According to Jordan :
Simple disk. Simple magnet holders under the disk, out of sight.

[ ... ]

Thanks, good ideas.

What do you think about aluminum-rimmed bicycle wheels? Big diameter
should be good?

Yes -- but the spokes and the nuts for adjusting them may cause
a cogging effect -- unless both parts are a non-magnetic stainless
steel.

For maximum effect, does the magnetic pathway need to be "through" the disc?
That is, with a horseshoe magnet over the edge, or 2 linked magnets?

That depends on the speed. Above a certain speed, the field
would not penetrate very far in thick aluminum.

A single magnet would be simpler. In that case, would both poles need to
be near the disc? Or just one, the other end remote from the disc?

Ideally -- both poles near the disc -- which would make those
recovered from dead hard disk drives very good choices, since that is
how each side is arranged in my experience.

Sorry to pester, but I haven't been able to find details by web-searching.

You *won't* find information to the level you want. This is the
kind of thing which you answer by experimenting, especially since the
magnets from the disk drives are somewhat of an unknown quantity. But
they typically are *very* strong -- so beware getting pinched between
them while separating them. They are typically bonded to mild steel
backing plates, and you might as well leave them on that for ease of
mounting.

Go to a place which repairs computers and pick up one or two
*dead* disk drives. (A couple of bucks each at worst, or free if you hit
the right place.) Or -- if you are willing to pay more, you can buy the
magnets already extracted from someone who does this as a business,
probably selling on eBay.

Go to a store which has specialty screwdriver bits (a good
electronics place is a good choice) so you have the necessary medium and
small Torx drivers to take apart the disk drives. Extract the magnets
and play with them to answer your own questions.

And note the brand and model of the disk drive which provides
the easiest to use set of magnets so you know what to look for when
collecting those for the final workpieces.

The greater the radius the greater the effect, of course, so you
may wind up spending more to have discs of aluminum cut to the size
which you need.

Good Luck,
DoN.
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