GregBBRD

The front lower arm bushing is vulcanized to both the lower arm and the aluminum outer mount. The rear bushing is vulcanized to to arm, only, but gets "pinched in position very tight, by the rear aluminum lower bracket (thus the need to tighten the rear bracket only after the vehicle is sitting at ride height.)

If you apply some of your engineering background to this, you will quickly realize that the rubber pieces are a MAJOR part of the "transitional" front spring rate. When the lower arms are bolted into the car by themselves, it takes hundreds of foot pounds of torque to move the arms up and down....and the farther they are moved, the amount of torque to move them goes up. (The rubber is not linear in the spring rate it applies.)

It's such a dramatic amount of the spring rate that 928s become very "evil" when only one of the front lower control arms is replaced....the cars end up with two completely different "transitional" spring rates because of the different amounts of preload and twist on the different rubber parts.

Note that substitution of an aftermarket bushing that moves freely will require RADICAL compensation of the spring rate...and it needs to be a progressive rate to match the original rate, provided by the rubber.

This entire "vulcanized rubber" being a major part of the front spring rate is something thst very few 928 "people" realize...or even consider.

merchauser

^^^should there be any lube, like white lithium used on those bushing to prevent noise? or is it a must that the those bits stay dry?

FredR

Greg,

Not sure what you understand by the term vulcanization but as I am concerned it is a process of chemically hardening rubber invented by Mr Goodyear for use on his excellent tyres. Whether vulcanisation plays a part in how the arms are fabricated is an interesting technical point in itself.

When we pressed the front peg out of the holder I expected the rubber to disintegrate and that happened to some extent on the inside surface- but on the outside surface the bush came out as though it was pressed in. This led me to conclude that the friction between the inner and outer surfaces is greater than the overturning moment resistance caused by the rubber. The front peg on the arm is a flush finish. The bush on the rear arm is a composite item and the forces applied by the clamp bite into the rubber and deform it to create the grip on that surface, the rear peg is serrated presumably to help the rubber grip and the rear bolts are torqued to 88 ft lbs presumably to control the grip applied by the mounting collar so not difficult to understand that the rear has to be tightened under settled conditions. Whether or not the front rubber has different properties to the rear most rubber I do not know but could not perceive such. Given that the car has to settle before the rear clamp can be made and it takes some time for that to happen that tells me that the front bearings alone are putting up a lot of torsional resistance.

In a recent discussion about the Powerflex bush concept I was taken by surprise when the Aussie chaps advised that it slips by design with little resistance. This indeed suggests that the resistance absorbed by the original arms must be transferred to the coilovers. What proportion of the load is involved remains to be seen - whether my Eibach springs [25% stiffer] are up to it remains to be seen but with the stock arms they are a bit harsh.

The lads down under seem impressed so I figured worth a crack. Another point of view is that if the stock bushes are age hardening as I believe they are, not only will they creak but they may also adversely impact the coilover performance for the reasons you mention.

For sure it is not a simple or straight forward matter and any modifications need to be well considered if they are to be beneficial.

GregBBRD

I interpreted vulcanized as "rubber permanently bonded to aluminum".....I'll substitute in "glued", if vulcanized is the incorrect word.

The front rubber bushing on the lower control arm is "glued' to the control arm and to the outer mounting bracket. The rear rubber bushing is "glued" to the control arm only. Both the front and the rear bushings do not "rotate" on either the inner or outer surfaces, but must "twist" or "deform" as the suspension travels. Note that under compression, this "twist" becomes part of the spring rate...and plays a significant role in shock dampening (which is one of the reasons that 928 shocks last so long.) In rebound, the "twist" is actually assisting the spring to return....and the shocks are valved to compensate for this.

At this point in time, it is extremely common to have one or multiple of these rubber bushings to be deteriorated. It is common for the front rubber to rip loose from the control arm....which means it no longer is performing the intended "spring rate" function. The rear control arm bushings will tear off in chunks. Note that this never seems to occur on both sides of the car, at the same time. Sensitive drivers (I have a few) will be able to detect that the spring rate is different from one side of the car to the other and be able to tell me that there is "something wrong" with the handling of the car.

For fairly obvious reasons (these rubber pieces harden and eventually will "take a set"), front control arms must always be replaced in pairs.

I have no idea of your driving ability. However, if you install lower bushings without "twist", you should need to not only compensate for the reduced spring rate, but the required shock valving will radically change, also.

FredR

On the unit I pulled apart I was looking for signs of a bonding material but could not see any whatsoever and that puzzled me. I had both the lower arms off the car last year when I did the upper arm bushes. I [crudely] inspected the lower arm bushes with a pointed tool just to see if the rubber was like concrete or whether it showed signs of elasticity and to my surprise at the surface level it seemed remarkably rubbery. Of course that does mean a whole lot but it was something I did not expect to see. I have mental visions of the substrate looking like crazy paving after all these years!

That the lower arm is causing the resistance to settlement is without doubt a sign of just how much resistance it puts up as it twists and that is when it is sat at ride height never mind under dynamic loading- my assumption is that this is intended by design to minimise nose dive under braking which the 928 does very well at. As to whether I am good enough to tell when something is going off in the handling department that is a moot point. When I first started doing the autocross events I was convinced I would never be able to tell the difference of what I did in the alignment department but I surprised myself in that I could actually feel some subtle differences so who knows? I do know that I have this mystery occasional creak that I do not like, as you might have seen from my post yesterday it is seemingly not caused by the [non existent] fingers catching the bracket so I am back to the lower arm as the likely cause. I suppose it could also be the rose joint in the Devek bar but they looked fine on inspection.

I am thinking of writing to Powerflex to get their take on the very points you raise about torsional stiffness of their bushes as it does concern me given they are reportedly meant to be sliding joints and that de-facto implies much less resistance to torsional movements. My interpretation of their design is that their front bush is not doing much in terms of torsional resistance but perhaps the rear bush does more when torqued down given the stiffer material they are made from. There has to be a reason why the rear bolts on the stock setup are torqued to 88 ft lbs when the front ones are 50 ft lbs and my presumption is that this is required to transfer torsional resistance into the bushing material. Something similar happens with the upper arm bushings in that until the end bolts are pinched up there is little resistance to movement but when fastened up the bush then resists the suspension travel.

I always enjoy trying to rationalise engineering issues outside my knowledge base and even more so if and when I happen to get such rationalisation correct!

GregBBRD

If you bolt a new lower control arm (all by itself) onto the front of a 928 and apply force to move it up and down, you won't need to call anyone to know how much the substitution of "free floating" bushings is going to alter the front suspension....your engineering background will instantly logic out the impact of such a change.

The forces involved are literally hundreds and hundreds of ft lbs. throughout the range of motion (the force also increases as the travel increases.)

Spring rate and shock valving are completely dependent on the condition of these small rubber pieces, which are largely ignored by the 928 community.

merchauser

not intending to hijack your thread, but will be installing crossmember and wonder about lube on those bushes

fred, did you get this resolved?