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



 
 
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  #1  
Old February 10th 05, 06:00 AM
Phil, Squid-in-Training
external usenet poster
 
Posts: n/a
Default "Catastrophic" failure of mountain bike fork lowers (Manitou Skareb Comp)

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.

--
Phil, Squid-in-Training



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  #2  
Old February 10th 05, 12:46 PM
Ride-A-Lot
external usenet poster
 
Posts: n/a
Default

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.

snip extensive research

Dude, way too much analytical time on your hands! You forgot to do a
gas chromatograph test on the magnesium dust. That should give you the
exact compound ingredients.

If that happened to me, I wouldn't waste time analyzing it. I'd just
buy a new fork and ride.

Too much flat Florida sunshine and college beer!

--
o-o-o-o Ride-A-Lot o-o-o-o
www.schnauzers.ws
  #3  
Old February 10th 05, 03:30 PM
Zilla
external usenet poster
 
Posts: n/a
Default

Hey you now have a lefty!

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

it.

--
Phil, Squid-in-Training


Word!

--
- Zilla
Cary, NC
(Remove XSPAM)


"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.

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.

--
Phil, Squid-in-Training





  #4  
Old February 10th 05, 03:39 PM
R Hace
external usenet poster
 
Posts: n/a
Default


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

snip extensive research

Dude, way too much analytical time on your hands! You forgot to do a gas
chromatograph test on the magnesium dust. That should give you the exact
compound ingredients.

If that happened to me, I wouldn't waste time analyzing it. I'd just buy
a new fork and ride.

Too much flat Florida sunshine and college beer!

--
o-o-o-o Ride-A-Lot o-o-o-o
www.schnauzers.ws


And not enough of what ever is in the bag in the last photo! Good write up,
I'm now heading out to the garage to inspect my forks a little closer.

Lance


  #5  
Old February 10th 05, 04:00 PM
justen
external usenet poster
 
Posts: n/a
Default

Phil, Squid-in-Training wrote:
The magnesium lowers of my 2003 Manitou Skareb Comp had begun to crack last


This is a farily well-known problem with the 2003 reverse arch forks.
Supposedly things were fixed for 2004. I have heard of failures of the
2003 forks, but not the 2004. FWIW.

justen

  #6  
Old February 10th 05, 05:23 PM
Phil, Squid-in-Training
external usenet poster
 
Posts: n/a
Default


"justen" wrote in message
...
Phil, Squid-in-Training wrote:
The magnesium lowers of my 2003 Manitou Skareb Comp had begun to crack
last


This is a farily well-known problem with the 2003 reverse arch forks.
Supposedly things were fixed for 2004. I have heard of failures of the
2003 forks, but not the 2004. FWIW.


Yup... all over MTBR. I'm surprised they didn't recall them all; I'd be
scared to know what a heavier rider could do... I'm only 135 lbs.

--
Phil, Squid-in-Training



  #7  
Old February 10th 05, 05:42 PM
G.T.
external usenet poster
 
Posts: n/a
Default


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

snip extensive research

Dude, way too much analytical time on your hands! You forgot to do a
gas chromatograph test on the magnesium dust. That should give you the
exact compound ingredients.

If that happened to me, I wouldn't waste time analyzing it. I'd just
buy a new fork and ride.


That's a pretty lackadaisical attititude towards something as important as a
front fork. I quit working at Answer when they had two rounds of recalls
around 1994. I saw a couple of pictures of riders' faces after their forks
catastrophically failed. They weren't pretty.

GT

--
"tilt the bottle,
**** the cup,
yeah" - Bottom 12


  #8  
Old February 10th 05, 05:57 PM
Ride-A-Lot
external usenet poster
 
Posts: n/a
Default

G.T. wrote:
"Ride-A-Lot" wrote in message
...

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.


snip extensive research

Dude, way too much analytical time on your hands! You forgot to do a
gas chromatograph test on the magnesium dust. That should give you the
exact compound ingredients.

If that happened to me, I wouldn't waste time analyzing it. I'd just
buy a new fork and ride.



That's a pretty lackadaisical attititude towards something as important as a
front fork. I quit working at Answer when they had two rounds of recalls
around 1994. I saw a couple of pictures of riders' faces after their forks
catastrophically failed. They weren't pretty.

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
www.schnauzers.ws
  #9  
Old February 10th 05, 08:29 PM
Phil, Squid-in-Training
external usenet poster
 
Posts: n/a
Default


"Ride-A-Lot" wrote in message
...
G.T. wrote:
"Ride-A-Lot" wrote in message
...

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.


snip extensive research

Dude, way too much analytical time on your hands! You forgot to do a
gas chromatograph test on the magnesium dust. That should give you the
exact compound ingredients.

If that happened to me, I wouldn't waste time analyzing it. I'd just
buy a new fork and ride.



That's a pretty lackadaisical attititude towards something as important
as a
front fork. I quit working at Answer when they had two rounds of recalls
around 1994. I saw a couple of pictures of riders' faces after their
forks
catastrophically failed. They weren't pretty.

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.


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.

--
Phil, Squid-in-Training



  #10  
Old February 10th 05, 09:36 PM
Jon Bond
external usenet poster
 
Posts: n/a
Default


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

snip extensive research

Dude, way too much analytical time on your hands! You forgot to do a gas
chromatograph test on the magnesium dust. That should give you the exact
compound ingredients.

If that happened to me, I wouldn't waste time analyzing it. I'd just buy
a new fork and ride.

Too much flat Florida sunshine and college beer!

--
o-o-o-o Ride-A-Lot o-o-o-o
www.schnauzers.ws


Nah, that stuff is all basic materials science/machine design stuff. I was
about to do it for him until I kept reading and saw he did it himself. Yay
ME students!

However, I do believe extensive research is needed to determine the contents
of that ziploc baggie...

Jon


 




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