the bearings that are commonly used in bicycle hubs are not truly sealed.
If they have contact seals*, they are truly sealed. The pressure differential needed to force fluid past the seal is obviously much smaller than it is for submarine prop shafts, but it still exists.
*And they do, for the most common cartridge bearings
If they have contact seals*, they are truly sealed. The pressure differential needed to force fluid past the seal is obviously much smaller than it is for submarine prop shafts, but it still exists.
*And they do, for the most common cartridge bearings