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"Catastrophic" failure of mountain bike fork lowers (Manitou Skareb Comp)



 
 
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  #21  
Old February 11th 05, 02:45 PM
GeeDubb
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cc wrote:
"GeeDubb" wrote in message
...
Ride-A-Lot wrote:
I'm a mechanical engineering student. The post was written more
for rec.bike.tech, but I guess I won't lose the group next time
with my techno babble.


Well, that explains a lot. I think GeeDub is a mech eng, too. But
I don't remember him using techno babble.


I'd never admit to being an engineer (glorified secretary),
especially a mechanical engineer. Lowest form of life next to Mike
Vandeman. Like Mike,


Your ignorance is fantastic. I see you like working with your hands
and not your brain. Sounds like you just don't like being wrong. It's
hard, but you'll get used to it.

cc


The fact that I'm able to work with my hands puts me miles ahead of most
ME's. Ninety percent of them don't know how to put things together and
can't admit to that. Both the brain and hand have to work together which is
why I've made a successful career out of both woodworking and engineering.
I'm wrong a lot and I admit it. Very few engineers I've worked with can
claim the same, even when somebody shows them their mistake.

Then there's the issue with dealing with an ME buying a house and thinking
metallurgical tolerances should (and can) be applied to wood and drywall.
Dumbasses!

Gary


Ads
  #22  
Old February 11th 05, 03:00 PM
Dave W
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On Thu, 10 Feb 2005 05:00:19 GMT, "Phil, Squid-in-Training"
wrote:

The magnesium lowers of my 2003 Manitou Skareb Comp had begun to crack last
year, about 6 months before I stopped riding it. I've kept it stashed away
for a while until today, when I was cleaning my room and found the lowers.
I gave the legs a light (maybe 2.5-4lb) squeeze, and they snapped. Although
I didn't mean to snap them, they lent some interesting insight into their
failure.

Necessary background:
Manitou hailed their reverse arch technology in 2003 as being revolutionary
because it allowed lighter forks, stiffer blah blah blah. The Skareb, Six,
and Axel were the forks I can remember off the top of my head as having
identical lowers. These bikes for 2003 were assembled and shipped with the
fork/stem turned backwards, which, traditionally, puts the arch towards the
back of the bike. But with the reverse arch, the arch faces forwards, and a
bike's weight is all put directly on the arch during shipping. This
stresses the fork in a manner that it was never meant to be stressed
(perpendicular to the axis of the stresses normally encountered on a MTB
fork). As a result, as these bikes were ridden, the lowers began their
downward spiral into separation.

My experience:
From the first day I bought the bike, the front wheel had always rubbed the
v-brake pads when turning. I thought this was just because it was a light
bike (23lb Giant XTC2). But as I continued to ride, my expectations turned
to frustration, as the larger tire that I had put on was actually beginning
to rub the arch when braking hard. This wasn't really a problem until I
began riding more aggressively. It was at that point that noticed that the
wheel was beginning to get cocked to the side with the mere application of
weight. I knew this was out of the norm and searched for the problem, which
I found in the form of a crack in the fork leg that contained the coil
spring. New updated lowers with a beefed-up arch/leg join were sent, and I
installed them without a hitch. After another 2 months of use, I broke the
damping assembly inside the fork. Rather than fix it or pay to have it
fixed (I wasn't working at an LBS at the time) I left it as it was,
spring-only. After another 3 months, I acquired my current bike (Giant
STP2) and a Fox Vanilla. The Skareb with the updated, uncracked, flexy
lowers was put aside.

Today:
I broke the lowers manually and was surprised to see what had transpired.
The crack was much more extensive than I had thought, making an interesting
path parallel to the casting wall.

