The (Not So) Evil Traction Magnet

By Michael Ashton

Like most hobbies, the slot car hobby has its share of ‘religious beliefs’ that are constantly being debated, often vehemently. Routed wood vs. sectional plastic track; analog vs. digital; silicone vs. rubber tires. And arguably the most controversial of all — running with vs. without a traction magnet. Visit any of the major Internet slot car forums or a retail establishment or club that has a slot car track, and at least one of these debates will inevitably flare up. All of these doctrines tend to divide slot car enthusiasts into two opposing camps that defend their respective positions passionately. This article will focus on the traction magnet debate, affectionately known as ‘mag vs. no-mag’. It is important to note that any discussion of slot car traction magnets applies only to track which uses metal rails that will exhibit some degree of magnetic attraction (i.e., magnets will have no effect on a routed wood track that uses copper tape for its rails).

The vast majority of ready-to-run slot cars come with a traction magnet installed, usually somewhere between the mid point of the chassis to just in front of the motor in sidewinder configuration or the rear axle in in-line cars. These magnets usually fit into an open chamber in the cassis and are held in place by small tabs. Occasionally, as is the case with Carrera cars, the magnet is held in place by a screw-in holder which is very secure.

The majority of modern slot car magnets are made of neodymium alloy which has very strong magnetic properties. The magnet is attracted to the metal rails of the slot car track, therefore, creating additional downforce. Its purpose is twofold:

1. Encourage the car to stay in the slot.
2. Increase traction in the rear tires.

This will tend to enhance the performance of just about any slot car, especially those which exhibit inherently poor handling characteristics such as narrow width, high center of gravity and uneven weight distribution. The downward force of the magnet creates a drag effect which tends to slow the car in the straightaway sections but allows significantly higher cornering speeds which, in turn leads to faster lap times. This does not necessarily make a car easier to drive because it becomes more difficult to judge the limit of adhesion, especially at high speeds. When a car equipped with a powerful magnet exceeds that limit the result is very often a violent, ’snapping’ de-slot which could possibly result in significant damage to the car.

Slot car traction magnets come in a variety of shapes, sizes, and levels of magnetic force.

The following photos illustrate some typical magnet installations, highlighted by the red arrows.

A large flat bar magnet installed in front of the motor in a sidewinder chassis.

A button magnet installed mid-chassis in an angle-winder chassis.

A bar magnet installed under the motor shaft in an in-line chassis.

An in-line chassis with two magnets installed, in front of and behind the motor.

What’s the Issue?

There is a myriad of things that can be done to a poor handling slot car to improve its performance. This is generally referred to as ‘tuning’. Three of the most common issues are: 1.) de-slotting too easily (especially in the corners); 2). fishtailing (wide swaying of the rear end, also in the corners); and 3.) a tendency to tip (again, in the corners) due to high center of gravity (COG). All of these issues indicate the need for increased downforce and/or better weight distribution.

These issues can be dealt with in one of two ways — either by adding weight (ballast) to improve weight distribution and increase traction, or installing a magnet that creates enough downforce to effectively mask the problems. Generally speaking, the magnets that are installed in ready-to-run slot cars at the factory tend to be very powerful and positioned to provide maximum downforce.

So, what to do? Mag or no-mag? This is where things tend to get very serious because otherwise mild-mannered, rational people begin to lose their composure and end up divided into two seemingly irreconcilable camps. There are those who believe that all slot cars must have at least one magnet so that they can be run at the fastest speeds that the laws of physics will allow and handle as a ‘modern’ car would. Then there is the camp that believes any slot car — no matter how unwieldy — can be tuned to run well by adding weight to various points of the chassis, or changing critical components such as the guide or rear tires, or in extreme cases, replacing the entire drive train and/or chassis itself! For this camp, resorting to the use of magnets is a sign of weakness that brings one’s very manhood into question.

The Path of Least Resistance

My preference is to try to run without the magnet, whenever possible and appropriate. Once I become sufficiently familiar with a newly acquired slot car, I like to place it in one of three categories:

1. Cars designed to run without a magnet.
2. Cars that should run with a magnet.
3. Cars that run poorly without a magnet.

