[wonko]

This is a continuation of a discussion begun in the following post:





Pablo's 968 Racer, 95 968 NASA GTS and/or PT Bound Racecar is Battle Ready



Unfortunately it has come to light that there have been several failures of the ball joint pin (or the pin that goes from the A-Arm to the spindle for the wheel) under race conditions. This has resulted in events as mild as our 968 Firehawk in which the wheel was pushed into the fender, but little damage was done, to Pablo's unfortunate (and catastrophic) introduction to a concrete barrier.



In discussions with Pete and in the above post, it has become apparent that there is an issue here that needs to be addressed regardless of manufacturer. The incidents have happened with Porsche factory racing, Fabar, Kokeln Racing, and Charlie arms. Others are almost certainly suspect as well.



Porsche A-arm pins. Discuss....

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[flash]

yup - good idea to start a new thread over here - this is a major concern - i don't think it is the control arm or ball joint at fault as much as the physics and geometry - this is a touring car that we are all trying to make do something it was never intended to do



i have been watching this carefully over the years, and seen quite a few failures like this, with varying brands of arms and such - it is something i am paying close attention to right now as i am preparing to build a race car myself - plowing a wall due to front suspension failure is not something i want to repeat - been there wrecked that - i ran into exactly this problem with another car many moons ago - i learned a lot from it in what to do and what not to do in setup



one problem is the increased friction and load from the newer and much stickier tires - wider wheels and tires also exacerbate this - this makes the connecting components work much harder, and the loads they must bear increase geometrically



it also confirms something i have been saying all along - geometry is critical - lowering the car too far creates a mechanical leverage increase on the ball joint - take a look at the control arm angle relative to the ground at rest - at most it should be parallel to the ground and NOT have the outside point (ball joint) higher than the inner mounting points - when it goes lower, you can see how it would increase load on the ball joint



lower is not always better



if you are going to lower the car so far that the roll center is that low, which makes for other geometrical issues to begin with but we can discuss elsewhere, you will also need to transfer load back off of that outside front corner by means of springs, much larger swaybars, and rearward brake biasing - you can't plow 3000 lbs of car onto one corner like that, without serious reinforcement, and expect it not to fail - unfortunately this is going to mask the larger problem, and really reorientation of the suspension mounting is the correct thing to do - this may push you out of class though



either way, it isn't easy, and there are going to be complications and limits that will be run into



choice of materials of the pins and arms is going to be critical - regular and careful inspection of the pins is going to be extremely critical, and something i would do after every race

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[wonko]

As a discussion starter, I was speaking with the Chief Mechanic for Corsa Racing in Utah. They will be announcing a major shift in their American LeMans effort tomorrow, as an aside. Anyway, the A-arm issue came up. He had seen similar things in other cars before, and cautioned against over strengthening the pins. The problem as he saw it was if that part isn't the weak point, other parts will fail instead, often with even more catastrophic and expensive results. Basically, if you beef it up too much, the A-arm will break instead, because something has to give. As the pin is the cheap part, let it be the weak point. The only answer is consistent and vigilant inspections and replacement.



If you are running the Fabcar arms, there is only one place to get replacement parts, and that is OG Racing. The pin can be found HERE



It appears that to truly inspect the part it must actually be removed. In two of the cases the pin had obviously been broken in two separate phases or incidents and could have been caught had the pin been properly inspected. The only problem with that is that since the actual fracture was within the actual spindle, the crack would not have been visible without removal. In short, your pins are not inspected if you didn't actually remove them at minimum from the spindle to confirm their state.



Here is a shot of our Fabcar pin. It has aged a lot and been handled so the lines aren't as clean as they were, but you should get the point:







I would like to hear of any other issues and/or solutions anyone has.



John

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[flash]

i agree with the idea that there is always a weak point - this is a problem with not doing the job thoroughly - when you beef up one thing, you have to beef up the rest, or you end up with exactly this kind of problem



it's either that or slow down, or remove the forces acting on those components



that's why i have been saying that the setup is key - too low is bad - a slightly higher center of gravity is not nearly as bad as a roll center below ground, and certainly not as bad as overloading components due to bad geometry



i see it all the time in amateur racing - unfortunately this is often the result before anybody figures it out - i know that is exactly what happened to me, and why i started drawing out the suspension on cars to verify the angles and such, before i started seriously monkeying around with making them go faster than they were designed to go



before i start building a race car, i'll be laying out drawings of the suspension geometry, and determining exactly how low the car can go before the geometry gets complicated, or the roll center goes below ground

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[wonko]

[quote name='flash' post='58568' date='Aug 28 2008, 05:08 PM']i don't think it is the control arm or ball joint at fault as much as the physics and geometry - this is a touring car that we are all trying to make do something it was never intended to do[/quote]



I think you have it right on. How many of us are running 18" wheels, or wider than stock wheels, or both. There is a price to pay for this on some level. In most cases it is acceptable, but we need to watch for unintended consequences and mitigate them.