Photos: (I'm a much better photographer than I am a rider)

Here it is a year ago when I first identified the crack:
http://plaza.ufl.edu/phillee/rbt/p1.jpg
http://plaza.ufl.edu/phillee/rbt/p2.jpg

Here is the upper half, the arch portion:
http://plaza.ufl.edu/phillee/rbt/u1.jpg
http://plaza.ufl.edu/phillee/rbt/u2.jpg

Here is the lower half, the leg portion:
http://plaza.ufl.edu/phillee/rbt/l1.jpg

Here are the component parts, for reference:
http://plaza.ufl.edu/phillee/rbt/o1.jpg

(note: shiny marks on the pointy parts of the cracks are due to my
matching-up of the two broken pieces, which, I assume, smoothed over the
apexes of broken edges.)

Analysis:
Dark corrosion shows fatigue cracks weaving its way through the magnesium
(if it really is magnesium) up to a certain point about 2mm away from the
surface. From that point, the cracks then take a 60-degree turn upward
towards the arch. This suggests that at least some of the stress
experienced by the lowers were shear stresses (45 degrees if fully shear).
The cracks actually occupy a rather large portion of the total cross
section. Failure would have been imminent had I continued to ride it. The
final fracture indicates brittle material, with a smooth sandpaper-like
surface. Beach/clamshell marks are not very prominent, but they are there.

Corrosion is also visible at the very inside edge (the portion with the
smaller radius) of the casting. This is particularly worrisome, as it
appears that the fork had begun to fail not just at the thick portion, but
at the thin portion too. If this were the case, failure would occur even
faster since the forces acting along the thin part of the cross-section
would guarantee high bending moments, and thus high tensile stresses. If
they were acting along the long part of the cross-section, the forces at the
ends of the cross-section would not be as high.

The location of the inside edge corrosion also suggests that the casting was
improperly radiused/chamfered, at least in the original design. These sharp
corners should be avoided.

Final word: I'm lucky these didn't totally fail on me while I was riding it.



Thanks for the write-up. Never liked the look of that funky
reverse-technology. I guess this fork does not get your recommendation
huh?

Thanks again...

Dave (proud member of Bomberhood since 02')
  #23  
Old February 11th 05, 03:05 PM
Dave W
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On Thu, 10 Feb 2005 19:02:58 -0800, "G.T."
wrote:


"Ride-A-Lot" wrote in message
...

Why is it "lackadaisical"? Because I would rather ride than figuring
out what to do with an obviously busted fork?


Because if forks of the same model are going to fail I'd like to know about
it. I'm grateful Phil took the time to analyze the break. It could have
been a defect in the casting just on that one fork and then he could have
put everyone at ease. Conversely, now we know what types of forks to avoid.

I wouldn't have gone to the trouble so his analysis is a lot better than me
posting a picture and saying Manitous suck, which is probably what I would
have done and then no one would have learned anything.

GT



Agreed! Thanks again Phil for the heads up!
  #24  
Old February 11th 05, 04:13 PM
JP
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Here it is a year ago when I first identified the crack:
http://plaza.ufl.edu/phillee/rbt/p1.jpg
http://plaza.ufl.edu/phillee/rbt/p2.jpg


You found a crack in an integral structural portion of
the front fork, the fork that hits everything first,
the fork that absorbs the impact on downhills,
log piles, rock gardens, running over wildlife.
Yet you continued to ride on it.

The mind boggles.
Are you campaigning for a Darwin Award?



  #25  
Old February 11th 05, 06:07 PM
D T W .../\\...
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"Ride-A-Lot" wrote in message
...
G.T. wrote:
"Ride-A-Lot" wrote in message
...

Phil, Squid-in-Training wrote:



snip


GT



Why is it "lackadaisical"? Because I would rather ride than figuring
out what to do with an obviously busted fork? I didn't say go and buy
another Manitou. Send it back, buy a new fork, and move on. When you
get it back from Manitou, sell it.

I have a Minute 2:00 on my Titus. I'll let you know what happens.
Right now I think it has the best feel of any fork I have ever used. I
would have bought the similar Fox, but it was too expensive.

--
o-o-o-o Ride-A-Lot o-o-o-o




Eyeball analysis of my Minute One shows a thicker cross section in this
area,

--
DTW .../\.../\.../\...

