That hole in the middle of this '70's era front wheel hub is a grease port:
And when grease is not being inserted in it, the black metal band can be moved over the hole, like so:
It's there so that a mechanic can refresh the grease inside the hub without taking the hub apart.
Here are the exit holes:
New grease goes in the center, old grease spills out the sides. That's how it worked back in the day.
Modern hubs don't have bleed ports. They're obsolete for a number of reasons:
1. Grease is stickier and doesn't run away like it used to.
2. Machining techniques have improved, closing the gap between the hub shell and the spindle, reducing the occurrence of road grime contaminating the grease.
3. Sealed bearing&race systems need to have their seals removed to refresh the grease, thus necessitating a hub overhaul regardless of whether there's a grease port.
4. You need a hub polisher to keep the center clean and protected.
5. People don't like them.
That last reason is a big one. Grease ports make a mess.
Last year I built a wheel around an old Campagnolo Record hub, and I've been riding it on my commuter every day since. The problem I keep running into, is that despite only refreshing the grease once or twice, the sun-of-a-gun just spills out White Teflon Lube. So much White Teflon Lube, more than I ever put in in the first place. It's like it's breeding in there:
Every few days I have to run a rag around the exit ports, or else they'll drool like a couple of SRAM engineers all over the place. Ugg.
So that's why people don't ride old junk anymore, except me, I guess. So to end, take a look at the hidden mysteries that reside inside a modern hub, with sealed bearings and no grease ports. Must be nice:
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