mumzer

dude;

I started out trying to provide some constructive criticism....two or three other people offered well-reasoned explanations of their concerns about your parts. I am not a practicing engineer... i spent 5 years building welded chassis parts for race cars, and have seen about every failure mode you can imagine...i assumed that M758 was an engineer....and cervelli has seen enough broken parts on enough racecars to have earned his "i'll know it when i see it" degree. It was all pretty civil.

You responded by assuring us that the material was so heavy, the design flaw was outside the realm of loadings that the material would see in operation. Well...ok....thats how Ford would do it too.

A number of people pointed out that your design incorporated an innapropriate use of a rod end. Your response is that it must be ok because Kokeln does it (or plans to do it) the same way.

Ignoring physics, materials science and the nature of bolted and welded joints will lead to failures. The fact that lots of people do stupid things doesn't make it (or them) an less stupid. An experienced chassis builder doesnt need to look at your facility to assess the merits of the arm, nor must they x-ray the part to know whether or not it will break. I have said at least three times that the fab quality was good. The problem is the design.

Using x-rays as a way to QC welded parts is fine for determining if the bead penetration was adequate. It doesnt tell you a damn thing about how the part will endure fatigue and fully reveresed loads in service. Nor will it set off a flashing warning light to tell you if you are loading a heim joint in bending in an unsafe way. Having said that, you have ignored the group of people here who have suggested that it is not a good idea, so im not sure what good a flashing light would be either.

Continuing to insist that it's ok to fab an arm the wrong way "because all the cool kids are doing it "just makes you sound silly.



It will never hold up in court? this is not about holding up in court...its about using a rod end the right way...and using it the wrong way after a bunch of experienced people have told you to change it doesnt really seem that wise. My point was simply that discussions about potential design flaws before you went to market with the part were the sort of things that can bite you in the *** in the event of a liability claim.

so i guess in the final analysis, whatever. Do what you want. Its your name you are putting on the part, so if you want to misuse the heim joint, super. I dont understand why you are not willing to take the advice of a group of experienced people to change the design of a preproduction part.

Sean Hall

I sent a pic of these to a suspension design engineer(who will remain anonymous). Here's the jist of what he had to say about the first revision of these arms:
- These arms are probably A LOT stronger then the stock a-arm.
- In the event of a crash, you need a weak point in the arm, so the arm will yield before the tub. In that respect, these arms are probably too strong.

None of the cirtics mentioned the second point. This lends me to believe the credibility of the posters who've criticized this design, should be scrutinized as well.

I say this because the engineers commenting on this are losing site of the big picture: the stock arms are dangerously ineffective. This is a FACT. Noone has yet to address this fact with a credible solution that's cost effective. Another fact.

Additionally, are you guys(the critics) racing your cars with the stock a-arm? If so, this makes you hypocritical, doesnt' it? I mean, you know the stock a-arm is dangerous, yet you're still using it. Maybe we should be asking you why you're racing your car on flawed a-arms? I mean, given that you're engineers and all, maybe you should know better, shouldn't you?

A lot of the criticisms/recommendations made, have been conjecture. ie. not backed up by data or had facts. So, why doesn't one of the "Engineers" load both this and the stock design in NASTRAN, and do the finite element analysis, then report back your findings? Or, do it the long way, and do the math and provide back some real data rather than conjecture.

okay, flame away. Bear in mind though, fallacy arguments will not be responded to, so you may not want to waste your time.

Sean

Russ Murphy

[QUOTE]okay, flame away. Bear in mind though, fallacy arguments will not be responded to, so you may not want to waste your time. [QUOTE]

The first fallacy to be introduced is: [QUOTE]I say this because the engineers commenting on this are losing site of the big picture: the stock arms are dangerously ineffective. This is a FACT. [QUOTE]

Where'd you come up with that? How many tens of thousands of car/hours have been run without a problem on the track with the stock arms. Lots. The stock arms are completely effective. The issues arise when you significantly lower the car and thereby alter the control arm geometry. It's not the fault of the design(ers) when a product is used in a manner for which it wasn't designed and then fails.

Sean Hall

Russ,
Irregardless of rideheight, a suspension is designed to travel within the upper and lower limits of the wheel. If the suspension binds when pressed to the upper limits. This is a poor/broken design. Its that simple.

If you don't agree to the above, there's no more discussion since my concept of acceptable levels of safety are totally different than yours.

Just to be clear, one does not "alter" the control arm geometry when you change ride height - the axis points, which determine the geometry, are static. Therefore, geometry will only change when the axis points are changed.

Sean

Russ Murphy

"Just to be clear, one does not "alter" the control arm geometry when you change ride height - the axis points, "

Gosh, you know I have no training in this field whatsoever, but I do have a noggin between my ears. I'm pretty sure that by changing the ride height you've changed the roll center thereby changing the "geometry" of the suspension. I get what you're saying about complete and safe suspension designs taking into account the full range of travel. It's kind of hard to imagine a project design group at Porsche making a sophomoric error of such gigantic proportions. If you think that you know more about suspension than Porsche circa 1984 than I'm afraid you're sadly mistaken.

