Thanks Harv, for turning me on to this site! Never have I seen a discussion board that would spend so much time on self holding cups (as opposed to cupholders)
I'm not much of a tractorer myself, but I can offer some insights about the magnets and the design.
Firstly, I'd skip the sign magnet material entirely, and use only a couple or 3 of the hard drive magnets glued to the outer edge of the bottom of the cup (3 would probably be best for stability). Then I'd go to the local supply shop and get some neoprene. My local bearing and belts shop sells the stuff in various thicknesses by the running foot, a couple feet wide. Cut a piece of that out the size of the bottom of the cup, and glue it over the magnets. That will keep it from sliding, and protect you beloved tractor. Double it up if its still too strong.
Not having a magnet in the center will allow the cup to conform to the contours of the tractor's fender. You'd probably have to alter your design to specifically fit the tractor in question.
As for the magnets themselves, we've recently done some lifting tests. Using two flat pieces of steel, hanging the upper plate securely and attaching a magnet to it, then attaching the other plate to the magnet. from the lower steel plate (suspended only by the magnet), we hang a 5 gallon bucket and fill it up with sand. When it falls, take a little sand out, and weigh the bucket.
Using this technique, we were able to determine that even the smaller hard drive magnets can lift a LOT! Item #21 (much smaller than the item #42 which Harv ordered) on our forcefield website was able to lift
more than 60 lbs.
The surplus hard drive magnets are definitely your best bang for the buck (free if you do your own salvage work). In addition to computer shops, you might also check scrap metal dealers (lots of aluminum in hard drives), and surplus electronics stores if there's such a place around.
On a related note, I thought y'all might find
this picture, which my mom put together, appropriate for this discussion board.
Happy tractoring!
-Matt B