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Spoke flange failure, Shimano FH-3300 cassette hub next to sprockets. From http://www.flickr.com/photos/65657096@N00/7014908657/ as of 2014/08.
No reports of rider injuries. Service history unknown except "five years". No notes on how the failure was discovered.
Typical flange tear-outs are from the spoke holes out. Discoloration on the inboard part of the crack suggests that the failure had been progressing on the inboard part for some time (that is, was not the last thing to fail). It is unclear from the photo whether the intial failure was outboard of the spoke hole and grew in, or whether it started inboard and grew out.
Note that the flange is undercut to clear the cassette driver, as shown here:
Via http://forums.adventurecycling.org/index.php?topic=11734.0 and http://www.flickr.com/photos/63373992@N07/8036521062/ as of 2014/08:
Shimano makes two styles of hubs those with aluminum splines for the cassette and those with steel splines. The steel spline hubs have a 2mm longer cassette and the flange is undercut to make room for it. [Without the undercut] adds 2mm of dish. On the M529 the did something different they used a steel spline with out undercutting the flange and combined it with an LX level cassette. The hub is advertized as a high torque hub [...].
[W]hich shimano hubs have the steel insert[?] [The] 756 does not. The 529  750 755 and HF 07 also have the steel insert.
Presumably another approach is to make the flange thicker where it is undercut; there is limited clearance to do this without hitting the inboard spoke heads, but a slight increase in flange diameter would allow thickening. Another alternative might be to increase the material outboard of the spoke holes, to better spread the load and thus reduce loads in the undercut area.
Shimano cassette hubs typically have the right flange further outboard than other hubs, leading to reduced wheel dish compared to other hub &mdsah;(which in turn improves wheel strenght and durability. A standard measure of flange placement is from the center of the hub to the center of each flange, oftren noted WR and WL. Direct comparisons among hubs are complicated when the flanges are different diameters because the spokes form a cone; however, approximating: an WTB (Wilderness Trail Bikes) hub based on a Shimano ratchet has WR ≈ 21 mm, while a Shimano FH-HF08 (48-hole cassette tandem hub, same dropout spacing 135 mm) at about the same diameter has WR ≈ 19 mm. (Although 2 mm may seem small, this is about 10% worse for the FH-HF08.)
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