There are a number of slot cars that, although come equipped with a traction magnet, are actually designed to run without the magnet. For example, Slot.it, NSR and Ninco cars fall into this category. They perform very well with the magnet out-of-the-box, however, they also exhibit superior performance without the magnet and with little to no tuning at all. I almost always run these cars without the magnet.

There are certain classes of cars that I feel should be run with a magnet because it creates handling characteristics that more closely reflect the actual (1:1) car that they represent. LMP, certain GT sports cars and modern Formula 1 are in this category. This relates more to creating a realistic racing experience than improving handling and performance. Remember that this is my opinion.

Lastly, are cars that are designed to run with a magnet installed. These cars could be from any manufacturer and they may be able to run without a magnet on smooth, flat track surfaces, but usually require significant tuning and/or modification in order to obtain an acceptable level of performance. This is the category where I have occasionally found cars that I simply could not get to run well without the magnet, even after some of the most aggressive tuning tricks were applied.

One slotting enthusiast once told me that he absolutely refuses to ‘give in’ to the use of a magnet. That there is no such thing as the car that he can’t tune to run well with a little weight added here and there. Then he revealed his secret: he glued half a pound of lead to the underside of the chassis and turned the track voltage down to 9 volts! This is ‘tuning’?

Then there’s the guy who runs with three magnets creating so much downforce that the car sticks to the track when held upside down. Track voltage must be turned up to 18 volts just to overcome the drag created by the magnets and the motors sometimes catch fire! But the car never de-slots!

Now I’m not disparaging either of these approaches to enhancing performance. And, yes I may have exaggerated a little, and everyone is entitled to enjoy this hobby in their own way. But people at the extreme ends of things tend to become somewhat evangelistic and can occasionally get a little emotional when another point of view is presented. Nevertheless, I believe that there is a middle ground.

I have found that it is possible to tune a slot car using one or more relatively mild magnets, resulting in performance and handling characteristics that are quite similar to that of running without a magnet. Moreover, this car will afford the average home racer a wider margin for error resulting in more fun and less stress. There is no special talent needed and no voodoo involved. In fact, some manufacturers provide this capability in their cars straight from the factory. Carrera typically includes two bar magnets in their chassis. A wide magnet directly in front of the motor and a shorter one just in front of the rear axle.

The red arrows highlight the position of the two bar magnets in the typical Carrera chassis.

Both magnets are secured by a holder and include a metal shim that allows the magnet to be raised about 1 mm. This may seem like a trivial distance, however, it has a measurable effect on the magnet’s downforce.

The Carrera bar magnets. Shown with holders and shims.

This combination of magnets and shims allows the chassis to be configured to allow nine distinct levels of magnetic downforce.

I have performed many tuning tricks on my Carrera GT cars to get them to run well without a magnet on my twisty, bumpy Ninco track. This included adding weight behind the guide assembly, sanding the top of the mounting post bases flat and loosening the body screws to allow the body to float, and installing high grip urethane rear tires. Still, I can only describe the results as marginal. The cars were drivable, however, the slightest amount of excess throttle in the turns would result in a de-slot. It is important to note that the Carrera chassis has fixed front bushings in contrast to the slotted axle holders that many cars have. The fixed bushings are not an issue on a smooth flat track surface. However, they tend to cause the front end to bounce upward on a bumpy, undulating track surface such as Ninco. So, I decided to see what the flexible Carrera magnet implementation could do for performance.

With both magnets installed in their lower position (Shims on top) the car was quite ’stuck down’ and performed like an LMP on the track. By removing the rear magnet (yellow arrow in the above photo) and placing the shim below the mid-chassis magnet (red arrow above), I was able to achieve handling that was surprisingly close to that without a magnet. The car could now be run with much more confidence in the turns and the rear end was still able to drift slightly when powering through corners.

You are not limited to the stock magnets that are installed by the manufacturers. There is also a wide range of aftermarket traction magnets available. I found two such magnets, available from BRS Hobbies to be particularly useful because they are relatively mild and can be ’stacked’ together in order to create different levels of magnetic downforce in the same footprint in the chassis. They are the BRS bar magnet and BRS button magnet, respectively.

The BRS bar magnet

The BRS bar magnet, at 19 mm x 3.2 mm x 1.5 mm, is an inexpensive, relatively compact, mild strength magnet that can be glued into a wide variety of locations in a typical plastic ready-to-run slot car chassis. By stacking this magnet one over the other, Magnetic downforce can be added in meaningful increments. This is especially useful when a magnet is obviously required for a particular car but the stock magnet proves to be too powerful.

