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Old 09-12-2012, 06:21 AM   #706
slide
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My statement is second hand based on what those who suffered these failures posted on this and like boards.

AFAIK, BMW have made no statements directly. One thing I feel secure about is that BMW have never acknowledged a defect of any kind in these forks.
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Old 09-12-2012, 07:25 AM   #707
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This picture alone suggests that the failure lasted over a period of time. The bike was moving, and If I am seeing what I think I'm seeing it continues upright spraying fork oil for maybe 50, 60 or more feet, before totally collapsing and flipping or rolling to a stop and draining the rest of the oil in one spot.

http://red-baroness.net/Motorcycling...Bike/road1.jpg

If I were guessing, and I am, the right leg gave up first, the left leg being stronger in that area held on for a bit longer and came totally apart when the loose right leg allowed more twisting of the wheel and caused the bike to flip, roll or slide to a stop.

In the pictures I have seen with both fork legs failing, the left leg shows more damage. I believe that is because the right leg always goes first because it is weakest, and even though the left leg might have deteriorated at a similar rate it only fails completely when the support of the right leg is gone. In my mind that helps to explain the mutiple pieces, tears, banana peel looking rips that are seen in some photos of left leg failures.

I don't see anything in this picture that indicates the results one would expect if there was a single blow sufficient to break a fork leg.
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Old 09-12-2012, 07:45 AM   #708
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If you look more closely at that failure you will notice the fork brace did not break and the LHS fork leg was not able to rotate anti clockwise with the wheel/axle attached after the RHS failed as other failures did hence the LHS axle mount peeled open like an orange

The difference in this machine to the others was it was fitted with a Wunderlich after market low guard which came with an additional fork brace
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Old 09-12-2012, 07:57 AM   #709
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How much rotation a single fork leg might have after the other is broken would certainly be dependent on several factors, including the type or style of forkbrace, fender, and probably other factors. The rider's input at and just after the moment of failure would be a factor as well. I think we can rest assured that with right fork leg broken and the axle still atteached to the other, it will have significantly more freedom to twist, rotate, and/or otherwise put a different kind of pressure on the left fork leg.

Not offering as fact, just an opinion.

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Old 09-12-2012, 10:54 AM   #710
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Re: The .pdf file on the NHTSA summary report.

Are there any engineering sorts here that have ever heard elsewhere of "forced fracture"? I have not in my 45 years in fatigue testing.
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Old 09-12-2012, 12:01 PM   #711
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Quote:
Originally Posted by Unca Fud View Post
Re: The .pdf file on the NHTSA summary report.

Are there any engineering sorts here that have ever heard elsewhere of "forced fracture"? I have not in my 45 years in fatigue testing.
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It essentially refers to a material failure caused by external, rather than internal, factors. Seems consistent with BMW's position that the fork failures weren't attributable to a material flaw or poor design but rather to some external factor beyond their control, in other words, "the crash caused the fork to break" rather than "the fork breaking caused the crash". Or, probably more appropriately, "something the user did caused the fork to break, causing the crash."

EDIT: It's something that probably never would have come up in the context of fatigue. It's a totally different failure mode (as stated in the report).
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Old 09-12-2012, 01:21 PM   #712
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Originally Posted by reenmachine View Post
It essentially refers to a material failure caused by external, rather than internal, factors. Seems consistent with BMW's position that the fork failures weren't attributable to a material flaw or poor design but rather to some external factor beyond their control, in other words, "the crash caused the fork to break" rather than "the fork breaking caused the crash". Or, probably more appropriately, "something the user did caused the fork to break, causing the crash."

EDIT: It's something that probably never would have come up in the context of fatigue. It's a totally different failure mode (as stated in the report).
Hypothetically speaking, if the forks were too weak to hold up under normal riding stress and they broke while riding down a straight paved road, could we say the failure was caused by a "Forced Fracture"?
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Old 09-12-2012, 01:57 PM   #713
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Hypothetically speaking, if the forks were too weak to hold up under normal riding stress and they broke while riding down a straight paved road, could we say the failure was caused by a "Forced Fracture"?
No. A forced failure is not due to inherent defects in design or material. I think you could make a case for a forced failure if a large rock hit the fork on or around the boss which started the series of failures which ended in the parting of the boss from the fork leg.

An often given example is a foreign piece of metal (such as a metal shaving) in a ball bearing which causes the failure of a race.
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Old 09-12-2012, 04:17 PM   #714
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Quote:
Originally Posted by Unca Fud View Post
Re: The .pdf file on the NHTSA summary report.

Are there any engineering sorts here that have ever heard elsewhere of "forced fracture"? I have not in my 45 years in fatigue testing.
MN MechEngr (retired)
RR Moore, huh?

A "forced fracture" isn't an engineering term. What they mean is an overload fracture, often called "gross overload", but that term isn't always accurate. It's also frequently called a "one-time" overload, but that's not always right either.

