My front roll center/bump steer solution
09-22-2017, 12:58 PM
#31
Race Car
Yes, Jarod @ SCCH manufactures the drop pins to have a 19MM OD at the top to fit a VW application (MKII maybe), but can also turn them down to 17MM (I think for MK1 VWs) so they fit the 944 spindle perfectly. The bottom end is 5/8" OD to fit off-the-shelf spherical bearings.
The best part is that you don't have to ream out the spindle to 19MM and weaken it further. The welding also means that you are not relying only on the pinch bolt. I think the Racer's Edge arms look fantastic, bit the longer pin relying on a weakened spindle and pinch bolt is sketchy at best . . . .
The best part is that you don't have to ream out the spindle to 19MM and weaken it further. The welding also means that you are not relying only on the pinch bolt. I think the Racer's Edge arms look fantastic, bit the longer pin relying on a weakened spindle and pinch bolt is sketchy at best . . . .
But, it's cool, I just needed to get the dimensions of the spherical. I'm a diagnosed Pegasus junkie, don't know why I never looked there.
But yes on the other points, that's why I am making my own billet arms. Don't want to weaken the spindle with the +2mm bore.
The next inevitable DNF won't come from a broken ball joint.
T
09-22-2017, 01:03 PM
#32
Race Car
The thing we're trying to accomplish is add strength but retain the 17mm pin and correct the geometry all at the same time.
With our own solution.
Other's mileage will vary, but I have free Solidworks pro and free cnc service from a friend.
Landed a single billet large enough to mill 2 arms from.
All told, I may have $500 in two billet a arms with pin setup that is stronger than the 19mm non-tapered pin.
T
09-22-2017, 01:57 PM
#33
Three Wheelin'
Thread Starter
I already contacted Jarod, no reply yet in a few days.
But, it's cool, I just needed to get the dimensions of the spherical. I'm a diagnosed Pegasus junkie, don't know why I never looked there.
But yes on the other points, that's why I am making my own billet arms. Don't want to weaken the spindle with the +2mm bore.
The next inevitable DNF won't come from a broken ball joint.
T
But, it's cool, I just needed to get the dimensions of the spherical. I'm a diagnosed Pegasus junkie, don't know why I never looked there.
But yes on the other points, that's why I am making my own billet arms. Don't want to weaken the spindle with the +2mm bore.
The next inevitable DNF won't come from a broken ball joint.
T
And yes, Pegasus is the best. Their stuff is all proven. Their uber-expensive double-walled air duct tubing is the the only kind I have tried on my brake cooling ducts that doesn't get shredded up in a few months. I usually avoid looking at their catalog because it just makes me want to spend money!
09-23-2017, 01:29 AM
#34
Burning Brakes
This is a very interesting thread - you guys seem to really understand what you're doing. I have a track-focused 968, which sits at about 25" (636 mm) from the bottom of the fender arch to the ground (not sure what the stock measurement is), which leaves my control arms inclined by 17.4 mm (about 11/16"). I have the Racers Edge heavy duty control arms which can accept longer ball joint pins. I originally had the car about 1/2" lower (based on Karl's original recommendation) than I have it now, which created a control arm inclination of 7/8". But when I asked Karl about whether I should get longer ball joint pins to level the control arms, he strongly advised against it, which I thought was interesting, considering the arms' ability to accept longer pins is one of the main points he emphasizes in the listing on his web site. Here is what he told me regarding the length of the pins:
All aspects of a cars suspension are interdependent and not easily changeable on a production vehicle. Your below ground front roll center is very common on these or any McPherson strut cars when lowered. The reason I didn’t recommend that you go to a longer front pin is multi-faceted but basically lies at the intersection of the fact that any change you make to the front should be replicated at the rear (not easily done) or else the roll axis will change and thereby change the relative roll resistance front to rear. The inherent mechanical stability that is built into production cars at the rear will be lost as the roll axis becomes flatter . Also if you go to a longer front pin then the front bump steer characteristic will change significantly (as you will now have a wedge shape of the control arm relative to the tie rod,… McPherson strut control arm and tie rod should be roughly parallel) so you would then need to fabricate a front tie rod link to get the bump steer back to as flat a curve as you want it to be. Lastly, the rules or the racing class you are in may or may not allow such changes. As I rule I recommend that most casual racer’s/drivers use the stock length pin,.. so as to not make the car worse, which is easy to do if you just bolt parts on without KNOWING what it is doing.
So, he's basically saying that changing the front roll axis will impact the rear, and correcting for this is not easily done. In post #6, the OP mentions that his short-term solution to the front/rear balance issue is going with stiffer rear springs, which seems reasonable. I'm currently running 700 lb springs in front, and 800 lb at the rear (inverted coilovers, like the OP). I've recently performed a corner balance, alignment, had the bump and rebound settings of my Moton Clubsports optimized, and I've softened the rear springs by 20%, and I raised the front ride height as part of this exercise. As a result of this work, the car is much more stable and forgiving, and much easier to rotate using the throttle. However, reading through this thread is making me wonder whether I should go ahead and flatten the control arms, which on my car would be a simple matter of installing longer ball joint pins, but this would be going against the advice of a very reputable Porsche tuner and racer. So, as you might expect, I'm confused and unsure of what to do.
