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Building A Slot Car Motor:


Slot Car motor building seems to be a black art to many racers. Not so. Common sense and some basics should help you work some magic of your own. You probably started out with a 16d slot car motor and may now want to advance to a Mura Challenger one arm. The advantage here is the way the wire is attached to the commentator tabs and a much tougher commutator and overall better armature construction.

We'll walk through a conversion and some tuning tips. Before taking your slot car motor apart, mark the top of the can and endbell with a file or dremel tool cut off wheel so you can put them back in the same location. Now take your 16d apart. But be sure you can see the paint mark on the right hand magnet looking at the top of the can with the endbell facing away from you before you remove the magnets. You must be able to put the same magnet back into the can on the same side as it was before. If not you will have reversed the direction the motor turns which you don't want. If you can find no mark, paint one on with some slot car body paint on just the right hand magnet. Then clean the can, the magnets and endbell out with some kind of mild solvent or hot water and soap. Dry it all well with mild warming or an air blast. Set the 16d arm aside for future use if still good.

Get a new challenger I arm and have the shop owner, if he has one, put it in his comm lathe and give it a skim cut to true it. I find many of the new arms right off the self are not true. Then make sure that the comm slots are cleaned out with the back side of a # 11 exacto knife blade. This is to avoid digging into the sides of the slots. Remove any small burrs or copper shreds. Then take the point of a very fine point ball point pen and rub the top edge of each slot to burnish it a bit. When done, you should see a small shiny area on each side of the slot. This will help eliminate brush shock, wear and bounce on the sharp edges. Avoid scratching the comm surfaces or getting greasy fingerprints on the shiny copper. Then, set aside.

For the next step, you need a brush hood alignment tool. Assemble the can, and endbell with no magnets by using can screws to keep it all in position. First, insert the alignment tool into the endbell brush hoods. Then insert the alignment rod representing the armature shaft thru the endbell and thru the alignment tool all the way thru the rear bearing. If it is all too tight, loosen the hood screws to ease alignment. It may be necesary to take a small rat tail file and open up one or more of the hood screw holes. The rectangular alignment bar should move back and forth very easily thru both brush hoods when they are tightened down with screws. Then the alignment rod should slide up and down easily thru the motor bearings and the hole in the rectangular alignment bar as well. It may take a bit of shimming or filing to get everything aligned in all directions. This will make a big difference in your motor performance.

Now assemble the can, endbell and the armature without the magnets. Make sure the arm spins very freely in the can bearings with no binding. If there is any binding you may have to realign the bearings either rotationally or perpendicular to the shaft. A little time here pays big dividends. Reassemble the can, arm and the magnets along with the endbell. Space out the arm in the can with just an oil slinger phenolic oil washer and a couple of .005 brass washers on the end bell side. Space out the rear of the arm and can bearing with brass washers to allow about .005 to .010 endplay. Make sure the magnets with keepers are centered along the whole armature stack. The arm, when centered in the can, should not be pulled fore or aft by the magnet alignment. If allowed by your rules, use a small amount of epoxy to hold the magnets in place so they can not move fore and aft. Anytime the arm is pulled one way or the other by the magnets it causes drag and a reduction of motor speed. You may have to shave off some plastic material from the back side of the endbell to allow the magnets to move far enough forward. But be careful to not remove the can to endbell retaining screw holes.

Now with the can assembled and the screws in place, we can start on the brushs. Get new Mura Bigfoot brushes. You can try the new softer ones but for the money difference I have found no big advantage to them. Also get two sets of new brush springs, one set of light and one set of extra light. First, take the brushes and with a # 11 Exacto knife blade and scrape lightly along the long side of each of the four sides of each brush to lightly bevel the edge. This prevents an interference fit between the edge of the brush and the corner of each brush hood side which could cause the brush to hang up. I also like to lightly scrape the front edge of each brush face to reduce the initial contact of the brush with the commutator for the break in period. Install the brushes and the extra light springs.

The little post that sticks up to hold the spring leg end in place should be vertical and not bent over. To keep the spring leg from sliding off I take a fine pair of needle nose pliers and just bend over (toward the motor) the top 1/32 of an inch to prevent the spring from sliding off. This makes it a lot easier to change springs later. And there is less chance of breaking off the small post by bending it back and forth to get the spring leg off.

Now you're ready for break in. Apply only about 6 volts to the motor and let it run in for about 5 minutes. Either hold it in your hand or rig a clamp lightly in a vice. Then apply 12 volts for about one more minute. Do this with the motor out of the chassis. Now remove the brushes one at a time and mark the top of each brush with a permanent marking fine point pen to make sure you put each back in place exactly as you took them out. Check the brush wear pattern on its face. If it is to one side or the other noticeably, you may have to realign the brush hood in the right direction. If it is centered you did a good job.

If you have an ammeter meter available, put it in series with one side of the power supply. The reading under no load should not fluctuate more than a very small amount at free run. Any noticible rapid flucuation of the current reading could indicate the armature may be out of round. Get it trued. The amp draw should not be over 1.90 amps at 12 volts. If under 1 amp you will need to use the light springs instead of the extra light to bring the performance up a bit more. Too high a draw indicates a drag somewhere or an arm problem. Remember the amp draw, 1.5 amps is a good start. Then install the motor in the chassis with gears and wheels. Check for drag or binds. The amp draw with the car ready to run should not be more than .5 amps higher than with no load. Any more than that and you need to track down a drag or bind problem. Get that gear mesh perfect. Oil the bearings lightly. Check the axle and bearing alignment.

These are the basics. They apply to almost any motor, whether you are just changing an arm, tuning a new motor or building a much hotter motor. As you progress from this type of motor, the basics remain the same. You will just spend more time and money achieving the same sort of performance. As with anything, the faster you go the more it costs in money or time and precision.

Larry Shephard

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