Rennteam 2.0 - EN - Forum - Pics of Shattered Dymag Wheel & Report of Problem - Page1

Nov 15, 2008 6:30:59 PM

cannga: Expert; Loc: Los Angeles's "twisties" :-) , United States; Posts: 1940, Gallery; Registered on: Feb 6, 2005

Pics of Shattered Dymag Wheel & Report of Problem

Dymag current and potential owners may want to check out the thread below. BTW, I didn't think about this before (duh) but the spokes/center of the dymag wheel -- the part that shatters -- is, of course, magnesium alloy. The alternative is using a center of aluminum alloy. Does such a beast exist? Not yet, but it will be: The new HRE - Dymag wheel.

http://www.6speedonline.com/forums/997-turbo-gt2/150498-lightweight-wheel-questions-2.html

(Please see thread for full details. I am quoting some relevant information only.)
I handeled the situation of the broken center for Dymag.
The wheel was broken at a track event in Mexico the customer runs full slicks all the time at the track and these wheels had about 60 full track days on them unfortunately they were not checked after each event and the spoke developed a hairline crack that started from the engraving on the back of the spoke. They stamp the back of the spokes with the model number of the mold. The crack propagated causing the spoke to break in half and transfering the weight load the the remaining spoke as he continued to run the wheel then broke as he was in a straight line braking zone at the end of a staright away.
The damage to the car was minimal about $10k and Dymag paid for the damage and the full set of wheels the car was back at the track in 10 days time and the customer was fully reimbursed.
The magnesium center was sent to a testing lab here in the U.S. to make sure that it had all the properties it should have for cast magnesium the test results came back with passing results and upon inspection of the wheel it was determined that fatigue was what caused the failure.......

.......Dymag will offer an upgrade to the Magnalium centers if the customer has issues. Not all wheels have had problems but in cases of future problems will will address these on a case by case scenerio. A letter wil be mailed to these customers in the next few weeks......

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Cargraphic Exhaust/PSS10 http://www.rennteam.com/forum/thread/449462/Bilste...

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Nov 15, 2008 6:32:17 PM

cannga: Expert; Loc: Los Angeles's "twisties" :-) , United States; Posts: 1940, Gallery; Registered on: Feb 6, 2005; Reply to: cannga

Re: Pics of Shattered Dymag Wheel & Report of Problem

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Cargraphic Exhaust/PSS10 http://www.rennteam.com/forum/thread/449462/Bilste...

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Nov 15, 2008 6:32:46 PM

cannga: Expert; Loc: Los Angeles's "twisties" :-) , United States; Posts: 1940, Gallery; Registered on: Feb 6, 2005; Reply to: cannga

Re: Pics of Shattered Dymag Wheel & Report of Problem

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Cargraphic Exhaust/PSS10 http://www.rennteam.com/forum/thread/449462/Bilste...

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Nov 16, 2008 10:58:40 AM

Moogle: Rennteam Master; Loc: Bay Area , United States; Posts: 5019, Gallery; Registered on: Aug 9, 2003; Reply to: cannga

Re: Pics of Shattered Dymag Wheel & Report of Problem

Crack originated from an engraving on the back?!

It is always the smallest straw that breaks the camel's back eh?

Ever since the dynamags I've been waiting to see this happen.

--
Tom

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Nov 16, 2008 11:23:56 AM

Joost: Rennteam VIP; Loc: Dordrecht , Netherlands; Posts: 8059, Gallery; Registered on: May 3, 2004; Reply to: Moogle

Re: Pics of Shattered Dymag Wheel & Report of Problem

Moogle,

In this case I think it wasn't so much as the weakest straw. Fatigue problems are often counter-intuitive and may result in weird failure behavior.

Fatigue problems are often found around stress concentrations. For example, a pit in a metal spoke that was caused by a pebbel or so might result in a stress concentration and a micro crack. Furthermore, alternating loads, such as braking and pulling up are also nasty. They may cause a metal object to fail at relatively small loads in the long run, where statically, the object was able to sustain the loads. (Think of what you would do if you had to break a paperclip; bending it twenty times will do the trick, but trying to destroy it in on time is quite hard).

The theory behind fatigue strength and crack growth is pretty well documented, but in real life, structures that are likely to have substantial fatigue spectra always have to be tested with a long-time test.

Interesting to see that also magnesium is susceptible for fatigue, didn't know that. Keep us updated Cannga!

-Joost-

--

Porsche, seperates LeMans from LeBoys

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Nov 16, 2008 12:10:54 PM

Eunice: Rennteam Administrator; Loc: Karlsruhe , Germany; Posts: 6155, Gallery; Registered on: Oct 27, 2008; Reply to: Joost

Re: Pics of Shattered Dymag Wheel & Report of Problem

If there is a minor failure in the material (metal is not 100% pure, or some minor scratches might be enough), current physics can not even calculate the durability.

According to what's current theory, an infenitesimally small crack makes the whole part break with an infenitesimally small force!

