A Cycling & bikes forum. CycleBanter.com

Go Back   Home » CycleBanter.com forum » rec.bicycles » Techniques
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

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



 
 
Thread Tools Display Modes
  #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



Ads
  #2  
Old February 10th 05, 06:22 PM
Dion Dock
external usenet poster
 
Posts: n/a
Default

Would a failure in use really have been a catastrophic problem? The sliders
are bolted to the fork at the bottom, so they won't fall off. Maybe braking
would have caused them to spin though.

-Dion

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





  #3  
Old February 10th 05, 09:22 PM
external usenet poster
 
Posts: n/a
Default


My riding buddy has the identical fork, and the identical crack. We
think it happened about six weeks ago, when he caught a stump and came
to an abrupt halt. I heard a pop, but he didn't have any broken bones
and the bike seemed okay. We rode the next week, and then the week
after that we went to a six hour race. As he was lifting the bike off
the truck he noticed that the arch was completely snapped.

Amazingly, he opted to ride the bike anyway (he's pretty stupid that
way). He made it through all of his laps without incident, but retired
the fork afterward.

The two lower legs were moving independently (obviously). I would have
expected the leg with the disc caliper on it to rotate around and cause
the caliper to bind, but that didn't happen. I guess the skewer was
doing the job of keeping the dropouts facing each other.

Now he wants me to find him another Manitou on eBay. Talk about brand
loyalty.

Dave
www.davewilson.cc/Bike

  #4  
Old February 10th 05, 10:44 PM
Werehatrack
external usenet poster
 
Posts: n/a
Default

On Thu, 10 Feb 2005 05:00:46 GMT, "Phil, Squid-in-Training"
may have said:

The magnesium lowers of my 2003 Manitou Skareb Comp had begun to crack last
year, about 6 months before I stopped riding it.


You knew they were cracked, and kept riding it that way?

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.


[mondo snip]

Looking at the pictures, I have just two things to say.

First, the failure looks very typical of what I've seen in alloys
which employed too much magnesium in an application where flexure was
going to be present. High-Mg alloys are *brittle*, as is Mg itself.
In small amounts, it can contribute to making a good, tough alloy, but
if too much is used, it's a bad thing in my experience.

Second, the area of the failure looks like it was poorly engineered in
my estimation. The bridge-to-tube transition forms a natural stress
concentrator which I would have expected to result in failure in
exactly the way that it did.

I'm not familiar with the features of the Skareb; is this one of the
forks which has the damping in one tube and the spring in the other?
If so, that would have hastened the cracking.

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


I can see several possible ways that such a failure could have gone
down; unfortunately, a number of them involve you doing a face plant
in the process. I agree; this was a failure which you are lucky to
have had happen where it did.

--
My email address is antispammed; pull WEEDS if replying via e-mail.
Typoes are not a bug, they're a feature.
Words processed in a facility that contains nuts.
  #5  
Old February 10th 05, 11:00 PM
Phil, Squid-in-Training
external usenet poster
 
Posts: n/a
Default

"Werehatrack" wrote in message
...
On Thu, 10 Feb 2005 05:00:46 GMT, "Phil, Squid-in-Training"
may have said:

The magnesium lowers of my 2003 Manitou Skareb Comp had begun to crack
last
year, about 6 months before I stopped riding it.


You knew they were cracked, and kept riding it that way?


I misstated that. It was 6 months after I got it that I noticed the crack,
and about 2 months after noticing before I was able to get a replacement set
of lowers, as it was during winter break, and I didn't get back to the LBS
until a month after noticing. I rode more gently, and not very often.

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.


[mondo snip]

Looking at the pictures, I have just two things to say.


Second, the area of the failure looks like it was poorly engineered in
my estimation. The bridge-to-tube transition forms a natural stress
concentrator which I would have expected to result in failure in
exactly the way that it did.


Yep - I think I mentioned that at the very end.

I'm not familiar with the features of the Skareb; is this one of the
forks which has the damping in one tube and the spring in the other?
If so, that would have hastened the cracking.


Yes, it is. The opposing forces made the fatigue stresses even worse.

--
Phil, Squid-in-Training



  #6  
Old February 11th 05, 11:07 AM
Marvin
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.

If you manage to lose an entire set of fork lowers in your room you're
very nearly as messy as I am :-)

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.


Just to add my voice, I've had one of these reverse-bridge forks fail
on a customer's bike. It doesn't seem to be safety-critical since he
only noticed it when the clunking noise and fork flex really got on his
nerves and he inspected it a bit more closely. Since the legs can't
fall off and the skewer is still tight, there's no short-term potential
for a catastrophic failure. More mileage would probably twist and snap
the QR skewer, at which point you really would be in trouble, but my
customer got round the rest of a fairly tough off-road ride with no
worse problems than his fork legs walking around and crapping up his
steering (disc brake still worked fine). As soon as we removed the
wheel the lower legs twisted around quite independently and freely, but
the axle and skewer seemed to hold them enough for him to get home.

It definitely looks like a design problem, right on that sharp corner
where cornering stresses and the asymmetry between spring and damper
cartridges combine. The fact that it also seems to be quite a flexy
design in a material that doesn't like being flexed suggests that
Manitou really weren't thinking all that carefully during the design
phase.

Bottom line: yes they're a bit of a suspect design, no failure wouldn't
necessarily have resulted in a faceplant.

  #7  
Old February 11th 05, 04:00 PM
Werehatrack
external usenet poster
 
Posts: n/a
Default

On 11 Feb 2005 02:07:18 -0800, "Marvin" may
have said:

Bottom line: yes they're a bit of a suspect design, no failure wouldn't
necessarily have resulted in a faceplant.


The tips of the tubes, where the QR clamps, are also mag alloy. I
suspect that they'd break off before the skewer would fail...but it
would take a while. The greater danger would be if the arch failed
during a bumpy downhill run; the asymmetric flex could put the wheel
into one tube enough to give just the added amount of unwanted braking
to produce an endo.

--
My email address is antispammed; pull WEEDS if replying via e-mail.
Typoes are not a bug, they're a feature.
Words processed in a facility that contains nuts.
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Road bike vs. Mountain bike Q-factor Ron Techniques 8 October 19th 04 08:25 PM
Still Looking for a bike [email protected] UK 19 September 5th 04 10:25 AM
aus.bicycle FAQ (Monthly(ish) Posting) kingsley Australia 3 February 24th 04 09:44 PM
How old were you when you got your first really nice bike? Brink General 43 November 13th 03 11:49 AM
First road bike: braking? Alan Hoyle General 47 September 28th 03 11:40 PM


All times are GMT +1. The time now is 06:16 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 CycleBanter.com.
The comments are property of their posters.