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mavic r-sys New Mavic r-sys wheelset
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The other thing I was a little skeptical about was how the aluminum drive spokes would hold up since they are now at lower tensions. The reason for the lower tensions was so they could make a lighter rim and because the carbon spokes could handle compression, but aluminum spokes can't. So in theory radial loads could significantly accelerate the fatigue of the drive spokes since they will not be able to handle compressive loads. The CF spokes (left) may be stiff enough that the tension in the aluminum spokes doesn't go to zero. Even if it did, the (thin, bladed) aluminum spokes wouldn't support tension, they'd buckle. Also, don't nick the carbon spokes or else they will be severely weakened. According to our mavic rep. they have already had a few cases of broken carbon spokes which they are still trying to determine whether or not they were caused from nicks in the spokes. I would wait a year before investing in these wheels. Besides impact/nick damage, either fatigue may be a problem or the spokes may fail under high radial impacts (potholes) depending on the layup schedules. My guess is, to get the stiffness, they're using primarily uniaxial CF, so these spokes may be very brittle to impact loads. Perhaps the CF spokes may buckle first, it'd take an analysis to determine that. <http://www.stormingmedia.us/97/9719/A971964.html Tension-Compression Fatigue Behavior of a Carbon Fiber/Epoxy (IM7/EPON 862) Composite Fabricated Using Vacuum Assisted Resin Transfer Molding Process Fatigue life curves were developed for stress range, maximum stress, and normalized stress. Specimens tested under tension-compression loading were shown to have lower fatigue life than those under tension-tension loading. In Google Book Search, see: Fiber-Reinforced Composites: Materials, Manufacturing, and Design By Pankar Mallick See page 257, figure 4.48 for S-N for compressive-compressive of various laminates. The accompanying text says that although those curves are for epoxy/E-glass, Similar trends are observed for T-300 carbon fiber reinforced epoxy systems . In short, not only does CF fatigue, it fatigues in compression, and fatigues even faster in compression-tension. Another interesting section on page 192 suggesting ways to improve the impact strength and avoid brittle failure of CFRP. <http://www.ornl.gov/~webworks/cpr/v823/rpt/106099.pdf Basic Properties of Reference Crossply Carbon-Fiber Composite Indicates that very good fatigue performance (~90% UTS @ 10^8) is achieved with 0/90 crossply, but more modest (~40% UTS @ 10^8) for +-45 crossply. Unfortunately, the 0/90 fatigues/fails to sudden failure. isn't it amazing how you can dig up relevant facts when you want to, and bullshit when you don't?
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mavic r-sys New Mavic r-sys wheelset
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so keep them in their bags when not using them - just like mavic say. that's why the bags come with the wheels! if you nick steel or aluminum spokes, they can break too! When I first heard about Mavic R-SYS wheels, I thought, they're Mavic, they're a totally inappropriate use of CFRP, they're a horrible, horrible idea
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mavic r-sys New Mavic r-sys wheelset
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Also, don't nick the carbon spokes or else they will be severely weakened. According to our mavic rep. they have already had a few cases of broken carbon spokes which they are still trying to determine whether or not they were caused from nicks in the spokes. I would wait a year before investing in these wheels. Besides impact/nick damage, either fatigue may be a problem or the spokes may fail under high radial impacts (potholes) depending on the layup schedules. My guess is, to get the stiffness, they're using primarily uniaxial CF, so these spokes may be very brittle to impact loads. Perhaps the CF spokes may buckle first, it'd take an analysis to determine that. isn't it amazing how you can dig up relevant facts when you want to, and bullshit when you don't? I back up my claims with citations. You don't.
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mavic r-sys New Mavic r-sys wheelset
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You'd need to know the materials & layup to accurately predict the spoke behavior, yes. That's always the big unknown with composites. _meta_ls are much simpler. not when you know about them properly they're not! far from it. Sure, a composite spoke with bonded ends and unspecified material and layup is not more complicated than a length of 302 wire bent, headed and threaded. OK, I'm convinced.
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mavic r-sys New Mavic r-sys wheelset
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there's three reasons we have tension on conventional spokes. 1. tension allows wire spokes to provide compressive support. 2. sufficient tension allows that support to be maintained. 3. sufficient tension allows that support to be maintained with sufficient integrity that spoke nipples don't unscrew. but, excess tension increases fatigue and decreases load capacity of the rim. and wheel strength does not increase as spoke tension rises above the minimum required for 1, 2 & 3. Without describing excess and minimum that statement is meaningless.
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The administrator has disabled public write access. |
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mavic r-sys New Mavic r-sys wheelset
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so keep them in their bags when not using them - just like mavic say. that's why the bags come with the wheels! if you nick steel or aluminum spokes, they can break too! When I first heard about Mavic R-SYS wheels, I thought, they're Mavic, they're a totally inappropriate use of CFRP, they're a horrible, horrible idea
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