For the engineers: http://en.wikipedia.org/wiki/Shear_stress (Stress = Force / Area -> Stress goes to infinity if Area goes to 0)

Obviously this is not true in real life, so they just adjust the theoretical calculations to match the real world experiences...

After all, the inability to compute that perfectly is part of the reason why craftsmanship really matters. (It would be really sad if the one with the biggest computer would build the best car, wouldn't it?)

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Nov 16, 2008 6:14:23 PM

Joost: Rennteam VIP; Loc: Dordrecht , Netherlands; Posts: 8059, Gallery; Registered on: May 3, 2004; Reply to: Eunice

Re: Pics of Shattered Dymag Wheel & Report of Problem

Hmm, how was that again? I thought there was some sort of treshold value for the magnitude of the load?

However, the loads spectrum, the amount of cycles etc. is an important part for the prediction of crack growth too. I guess a company like dymag will have it's norms for material, so it will be interesting to see whether the material was not up to spec, or that there was something els going on.

What I do like about the story is that dymag didn't hesitate too give their client a full refund and started investigating right away. that's the way it should be done.

--

Porsche, seperates LeMans from LeBoys

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Nov 16, 2008 6:23:35 PM

Eunice: Rennteam Administrator; Loc: Karlsruhe , Germany; Posts: 6155, Gallery; Registered on: Oct 27, 2008; Reply to: Joost

Re: Pics of Shattered Dymag Wheel & Report of Problem

I thought there was some sort of treshold value for the magnitude of the load?

Yes there is, but it is just a rough estimate. The prediction of crack growth is not perfectly understood in physics.

In theory, the tiniest crack can grow with very small forces, so every material with a slightly rough surface (all materials) would break at a very low force.

This is not true in reality, so the crack growth is ignored in calculations (or at least replace by a thumb value). But occasionaly it still can cause major damage!

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Nov 16, 2008 8:13:38 PM

Joost: Rennteam VIP; Loc: Dordrecht , Netherlands; Posts: 8059, Gallery; Registered on: May 3, 2004; Reply to: Eunice

Re: Pics of Shattered Dymag Wheel & Report of Problem

OK, will ask the fatigue experts at my work for it, might have some interesting info.

Back to the wheel, as I understand it, this is not a problem of the carbon part, but of the magnesium. How is the magnesium mounted onto the carbon? And why isn't the wheel totally out of magnesium,. or totally out of carbon? There are more wheels made out of magnesium, arent there? Is this a different alloy then?

-Joost-

--

Porsche, seperates LeMans from LeBoys

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Nov 16, 2008 8:21:12 PM

Eunice: Rennteam Administrator; Loc: Karlsruhe , Germany; Posts: 6155, Gallery; Registered on: Oct 27, 2008; Reply to: Joost

Re: Pics of Shattered Dymag Wheel & Report of Problem

The magnesium seems to be glued to the carbon, but I don't think that was the problem.

It's more like the crossings broke at the center of the wheel. The next question is why all of them broke there? If it was one crossing failing, I would expect the other crossings break at a different spot.

This is highly mysterious, most propably the magnesium was faulty altogether, or was made at the wrong temperature or something..

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Nov 17, 2008 3:43:10 PM

Joost: Rennteam VIP; Loc: Dordrecht , Netherlands; Posts: 8059, Gallery; Registered on: May 3, 2004; Reply to: Eunice

Re: Pics of Shattered Dymag Wheel & Report of Problem

No, don't think the connection was the problem either, that was just a sidestep out of curiosity.

It makes sense though that the other spokes broke "in the same place". When one spoke breaks, the others have to sustain more load. The spokes just broke at the load introduction points, so at the rim and at the hub. Remember that the wheel probably still was turning when the first spoke failed. Wa this a car with a limited slip differential? If it was not LS, all the torque would have been sent to the failed wheel, wouldn't it?

-Joost-

--

Porsche, seperates LeMans from LeBoys

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Nov 17, 2008 7:35:20 PM

cannga: Expert; Loc: Los Angeles's "twisties" :-) , United States; Posts: 1940, Gallery; Registered on: Feb 6, 2005; Reply to: Joost

Re: Pics of Shattered Dymag Wheel & Report of Problem

Joost:

OK, will ask the fatigue experts at my work for it, might have some interesting info.

Back to the wheel, as I understand it, this is not a problem of the carbon part, but of the magnesium. How is the magnesium mounted onto the carbon? And why isn't the wheel totally out of magnesium,. or totally out of carbon? There are more wheels made out of magnesium, arent there? Is this a different alloy then?

-Joost-

--

Porsche, seperates LeMans from LeBoys

I have seen pictures of an all carbon wheel but never seen one used. What this tells me is the center section has to be non-carbon. Interestingly enough, there willl be 3 versions of cabon wheel:
Old Dymag: Carbon barrel, magnesium center (the fractured wheel).
New Dymag: Carbon barrel, magnesium-aluminum alloy center.
HRE-Dymag: Carbon barrel, aluminum center.

