GregBBRD

You'd be amazed at what I see, on a daily basis.

FredR

This is the crux of the matter and precisely why I took the view that the Powerflex option, although interesting, is not technically viable with a stock suspension setup. Your use of the term "progressive" is also key to the concept as that is exactly what it is- progressive- not linear. What I noticed more than anything else first time I drove a 928 over 20 years ago was how little the car dives under braking. At the time I had no idea why this was but when I started playing around with the suspension doing upgrades and the like a few years later it became obvious the role that the arms play in the process. I also noticed how over the years it took less and less time for the suspension to recover after a "wheels in the air" event.

As the arms age they will invariably develop micro cracks in the rubber material and somewhat ironically, the nature of rubber being what it is, lack of use will doubtless accelerate this process as rubber tends to take a permanent set when it is not flexxed- leave it too long without movement small cracks will initiate when subsequently flexxed - nothing dramatic but nonetheless degredational in nature and that can only accumulate. Same process applies to tyres and [timing] belts.

An interesting aspect is the difference between the front bush design and the rear bush design- i have never seen this aspect discussed in this list. I have seen opinion expressed that the bushes are bonded but having removed both front and rear bushes from an arm it was very apparent that they were not bonded in place. The rear bush is indeed pressed into position [or so I concluded]. The spigot has serrated rings and the outer surface of the bush is also serrated. The external clamp is plain. With age the external surface of this bush deforms and the rubber "flows" into the small gap between the clamp and the body the arm seats in. When removed one sees two peaks either side of the bush so the position of the arm is now specific in terms of rotation. Thus the "peaks" on the rear bush develop with use and as can be seen such are not cast into the original fabrication. Once those peaks form the arms are no longer the same both sides and thus develop a form of handedness making them unique to the side they were originally mounted on thus if removed critical that they be remounted on the correct side..

The front bush is manufactured unipositional with the axis of the mountng bolts perpendicular to the axis of rotation such that when the arm is horizontal the bolts are vertical and thus when the arms are new they are the same both sides [same part number] irrespective of which side it is mounted- i.e. the arms are geometrically identical and not handed. Thus once "run-in" they are no longer identical and are then in effect unique to one side only. I'll bet Mark Anderson can look at a set of arms removed from his donor cars and can tell immediately which side they were originally fitted on.

Now the "interesting bit". Why did Porsche make the mounting design different "front to rear"? The front mount has two bolts [same size as the rear bolts] but the front torque is 50 ft lbs and the rear bolts are 82 ft lbs [or whatever the spec says]. It would seem somewhat obvious that the rear bushes are taking much more of the lateral load given the way the spigot is 50% captive in the body half of the mounting frame thus the front bushes probably peform a stabilising role as muc as they do rotational resistance. The notion that the front bushes are bonded is not correct and as stated the rear bushes are a press fit on th spigot. That the front bushes are simply a "press fit" is the bit I have a problem with. If both bushes are a press fit why does Mark have to send them to Germany for manufacture? Maybe it is simply a quesiton of "they are the only folks who make such bushes" and thus have the market cornered. However careful inspection of the front face of the front bush reveals what appear to be some "flashes". In an earlier thread there was some discussion about how the front bushes were fabricated and "cast in situ" seemed to gain some traction- why I am not sure but it made much more sense than "bonded". However they are made is of interest but not really relevant to the discussion, the pertinent question being "how do they work?". The bushes on the upper arm "do not work" until the bolts are tightened and the bushes on the rear of the lower arm do not work until the cap is tightened up. The pivot angle is the same front and rear so one can only conclude that the front bush loading is much lower than the rear bush loading- maybe the front rubber is a different spec to the rear bush and has less resistance to flexure?

Whatever the dynamics, the process is somewhat complicated and the original design is very clever irrespective of its limitations. As I studied the system I concluded that the lower arms probably have a service life of 20 years more or less irrespective of mileage but after that it is a crapshoot. I am pretty sure my nearly 30 year old lower arms are well past their best and the fact that they are still serviceable is problably down to having the stiffer suspension units that will compensate to some extent for the loss of spring resistance in the bushings.

The possible "downside" of the stock system is the impact it has on the ability of the arm to track the bumps it hits. This is where I suspect the Powerflex bushing system may well have a distinct advantage. By nature race tracks are smooth pit hole free surfaces so the suspension does not travel the same. Cornering and braking loads are higher as is their frequency but stiffer springs can take care of that. Whether or not my stiffer springs can mitigate the Powerflex approach is the point that intrigues me. If I were to fit refurbished stock arms would the impact be "too much"? When I first fitted the upgraded suspension system it was on my late S4 that was already some 12 or 13 years old and for sure one felt the difference with some comfort being traded for peformance.

Finally, good to see that Kevin has taken the "smart approach". I suspect that just about every 928 out there that has original lower arms is in effect "naturally downgraded" to some extent or other in handlnig performance terms no matter how well they have been looked after. For sure Kevin's current lower arms are well and truly knackered and will remain so until replaced

Speedtoys

This is also highly likely, the "spring sag" people are on about.

Your arms are slowly failing..failed..and there is 100s of lbs of spring rate at the bushings. Greg threw a strain gauge in there...its a lot.

FredR

Jeff,

I certainly believe there will be quite a large torsional reaction when the arm is compressed severely and similar if the car goes airborne but when the car is static there should be no reaction from the bushes unless the car is "stoopid low" as in the kind of setup Mark K [understandably] deploys for his track work.

