Carl Fausett

This is my third post on polyurethane bushings for the rear suspension of the 928.

The other two were the lower link bushings:
https://rennlist.com/forums/928-foru...-bushings.html

and the Weissach spring bushing:
https://rennlist.com/forums/928-foru...available.html

The basis for these products is that we believe the Weissach rear link system on the 928 is a good thing, and I think it's a shame to pin it or remove it. Mark Anderson taught me that his White Zombie was indeed faster with all the bushings removed and replaced with spherical rod ends - but at the loss of forgiveness. He said it was much harder to drive fast and easier to get into trouble with the Weissach gone.

On the other hand, there are ample reports from many knowledgeable racers and performance enthusiasts to state that the Weissach can and does over-react when the 928 is fitted with wider tires or a wider track or both. These items (spacers, more negative wheel offset, and/or wider tires) increase the loads on the old rubber suspension bushings and are responsible for the Weissach moving too far, too fast.

So my idea was "Can't we find a compromise in the middle somewhere?"
I did not want to pin the Weissach, yet I did want to equip it to handle the wider tires and wider track a little better. And something easy for the average guy to install, with no welding or permanent mods required.

That's what brought me to find and fit the polyurethane bushings in to the 928 rear links.

This item is actually the first polyurethane rear link I developed - I had it with us at Road America in October 2012 for testing, but we did not get a chance to put it in. I have been driving these parts on the street since then, however. I like them and I have not had any trouble with them.

You may think these links are a little pricey ($240 to do both sides), and it is because we have to re-machine the stock links to accept the poly bushings. Then we assemble them for you (pressing the bushing and center sleeve into position). The new poly bushings are Shore 88-A and graphite-impregnated. I like the graphite impregnated bushings because they are self-lubricating and do not squeak.

Not for everybody, I am sure, but here they are none-the-less.

Complete information and pricing are available on our website here:
http://www.928motorsports.com/parts/banana_link_kit.php
We are charging a refundable core charge so I can keep a steady supply of stock upper links on hand.

They are very simple to install. Both sides can be done in about 1 hour. The installation instructions are available for your review here:
http://www.928motorsports.com/install.php

Thank you for your interest.

Lizard928

Carl,
I really don't understand how you can make products like this......

The factory had a ball cup/conical washer design to allow for aft movement.
Your design eliminates the possibility of that movement.
So as the weissach axle articulates forward (or rear) the upper links will no longer be able to rotate forward. This will then put stress through the upper link. Eventually the upper link will snap in half from these forces.

This is by far the scariest part you've made yet......

Carl Fausett

Again, I did not list this until I had been driving it for quite a while. Its on our Stage III prototype car and seeing plenty of HP.

Yes - you are right - the stock rubber bushing is semi-spherical in shape and allows for easy movement of the link when the lower Weissach arm moves.

And that is exactly why we changed from a semi-spherical bushing to a straight-sided bushing - because the link was moving too easily and we want to firm it up. This does not eliminate that movement, there still is compliance in the system. There is just less compliance.

But there is no evidence that the upper link is going to snap. That's conjecture on your part. If uncomfortable, don't buy them, wait for me and others to have them on our cars for a number of years first. That's usually how it goes.

Lizard928

If you want them more rigid, ok.
But keep them spaced out on the shperical style ball. Eliminating thus reduces their designed ability and is dangerous.

Carl Fausett

I agree with you that eliminating this links ability to move in response to the Weissach trailing arm's movement would be an error. That would cause an increase in stress on other members of the Weissach system, resulting in a possible failure of one of those links/members. That is why I have been careful not to eliminate its ability to move.

With respect to the rear link breaking, I can only report it has not. Granted, this car is equipped with all our polyurethane rear bushings, each with a subtle increase in resistance (but not the elimination of) Weissach movement.

I respect that you have not driven this car or any other with this modification, and I wish you knew how good it feels on corners. It's awesome.

Vilhuer

Both rear subframe and hub have conical mounting surface for conical alu piece at center of stock bushing. Does flat bushing mean that one side of polyurethane is pushed into this conical shape when nut is tightened?

Vilhuer

Based on installation instructions answer is no. There is flat washer between hub and poly bushing. I see problem in this. Bolt and nut are pulling things together and outer edge of that washer is taking up load leaving air gap around bolt shank. Way to fix this to add extra conical washer and possibly another flat washer to take up slack so that space is completely filled up.