Now let's look at what xy500 posted.

"This failure displays no sign of being a compressive, tensile, shear or buckling failure though (you didn't even mention shear or buckling). It is most likely impact failure."

First, metallurgically most "compressive" fractures are shear fractures, along a 45 deg. plane from the load direction. Note that this is different from a mechanical shear fracture--think of a shear pin/bolt where the load is "sideways". When stressed in tension or bending, a fracture is either ductile (metallurgical shear--good) or brittle (grain cleavage or intergranular--bad). Now it gets a bit more complicated. The opposite of brittleness is not ductility, but is toughness. Toughness is the combination of both ductility and strength. Pure gold is extremely ductile but its very low strength means it isn't very tough.

"Buckling failures" occur in columns--easy to predict if buckling is likely. "Impact failures" imply a rapid rate of stressing, and just how rapid can have a big effect. They are not a mode of fracture, but can be a mode of failure.

So, repeating what I've posted several times in the old thread, most cast aluminum alloys are relatively brittle, and have the same "look" whether loaded slowly or quickly to the point of fracture--except when the rate of stressing is at very high rates, such as those involving very close explosions. Therefore it cannot be concluded that any impact was involved.

OK, are brittle materials always bad? Hardly, there are millions of brittle cast iron engine blocks and cylinder heads which survive millions of cycles of rapid stressing. If stressed to the fracture point, the part breaks. Had the part been made from a tough material and been stressed to a high enough level, it probably wouldn't fracture but could distort enough to make it useless.

Time for dinner.
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Old 09-14-2012, 04:21 AM   #715
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huh?


................"Buckling failures" occur in columns--easy to predict if buckling is likely. ..........

................ If stressed to the fracture point, the part breaks. .....................

.................Had the part been made from a tough material and been stressed to a high enough level, it probably wouldn't fracture but could distort enough to make it useless...................

And hard to predict if unlikely????

Hmmmm, yea, fracture = break, I got it

I think I know what you meant in the last sentence quoted, but it doesn't make sense as written, when reading in context of the sentence if front of it.
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Old 09-14-2012, 05:02 AM   #716
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And hard to predict if unlikely????

Hmmmm, yea, fracture = break, I got it

I think I know what you meant in the last sentence quoted, but it doesn't make sense as written, when reading in context of the sentence if front of it.
When brittle parts like these forks break, they do not physically distort much at all. Tough parts do distort, and in many cases the distortion makes them useless--before they actually break.
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Old 09-14-2012, 07:42 AM   #717
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Interesting video

I found this interesting video:

http://www.youtube.com/watch?v=vfiQXxKN2VY (skip the add)

I checked the unpacking instructions in the Owner's Manual DVD for both the "Returnable" and the "Non Returnable" crates. The packaging for shipping is not the culprit here. In both cases the bike is not secured using this method.

But I wonder if any of the dealerships secure the bikes in their showrooms this way. It does look sexy and the forces are probably minor, if the bike is perfectly balanced. But, if anyone climbs on the bike, I cringe thinking of the loads imposed on the front fork bosses.

Not saying I found the answer here, but it sure makes one think...
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Old 09-14-2012, 09:19 AM   #718
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Originally Posted by windowto View Post
I found this interesting video:

http://www.youtube.com/watch?v=vfiQXxKN2VY (skip the add)

I checked the unpacking instructions in the Owner's Manual DVD for both the "Returnable" and the "Non Returnable" crates. The packaging for shipping is not the culprit here. In both cases the bike is not secured using this method.

But I wonder if any of the dealerships secure the bikes in their showrooms this way. It does look sexy and the forces are probably minor, if the bike is perfectly balanced. But, if anyone climbs on the bike, I cringe thinking of the loads imposed on the front fork bosses.

Not saying I found the answer here, but it sure makes one think...
Nice visual. Look at the size and mass of the fork brace then look at the size and mass of the axle lugs. Which do you think will break first?
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Old 09-14-2012, 11:21 AM   #719
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Come on, Those lugs will get way more stress on a mild, slow, off-road ride than the fattest idiot in the world could cause by climbing on with the bike secured like this.
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Old 09-14-2012, 12:07 PM   #720
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Come on, Those lugs will get way more stress on a mild, slow, off-road ride than the fattest idiot in the world could cause by climbing on with the bike secured like this.
Are you serious? I am not talking about an up and down force from hitting bumps on the trail (or a down force from the weight of someone climbing on the bike). The sliders are presumably designed for this (I say "presumably" because of the failures). Also, the shock absorbers obviously help here. I am talking about the force of a bike leaning to one side or another against the axle firmly fixed by the brace with someone actually disturbing it by climbing on it... Think of the arm. This is a pretty heavy bike too. I don't think the sliders were designed for this kind of stress. Anyway, when the bike is free of such a brace and is ridden, the forces act in a totally different direction.

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