The back of the hubs on my trailer have some "bumps" which I'm not sure what they are for.
The hub is a 5 lug x 5½" bolt circle diameter. From the view in the photo, the lugs are the outermost bumps which I've marked with "L".
The other bumps marked with "?" are what I can't identify. Measuring them as though they were lugs, they seem to have a 4½" circle diameter.
Note that I've been told these might be agricultural rims & axle, although this is just a normal non-ag utility trailer.
Thanks!
They may just be an artefact of the manufacturing process.
The hub looks like a casting, and if it was cast with a central hole to save machining time later, you would need to pour the metal evenly around the circumference to avoid eroding the mould at one point and producing an unsymmetrical part.
The bumps are probably the bottom of the "risers" and "feeders" used to pour in the metal and ensure there was enough molten metal to completely fill the mould as it cools and the metal contracts.
If the view in your photo was the top when the hub was cast, these would originally have been several inches tall. Cutting them off flush with the surface of the hub would require machining the whole surface to remove any damage caused when cutting them, which would be a pointless waste of time. So they were cut off leaving the slight bumps you can see.
I suspect the same hub can be configured for a five OR four lug pattern. Trailer parts are often made as cheaply as possible from the lowest bidder.
