Spacing Out With CDROM Cans: The Strong Magnet Spacer Approach - RC Groups
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Dec 14, 2005, 01:12 AM
Dave North
timocharis's Avatar

Spacing Out With CDROM Cans: The Strong Magnet Spacer Approach

After I posted a thread on winding a basic 17-turn motor, a number of people asked for tips on placing the magnets.

I know of three techniques: using a template, using something (rubber bands, plastic, paper clips) of the correct width between each magnet, and the Strong magnet spacers available from

I've done all three, and I'm pretty much a convert to using the Strong spacer.

For this example bell I'll be using N50 curved mags designed specifically for these bells and sold by Don Armstrong. He also has spacers for flat magnets, 20mm bells, GBx, you name it. They're generally about $3.

Before you start, gather up your magnets of either pole and mark the outer edge, leaving the other polarity blank. This will help you keep track of what you're doing. At this stage, I keep them stuck to the bell and pop one in to make sure the carboard spacer is putting it at the right height. Yup!

But first, a note about yet another problem: the new N50 magnets (use them!) tend to "creep" toward the front of the bell, almost no matter what you do. This is particularly true of the curvies (more on that in a bit). You actually want them as far from the front of the can as possible. The front of the can is just clearance for the wire; the back of the stator should be flush with the back of the can, as should the back of the magnets).

The only cure I've come up with is using a bit of paper card stock as a spacer in the front of the bell. It should be about 3-1/8" long and about a millimeter wide. Too wide is better than too narrow, since you can jam the magnets into it (it's paper, right?). Thickness is not very important, but it should be absorbent and flexible. I find the header cards from Dubro Mini Hinges are perfect (they're 3-1/8" wide! No measuring).

If you look closely in this first picture, you can see the little cardboard ring.
Last edited by timocharis; Dec 14, 2005 at 01:16 PM. Reason: really lousy grammar error
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Dec 14, 2005, 01:13 AM
Dave North
timocharis's Avatar

Drop In All Your Marked Mags

First drop the spacer in. I prefer to work without the screw in it because I have hammy fingers. Don suggests leaving it in (partly so you can raise it a bit higher).

Then put your six marked magnets in every other slot. That should be fairly easy, as they're somewhat distant from each other. Then start putting in the other six.

This will be a bit harder, since the magnet you're trying to put in will "jump" toward one side or the other if you're not precise in your placement (and I'm not).
Dec 14, 2005, 01:14 AM
Dave North
timocharis's Avatar

Straighten the Strays As You Go

When this happens to you, a pair of needle-nose pliers can come in handy. Don also uses a wooden skewer, and toothpicks have been known to do the job. A handy bit of carbon rod.

If you get it straight and centered, it will sort of just pop into the slot.

Normally I would hold the spacer in place while doing this, but I'm also trying to shoot the picture at the same time. Try it; it ain't easy!
Dec 14, 2005, 01:14 AM
Dave North
timocharis's Avatar

All Magnets In

Should end up looking about like this.
Dec 14, 2005, 01:15 AM
Dave North
timocharis's Avatar

Get the Spacer Back Out

Retracting the spacer is a cinch. Just turn in the supplied screw and pull it gently out.
Dec 14, 2005, 01:16 AM
Dave North
timocharis's Avatar

Turn It Over

On the back is a wood "lift" with a nice big hole in the middle that misses the flange in the middle of the bell. It's also threaded, so you can stick the screw into it.
Dec 14, 2005, 01:17 AM
Dave North
timocharis's Avatar

Even Up The Mags

Then you can stick the spacer back in and run it all the way up and down the can. This will take out any unevenness in the spacing (the magnets can still be slightly tilted).

For the truly anal, raise the spacer up about halfway to the top, then rotate the screw lightly. This will even up the spacing between the magnets just a hair more (since they could be anywhere in their alloted slot).
Dec 14, 2005, 01:17 AM
Dave North
timocharis's Avatar

Press The Magnets Flush With The Can

I don't trust my eyes to get the magnet level exactly right, so at the end I like to turn the can over and gently press it to a flat surface. After that, the mags should be solidly seated into the paper and all at very close to the same height.

