Being skeptical of a design is fine. Smart in fact.
Well we agree on something!
Making an assumption that a given product is possibly inferior due to the material isn't.
Says who? Perhaps you should have prefixed that statement with "In my opinion... " I mean, come on, if the part were made from say, ice, would (in my mind) disqualify it from being considered as a part of
my truck, but not according to your statement. For my particular use I would prefer a steel control arms for some of its other properties including better ability to handle being repetatively dragged across sharp rocks. And, in my opinion, the product is inferior for me because of its needlessly high cost.
If aluminum were not a viable material, then every dirt bike out there would have rear tires laying behind the rider after every jump.
I am confused, I thought I made it quite clear from the beginning that aluminum is a viable material, in the right hands. That being said I have seen many a cracked swing arms and cracked frames on mountain bikes, usually due to poor design.
CNC maching of steel, is slooowww, and hard on tools relative to aluminum.
Agreed, and this was one of my main points as to material choice I made in the thread several posts ago.
In any given design, the pro's and con's of each type of material, manufacturability, and price have to come into play.
Exactly, and it was precisely the chosen combination that made me question the product. It would appear they didn't consider cost because of a penchant for material/manufacturing choice.
After prototypes have been made, design validation takes many forms. Is racing a 1000 mile race meaningfull? Sure it is.
Respectfully, I will disagree with you. As I wrote in a previous post, I have witnessed tests to failure and your conclusion that a 1000 mile race is sufficient is simply not scientifically justified. It might satisfy you personally, but it is in no way a suitable technique to claim fatigue resistance. Imagine this, me standing in court with my fatigue diagram and load-cycle data, citing engineering text books with good science backing me and you with "It survived Baha, here's your t-shirt" who will the jury believe?
Every mile I drive is further validation for the strength and resilience of the KORE shock tower.
I agree with you on this point, sort of. If it were steel you could get to 1,000,000 cycles and you could claim success, with aluminum you might get to 2,000,000 and still fail if it is not designed properly, I said
if. Do you happen to know what the design criteria is that North American auto manufacturers use for fatigue design? I know for most parts it is 1000 hrs. use but don't know what that translates in to in terms of cycles.
Is JOblenes concerned that the bearing caps on his shocks, are bling bling red anodized aluminum? Probably not.
Well actually out of ignorance, I was, but have since learned that the various shock manufacturers strap their shocks into a shock dyno to test them, and in addition to doing this to evaluate the damping vs. velocity curve they also do longevity testing of their seals, bearings and wear elements and are at the same time doing a fatigue evaluation of the entire shock. Thats the nice thing about the shock dyno you can get many cycles into the product in a controlled fashion and observe the results. Do I feel this way about all of the products I buy? No, I am not always such a skeptic. For the largest of manufacturers I do beleive they have sufficient pocket books to do all of the required testing, but I am much more skeptical when it comes to small companies.
I am not here to pick on JOblenes, blind assumptions are not at all uncommon when it comes to materials in the engineering world.
Its OK i do not feel picked on, I feel I am on firm ground technically. As to blind assumption I would say it is
not common in the engineering world I work in, for example no one guesses at an ASME rated pressure vessel. True some characteristics are not always known but that is why a rigorous testing protocol must be used to qualify the finished product. The truth is fatigue resistance is relatively easy to prove to a fairly high degree of certainty. Like the forklift example I provided above, just do the test, its not hard, and prove it, simple as that.
Thin skins of aluminum with rivet holes all throughout them(like a wing), are good candidates for stress fractures. Big chunks of 7075-T6 for control arms really aren't.
This is patently untrue, I will post a scanned excerpt from a text book as evidence tomorrow. I have personally observed chunks of forged steel approximately 3 foot long, 2 feet wide and 1 foot thick, split in two by fatigue failure that started by nothing more than a scuff mark where two components rubbed together (fretting). Large (thick) parts often fail from fatigue, just pick up any introductory textbook on the subject and they are full of pictures of large parts that have failed. You seem to be speaking from experience, but perhaps your experience is limited.
I trust that the engineering behind them was performed in a professional manner. Until I hear of failures in the field, I will not be concerned by any engineer, designer, fabricator, or other random critic's claims of inferiority.
I guess that gets to the crux of the matter for me, I am somewhat more skeptical of the "engineering" that went into the product and that was the initial reason for my post. I have a few friends who are machinists and we are always ribbing each other over the best way to "design" something. My job is to assess the suitability of a part as it functions in its intended role. Through the machinist's eyes they assess "design" by how easy it is to manufacture. Some machinists are also good designers, but some will always stick to what they know. For many machinists who have access to CNC machines, they CNC everything, heck I have one buddy who won't even drill a hole on a drill press any more, he sticks it in his CNC milling machine, I am not kidding. It just struck me that the product being discussed here may be suffering from the same attitude. Aren't the originators of the product line machinists based in the aerospace industry? My buddy is a great machinist and can turn out beautiful parts, but knows squat about fatigue. Though the uninitiated often assume that because it looks well machined, it is well designed.
But don't doubt the engineering behind any product because of any one man's rantings.
Why not? Isn't that the point of this web site, to have discussions about all things relating to our Dodge Cummins trucks? We discuss openly some of the crazy decisions Dodge makes, why not an aftermarket vendor of Dodge related parts? Though it is rare the discussion ever gets as in-depth as it is in this thread, frankly I wish it would more often, but I am sure people are already tired of reading my typing (ranting as you call it).
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