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



 
 
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  #11  
Old February 10th 05, 10:01 PM
Ride-A-Lot
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Phil, Squid-in-Training wrote:
"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.


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

--
o-o-o-o Ride-A-Lot o-o-o-o
www.schnauzers.ws
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  #12  
Old February 10th 05, 10:53 PM
Phil, Squid-in-Training
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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!


Hehe... I was hoping you'd notice.

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


LOL... if you really want to know, they're pumpkin seeds.

http://plaza.ufl.edu/phillee/rbt/itspumpkinseedsduh.jpg

--
Phil, Squid-in-Training



  #13  
Old February 10th 05, 11:07 PM
Ride-A-Lot
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Phil, Squid-in-Training wrote:
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!



Hehe... I was hoping you'd notice.


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



LOL... if you really want to know, they're pumpkin seeds.

http://plaza.ufl.edu/phillee/rbt/itspumpkinseedsduh.jpg


I think you just squashed everyone's hopes. Like a pumpkin!

--
o-o-o-o Ride-A-Lot o-o-o-o
www.schnauzers.ws
  #14  
Old February 10th 05, 11:15 PM
Gwood
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Any recourse via Manitou ? Hoping my old trusty SX-R will give me another
couple of years...

"Phil, Squid-in-Training" wrote in
message ...

"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





  #15  
Old February 10th 05, 11:38 PM
GeeDubb
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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,
mechanical engineers think they know all that there is to know until you
point out their obvious flaws......which they still insist is
correct.....until it breaks like the manitou fork.......and for craps sake,
stop trying to re-invent the freeken wheel.....and admit when you're
wrong!!!!

They don't call it a BS in Engineering for nothing. (;-}

Gary (Williams Woodworking, Inc.!)




  #16  
Old February 10th 05, 11:50 PM
Phil, Squid-in-Training
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"Gwood" wrote in message
...
Any recourse via Manitou ? Hoping my old trusty SX-R will give me another
couple of years...


The SX-R has the normal arch, doesn't it? If so, it's probably fine.

I sold the fork with the new lowers for $15 on eBay. I kept the old lowers,
which are the ones I broke fully yesterday.

--
Phil, Squid-in-Training



  #17  
Old February 11th 05, 03:14 AM
cc
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"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


  #18  
Old February 11th 05, 04:02 AM
G.T.
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"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
--
"Once you have something that grows faster than education grows, you're
always going to get a pop culture" - Alan Kay


  #19  
Old February 11th 05, 05:08 AM
Slacker
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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 th=

eir
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 havin=

g
identical lowers. These bikes for 2003 were assembled and shipped wit=

h =

the
fork/stem turned backwards, which, traditionally, puts the arch toward=

s =

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 M=

TB
fork). As a result, as these bikes were ridden, the lowers began thei=

r
downward spiral into separation.

My experience:
From the first day I bought the bike, the front wheel had always rubbe=

d =

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 tha=

t =

the
wheel was beginning to get cocked to the side with the mere applicatio=

n =

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 brok=

e =

the
damping assembly inside the fork. Rather than fix it or pay to have i=

t
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 (Gian=

t
STP2) and a Fox Vanilla. The Skareb with the updated, uncracked, flex=

y
lowers was put aside.

Today:
I broke the lowers manually and was surprised to see what had transpir=

ed.
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 t=

he
apexes of broken edges.)

Analysis:
Dark corrosion shows fatigue cracks weaving its way through the magnes=

ium
(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 upwar=

d
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-lik=

e
surface. Beach/clamshell marks are not very prominent, but they are =


there.

Corrosion is also visible at the very inside edge (the portion with th=

e
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 e=

ven
faster since the forces acting along the thin part of the cross-sectio=

n
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 casti=

ng =

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 ridi=

ng =

it.

--Phil, Squid-in-Training




Excellent work and pix... Thanks. This gives me 2nd thoughts about =

defecting from the 'hood.

Glad the failure didn't happen while you were riding.
-- =

Slacker
  #20  
Old February 11th 05, 12:31 PM
Shaun aRe
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"Phil, Squid-in-Training" wrote in
message ...
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!


Hehe... I was hoping you'd notice.

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


LOL... if you really want to know, they're pumpkin seeds.

http://plaza.ufl.edu/phillee/rbt/itspumpkinseedsduh.jpg


Yeah, they're pumpkin seeds you bunch o' eejits - Phil keeps his weed stash
elsewhere so NER!


Shaun aRe
--
Sometimes, the true and living thought bubbling rapidly up from the
depths, surfaces with a severe case of the Benz.

--
Life is the dream you wake up to.


 




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