They are almost totally different systems with the only things in common are filling the hub with grease by means of a Zerk fitting.
is a spring-loaded grease cap on the hub. One fills the hub with grease, from the outside in, using the Zerk fitting until the hub is filled (I don't know where the air goes..). The spring keeps the hub under positive pressure, so that when a warm hub is dunked in cold water, the shrinkage won't pull in water past the seal and grease cap to contaminate the grease. There is no way for fresh grease to lubricate the inner bearings unless the seal leaks
. Under hot highway conditions, expanding grease has to push the spring back or exit through the seal in older models; newer models may have pressure relief valve to prevent this. Newer BBs use a special seal
to prevent overpressure, obviously requiring disassembly of the bearings to install.
BBs are also subject to loss by getting knocked off, excess tire bounce from unbalanced tires
and theft, according to the BB site.
OTOH, the EZLube
, AG Hub
systems have no pressurization except when pumping grease. The axle
spindle is hollow and has a Zerk fitting on the outer end, covered by a rubber cap. The inner end of the spindle has holes between the inner bearing and the seal. With the rubber cover removed, fresh grease is pumped into the spindle (by hand pump, with wheel jacked up and slowly turning, according to Dexter). The grease goes down the spindle, comes out the holes, goes through both the inner and outer bearings and comes out next to the Zerk, forcing the old grease out. Once the hub is greased, if the grease expands, coming out through the rubber cap is easier than getting past the seal. (I recommend scooping out some of the excess grease under the cap to prevent grease fling, but be careful because the inner edge of the metal cap is *sharp* and fresh blood looks really strange on greasy fingers...).
With the spindle-lube systems, one can also choose to hand pack bearings in the conventional manner, esp if one doesn't like having a hub full of grease.