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Bent 'blades' on rear lower control arms

 
Old 07-11-2019, 04:47 PM
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skpyle
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Default Bent 'blades' on rear lower control arms

Hello Gentlemen,

I need advice on the rear lower control arms for my Red Witch. Last year, I bought the complete rear suspension from a 60,*** mile 1988 S4. All of its components are in better shape than what is under my 928. I am using all the new parts to reassemble the rear suspension.

However...the 'blades' on the lower control arms of the S4 suspension are slightly bent. Not terrible, but not straight. IIRC, past threads on Rennlist have said "THIS IS BAD." The 'blades' on the lower control arms that came out of the Red Witch are perfectly straight. Unfortunately, one control arm has a stuck shock pin, and both have seized adjusting bolts at the front in the mounts. I THINK I can get them all out with select application of violence.

I was operating under the pretense that the bushings and such in the 60,*** mile 1988 lower control arms would be in better shape than that of the 168,*** mile 1986.5 lower control arms. But, the bent 'blades' give me pause.

So...do I use the 1988 lower control arms or deploy violence and use the 1986.5 arms?




Passenger's side rear lower control arm. Best view of the bend in the 'blade.'





Driver's side rear lower control arm. Best view of the bend in the 'blade'.




























Thanks for your time, opinions, advice, insults, and brickbats. Try really hard with the insults, because my wife hates me and my mother never loved me. So I am already accustomed...
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Old 07-11-2019, 05:20 PM
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I would say those are compromised, and not use them. They'll never be really.."straight" again.
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Old 07-11-2019, 05:44 PM
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Agreed. I would not use them either. Once bent, I would think that the properties of the metal are compromised.
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Old 07-11-2019, 05:49 PM
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Seth, my PS LCA was bent from the shippers using it to chain down the car on the flat bed when I first had my '84 delivered (). I asked the same question you did and the consensus was pretty much that it should be replaced with a non-bent one. Here is the thread about it.

https://rennlist.com/forums/928-foru...ntrol-arm.html
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Old 07-11-2019, 06:41 PM
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I used to think they were spring steel and once bent, they were junk.

I no longer believe that...welding to spring steel turns it into scrap metal....and there are giant welds on these pieces. I've straightened several, with minor bends like yours, without any issues.
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Old 07-12-2019, 01:40 AM
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Thank you, gentlemen.
I will begin imparting violence onto the original control arms from the Red Witch today after I finish assembling the parking brakes/flange hubs to the rear wheel bearing carriers and the Koni/Eibach rear shock/spring combos.

I will use the control arms that came with the Red Witch and hold onto the 1988 S4 control arms for whatever.

Thanks!
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Old 07-12-2019, 04:50 AM
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Originally Posted by skpyle View Post
Thank you, gentlemen.
I will begin imparting violence onto the original control arms from the Red Witch today after I finish assembling the parking brakes/flange hubs to the rear wheel bearing carriers and the Koni/Eibach rear shock/spring combos.

I will use the control arms that came with the Red Witch and hold onto the 1988 S4 control arms for whatever.

Thanks!
Seth,

If your original arms are straight then simply refurbish them with new bushes and be done with it.

Regarding the S4 arms the question I would ask is how did you end up with slightly bent arms on a spare part you purchased assuming that is not too painful to answer? These arms do not wear out and s/h units are plentiful in your part of the world- on a good Ebay day or whatever you can probably pick them up for $100 or less so in that context why would one ever bother considering repair?

With respect to repair, for purposes of discussion and especially for those "on the other side of the world" where s/h parts are simply not available, if my rear lower arms were like those S4 arms you have, if I needed my car and that was all I had at my disposal I would repair them without hesitation.

The logic is quite simple- as GB notes above the arms are welded so the notion that a "spring steel" was used is engineering folly or so I would think- one does not weld spring steel- no need for debate on that- either one understands such or one does not. That welding has been used tells us that Porsche probably used a reasonably fine grade of carbon steel given the arm is a safety critical element that has a dynamic response. Unfortunately we do not know the grade of steel but I do not think that is a major issue. Cold bending of carbon steels is not an issue within certain bounds and I would say that the cold bending that was applied to those arms is of no consequence. In the process industries we routinely cold bend pipeline to defined [conservative] limits and they can carry some seriously dangerous cargo! If we need tighter bending radii hot formed induction bends are specified. Thus on a simple system like this apply some heat [grey/red] across the section to be bent, quickly put the arm in a large vice and close the jaws over the slightly kinked section.

