I have always used them as they keep more grease than the original cup. Also inside its spring loaded so when its full there is always grease being pushed on the bearings. If you take the cup off that came with the trailer the grease always ends up around the cup.
Where they are really useful is on boat trailers. You get your bearing nice and hot on the highway and then you back it into cold water!!! If your seals leak, your bearing is in trouble but the bearing buddy keeps the water out by forcing grease around the bearing.
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