Think of it this way: a standard bearing is capable of holding your axle as if you were holding a stick with the tips of your first finger and your thumb in both hands. A tapered bearing would be analogous to you holding the same stick except with your whole palm and fingers wrapped around them.
A standard bearing can not handle more than about 600 lbs of side load, like if you slid around a hard corner, or even into a tree. A tapered bearing can handle about 6000 lbs of side load (I'm not kidding) from the same impact. The reason is that the tapered design takes a side load and converts a portion of that load into a vertical load which the carrier would absorb. The hearings ride on a surface that is somewhat perpendicular to a side load, therefore, the load is actually on the carrier instead. The carrier is capable of handling a huge amount of loading, so it ends up being about 10 times stronger than a standard set of bearings.
Sorry for the long explanation, but I wanted to get some gears spinning for anyone who is interested.
Keep in mind that having an external grease zerk is like getting a $1000 bill for your birthday, nice and handy.
Joel Morris
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Purdue University Graduate Student
Mechanical Engineering Technology
1985 ATC70
web.ics.purdue.edu/~jsmorris