Does anybody know what the name of this feature is?
This is the back side of a hyperglide hub where the hub body engages the wheel hub. I’m trying to upgrade to 10 speed. Original on the left, new part on the right. There’s 10 teeth on the old one and 15 on the new one.
New one obviously won’t fit due to the difference in the number of teeth, but adding 10 tooth to my searches doesn’t bring up anything helpful
Those teeth are not involved in the ratcheting system. They are only how the freehub body attaches to the rest of the hub. The pawls are inside the freehub.
One additional consideration you need to account for is the chain stay width. 7s and 10s are not the same. You can spread a steel frame to fit, but it isn’t easy.
I tried to do what you want by disassembling the freehub body and keeping the base that fit my hubs, but it didn’t work. So I stopped at 9s.