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#1
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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 |
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#2
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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
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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
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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
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"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
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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. 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
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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. |
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