Incidentally, the following was posted on that same thread. I believe Dymag is offering some sort of center replacement for owners. If I own Dymag, there is no question I would not use the wheel until then.

>>>>>>>>>>>>>>>
Dymag has changed the center disc material to a forged exotic metal called Magnalium this is a hybrid of Magnesium and Aluminum and contains the best properties of each. The magnesium offers the lightweight and strength and the forged aluminum offers the non-corrosive property that is one of the downfalls of magnesium.

This new center disc material will be on every new Dymag wheel starting delivery next month we stopped taking oders in August so that we could be sure of the testing of the new material I will attach the updates I recived on the testing but you can be sure that DYMAG wheels are properly tested to the multiply requirements of TUV,JWL,DOT and they have passed independent test by General Motors in both the Cadillac and Corvette divisions as well as Mclaren.

--

Regards,
Can
997 Turbo + Cargraphic Exhaust (Exhilarating Sound!) + Bilstein PSS10 (Review)

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Nov 17, 2008 7:37:25 PM

cannga: Expert; Loc: Los Angeles's "twisties" :-) , United States; Posts: 1940, Gallery; Registered on: Feb 6, 2005; Reply to: Eunice

Re: Pics of Shattered Dymag Wheel & Report of Problem

Eunice:

I thought there was some sort of treshold value for the magnitude of the load?

Yes there is, but it is just a rough estimate. The prediction of crack growth is not perfectly understood in physics.

In theory, the tiniest crack can grow with very small forces, so every material with a slightly rough surface (all materials) would break at a very low force.

This is not true in reality, so the crack growth is ignored in calculations (or at least replace by a thumb value). But occasionaly it still can cause major damage!

I thought you were "just" the software guy? Didn't know you were gonna jump into a thread and talk cars. Smiley

--

Regards,
Can
997 Turbo + Cargraphic Exhaust (Exhilarating Sound!) + Bilstein PSS10 (Review)

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Nov 17, 2008 7:50:47 PM

Eunice: Rennteam Administrator; Loc: Karlsruhe , Germany; Posts: 6155, Gallery; Registered on: Oct 27, 2008; Reply to: cannga

Re: Pics of Shattered Dymag Wheel & Report of Problem

well, software isn't my only interest Smiley

But I have to admit that I have to use my playstation to drive the cars of my dreams... (I was on a race track only once)

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Nov 17, 2008 10:55:20 PM

Joost: Rennteam VIP; Loc: Dordrecht , Netherlands; Posts: 8059, Gallery; Registered on: May 3, 2004; Reply to: cannga

Re: Pics of Shattered Dymag Wheel & Report of Problem

Thanks for the heads-up Cannga!

One of the "problems" with carbon fibre always is load intorductions; how to get the load into the part? You'll always have o create some sort of lump of material, which makes you wonder why you didn't use a metal in the first place ;-)

I understand the choice of an alloy which is more corrosion resistant than magnesium. Corrosiion tends to result in corrosion pits, which may in the end be just that kind of stress concentration that reduce the fatigue life of your wheel... wise coice I would say!

BTW, How much lighter than a normal aluminium wheel is a CFRP/Magnesium wheel? Is it really usefull, or does it just save you a couple of grams? I know that there is something about unsprung mass and rotational mass and that it therefore pays to have very light wheels, but not how the theory behind that is exactly. Also, when does a weight decrease start to count? 10 gramme? 100 gramme? 500 grammes? Surely at a certain point the mass-decrease doesn't outweigh the costs anymore...

-Joost-

--

Porsche, seperates LeMans from LeBoys

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Nov 17, 2008 11:52:51 PM

grease: Newbie; Loc: Austin, TX USA , United States; Posts: 45, Gallery; Registered on: Jul 31, 2005; Reply to: cannga

Re: Pics of Shattered Dymag Wheel & Report of Problem

Any vehicle that does 60 track days should have regular tech inspections of high stress components. Wheels are high stress components, especially with Hoosiers on them.
--
Best regards, Gordie Austin, TX ---------- '04 Boxster 550 SE (for sale) '09 C2S Cab (on order)

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Nov 18, 2008 7:57:58 PM

fritz: Rennteam VIP; Loc: BaWü , Germany; Posts: 10248, Gallery; Registered on: Jul 23, 2003; Reply to: grease

Re: Pics of Shattered Dymag Wheel & Report of Problem

grease:
Any vehicle that does 60 track days should have regular tech inspections of high stress components. Wheels are high stress components, especially with Hoosiers on them.

Smiley

Magnesium is not normally the "material of choice" for long-life high-stress applications. I believe mag wheels are normally used on rally cars in high-level competition, where they are routinely replaced at short intervals. Teams will keep track of the "life" of each wheel to ensure that they are scrapped after a certain number of kilometers or hours.

If the failed wheel had not been subjected to excessive stress (e.g., due to kerb contact) resulting in premature failure, maybe it had just passed its "normal" service life.

In that case, Dymag would have covered the costs (incl. "collateral" damage) purely as "goodwill" to avoid scaring off other buyers. Smiley

--
fritz

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