Any advice how I can identify the „ideal“ bump steer correction for my car? I assume I cannot just take the 42 mm and apply it to my car but rather need to adjust it according to my ride height. Thanks a lot for any advice!

It depends on the ride height and tire spec





stock ride height spec is 108+/5m a - b, call it 110mm for this discussion

b is the actual height of the chassis

a is the axle height which is determined by the tire, lets say this is fixed @ 306mm+/- which is what a 205/55 x16 tire would provide.

so 306 -110 = 196mm for chassis height

c & d, I need to measure but they should be roughly equal stock

lower the car from stock by say 20mm


now 306 - 130 = 176mm for chassis height

here is where the 19mm raised spindle comes in


What the raised spindle does is restores 20mm of bump travel and changes the angle of the A-arm which determines where the instant center is, which in turn determines the roll center height, which wants to be under but as close as possible to the CoM height, horizontal is the lowest angle that you want the A-arm to be, this is bad but it's worse when the outer end is above the inner end


this diagram compares 2 different ride heights, w/ 2 different A -arm angles. The top one is preferred as the roll center is closest to the CoM, A 1 and A2 are the critical datums. A1 < A2 which is a good thing.



c is now the same as before but d is down 20mm this makes the arcs vary more which is where roll steer comes from

so if you add a 12mm rack pacer d is now only down 8mm

to eliminate that last 8mm or more, you need an adjustable height steering knuckle

I good starting point to reduce roll steer would be roughly equal heights for c & d, though a full bump sweep is necessary to see exactly what you have .

[Jonny042]

Originally Posted by DennisMenace

I am about to install a KW V3 with 19 mm raised spindles. Just ordered the Tarret Bump Steer Kit. Any advice how I can identify the „ideal“ bump steer correction for my car? I assume I cannot just take the 42 mm and apply it to my car but rather need to adjust it according to my ride height. Thanks a lot for any advice!

It's easy enough to DIY as long as you have a means to accurately and repeatedly measure toe, such as toe boards.

Ideally what you'll do is set your ride height to your liking and set toe to zero. Then back off the ride height adjusters so the ride height drops below normal by some convenient amount, say 1-1/2" check toe at that point, then raise the ride height above normal by a similar amount with a floor jack and measure again. This will give you an idea where you are at, and hopefully, inform you as to what you need to do to resolve the bump steer. The process will tell you you the overall toe change over 3" of travel which is plenty.

When you make a change to the bump steer spacers you'll need to reset the basic normal "zero" point then start over.

It's relatively time consuming and requires patience but can be done. The alternative is to take it to a race prep shop with alignment equipment and a bump steer gauge and have them do it for you.

I did a post on it a few years back, I'll see if I can find it. (EDIT - see the second post of this thread!)

[stownsen914]

As mentioned above, the way to optimize toe change / bump steer is to measure it. You can get in the ballpark though by adjusting the rack position and/or outer ball joint placement to get the tie rod more or less parallel with the A arm.

[bdonally]

Maybe I missed it in reading this thread, but what ride height resulted in needing 42mm correction with 19mjm raised spindles.
By ride height I mean dimension C, from the manual.

[Bill Verburg]

Originally Posted by bdonally

Maybe I missed it in reading this thread, but what ride height resulted in needing 42mm correction with 19mjm raised spindles.
By ride height I mean dimension C, from the manual.

It's sort of unfortunate that Porsche chose to use the a -b metric for ride height. It conflates 2 related but different issues. You'll note that they only did this for 911 and went to a more conventional and simpler metric starting w/ 964.


The actual chassis height is best described by b, It's easy to measure and is repeatable and measures the actual height of the chassis.

stock b for all 911s w/ 185/70 x15 front tires was 212+/-5mm, this went to 197+/- 5mm w/ the 205/55 x16 Row cars, US '76 -83 w/ 205/55 x16 were 206 +/-5 mm.

any way back to your question,

if the car is lowered from b of say 195mm by 20mm to 175mm, then you would want to correct 2 things
1) compression travel, This started at ~90mm and is now reduced to ~70mm, to correct for this raise the spindles 20mm. This has the additional salutary effect of correcting roll center height
2) additional steering arm angularity which leads to additional roll steer & extra effort on the part of the driver, There are 2 ways to correct this
a) raise the steering rack 20mm
b) lower the steering knuckle 20mm

if the car were lowered by 45mm to 150mm the corrections would be
1) raise the spindle 40mm, assuming 17" wheels the other 5mm you can live w/
2) raise the rack 20mm(this is the max) and lower the knuckle by 25mm or just lower the knuckle by 45mm

[ccallana]

Originally Posted by Bill Verburg

It's sort of unfortunate that Porsche chose to use the a -b metric for ride height. It conflates 2 related but different issues. You'll note that they only did this for 911 and went to a more conventional and simpler metric starting w/ 964.