I don't think it means we need to rethink our whole setups in all cases, really this comes down to a maintenance issue. You just need to be aware and willing to inspect them regularly. In my case, I have pretty much come to the conclusion that we are going to replace the pins annually regardless, and they will be inspected after any significant front end impact.



In my dad's case he hit the tire barrier and didn't do any sheet metal damage, but did tear off the front splitter and batwing. That should have initiated a much more thorough inspection than I gave it at the time. So don't think significant necessarily means sheetmetal damage. I mean any off that significantly over stresses the suspension. As I don't have any way to give a specific amount, we will probably each have to figure out our own threshold of pain on that, but stick to the regimen regardless.



John

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[wonko]

[quote name='flash' post='58573' date='Aug 28 2008, 05:26 PM']i agree with the idea that there is always a weak point - this is a problem with not doing the job thoroughly - when you beef up one thing, you have to beef up the rest, or you end up with exactly this kind of problem

...

before i start building a race car, i'll be laying out drawings of the suspension geometry, and determining exactly how low the car can go before the geometry gets complicated, or the roll center goes below ground[/quote]



OK, now you are intriguing me. I'm guessing boring a lot of other people, but intriguing me. For me the question is then, how do you confirm this as a layman? I'm just figuring out a lot of this stuff, and need it in simple terms.



John

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[flash]

that would be a lot easier to explain in two different methods, neither of which would be easy in a post



i could do it over the phone (pm with phone number on the way now) or with drawings i have not yet done for this car

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[wonko]

[quote name='flash' post='58579' date='Aug 28 2008, 05:53 PM']that would be a lot easier to explain in two different methods, neither of which would be easy in a post



i could do it over the phone (pm with phone number on the way now) or with drawings i have not yet done for this car[/quote]



I'll look forward to the conversation.





John

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[Eric_K]

I don't think this is a geometry problem, i.e. the stock pin binding on excessively lowered cars. Many of these aftermarket arms offer longer pins for roll center correction on lowered cars. Look at the length of the pin in Pablo's pictures. Mine was also a longer pin.



I believe mine was cracked from earlier contact but was not caught. I will post pictures of mine when I get home next week, but it looks very close to what John posted. That is something that happened over a period of time.



Eric

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[flash]

a longer pin, while moving the control arm back down, adds even more leverage load to it in doing so



the inner mounting locations need to move upward if you are going to lower the car that far, and want to maintain proper geometry

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[Cloud9...68]

Very interesting discussion, as usual. I would think experience with 944's, particularly turbos, would serve as a good database for this problem. Is A-Arm pin failure common on these cars? Granted, they are a bit lighter than the 968, but they're also older, and there are vastly more of them. Thanks.

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[PorscheDude]

Not really per se, more so for cars that are driven hard (racing) with wide sticky tires.

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[Cloud9...68]

[quote name='smokiemon968' post='58645' date='Aug 29 2008, 05:27 PM']Not really per se, more so for cars that are driven hard (racing) with wide sticky tires.[/quote]

That's actually what I meant - 944's are very popular in racing because they provide so much bang for the buck, and with so many of them having been raced over the decades, I would think they would provide a good indicator as to how big of a problem A-Arm pin failures are likely to be in tracked 968's.

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[333pg333]

Link from RacersEdge:

http://www.racersedge-inc.com/racersedge/r...n&id=10.3.5

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[wonko]

[quote name='333pg333' post='58687' date='Aug 30 2008, 04:15 PM']Link from RacersEdge:

http://www.racersedge-inc.com/racersedge/r...n&id=10.3.5[/quote]



I appreciate that post. I have the Racer's Edge A-Arms on the Yellow car that was just totaled. In moving them over to the new tub, I will have to take that into account.



It is also interesting how they make such a point of dealing with the specific geometry issues Flash mentioned.