If steels with really high tensile strength values were used I might be a bit wary about trying this but then such steels are really difficult to weld and using such on this application would not make sense.

Would Porsche sanction such repair?- of course not but then they would not hesitate to charge you $3000 or whatever for a new arm.
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Old 07-12-2019, 11:02 AM
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Originally Posted by FredR View Post

The logic is quite simple- as GB notes above the arms are welded so the notion that a "spring steel" was used is engineering folly or so I would think- one does not weld spring steel- no need for debate on that- either one understands such or one does not. That welding has been used tells us that Porsche probably used a reasonably fine grade of carbon steel given the arm is a safety critical element that has a dynamic response. Unfortunately we do not know the grade of steel but I do not think that is a major issue. Cold bending of carbon steels is not an issue within certain bounds and I would say that the cold bending that was applied to those arms is of no consequence. In the process industries we routinely cold bend pipeline to defined [conservative] limits and they can carry some seriously dangerous cargo! If we need tighter bending radii hot formed induction bends are specified. Thus on a simple system like this apply some heat [grey/red] across the section to be bent, quickly put the arm in a large vice and close the jaws over the slightly kinked section.

If steels with really high tensile strength values were used I might be a bit wary about trying this but then such steels are really difficult to weld and using such on this application would not make sense.

Would Porsche sanction such repair?- of course not but then they would not hesitate to charge you $3000 or whatever for a new arm.
This is what I am worried about - we do not know the properties of the steel used, and re-bending it, especially done without heat, could cause small fractures that manifest in a failure when you least expect it. If I were to repair them, I would use heat as Fred suggests. Do not simply re-bend them back. I did not think they were spring steel. I just don't like re-bending steel without doing it the right way and knowing what I'm working with, especially on a suspension part. This is a plentiful and inexpensive used part, so using the one off the red witch or getting another from 928 Intl or 928 Specialists would be what I'd do.

One additional comment to Fred's excellent post - cold bending steel as described is not quite the issue here - in essence, it was "cold bent" already when it was originally deformed. Re-bending cold bent steel is where the concern lies. Bend once - okay (within proper limits). Bend back again, now there are stresses that need to be relieved (if I remember my metallurgy science from my college and manufacturing engineering days).
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Old 07-12-2019, 11:11 AM
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I'm sure those get bent from people hooking tow ropes or tie downs around them.

A side benefit of Carl's front end protection plates is that they provide a solid, non-damaging place to tow or tie down to, along with his lower suspension mounts in the rear and one of the reasons I have both.





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Old 07-12-2019, 12:30 PM
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Originally Posted by linderpat View Post

One additional comment to Fred's excellent post - cold bending steel as described is not quite the issue here - in essence, it was "cold bent" already when it was originally deformed. Re-bending cold bent steel is where the concern lies. Bend once - okay (within proper limits). Bend back again, now there are stresses that need to be relieved (if I remember my metallurgy science from my college and manufacturing engineering days).
Ed,

That is spot on the money. I learned that lesson the hard way on my Triumph 650 way back when - bent a foot rest on the thing- no sweat- piece of scaffolding on the end and was about to give it some welly when my friend [a well versed motorcycle rider] screamed out to stop what I was doing. He barked in my ear "if you do that it will snap". He had a torch in his shed- heated it until it was glowing red and said now reform it- it fell into place like putty. Like as in the piping, it can cold bend in one plane but trying to counter bend the crease risks inducing micro cracks and ultimate failure somewhere further along the line.
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Old 07-13-2019, 06:13 PM
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Thank you for all your advice. I am going with the arms that came on the Red Witch. I will probably do nothing with the bent S4 arms. I am not much of a fabricator or one who works with metal.

Thanks!
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