The actual chassis height is best described by b, It's easy to measure and is repeatable and measures the actual height of the chassis.

stock b for all 911s w/ 185/70 x15 front tires was 212+/-5mm, this went to 197+/- 5mm w/ the 205/55 x16 Row cars, US '76 -83 w/ 205/55 x16 were 206 +/-5 mm.

any way back to your question,

if the car is lowered from b of say 195mm by 20mm to 175mm, then you would want to correct 2 things
1) compression travel, This started at ~90mm and is now reduced to ~70mm, to correct for this raise the spindles 20mm. This has the additional salutary effect of correcting roll center height
2) additional steering arm angularity which leads to additional roll steer & extra effort on the part of the driver, There are 2 ways to correct this
a) raise the steering rack 20mm
b) lower the steering knuckle 20mm

if the car were lowered by 45mm to 150mm the corrections would be
1) raise the spindle 40mm, assuming 17" wheels the other 5mm you can live w/
2) raise the rack 20mm(this is the max) and lower the knuckle by 25mm or just lower the knuckle by 45mm

This is the best/easiest to understand description of ride height and side effects that I have seen - thanks Bill!!!

Could also shorten your shock by 20 mm instead of raising the spindles?

[nickelplated5s]

No raised spindles;

[Bill Verburg]

Originally Posted by ccallana

This is the best/easiest to understand description of ride height and side effects that I have seen - thanks Bill!!!

Could also shorten your shock by 20 mm instead of raising the spindles?

In an extreme case, yes

I recently went through this w/ my '76 C3 hotrod, where I am running at a b of 115mm w/ 225/45 x17 front tires

This is lowered by ~100mm from stock, to return compression travel to near oe spec I raised the spindles 40mm and shorted the insert by 25.4mm, i.e. I got back 65mm of travel, this is quite livable as the oe was ~175mm w/ 205/50 x15 tires and no compensation.

I did a ton of research on these w/ a lot of help from Chuck @ Elephant and Bilstein Racing

There appears to be 2 sizes of Bilstein inserts for Bilstein struts
most b6 or b8 are 21.36" long and there is one version of b8 that is 22.07" long, I believe that this later was made for the Safari cars.

Elephant has a service where they can cut down the insert to you spec, this combined w/ a shorter wiper seal gets the extra compression travel back.

Something I need to emphasize here is that there are 2 factors affecting the steering knuckle heigh you want to use
1) chassis height b
2) is spindle height

changing either of these implies that the steering knuckle height will need to change

i.e raise spindle 20mm by itself w/ no other change also raises the steering knuckle height by 20mm

lower b by 20mm alone w/ no other changes also raises the steering knuckle height by 20mm

do both and you need 40mm total adjustment to steering knuckle height, this can come from just the outer end or a combination of outer and inner end ajustments.

[bdonally]

Thanks for the information.
The principles are well understood, but it would be good to hear from some of the people in the thread who made specific adjustments, with information about the spindle height and dimension C. ALso info about dfender height.
Personally I like to work with "C" because it is independent of tire size. My street tires are 205/55; track are 205/45, a 1.6" (41mm) difference in diameter. Messes up just looking at chassis height.

[Bill Verburg]

Originally Posted by bdonally

Thanks for the information.
The principles are well understood, but it would be good to hear from some of the people in the thread who made specific adjustments, with information about the spindle height and dimension C. ALso info about dfender height.
Personally I like to work with "C" because it is independent of tire size. My street tires are 205/55; track are 205/45, a 1.6" (41mm) difference in diameter. Messes up just looking at chassis height.

here are the a spec ranges for the most common 911 front tires
185/70 x15 310 - 322 mm
195/65 x15 302 - 308 mm
205/50 x15 279 - 292 mm
205/55 x15 290 - 303 mm

205/55 x16 303 - 309 mm

205/50 x17 308 - 319 mm
215/45 x17 297 - 305 mm
225/40 x17 297 - 314 mm

avg low end is 298mm
avg high end is 309mm

not really much variation especially if the 185 & 195 x15 outliers, which aren't real common these days, are excluded

the biggest impact is the wheel,
15 allows for 19mm spindle rise
16 allows for 30mm spindle rise
17 allows for 40mm spindle rise

there is a significant difference in potential to reclaim compression travel w/ only a minor variance in a

fender heights are unreliable metrics, I used to try to collect and correlate fender heights, the results are all over the place so you can't infer much from them except in the broadest terms.

If you care about this stuff take the time to measure b

you could even measure it 1/2 assed by going from the floor to the bottom of the t-bar instead of the center as the difference is another reliable, repeatable quantity.

either of these can be used to do the measuring

[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.

[nickelplated5s]

My question is what % of A to C?

[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.