archman350z

Dmitryz, thanks for the correction...it's been a few years since I've had a Materials class. I would like to quibble over one small issue however: I'm not really sure that cast aluminum can be labeled "ductile." From my school days, I distinctly remember that the % elongation for cast aluminum was much less than for forged aluminum. I also remember the BANG cast aluminum made when the sample finally failed! Anyway, yes, when you forge a metal you're actually forcing the grains of the metal to align and remain continuous through changes in geometry.

I would still venture to say that the failure was due to stress cracks/fatigue. It could have started from something as simple as a nick on one of the spokes. Dmitryz has the right idea...get a good picture of the fractured area, if it was fatigue you should be able to see "rings" from where the crack propagated. If there were any trapped air bubbles in the casting, then this would also show them. Also, since a wheel loses much of its strength when a spoke is broken, the other spokes may have successively failed soon after the first. If you look close, you'll notice that the fractures are still located in a stress concentration (the spoke is getting narrower as it leaves the hub). Bottom line: there WAS a reason why it broke...it would be nice if we could figure out why.

Finally. If you guys are breaking wheels on a regular basis, then I strongly suggest something other than a cast aluminum piece. I think you got lucky that the car didn't flip this time...you may not be so lucky next time. If you really need something cheap, use a steel wheel. Yeah, they weigh a ton and ugly as hell, but there're cheap. Even for forged wheels, your life is worth far more!

DmitryZ

Err. I apologize for the long post and very sorry for stealing the thread but hopefully someone will find this informative… (If not, the mods can delete this)

It basically depends on how you define ductility, as it is a relative term. I will admit, I was using the term ductility incorrectly – guilty as charged. Ductility, by definition refers to the amount of elongation a material can sustain before fracture. You are correct, % elongation for cast aluminum is much less than for forged aluminum. However, both materials still have the same modulus of elasticity which means they are still far away from being considered brittle. The materials have the same ductility in the elastic range but in the plastic range the forged stuff is more ductile but neither material is brittle. The forged stuff is tougher, hence it can withstand higher load before yielding or breaking.

Putting this in english for the sake of the others in this thread (or at least I will try my best): Imagine you are the unfortunate owner of a Z and have 2 types of wheels - wheel C is cast, wheel F is forged. You put the wheels on a Z and measure the diameters very closely with some super duper highly accurate measuring tool.

Note all of the following numbers are made up for the sake of the argument!!!

You will see that both wheels deformed and have an egg shape to them. This deformation will be on the order of .00002in (very very small) or something like that (horizontal diameter 18.00002 in, vertical diameter 17.99998) for both wheels. This deformation will also be nonpermanent i.e. you can leave your car like that for 100 years and when you take the wheels off, the diameter will return to 18.0000 all the way around – nothing to worry about. At this point both wheels have the same ductility.

Now lets imagine that some a** loaded a Z with a crapload of bricks while the unlucky owner was sleeping! You measure the wheels and find that both wheels are deformed even more but this time wheel C (horizontal diameter 18.00006, vertical diameter 17.99994) is more eggshaped than wheel F (horizontal diameter 18.00004, vertical diameter 17.99996). The owner, cursing and screaming, quickly unloads the bricks and takes the wheels off the car. To his relief wheel F returned to its original size of 18.0000 all the way aroung but wheel C is FUBARed and remained deformed after removed from the car (18.00004 horizontal, and 17.99996 vertical). Wheel F has a higher yield point than wheel C.

The owner goes to sleep thinking he is gonna kill the sob that did this and his insurance will buy him brand new Nismo wheels… However in the morning he finds that someone has loaded his car with lead!!! To his horror he sees that wheel C broke after some severe deformation. Wheel F is still intact but when the owner removes it he finds that it is still eggshaped and is FUBAR. Wheel F is tougher than wheel C because it can withstand more load before breaking. Overall wheel F is more ductile than wheel C but when wheel C broke it did so after some severe deformation hence it is not brittle

The moral of this story is…that Dmitry has waaaay too much time on his hands…
and that when you start noticing the difference in ductility both wheels are already too deformed to be useable.

Back to the problem at hand…Ductility has absolutely nothing to do with this failure as it is a brittle failure. WTF-you just told us that Aluminum was not brittle!!!! That is still true but Aluminum, forged and cast, can still fail in a brittle manner – through fatigue. After thinking about this, I will have to change my mind and agree with archamn that fatigue is probably to blame. Whether the failure occurred through surface flaw or internal flaw is a very hard thing to determine and usually involves some expensive equipment like an electron microscope…

If I had a wheel company, I would give you a set of wheels for free just so I can get my hands on the damaged set and analyze the to see what the problem is. This could be caused by powder coating, but IMHO Enkei let one bad wheel slip through their system.

Damn this was long post,

Dmitry

speedform

archman350Z and DmitryZ, I'm forwarding your posts to Enkei. You should expect a called about a job offer shortly thereafter.
thanks for the informative posts all.

archman350z

Oh, that's TOO funny!

Great explaination, Dmitryz. The moral of all of this geek-speak is to let people know that there is more out there than you might initially think. No single person can know it all...hence the word raceteam, but you can at least know enough to be safe.

Here's an idea: take your wheel into some metallurgy professor at a nearby college. They usually like to geek out over this sort of stuff. They may have the equipment/knowledge to tell you what caused the failure. We had a professor that we took stuff to all the time to see if we could ever stump him: we got FREE failure analysis, and he got to pride himself in finding it.

Hmm, a job at Enkei...well, here's where I discredit everything that I stated in this thread: I'm an Electrical Engineer. I design engine management controllers for a living. But in college, I loved statics, materials and thermodynamics. Over the years, hanging out with the mechanical guys in the auto industry you tend to pick up a few things...I pride myself in being able to fool mech. engineers into thinking I'm one of them! If I can learn this stuff, anyone can learn it...

I just hope we've gotten a few people to look at wheels in a totally different way now. Just think, when you're pulling 1.0g in a turn, you're effectively putting the entire weight of the car sideways onto the wheel!

Lucino

I am so glad that I found this thread. I have been looking for info's on 17's rims.

I know there has been any posts since June. But, can somebody please confirm that 17x8 with 35mm will clear the brembo?

Thanks.
L