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"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 |
#2
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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
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"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 |
#4
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"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 |
#5
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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 |
#6
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"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 |
#7
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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 |
#8
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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 |
#9
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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! |
#10
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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. |
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