Don't twist the can when you do this!
Dec 14, 2005, 01:18 AM
Dave North
timocharis's Avatar

Almost Done!

The tension among the magnets, and their attraction to the flux ring, will hold them neatly in place as long as you're reasonably careful about handling the can. This is a bad time to drop it.

Now you'll need to glue them in place. I like to run a small drop of CA between each set, letting it bleed into their edges and under them a hair. The excess will run down into the paper spacer and turn it into a solid magnet grabbing monster. This is a good thing.

Set it aside and let it dry.
Dec 14, 2005, 01:19 AM
Dave North
timocharis's Avatar

A Comment About The Magnets

I'm a compulsive CDROM motor part tryer outer. These didn't seem to be run-of-the-mill curvies (which usually don't have quite the right air gap), so I was intrigued.

In later email, Don told me they are in fact custom designed and ordered by him, which explains why they fit so well.

But does that mean anything to the pilot? If my first impression running this can is any indication, the answer is: yup.

I tested it on a 14-turn 24awg 22.7mm stator that I have previously run with N50 flat magnets and N50 curved magnets I was able to acquire, but that were not a precise fit. By comparison, I'm getting obviously better performance (based on my subjective opinion while flying the plane) and probably better efficiency (based on mah put back in / time flown).

I think they're great, one of the easiest and best hop-ups I've done.

And yes, I paid for my parts!

Dave North
Dec 14, 2005, 01:52 AM
Flying motor mount master
fly_boy99's Avatar
You know tim I own a couple of spacers from Don but I've found that using rubber bands is just as effective and fast!!!

Dec 14, 2005, 03:33 AM
Good Better Best quest.
olmod's Avatar
mmm ive been using Dons spacers for some time now ,but i fit an alloy tube to the centre as i do the magnet installation with the shaft in, so as soon as i add the loctite i can spin them in my dremel inside a toilet roll cardboard tube to get them ballanced and spin off the excess loctite. i tried every other way imaginable but nothing beats the time saving of using the spacer. IMHO.
also spinning really gets the loctite all the way across behind the mags' i know coz ive had to remove some
Dec 14, 2005, 09:09 AM
Why aren't you out flyin?
colbyweb's Avatar
Olmod, what kind of loctite are you using?
Great thread Dave! I've never broken down and ordered a spacer and I've found that rubber bands work great for me.
Dec 14, 2005, 10:00 AM
Nice thread Dave. Thanks for posting and sharing your experience with STRONG Magnet spacers.

Olmod, I have taken your idea of using an Aluminum tube for the center and used it on the the spacers for GBx cans. Most guys are now buying the GBx endbells pre-pressed on the shaft, so using the tube you can slide the spacer right over the shaft. I like this so well the other spacers will also be made this way in the future.

I would welcome others who have used the spacers to share their experience and suggestions here also. You can help others learn about placing magnets and, I am always looking for ways to make better products.

Readers of this thread may want to vist the Instructon Documents on the web site. Here are links to two related documents.

Handling and Installing STRONG Magnets

STRONG Magnet Spacers
Last edited by bz1mcr; Dec 14, 2005 at 10:11 AM.
Dec 14, 2005, 11:23 AM
Master of 1 point landing
Originally Posted by colbyweb
Olmod, what kind of loctite are you using?
Great thread Dave! I've never broken down and ordered a spacer and I've found that rubber bands work great for me.
I use red locktite and never had an issue. The key with locktite is to make sure there is no grease on either part. Apply gobs and spin to distribute. I have also used CA with good success - only breaking mags on one point landings.

I have attempted to use thermal transfer epoxy, just for grins. The stuff I had was like working with JB Weld, very messy, very thick. I soaked the can in MEK to break the bond as the mags were high, then went back to locktite.

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