John

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[333pg333]

Here is another version. This is on a pretty tricked up 968 turbo. Worth checking out on Pelican forums.

http://forums.pelicanparts.com/showthread.php?t=426883

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[flash]

yeah - lengthening the pin could actually make things worse - it would ADD stress to the pin, because the sheer load is now moved away from the fulcrum point, and out onto a lever - a longer pin wold need to be bigger and harder to handle the same load, let alone an increased load - not sure if they did that or not



yes the geometry is improved for purposes of roll angles and such, and this is VERY important, but the loads on that component are increased - i'll let somebody else do the math on how much goes where though, and what needs to be beefed up and by how much to accommodate it



as i said, the perfect answer would be to move the inboard points upward - this would likely be very complicated though, and would certainly bump people out of class

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[333pg333]

The pin with the REdge arms is bigger in diameter. We had to tap the spindles to accept it. I get what you're saying Flash too. It would be interesting to hear Karl's take on this.

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[frogisland]

Have been thinking about this today – I have the Kokeln A-arms with 17mm pin, hence my interest.



Firstly, are we experiencing the same failure mode on both arm types – standard and aftermarket? I understand that the aftermarket arms extended pins are shearing, but what is occurring in the case of standard arms? Is the aluminium arm or more specifically the area around the pin failing, as opposed to the pin itself shearing, or are the actual pins on the standard arms shearing also?



Secondly, although I only have a basic understanding of stress, failure etc, I concur with Flash re pin length. If it is fixed at one end, the force required to shear it reduces as you travel further along it. If it is fixed at both ends, as you increase the length of it the force required to induce failure through the bending moment is reduced. (try holding a pencil on one end and attempt to brake it from half way along, then at either end)



Therefore, it is quite simply a case of the forces being transmitted through our hubs/spindles, and through the pin, under our normal (and somewhat exaggerated from the race track) cornering and knocks etc are now being applied through a longer pin, and hence have a much more dramatic effect i.e. the threshold to shear greatly reduced.



I am certainly not capable of undertaking any calcs on this – is anyone else? Have the designers/manufacturers undertaken any? Is the 19mm pin much less prone to shearing, or is it just a little less prone to shearing?



If the standard a-arm failure mode is generally not pin shear (views sought), and the aftermarket arms is – then there is something blatantly wrong here, potentially some rather naïve design.



Again, if I am off in the wrong direction I will hold my hands up – I do have some genuine concerns though since I use these arms, regularly track my car for prolonged periods of time, with wide sticky tyres, no rubber in the suspension…and it is pretty low (and regularly use ALL the track including rumble strips and occasionally the rough ground just off the track, both of which knock the car around somewhat)



JP

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[wonko]

JP,



There are many far more qualified than me that can answer this, but my understanding is that the reason for going to the aftermarket A-Arms is that the arms themselves were failing under racing conditions. The aluminum arms just can't cope with the repeated stresses of racing. As I said before though, if you strengthen one part, some other part has to take the stresses. It appears that the next link in the chain is the A-Arm pin. I don't believe that this is bad design or even undesirable. If something is going to break, have it be the cheap easily replaced part.



That said, it does put a burden on the car driver/owner to make sure that the pins are properly maintained. Even the beefier Racer's Edge pins become an unknown, though at this point I have not noted any failures of their pins. They will most certainly take more abuse because of the additional material. The question is how much.



I'm currently processing on my own opinion regarding the arms I have on my two Firehawk cars. One has the RE arms, the other has the Fabcar. I am balancing several issues as I ponder:



1. Re: Racer's Edge parts. These are extremely well made, and the extra material should add additional strength. Unfortunately with that strength you lose compatibility with "stock" parts due to the need to bore and tap the spindle. It also adds complication to the inspection procedure. Yes you can probably get away will checking them less often, but when you do it becomes much more of an ordeal to get things apart. The other issue is that it limits my ability to scavenge parts off of my other car to repair this one should the need arise at an event. However unlikely, I can't just get a stock spindle and slap it on. I must have it machined to accept the pin. This also means that the spindle and A-Arms, for all intents and purposes become a unit for any salvage value.



2. Re: Fabcar arms. Also well made they have the benefit of using the stock interface to the spindle. Removal is almost trivial. Changing out the broken pin, and the good pin on the other side of our other Firehawk was very quick and easily done. In a pinch, should it be necessary, one can easily replace any of the parts with stock parts on a whim. The down side is that, obviously, one broke. I very easily chalk this up to lack of maintenance and naivete on my part. In other words, I don't blame the part. It broke over several distinct instances, and of which I had the opportunity to catch the crack had I just checked it properly. The down side as I see it is that they will need to be checked more frequently due to the smaller diameter.



So, check less often with more difficulty, or check more often with relative ease. Have flexibility should you need it, or be somewhat limited in options with less likelihood of needing the option. I'm not sure where I fall out on this one.



The one thing that is for sure is that you MUST upgrade your arms if you are going to track your car regularly. I can make good arguments for stronger pins or stock diameter pins. Just know which choice you have made and maintain accordingly.



John

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