I've spent most of my money on mountain biking and windsurfing.
The rest, I've just wasted.




  #26  
Old February 11th 05, 10:49 PM
Phil, Squid-in-Training
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"JP" wrote in message
news:cY3Pd.23899$s16.11910@trndny02...

Here it is a year ago when I first identified the crack:
http://plaza.ufl.edu/phillee/rbt/p1.jpg
http://plaza.ufl.edu/phillee/rbt/p2.jpg


You found a crack in an integral structural portion of
the front fork,


Sure did. Not sure about the back fork though.

the fork that hits everything first,


If you're a smooth rider, it doesn't.

the fork that absorbs the impact on downhills,
log piles, rock gardens, running over wildlife.


Where did I say I did all this? Does riding a mountain bike *require* doing
all these?

Yet you continued to ride on it.


And...

The mind boggles.


Yeah, it'd boggle my mind too if I were small-minded.

Are you campaigning for a Darwin Award?


Nope - I wanted to ride. If you can't accept a certain level of risk, go
cry back home.

--
Phil, Squid-in-Training



  #27  
Old February 12th 05, 10:06 AM
Duncan
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"Phil, Squid-in-Training" wrote in
message news
The magnesium lowers of my 2003 Manitou Skareb Comp had begun to crack

last
year, about 6 months before I stopped riding it. I've kept it stashed

away
for a while until today, when I was cleaning my room and found the lowers.
I gave the legs a light (maybe 2.5-4lb) squeeze, and they snapped.

Although
I didn't mean to snap them, they lent some interesting insight into their
failure.


I friend of mine had her skarebs snap in the same place. She's small and a
fairly timid rider so they had a very easy life.
LBS said he knew of at least three others going in the same place.


  #28  
Old February 12th 05, 04:33 PM
Bill Sornson
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Duncan wrote:

I friend of mine had her skarebs snap in the same place.


Now she does three shows nightly in Las Vegas.

Bill "rainy morning" S.


  #29  
Old February 12th 05, 08:28 PM
laffing@you
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"Phil, Squid-in-Training" wrote in
message ...
snip

Are you campaigning for a Darwin Award?


Nope - I wanted to ride. If you can't accept a certain level of risk, go
cry back home.

Exactly!!! I blow dry my hair, while standing in a puddle of water.

  #30  
Old February 12th 05, 11:08 PM
JP
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"Phil, Squid-in-Training" wrote in
message ...
"JP" wrote in message
news:cY3Pd.23899$s16.11910@trndny02...

Here it is a year ago when I first identified the crack:
http://plaza.ufl.edu/phillee/rbt/p1.jpg
http://plaza.ufl.edu/phillee/rbt/p2.jpg


You found a crack in an integral structural portion of
the front fork,


Sure did. Not sure about the back fork though.

the fork that hits everything first,


If you're a smooth rider, it doesn't.

the fork that absorbs the impact on downhills,
log piles, rock gardens, running over wildlife.


Where did I say I did all this? Does riding a mountain bike *require*

doing
all these?

Yet you continued to ride on it.


And...

The mind boggles.


Yeah, it'd boggle my mind too if I were small-minded.

Are you campaigning for a Darwin Award?


Nope - I wanted to ride. If you can't accept a certain level of risk, go
cry back home.

--
Phil, Squid-in-Training


For me the risk is inherent in the activity.
Then again my wheels go over bumps.
I don't use equipment I can't trust.

But please for the benefit of my narrow mind..

if you are such a smooth rider then why do you need
suspension in your front fork?


I'd hardly be the one crying back home with a shredded face
and dislocated shoulder resulting from the inevitable endo
when the fork failed. I'd never trust a casting with a crack.
Using the words "catastrophic failure" is a stretch.
Failing with no warning in use is catastrophic.
Failure after six months advance warning is rider error.
Failure after the series of events you described is predictable.

Oh right, you're a student.
You haven't yet learned that you can be seriously, irreparably damaged.
Blind faith in your own indestructable immortality.
You were lucky.





















 




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