As I said, the car drives much better than before, but there's still a fair amount of sway, so I would be open to further improvements as long as a) It isn't too expensive (I've spent WAY too much on this car already), and b) I don't create the problems Karl is warning about. Any guidance through this dilemma would be greatly appreciated.
All aspects of a cars suspension are interdependent and not easily changeable on a production vehicle. Your below ground front roll center is very common on these or any McPherson strut cars when lowered. The reason I didn’t recommend that you go to a longer front pin is multi-faceted but basically lies at the intersection of the fact that any change you make to the front should be replicated at the rear (not easily done) or else the roll axis will change and thereby change the relative roll resistance front to rear. The inherent mechanical stability that is built into production cars at the rear will be lost as the roll axis becomes flatter . Also if you go to a longer front pin then the front bump steer characteristic will change significantly (as you will now have a wedge shape of the control arm relative to the tie rod,… McPherson strut control arm and tie rod should be roughly parallel) so you would then need to fabricate a front tie rod link to get the bump steer back to as flat a curve as you want it to be. Lastly, the rules or the racing class you are in may or may not allow such changes. As I rule I recommend that most casual racer’s/drivers use the stock length pin,.. so as to not make the car worse, which is easy to do if you just bolt parts on without KNOWING what it is doing.
So, he's basically saying that changing the front roll axis will impact the rear, and correcting for this is not easily done. In post #6, the OP mentions that his short-term solution to the front/rear balance issue is going with stiffer rear springs, which seems reasonable. I'm currently running 700 lb springs in front, and 800 lb at the rear (inverted coilovers, like the OP). I've recently performed a corner balance, alignment, had the bump and rebound settings of my Moton Clubsports optimized, and I've softened the rear springs by 20%, and I raised the front ride height as part of this exercise. As a result of this work, the car is much more stable and forgiving, and much easier to rotate using the throttle. However, reading through this thread is making me wonder whether I should go ahead and flatten the control arms, which on my car would be a simple matter of installing longer ball joint pins, but this would be going against the advice of a very reputable Porsche tuner and racer. So, as you might expect, I'm confused and unsure of what to do.
As I said, the car drives much better than before, but there's still a fair amount of sway, so I would be open to further improvements as long as a) It isn't too expensive (I've spent WAY too much on this car already), and b) I don't create the problems Karl is warning about. Any guidance through this dilemma would be greatly appreciated.
Last edited by Cloud9...68; 09-23-2017 at 01:58 AM.
09-23-2017, 09:10 AM
#35
Nordschleife Master
The main problem with non-parallel arms is the increased chance that the steel pin will bind on the aluminum socket and cause it to fail. The RE arms have spherical bearings which have more articulation and the arms are more forgiving as they are made of steel. That could be why you were given the advice not to fit longer pins on non-parallel arms. Also, there have been a number of rear torsion bar carriers and rear control arms made in the past that were designed to correct the roll center geometry caused by lowering these cars and addressing the front/rear issue.
09-23-2017, 11:40 AM
#36
Rennlist Member
Anyone curious to see how the factory engineers did it more than 35 years ago? Note the raised steering and raised control arm inner attach points. Want to see the rear too?
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willrobinson (11-26-2021)
09-23-2017, 01:15 PM
#37
Burning Brakes
The main problem with non-parallel arms is the increased chance that the steel pin will bind on the aluminum socket and cause it to fail. The RE arms have spherical bearings which have more articulation and the arms are more forgiving as they are made of steel. That could be why you were given the advice not to fit longer pins on non-parallel arms. Also, there have been a number of rear torsion bar carriers and rear control arms made in the past that were designed to correct the roll center geometry caused by lowering these cars and addressing the front/rear issue.
1. The diminished camber change mentioned by the OP in his first post
2. The longer roll couple, leading to a tendency to sway
3. The strain on the ball joint pins you mention
4. The tendency to bump/roll steer caused by non-parallel tie rod arms
Karl was primarily concerned with the mismatched front-to-real roll couple that could result from correcting the issue in the front without making similar corrections to the rear. And yes, I've read through threads discussing various solutions to the rear roll center geometry. While these solutions are fascinating, this is more than I'd like to take on right now.
My dilemma is that I'm seeing very knowledgeable and experienced people giving advice that's 180 degrees apart. My goal is simply to make my car as stable, consistent, communicative, and capable as possible. I don't race competitively yet, but plan to soon (most likely not with this car, though), so I look at it primarily as a training tool. I don't want to go too crazy with additional mods (I already have one of pretty much everything RE sells), but if there's an opportunity to address a known flaw without creating unintentional consequences that are just as bad without having to spend too much time or money, I'd be interested in pursuing it.
If I decide to correct my front-end geometry, are there any rules of thumb as to front-to-rear spring rate relationship, sway bar settings, tweaks to the alignment, etc. to follow to minimize the front-to-rear roll center mismatch without going to the modified rear torsion bar carriers? Thanks.
