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Comment by deletes | Hacker News Reader

How do you prevent rotation of the magnets when they are stuck together. The light on a bike will experience a force in that direction sooner or later and move.

When I saw the dimple I though that was the solution, but it is only a marker for the north side. Maybe the solution would be to apply a thin layer of substance with high friction between the magnets.

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As you might imagine, others have encountered and solved this problem already with stick on high friction disks. They've also used cheap, steel cups to direct and strengthen the magnetic field:

http://www.leevalley.com/US/wood/page.aspx?p=32066&cat=1,423...

Also, I've used those nice shiny magnets in outdoor applications and can attest that they rust in fairly short order. But one might be able to address this by enveloping the magnet in sugru. Of course, this too is an already solved problem - plastic and rubber coated magnets:

http://www.kjmagnetics.com/products.asp?cat=164

Given all this, I'm not sure what this use of sugru is "hacking". It's performing the function of glue? Expensive glue in a nice package.

Glue? Gluing the magnet to the light is tricky unless you get the right shape case and the right glue. Gluing the other magnet to the bike is tricky for the same reasons.

Sugru solves both of them.

Epoxy putty, caulk and silicone adhesive also solve the glue shape issue. All available at your local hardware store for less than 20% the cost of sugru and with a shelf life greater than 6 months.

You're gonna secure something to a bike with caulk? Not so sure about that...

Easy of use, Sugru is trivial to apply with minimal effort.

It is a user interface victory. Pay more for easy of use.

You could do it with the sugru itself, no? Have one side with indentations, make the other side align with those indentations. You don't need it to resist forceful rotation so much as casual bumps. Or just include a small pin or dowel with one side that sticks into the other side. You can attach more than just magnets.

You can force that with diametrically magnetized discs[1]. It's not the usual way these discs are magnetized, but this will not allow rotation under light forces. On the other hand, if you manage to rotate them 90 degrees, they will pop out, as the magnets will start to repel.

With big discs, the force needed to rotate even axial magnetized discs (like the ones they seem to use) is not trivial if the magnet surfaces are clean. By the size of the discs they use, their adhesive force can be ~3-5 kg, and it's not easy to rotate such magnets. You can add a layer of thin cloth over the magnets to increase the friction, and it will also protect the magnets from scratching and damp the hits when sticking.

No idea what kind of magnets they use, but treating them like they do in the presentation (throwing them one against another, or against objects) is a sure way to crack the nickel plating in the long run.

[1] like this one: http://www.supermagnete.de/eng/data_sheet_S-12-06-DN.pdf

Yes, you can force that with magnets designed for that, but these appear not to be magnetised like that, so if you want to avoid the rotation, you need to do something about it.

As I say in my other comment, easy enough to do with non-symmetrical mating surfaces.

The hack in picture 1 has semi-circular magnets. If you use two magnets with opposite polarities then you can get it to be able to lock in only one position. Also, easy enough to make "wavy" mating surfaces to avoid rotation.

Use two pairs.

On a thin bicycle handle? You could if the flashlight is wide enough. ( two pairs seems like a waste )

They should include a pair of slightly equally dented magnets or magnets with a little dent like the one that marks the north, but this on is positioned with an offset from center.

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