It is quite obvious that the suspension design considers "normal ride height" to be when the lower arm is horizontal and presumably loaded with fuel and a driver give or take a bit, if this was not the case then there would be different arms for each side. This in essence was the problem I had when refitting the arms after I had them off the car a couple of years ago when I wanted to inspect them [following a mysterious creaaking sound] at the same time as I was changing out the upper arm bushes and fitting a used upper arm [with new bushes] on one side where the ball joint was shot. Then I opted to drive around [slowly as in 50 kph or less] over as many speed bumps as I could find with the rearward bush mounting cap held in place by bolts that were little more than finger tight and doing my Fred Astaire impression on the brake pedal - all because I had no possibility of pulling the car down at the time and did not want to attempt torquing the thing up out of position. Not following the pull down procedure could easily wreck the rear bush [or so I would think] and in the case of a newly refurbed arm- put a permanent set that is "incorrectly positioned".

Kevin in Atlanta

I have Mark's bushings in hand. I am about to drop the LCAs.

Looks like all I need to do is remove the two bolts that secure the articulated carrier (ball joint) and the LCA should just drop, right?

I scribed around the eccentrics and the allen key hole so I can it close enough to drive the 10 miles to my alignment shop. Did I miss anything?

The DS front bushing is secure so it is easy to mark the orientation of its housing. The PS is trashed. So how do I know where the bushing housing is relative to the LCA.

From the DS the front housing is a few degrees of the LCA. Assuming this is correct, that would seem to preload the LCA. Do I invert /match the PS orientation?

Newbie at this.

Kevin

FredR

With new lower arms you really need to deploy the pull down procedure before torquing up the rear most bushes.

Kevin in Atlanta

In my case the LCAs are not new, the bushings will be pressed on to the LCAs by my shop. I was interested in was are the housings square with the LCA or off angle.

I need to communicate that to my shop.

FredR

I was not aware that the bushings are available as separate items and that explains why I did not fully understand your query.

The arms are identical part numbers and the front housing therefore must be square to the horizontal axis of the arm. Indeed someone actually posted a picture of some refurbished arms and on one of them there was a very noticeable offset as in like 30 degrees. I pointed this out to the OP of the thread but heard nothing further.

I am planning to do my lower arms next cool season thus my interest in your exploits. If you have the facilities to do the pull down procedure all well and good -unfortunately I do not at the moment. However I have been thinking about the problem and suspect it may be possible to achieve the same end by compressing the coilover spring, mount the arm horizontally, torque the arm up, fit the bolt on the bottom of the coilover and release the spring compressors. Seems reasonable that Porsche intended that arm to be horizontal when normally loaded [driver/full tank of fuel- maybe a passenger?] so reasonable that the rear bush should be nipped up with the arm in that position to avoid pre-load. If that was not the case then there would be handed variants with a designed in offset but of course that is just not the case.

Geo55

Kevin, If you look at pics of new or rebuilt LCA bushings, you can see that they are slightly askew. would be nice to know what that angle would be. Keep us in the loop please.

Kevin in Atlanta

Here's a picture of Mark Anderson's offering:

DeWolf

I've done the Powerflex front bushes. All I can say is if you haven't driven a car with them....well.

Tony

If your conrol arm is to the point where you can pull the old ones off, id say your control arms are shot. The aluminum gets worn and polished. My pax side was bad. It would squeak and when I went to take it all off, the bushing pulled off with no effort.

i bought two “new” arms from mark.

i alos scribed the eccentrics for a ball park drivable alignment.

Tony

..as far as the pull down. I just left the rear lose..or snug..however you want to view it. (or was it the front ..cant recall..) then drove around the block a few times ..nothing remotely crazy...in and out of the driveway several times...then went underneath with out lifting the car and torqued them to spec. You want to avoid preload and have them loaded in a natural postion.

GregBBRD

Sometimes a little bit of knowledge is less than desirable.
Almost every 928 owner and mechanic, on the planet, is absolutely clueless about the intricacies of the 928 suspension.
(Sorry, but it's true....it's complex!)

"Pulling down" and where/when the rear lower "cap" get tightened down are totally dependent upon ride height.
Read the above again.
......Mixing factory instructions/pieces when doing something and using other than factory "set-up" specifications, is almost never correct.....and this job is the "poster child" for this.
Read the above again.

The factory front ride height is 183mm+/-10mm.....and there's a really good reason they use this specifications. I'll keep it simple, for this discussion, and just say that this is where the suspension was designed to work.
If you are running this ride height, use the factory instructions on how/when to tighten the rear lower cap. Use the factory suspension pieces, which were designed to "work" at this ride height.

.....Most people think that the 928 looks silly even at 173mm (I agree) and they set their ride height lower than this.
While not getting into the ramifications of setting the ride height lower (and the limits of where the ride height can be set without making modifications to the pieces) I'll just say that setting the ride height lower than 173mm requires a different procedure to tighten the rear lower caps on the front control arm. It also changes where the front lower control arm rubber bushing is "set" initially....along with a few other really important changes.
If the "target" ride height is 183mm +/- 10mm, the front rubber bushing works great starting out level with the control arm.....because that is where the rubber needs to be at that ride weight!
If the target ride height is lower than 173mm, all bets are off and the front aluminum outer housing no longer can be level with the control arm and needs to be changed! (Right and left lower control arms are no longer interchangeable, but specific to each side of the car.)

An easy way to wrap you head around this aspect of the 928 suspension, is to remember that the "goal" should be to have the front lower control arm tightened down so that the rubber bushings have no preload (in either direction) at ride height, so that they can have bump and rebound in equal amounts. This allows the rubber to articulate in both bump and rebound, without the rubber being overly twisted beyond its design limits and be weakened/torn.

Darklands

Very good explanation Greg! Thank you!