While dictating the limits of discussion might be a useful tool at times, it rarely helps in coming to a full understanding/getting to the bottom of things. If the control arms are so flawed, than why aren't spindles falling off all these 150,000 mile plus cars out there? I mean, they do have much more compliant suspensions (than most tracked cars) and the roads (potholes) are pretty bad out there. Yet, the anecdotal evidence is that they(spindles) only fall off after the cars have been lowered and are being hammered around the track. Hmmmm.

Since you've gone to the trouble to be so precise, I feel I should mention to you that "irregardless" isn't a word. At least not a real one. And if it was you would have used it incorrectly. I believe what you mean to say is "without regard to ride height". The word for without regard is "regardless".

Sean Hall

I said I wouldn't respond to fallacious arguments. Therefore, I have no comments about the points you made.

FYR, the definition of irregardless can be viewed at Dictionary.com:

http://dictionary.reference.com/sear...q=irregardless

Sean

Russ Murphy

From your link:

Usage Note: Irregardless is a word that many mistakenly believe to be correct usage in formal style, when in fact it is used chiefly in nonstandard speech or casual writing. Coined in the United States in the early 20th century, it has met with a blizzard of condemnation for being an improper yoking of irrespective and regardless and for the logical absurdity of combining the negative ir- prefix and -less suffix in a single term. Although one might reasonably argue that it is no different from words with redundant affixes like debone and unravel, it has been considered a blunder for decades and will probably continue to be so.



P.S.
It's fallacious reasoning to state that Porsche as an engineering entity knows more about suspension than you? Puhleezz.

Skip Wolfe

As an engineer, I cannot stand the argument of "since Porsche designed it then it must be fine."

As mus as I hat to admit it, engineers make mistakes and these sometimes make it to the market. I am an engineer who works for a manufacturer and I/we have made mistakes that have resulted in a part that prematurely fails in the market. It happens to every company regardless of size and even the almighty Porsche has these problems.

- #2 rod bearing that fails
- powersteering hoses that leak
- '86 headers that crack
- Timing belts that fail
- ring and pinion in 968 6 speeds

These are couple of the well documented problems that our cars have had. Some of them could be due to a mistake in the design, other could be due because they had to compromise because of cost, or supply issues or because of any other of a hundred issues that have an effect of a product design. If there was ever any problem then there would be no such thing as recalls, or lawsuits.

While I am a mechanical engineer I am not a suspension engineer and really cannot support either side in this argument. But PLEASE do not use the "Porshe designed it so its fine" and please do not assert that they are fine for regular track use. Are they fine for the occasional DE're with a stock suspension - probably, but I have seen to much documentation of the arms failing under track conditions, and I have seen one fail in person and it wasn't a pretty sight to really trust the stock arms on the track.

Russ please don't take this as a personal attack because its not. I just get tired of non-engineers making the"if manufacturer designed the that way than they must be perfect" statement and had to jump on my soapbox.

Russ Murphy

Skip,
Thanks for your insight. Valid points all. My take on the arms boils down to this: I find it highly unlikely that the ball joint binds on the stock arms at any point in the suspension travel with the car close to the stock ride height and with wheels/tires with the same overall diameter as stock 205/55/VR16. I agree that there's lots of evidence of on-track failures and I'll bet virtually every one was on a lowered car or altered wheel size. Does anyone know of a failed ball joint/arm from a non-lowered streetdriven only car? I'm not being cynical, really, I asking because I don't know. My ignorance relating to chassis engineering is pointedly obvious so please bear with me, but if it was purely an issue of the ball joint not having enough range of motion for the travel of the suspension design then I would expect there to be scads of issues with the soft stock suspension and potholes which seem to me to be a hole helluva lot rougher than directional change on the track.

Tomas L

I don't know if I dare to enter this already overheated discussion....
The use of a rod end loaded in bending like this is a VERY BAD DESIGN!!
However it may (or may not) hold for centuries if it's oversized enough.
The point here is that this is a part that the good health of the driver is dependent on, and anyone that intends to sell a piece like this should have made a serious effort to ensure that it is up to it's task. This could for instance mean FEA calculations or fatigue testing. It seems to me that no such effort has been made in this case, and it is definitely not some engineer in this forum who should do it. Where's the logic in that Sean??
As to the price of these I can only say that it once again proves that pricing of aftermarket performance parts has nothing to do with manufacturing costs....

TurboTim

We are not going to use the heim joint afterall.We found a threaded ball-joint from AFCO that we will be using.It will thread into the arm just like the heim but it is a true ball-joint.It designed to take the loads and you still can make adjustments to the camber.

M758

Tim,
The heim joint itself has never been the problem. It has always been the they way it was connected to the arms. It was a threaded connection loaded in bending. I believe the ball joint solution has the same weakness as heim joint. The bending loads placed at the threads. Simple axial tension or compression is far safer for these types of joints.

SANDOVAL

Hey Tim is Lindsey Racing carying your Arms? Page 34 of the latest Excellence page 34 it looks like they are advertising them!

TurboTim

Eric,


Those are the Kokeln units.

Matt H

Skip-
power steering hoses that fail and timing belts that break arent design flaws. They are a byproduct of parts wearing out over time. All hoses will fail over time and any rubber belt will eventually break. Those may be a nuisance to you but they are not flawed.

Rear wheels falling off Focus(es), who knows what the plural of focus is, is a design flaw. Parts failing do to time are normal wear describe basically every part on a car.
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