Hub Spline Repair Question

That hub spline looks pretty bad. It could fracture under load, and you could have an accident. Just replace it.

The way it's worn it could have been used heavily for criterium races. Hill climbs would have worn the spline closer to the center of the hub. Long, steady miles such as triathlons are not going to leave deep teeth marks in the spline. I have a Shimano brand hub with nearly 9000 miles and no wear to the splines. The material could be softer (aluminum is softer than steel). It could also be the cogs have a lot of rotational play, so under initial acceletation they slip a little until they hit the spline with speed and force, like a jackhammer.

That's my guess too.  I surprised how bad the damage is.  I don't see any way this much localized force could have been applied unless the gears were not properly torqued.

This is a very common problem with aluminum freehubs - particulary after Shimano went to the revised spline when they released their previous generation 10s groups.  Essentially, the cassettes were designed for the deeper 10s freehub bodies, but used the same key patterns to retain backward compatibility with 8 and 9 speed freehubs.  These revised cassettes didn't have as many splines, and tended to fit a bit looser, causing the individual cogs to 'dig in' to a soft aluminum freehub body.  I've even noticed a degree of this problem on my ti freehub 9speed DA hubs with 10s Ultegra cassettes.

American Classic came out with some clip/pin sets that lock the individual cogs together and fill the gaps in an attempt to address the problem (google American Classic 10 speed clip kit) - not sure if they'd work with a non-AC freehub, but it may be a 15 dollar insurance policy worth looking into.

SYJ