Balancing Acts – Generalities

Armature Balancing is an operation that is misunderstood by most racing enthusiasts. It may seem like a simple operation, but to be done right, it is not! In slot car motors the rotor (armature) is very small. Not only in length, but most critically in diameter. Its this small diameter and light rotating weight that makes it difficult to balance this tiny piece–if its size were not enough, slot car motors usually operate at very high RPM. All these items make proper balancing very challenging.

A further consideration is the equipment being used. If you try to use a machine not up to the task of reading a rotor as small as the one in a slot car motor, you may as well not bother. Most balancers are designed for rotors of larger size. Larger machines that are converted for doing slot car armatures usually don t work very well. The brand of balancer can be just as important, as can the salesperson. Some machines are simply not up to the job many users put them to. It is not always the equipment s fault, but rather the buyer looking for a bargain or the seller pushing off a piece of equipment not up to the job. A novice user must take the sellers word that the equipment is up to the job. Rarely does the seller offer to take back the machine if it doesn’t do the job right. Often the buyer doesn’t realize the equipment is not up to the intended job.

All aspects of the machine must work together to get the operation done properly. The pickups must be very responsive and light in weight. The filter must be capable of adjusting to the rotational speed of the rotor during balancing. The amplifier must have enough gain to make the ‘out of balance location’ easy to discern. The machine should also be easy to use so a highly trained operator is not required to use it.

How the balancer is maintained is important for proper extended operation. When the machine is used for long periods without proper maintenance, it can loose sensitivity and positioning may go out of adjustment. This can lead to armatures that will not perform as expected.

To Add or to Remove? The high RPM encountered with slot car motors usually indicates the use of drill balancing. High centrifugal force and the elevated temperatures encountered in slot car armatures precludes the use of epoxy balancing. It would be nice if it could be used (maybe someday) when the materials are better up to it. Drill balancing must be done carefully. If the hole is off center, part of the crown of the rotor may fly off. If this happens the armature is ruined. Precise positioning of the original balancing holes is very critical because any further drilling will cause more damage if the hole was off center to start with. The same can be said of any new balancing holes. They too must be centered in the crown of the rotor for damage not to occur.


Unbalance is solely due to the rotor’s mass distribution about its bearing axis. The unbalance of a rotor does not change with speed. It is the centrifugal force generated by the unbalance that increases as rotational speed increases. The balance tolerance must therefore be determined by the maximum operating speed of the rotor. Got That? As operating speed increases so does the apparent amount of unbalance. This is why balancing tolerances must also be tightened as speed goes up. Rotors of this type are not typically balanced at operational speeds due to operator safety and machine considerations. Such high speeds can slow down the overall operation of the balancer and may be reason for operator safety concern.

Higher balancing speeds can however yield better results with small rotors such as those used in slot car motors. The RPM used for Hi-Speed balancing might not be as high as operating speed, but can be considerably higher than what is used for normal balancing. This increases the centrifugal force caused by the unbalance. The result is higher signal amplitude. This makes it easier to read the out of balance position, because the signal you are trying to read is raised above the background noise level. Centrifugal force is what causes the vibration in an out of balance armature. If the rotor is not turning then no force is exerted, therefore no vibration. As RPM increases the amount of vibration increases greatly. Centrifugal force increases at the square of the speed increase. Double the speed, quadruples the force. Triple the speed, increases the force by nine times. This fact is what makes balancing very critical when motors are operated at very high RPM.

Stack length variations also affect how armatures are balanced. Not the balance specifically, but how the machine is changed or not changed when arms with different stack length are balanced. Does the operator alter the machine to properly read the different lengths? Failure to do this can lead to less than expected results.

Practical Considerations……

All this theory is nice, but what happens in the real world? This is where we get down to individual machine setups and what one person or another makes work. There is many ways this can be accomplished.

Some manufacturers have been around long enough to have developed processes and techniques that work time and time again. Experience is very important in this job, as is an up to date piece of equipment. Put all these items together and you can have a remarkably powerful combination. Other companies may say they balance to lower numbers than armature manufacturers. This comparison is very difficult unless the machines are setup identically, including RPM and the pickup locations. When we check armatures balanced by other companies, we also find higher numbers than we expect. While we are able to reduce these numbers by further balancing, we find the numbers initially read are generally within acceptable limits.

It s a common misconception by the consumer public that major manufacturers don’t have the time to balance properly, but they not only have the time, certainly with their experience, they do it faster and accurately. Manufacturers base what they do on their many years of experience and testing in this field.

Racers tend to judge balance by how smooth running their armatures are at low RPM. This kind of test does not always give reliable results, as the armature is not raced at this same low RPM. It is important for the armature to perform correctly at the highest operating speeds, as that is where any out of balance is most critical. Our machine sensitivity is -> 10 micro inches. “Moment Value” is the term used to describe the calculated value of imbalance. Moment Value is expressed as milligram/inches. We balance our .513 dia. arms to below .256 mg/in., our .480 arms to under .120 mg/in., and our Super16-D to less than .52 mg/in. This is well below acceptable limits based on all our known balance affecting factors.

The purpose of balancing is to reduce the rotors vibration. Vibrations come from many sources, including but not limited to gear mesh, tires, air flow over the armature, brushes sliding past the comm slots, magnetic cog(magnets pulling against the pole pieces), out of round and imbalance of the shaft and ball bearings, clearance between armature shaft and ball bearing, etc. Reduction of vibration by balancing the armature to a lower number, is only effective if the amount of unbalance is large compared to the other vibration sources. If the imbalance is very small, little improvement is likely.

Removing material while drill balancing should be done carefully. Over drilling must be avoided. Trying to balance an armature to ‘0’ is not usually necessary and often leads to an excessive amount of holes and/or dangerously deep holes. Excessive removal of magnetic material harms performance in ways you can t possibly imagine and reduces reliability as well.

Shaft roundness is one of the most important details in obtaining the best balance. If the shaft is not round it matters little as to how good the balancer is, it will be hard to get a really good balance. Straightness of the shaft is important, but the roundness of the shaft is what will ultimately determine how low the armature can be balanced. Some say they straighten shafts While this is possible to a limited degree with non-drill blank shafts, arms with drill blank shafts are almost impossible to change. This type of heat treated material usually breaks before it bend

Location of the pickups is likewise important. The operator should have the means to relocate the position of the pickups when armatures of different overall and stack lengths are worked up The ability to center the stacks over the pickups is something that can be done with some balancing machines. It is definitely preferred to have the pickups located directly under the stacks for the most accurate balance and best operation.

Stack diameter also affects balancing, especially if the operator fails to compensate for the diameter change. It can lead to over drilling, improper readings and an overall poor balance job. Many operators probably do not realize how much stack diameter affects the overall balance operation.

Direction of rotation is something else few users think of. Although direction does not seem like anything to be concerned about, at the speeds slot car motors operate, it is. The air moved by the armature at high speed can be considerable. Another subject that presents itself is harmonics. As the armature rotates, it generates sympathetic vibrations called harmonics that affect the balance. Balancing in the normal direction of rotation allows the balancer to react to these harmonic vibrations in the normal way, as vibrations are not always the same in both directions. With all other factors thrown in, direction can play a considerable role in accurate balancing.

Please understand, that manufacturers DO have the time to balance properly and that they DO balance properly. We likewise, balance to excellent specifications. Being a manufacturer or production house gives us the experience to produce performance racing components of very high quality.

As you can see from this article, we have arrived at our final balancing specifications by following the above technical aspects, considerations and calculations along with many other machine setup processes that we have not mentioned.