Two BRS bar magnets hot glued in a stack.

Similarly, many slot cars come fitted with a full-size button magnet which tends to be very powerful and cannot be easily raised using a shim. The BRS has the same 8 mm diameter as the full-size magnet so it fits into the same cylindrical compartment as the larger version. However, the BRS version is only 2 mm thick and has only about 1/3rd of the larger magnet's downforce. It too can be stacked for incremental levels of downforce.

The BRS button magnet

The Magnet Boneyard

Finally, don't forget about all those magnets that were pulled from cars that are now running no-mag. If you want to find out just how much of a purist a no-mag racer really is, ask him if you can have all his leftover magnets.

My leftover magnet stash proved once again to be a valuable asset. I recently acquired a Scalextric BMW E30 M3 as a companion to the same model that a slot racing friend has. Straight out of the box with the stock traction magnet in place, the car runs like an LMP. You can almost complete an entire lap at full throttle. We decided that we would like to run them no-mag in order to create performance that is closer to that of the actual car. However, when the magnet was pulled, initial testing indicated that this was going to be a significant challenge. The model is quite attractive and almost perfectly accurate to scale. Ironically, being faithful to the actual car's surfaces and dimensions makes this car very difficult to tune for no-mag racing.

As a slot car the BMW E30 M3 has many of the characteristics that virtually guaranty poor performance without a magnet:

• The car is tall and narrow, which makes for a high center of gravity.
• The front end is light with a relatively shallow guide flag.
• Rear tire grip was marginal, resulting in fishtailing.

With the magnet removed it was difficult to complete a lap without extraordinary caution and concentration. The first attempt at tuning was comprised of adding weight (about 15 grams of copper — don't like that lead!) behind the guide assembly to help keep the front end down and in the conduits on either side of the DPR chamber to address the tipping. Urethane rear tires reduced the fishtailing, however, the higher grip tended to exacerbate the tipping, which was the car's predominant problem.

BMW E30 M3 chassis with weight added (red arrows).

This made a marked improvement but the car was still too top-heavy. The rear end could not slide out in the corners and the car would tip far too easily. Not wanting to add any more weight I decided to see if a magnet could overcome these problems.

After experimenting with several magnet configurations using magnets from the stash, the best solution turned out to be a pair of Ninco bar magnets taken from Ninco 1 cars. Now these bar magnets are quite strong. Probably as strong as the stock Scalextric magnet that comes with the car. However, by carefully positioning the magnets and making them adjustable to control downforce they turned out to be ideal for this situation.

The following photo shows the area of M3 chassis where the replacement magnets will be installed.

The red arrows point to what are effectively two three-sided chambers on either side of the motor shaft. The Ninco bar magnets fit perfectly (snug) into these chambers. The next photo shows how the magnets were installed.

A magnet holder (strap) was created from thin, black styrene sheet. The holder is glued into the chamber and is tall enough to allow the magnet (red arrow) to be raised and lowered using shims (yellow arrows) and also to be easily removed from the holder. The magnets are positioned far enough to the sides of the chassis to avoid any significant drag while running on the straight track sections. However, when the rear end starts to drift in the corners the inside magnet shifts closer to the rails and downforce increases. The height of the magnets from the track surface can be adjusted to provide just enough downforce to allow the rear end to drift while keeping the car from tipping. Now this car can truly be powered through the corners without fishtailing or tipping.

Another benefit of this type of magnet implementation is the ability to tune two or more cars configured this way to have virtually identical performance as a racing group. By using the shims to change the height of each magnet in very small increments, lap times can balanced easily within tenths, if not hundredths of a second. This can provide very competitive and exciting racing.

Here is a short video clip of the E30 M3 configured with the dual traction magnets running on Ninco N-Digital  track.

So there. I admit it. I have more than a few cars that owe their enhanced performance to the judicious use of magnets. Try as I might, I could not get them to perform in an acceptable way (for me) by adding weight. But they all perform quite closely to similar cars that can run without the magnet. And now when they run around the track, instead of clenching my teeth as I approach each corner, I'm having fun. 


_Michael Ashton