EditTesting the Strength of your Motor Magnets
One of the more important things about magnets is that they are 'matched'.
Matched magnets are a pair that have generally equal strength. If they aren't matched, even a balanced armature will be rendered ineffective because the stronger magnet will always pull the closest wind harder to that end of the chassis creating vibration and unstable operation.
EditCurie Temperature
Do your slot cars run hot? Do you worry that it's affecting your magnet strength?
Take a moment to educate yourself about Curie Temperature. Curie Temperature (CT) is the temperature where magnets begin to lose their strength.
Did you know that ceramic magnet material is available in grades 1-8 just like bolts?
Ceramic material is usually baked as a block at about 2000 degrees then machined to shape. A typical ceramic magnet has a CT of 450º with a maximum operating temperature of 300º.
Neodymium (NEO) magnets are typically pressed into a shape from powder and therefore are far more brittle. The Curie Temperature of Neodymium magnets is lower than that of ceramic magnets at about 300º with a maximum operating temperature of around 220º.
EditTesting with a Compass
The following method is quite accurate. You need a compass (even a cheap one will do) a sheet of paper, a ruler and a pen or pencil.
Fasten the sheet of paper to a wooden table or workbench, well away from steel objects (including nails in the woodwork). Lay the compass on the sheet of paper and establish the North/South line. Use the ruler to draw that line from one side of the paper to the other. Measure and mark the center of the North/South line and draw a second line at right angles to the North/South line.
Place the compass in the middle of the North/South line. Place the magnet you want to test on that line, facing the compass, but as far away as possible. Slowly move the magnet towards the compass. When the compass reverses, mark the position of the magnet on the paper.
If the magnet does not reverse, place the magnet at the opposite end of the North/South line, and slowly approach the compass from that direction. The distance from the center of the compass to the position of the magenta when the compass reversed is a measure of the strength of the magnet. This distance will be greater with a stronger magnet, less with a weaker magnet.
You see, when the compass reverses, the magnetic field of the test magnet at the location of the compass is just a tiny bit stronger than the earth's magnetic field. In theory, you should get the same distance measurement even if you use different compasses.
But the distance measurement will be different at different locations on earth. That's because the earth's magnetic field varies in strength with location. (It also varies in direction. Magnetic north is not the same as geological north, and there are magnetic anomalies in some locations that keep a compass from pointing towards true magnetic north. Also, at higher latitudes the magnetic field develops a significant 'dip'. At magnetic north the field actually points straight down.)
This test is a reliable way to rank your magnets, from weakest to strongest. It also gives you a numerical measure of the strength of the magnets that should be repeatable as long as you perform the test in the same location.
One of the (HO-DL) list members once checked the accuracy of this test versus a quality gauss meter. He found he could compare the results of the two methods within 5%.
EditMeasuring & Matching Slot Car Magnets
The most important - MOST IMPORTANT - thing about magnets is that they are 'matched'. Matched magnets are a pair that have generally equal strength. If they aren't matched, even a balanced armature will be rendered ineffective because the stronger magnet will always pull the closest wind harder to that end of the chassis creating vibration and unstable operation at higher speeds.
Another thing to keep in mind is that depending how Magnet pairs were stored and also how many 'cool-hot' cycles they've gone through, their strength may of lessened over time. Technically, things that permanently affect magnetic strength are physical shock (when point of impact is opposite or at a right angle to the earths natural fields), heat ("curie temperature" - typically about 300 degrees for ceramic magnets) & long term storage against an opposing (repelling) field.
How to Measure Magnet Strength
There are many schools of thought here, because a single magnet may be stronger on one end that the other. My experience is that that while this is true, the difference in most cases is negligible.
Unless you have access to a $170 TrikTrax slot car magnet tester or a scientific grade gauss meter, you may feel like you are out of luck. However there are a few ways to measure "on the cheap". If you use the above information as a guide, you should be able to get more than reasonable results for a few bucks. Remember, a weak matched set can sometimes be better than a stronger mis-matched set. A strong matched set is typically best.
Magnet strength is measured in GAUSS, or MGO. We aren't concerned so much with measuring outright gauss strength as much as we are concerned with being able to comparison test strengths. So...
EditMagnet Matcher Options
BRP Magnet Tester - about $60 Bucks, available at their web site. Performs comparison tests.
An Analog Gauss tester - eBay about $30-$50. Not as accurate as digital, but usually cheaper! Picture and opining below.
BB's. Yep, BB's. $2 worth of BB's goes a long way to matching your magnets. Small ball bearings, steel nuts - you choose. Smaller the better. When 2 magnets pick up the same amount of BB's, voila! You have a matched set. The more BB's they'll pick up, the stronger they are.
Easier and cheaper than you thought, huh?
Remember, the stronger the magnet pair, the less your car will "coast" when you let up on the trigger - this is important - if you are a driver who only has 2 trigger finger speeds (on and off) you probably don't want the strongest magnets money can buy, you'll wipe out in the turns too easily.
Think about it, unless you are drag racing, the race is never won on the straights - but the straights are a place to catch up when you are losing in the turns. Driving style counts!
Environmental Magnetism
Without going through an entire picture spread, we can tell you that spinning a directional meter a few degrees in any direction will also change the needle slightly, again due to environmental magnetism (Magnetic North, right?). For this reason you should always test pairs with your meter, any meter, in a stable directional position.
Even Face Strength
Another factor is variance in strength along the face of the Magnet. The TrikTrax is supposed to test for this, however most testers will give you some idea of variances along the face. Yet another way to test is to take a T-jet or A|FX magnet and an opposing Super II Magnet and "feel" them as they come together. Because a SII magnet is fully oriented, it should act as a good baseline of evenness. If your T-jet magnet wants to "creep" to one end of the SII as they come together, it probably has uneven strength along the face. I actually have a white A|FX magnet that is so incredibly uneven that it rests on the end of any opposite pole magnet I "stick" it to.
As an aside...
If you have two equal strength magnets that both display stronger qualities on one end than the other and you install them into your chassis with the stronger ends "kitty corner", you will be effectively changing the "timing" of the armature. We'll go into timing a little more in a future page, but just want to make you think!
If you want to build a tester of your own, or maybe even purchase a full blown Digital Gauss tester, do a search on eBay for "Gauss" - usually, many gauss testers will show up. Find one that suits your budget. Then get yourself a few pair of JL or Super II magnets to use as a calibration baseline. We recommend a tester that allows you to re-calibrate, as a tester will lose calibration over time.