AO

I'm interested in the bushings, but more curious where this thread goes...

Lizard928

Ok,
I will expand,
By bolting these with a washer directly to the hub you're removing the links ability to move for/aft.
When the weissach axle moves forward or rearward, the upper link must move for/aft as the hub does not have the ability to rotate due to its attachment to the lower control arm. Put another way, the hub moves for/aft with the lower control arm.
By eliminating the cone and standoff, you prevent the for aft movement.
If the axle is pinned this MAY not be problem. Unpinned and this is a accident waiting to happy, not if, but when.

Tony

What "Almighty O" said.
I have my entire rear suspension out right now and all of these mods would be easy to do. But, its hard to complain with a 179K miles on them..they work and have not failed in 25 + yrs. . I know every thing is worn..it has to be.

I guess the question I have is why not put the resources into replacing it all with new "stock rubber and bushings//conical washers etc etc" as it came from the factory and offer a "street" kit.
99% of us don't push our car any where near the limit Carl or any other person that tracks the 928 does. Im just looking for a top notch replaceable "stock" type part/bushing. I would love to replace every piece of rubber on the rear suspension...with new stock rubber/washers etc and use it as it was designed and intended.

I honestly don't know the nitty grittys on how the weisach rear works but I think firming up one area or restricting its movment only transfers stresses else where along with perhaps unintend handling results? I don't want a firm bushing here...then a 177k/25 yr bushing there etc.

Some of you guys really know these cars...I wouldn't have even thought about what Lizard of Vilhuer said.

Mrmerlin

..

Carl Fausett

Errka: Thank you for downloading the installation instructions Erkka. I was going to answer your question, but see you already found the answer yourself.

Once again, if you would like to download and review our installation instructions for this part, they are available here: http://www.928motorsports.com/install.php

Tony: I have been watching the availability of replacement suspension bushings at Porsche for more than a year now... frankly, if all bushings that we needed continued to be available, then the need for after-market replacement bushings was reduced and I might not bother to come up with a replacement. I have seen them go from "none in stock with no specified delivery date" to suddenly "there are 40 pairs in stock", as an example. They do seem to be trailing off, some bushings are at zero stock (last time I checked) and others are at low stock. So I decided to proceed with making these poly replacements as I feel that is is somewhat inevitable that the OEM parts will be leaving us.

That said, this particular link is the most aggressive change I have made. The others (rear lower link and Weissach leaf spring) bushings were pretty straight-forward. A rubber part for a polyurethane part with virtually same dimensions. (Items 39 and 40 in the exploded view on Post #1) But this one replaces a semi-spherical bushing with a cylindrical bushing and the aluminum link is re-machined to receive the cylindrical bushing. So I understand and expected some concerns from others.

I like them, and I am enjoying them. But no doubt not everyone will feel the same way. You may want to call your favorite supplier soon and try to grab some OEM bushings while there are a few available.

Carl Fausett

I disagree. I would agree that we are adding resistance to the for-and-aft movement of the link (that is what I am trying to do) but in no way are we eliminating its ability to move.

Colin: what do you feel is the range of motion of the upper link? From the rear tires set at factory alignment spec (+10' plus or minus 5' ) to full Weissach movement during throttle lift? I can't find a spec for it, but it seems to my measurements that the Weissach allows a max of about -10' toe in, for a complete range of motion of 20'

Translated to the upper link, the range of fore and aft movement is not that great, and certainly obtainable by these bushings. Remember, I have not locked it down like Mark A did with his solid welded links (which also did not break) and we have more compliance in our system than he did.

Lizard928

Carl,

The solid upper links that Mark A and Mike Simard use have spherical rod ends which do not restrict for aft movement. On top of that, both of these gentlemen have their weissach axles pinned.

The fact of the matter is that you are changing the loading point and how it loads. How you have changed it is dangerous if the weissach axle is left unpinned.
Your change allows an up down motion, fair enough. But the for aft motion is no longer there. Instead you're putting a great deal more stress on the bolts, and the "banana" link.
If you add a conical washer and space the link back out like the factory did you will reduce the stresses greatly!

Carl Fausett

I can look in to that. And I agree that would reduce the stresses on the link. But it would also reduce the resistance to movement that is the point of all of this. In other words - regards limiting the Weissach's motion, it would then be less effective.

I don't often quote or refer to Wikipedia, but this quote from this page
http://en.wikipedia.org/wiki/Weissach_axle
is interesting: