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What do you mean by the term "rocker bar"?- never head that one used before. If you are talking about the upper rear link Roger does these and as I remember Carl at 928 Motorsports does a kit of some kind as do Powerflex.
If you are talking about the main bush in the rear arm as I suspect you are, I understand Carl does a urethane alternative for this and when I spoke to Powerflex recently whilst in the UK they advised that they are developing a rear arm bush kit.
That is what I [correctly or incorrectly] refer to as the Weissach joint and as I am aware there is no component available in the market to replace it. There seems to be a consensus that it is not really a wear part but whether that is factual or a myth I have no idea.
Mine seems to have a fair amount of movement in it. I hope that Powerflex hurry up with that kit. It would be good if they come up with a bush solution for the rocker bar, or least that's what it's called in PET. I've already got the sway bar and link bushes.
Caution when replacing the Weissach joint with other materials. This bushing helps control the Weissach system, and replacing it with a harder Shor-rated material (like polyurethane) will affect how the Weissach behaves in corners.
Here is a short review of how the Weissach passive rear steering system works:
Powerflex are looking at the bush part number 11 on the PET diagram, not the item you are concerned about.
Presumably the assembly you are concerned about is designed to flex to some extent- are you sure you understand what is normal and what should be considered excessive?
Got an email back from Powerflex and they are / have got the whole rear suspension under development including the joint.
Hi Scott,
Thanks for your email. You’re in luck! We’ve actually got some rear parts in development for the 928. We’ll be offering the following parts, including a CNC machined replacement rocker arm inc, bush
REAR TRACK CONTROL ARM FRONT INNER BUSH REAR TRACK CONTROL ARM FRONT ROCKER ARM & BUSH REAR TRACK CONTROL ARM OUTER BUSH REAR TRACK CONTROL ARM REAR BUSH REAR UPPER CONTROL ARM OUTER BUSH
We’re still a couple of months away realistically but this should pretty much cover everything on the rear for you, completing the full vehicle range of bushes.
I sent Powerflex another email with the concerns raised by Carl and promptly got a response. I'd imagined they'd know what they're doing given this is their business and has been for years. I'm actually itching to get these in and drive the car. Like I said before the difference in my M3 was amazing, so if that's anything to go by I should have a great driving car again.
Hi Scott,
The image I sent you was a concept rendering to show our intention, not an exact illustration of how the end design of the Powerflex Rocker Bar and Bush will be. We are mindful of the Weissach set-up and the requirement for some movement to ensure we retain some of the passive rear steer and the positive effects of this system. The concept of our bush/bar is actually not that different to the original in terms of its functionality, apart from it uses polyurethane rather than rubber. The CNC machined bar will have an outer polyurethane ‘sock’ very similar to the original in its design and then we will be using a polyurethane bush at the end of the bar. The OE rubber bush is actually very stiff as standard so it’s not something that we’re looking to go drastically harder with. Our goal is to offer slightly less compliance, but with enough movement to ensure we are still seeing wheel toe-in as the Weissach axle was originally designed to do. It will be a replacement solution to the OE part that is no longer available.I hope this helps explain it better.
On the face of it that seems to make a lot of sense. Over the years there has been a lot of discussion regarding the pros and cons of pinning the Weissach. With modern tyres being more grippy and some of us fitting ever wider rubber the concern was that of producing too much reaction so maybe there is a case for a replacement that is a bit stiffer as it were. Unfortunately I do not have the ability to tell whether my setup is overreacting or not all I can say is if it is, it sure as heck has fooled me.
It would be interesting to know if there is a test or method to determine whether this item is shot [or not].
As for functionality of that particular part, kindly consider that the offset of the rear wheels is perhaps the biggest wildcard variable in how much steering correction is made. To my knowledge, the factory added no extra consideration even as various wheel offset and spacers(!) were offered in factory-delivered cars. Now as we see wider and wider rubber, the effects of the changes in offset are multiplied. There's a reason why racers pin the bushings when they install wider stickier tires on wheels that stick out further.
in this case it means drilling a hole though the pin so that there are 2 bolts going through the "pin" and it can't move. This will disable the Weissach rear steering. With really wide 335's on the back I have thought about it more than once.
in this case it means drilling a hole though the pin so that there are 2 bolts going through the "pin" and it can't move. This will disable the Weissach rear steering. With really wide 335's on the back I have thought about it more than once.
SG: Don't know why this casual thought popped into my head -- There's no torque-steer with stock the Weissach setup until the tires stop spinning. I'm not sure I've seen your car under that condition except while it was parked. I have a limited exposure period. Seemed more like you were throttle-steering...
01-03-2018, 09:06 AM