09-23-2017, 02:32 PM
#38
09-23-2017, 06:58 PM
#39
Race Car
Yup, I'd do that too but rules state "factory suspension pickup points remain unchanged".
Me and Max were just talking about this on the phone an hour ago.
T
09-23-2017, 11:28 PM
#42
Rennlist Member
Cool car Michael! Is that yours? From what I can see the front doesn't look too different to how many modify their lowered cars. The rear is a bit different. Did you ever measure motion ratios and front / rear roll centres of this car?
09-24-2017, 01:23 AM
#43
Wow. A 933! I originally thought it was a GTR or other variant. Thanks for sharing these pictures MM.
I don't think too many are still original.
Does the 933 use torsion bars and coils or pure coils in the rear?
I don't think too many are still original.
Does the 933 use torsion bars and coils or pure coils in the rear?
09-24-2017, 01:54 AM
#44
Three Wheelin'
Thread Starter
It would be simple enough to take some measurements and relocate the forward inner control arm mounts by drilling new holes in the crossmember. The rear mounting points would be more difficult as there is no room to raise these, but from the first pic it looks like they actually moved the caster blocks to mount on the inboard side of the chassis rails . . . . . quite trick! I would be interested to know what this would do to the caster setting, as the forward mounting points are also further inboard . . . . but this would also reduce the track width at the hubs, unless wider control arms were used. I would be interested to know more about this setup.
Also, I'm not sure there's enough room under the 944 sump to move the steering rack that far up, but I'd have to double-check that . . . .
Last edited by Droops83; 09-24-2017 at 02:55 AM.
09-24-2017, 02:18 AM
#45
Three Wheelin'
Thread Starter
Thanks; what you say makes sense. However, from what I gather, there are four issues related to front-end geometry associated with lowering these cars:
1. The diminished camber change mentioned by the OP in his first post
2. The longer roll couple, leading to a tendency to sway
3. The strain on the ball joint pins you mention
4. The tendency to bump/roll steer caused by non-parallel tie rod arms
Karl was primarily concerned with the mismatched front-to-real roll couple that could result from correcting the issue in the front without making similar corrections to the rear. And yes, I've read through threads discussing various solutions to the rear roll center geometry. While these solutions are fascinating, this is more than I'd like to take on right now.
My dilemma is that I'm seeing very knowledgeable and experienced people giving advice that's 180 degrees apart. My goal is simply to make my car as stable, consistent, communicative, and capable as possible. I don't race competitively yet, but plan to soon (most likely not with this car, though), so I look at it primarily as a training tool. I don't want to go too crazy with additional mods (I already have one of pretty much everything RE sells), but if there's an opportunity to address a known flaw without creating unintentional consequences that are just as bad without having to spend too much time or money, I'd be interested in pursuing it.
If I decide to correct my front-end geometry, are there any rules of thumb as to front-to-rear spring rate relationship, sway bar settings, tweaks to the alignment, etc. to follow to minimize the front-to-rear roll center mismatch without going to the modified rear torsion bar carriers? Thanks.
1. The diminished camber change mentioned by the OP in his first post
2. The longer roll couple, leading to a tendency to sway
3. The strain on the ball joint pins you mention
4. The tendency to bump/roll steer caused by non-parallel tie rod arms
Karl was primarily concerned with the mismatched front-to-real roll couple that could result from correcting the issue in the front without making similar corrections to the rear. And yes, I've read through threads discussing various solutions to the rear roll center geometry. While these solutions are fascinating, this is more than I'd like to take on right now.
My dilemma is that I'm seeing very knowledgeable and experienced people giving advice that's 180 degrees apart. My goal is simply to make my car as stable, consistent, communicative, and capable as possible. I don't race competitively yet, but plan to soon (most likely not with this car, though), so I look at it primarily as a training tool. I don't want to go too crazy with additional mods (I already have one of pretty much everything RE sells), but if there's an opportunity to address a known flaw without creating unintentional consequences that are just as bad without having to spend too much time or money, I'd be interested in pursuing it.
If I decide to correct my front-end geometry, are there any rules of thumb as to front-to-rear spring rate relationship, sway bar settings, tweaks to the alignment, etc. to follow to minimize the front-to-rear roll center mismatch without going to the modified rear torsion bar carriers? Thanks.
His comment about the roll couple distribution change is spot on. As I mentioned earlier in the thread, I can feel a slight disconnect between the front and rear of the car, but I was 100% expecting this, and I hope to find a Kokeln rear end or similar at some point of balance it out. But, the car's previous setup was so front end-limited, and its current state of handling is light years ahead of how it was before, so I am happy for now. The rear is still predictable and has plenty of grip, but it simply doesn't react as quickly as the front. I went with stiffer springs back there, which is a good band-aid for the time being.
Oh, and as for bump steer, I have never actually measured it on my car, but I never felt any bump steer even when my car was lowered with severe control arm angles. The arms stay relatively parallel with the tie rods in the 944 regardless of chassis height (check the very first picture in this thread!), unlike some other chassis (early 911, for example). The only reason the bump steer tie rod kit is needed is to bring the tie rods back in line with the control arms once the ball joint pin is lengthened.
Last edited by Droops83; 09-24-2017 at 02:59 AM.