Strap Motor Setup by Kimmo Rautama and SlotSide

I will discuss the subject of setting up a strap motor from my point of view, which is setting up Gp.7 and Gp.27 motors. Some details involve parts from certain manufacturers. The set-ups I will be discussing are RL (Rolf Lundberg), but the instructions can be applied to all makes.


There is always the possibility to decide whether you buy an RTR motor, ready assembled set-up or if you just want to buy all the parts and make it yourself. When you start to build a motor from the beginning, you can make sure every component is of the best quality. Many manufacturers also make excellent RTR's, but naturally they cannot be made with the same care as you probably want to give to your motors.

The main goals in building are cooling and durability for race motors and performance for qualifying motors. Depending on the power used in qualifying, the other aim is for a good magnetic field. If you want to get serious about qualifying, you need to make your car light. The motor has some weight that can be taken off. The first chapter describes how to build a good and light endbell for a qualifying motor. For race motors, omit the drilling of the endbell. If you already have a ready set-up, go directly to ASSEMBLY.


Lightness requires a lot of work on the components. The endbell has a lot of extra aluminum that is not needed, in qualifying you need not go for maximum cooling surface. Thus, corners may be cut off, a section may be cut off between the can attachment holes, and some additional holes can be drilled. Here, a warning is in place. Remember that aluminum endbell is anodized. The anodization is very thin, and even a scratch from your knife will penetrate the surface. This will inevitably lead to shorting, should the brush or brush hardware touch the scratch. This is why in RL set-ups there are thin plates between the endbell and the brush hoods (and brushes). This enables me to drill material even from behind the brushes. The thin plates are insulated, so there is no need to be that careful with the anodizing. After removing all the burrs, the brush hoods receive similar treatment. The brush hoods are the secret of success. I use RL brush hoods, because they are the best fitting hoods for Koford brushes. No play whatsoever, just enough for the brushes to move. The key word is ALIGNMENT. This has been emphasized in every article regarding motor building, and that is not by accident. If you do not have proper brush alignment tool, buy one. The best are Slick7 and Magnehone (diamond coated). They have the hole in the middle, which is of premium importance. When you assemble the endbell, put the bearing in with glue (or, better if already glued), put the brush hood tool in place, push the axle through the tool and the bearing, and start assembling the hardware. If you are using an old endbell where you first have removed the old bearing, make sure you have removed all the glue from the bearing seat. If there are clusters of glue left you will not get the bearing straight, even more so if the bearing is loose in the recess. Assemble the endbell with your favourite epoxy, I use quick-setting (5-minute) epoxy, which sometimes sets a bit too quickly... After this has been done, check the alignment of the brush hoods. The tool should move freely. If not, use Magnehone brush tool to straighten, or if you have aluminum hoods, you can gently twist them. The final judgment can be said only after running the motor.


The next step is the can. If you use a can which has been pressed (like most cans on the market), you need to spend some time with it. RL cans are lathed, so they are absolutely accurate. Pressed cans often come little out of form, so some bending is required. First, measure the O.D. of your endbell (from the surface that fits into the can). This is the I.D. of your can. Then, measure the can, whether it fits the previous dimensions. It probably is little wedged, so first bend it square so that the I.D. is constant and the sides are parallel. If the dimensions match, you're OK. Next, screw the can and endbell together. Make sure the holes match and no screw will bind. If this happens, check which hole either is misplaced or too small. Enlarge the hole in question. Now, measure the distance between the sides of the can. Both should be identical. If not, you need to twist the can in an awkward way. Grab the can with pliers over the bearing hole. Then press the screw end on table and push. The end of the can should turn some angle. When you get the same distance between the sides, the operation is done. Then, remove burrs from the holes. The can is ready.


Choosing magnets has become confusing. Many manufacturers offer a line of different magnet shapes and sizes. What I use is standard RL Tall magnets (U-402) and RL quad magnets that are not any higher. They work really well on any power and do not break that easily. For qualifying, many prefer a more moderate magnetic field, not going for maximum strength. This gives you a more driveable motor, and with high power you will then get better grip. With my car, however, I get lots of grip so I usually go for maximum power to get good kick in the exits. I get my magnets matched from Lundberg with Gauss readings, so I can select the kind of field I want. This is really important to get consistent results. The secret behing magnets is to get an even field. Then, you need to glue the magnets. Any commercially available magnet glue is OK, but I use RAM Speed (German glue) that sets at 320 Fahrenheit in a half hour. Glue the magnets with a jig. Then, when magnets are in place, solder/glue the can bearing in, with a jig. One of the secrets of good motors is the air gap. With a straight can, the gap should be good already, but it never hurts to use your favourite Magnehone magnet tool. I use .470 for my .459 motors. Now, the components are ready for assembly


Choosing an armature is always Russian roulette. There are far too many things that cannot be seen, even though you were an expert in armatures. The biggest problem with armatures is crooked shafts. This is something I would like the manufacturers to improve on. I do not know whether this is caused by bad quality shafts or does it occur when the stack is attached. However, with open arms this seems to be a bigger problem than with Gp.27 arms. Arm that has a crooked axle can never be balanced correctly. It is as simple as that. To my surprise, over two thirds of my arms were on the bad side, this included my qualifying it does not mean you cannot use them. Another thing is that crooked axles cause bearing failures. You should always race with reconditioned arms. Well, the things you can look into are the tightness of the winds, stack integrity, balancing holes and commutator. The balancing holes tell you quite a lot. If some holes are much larger than the others, this may indicate either a bad balancing job or crooked shaft. The comm should be neat, straight (not conical) and the slots should be clean. When you have many arms and want to find the best ones, for group arms there is a decent solution. You can measure the resistance of the coils on each pole. With open arms this is virtually impossible, as the resistance is so low. But with group arms it can be done, if you have a decent power supply or a very accurate ohm-meter. With power supply it goes as follows: Turn the voltage to very low, around one volt. Then, grab the arm and attach the power supply output cables to two comm lamellas in the arm. There will be a short circuit through one pole, and the wire will get hot in just a few seconds. Check the amperage at the power supply readout. It will change when the wire gets hot, so be quick here. Then, do the same for the other poles. A good arm should have high amperage and it should be consistent between the poles. This way of measuring is as accurate as you make it, so it is down to equipment and the skill of the executor. It is not the best way to do it but just about the only possibility for average racers.


If you use an old set-up, you need to clean it. If possible, use ultrasonic cleaner. You can never get the parts as clean with conventional soap-and-brush methods. If not, just clean the set-up as well as you can. Check the bearings. Put an arm in the set-up and try the play in the bearings (sideways). If you can feel it, change the bearing. If you try to save here, you have to pay the bill later. A motor with bad bearings does not run well.

Put in the brush tool. Insert brushes and springs and hone the brushes. Mark the brushes, so you know which is which and which way they sit. The arm needs to be positioned in the middle of the magnetic field. This is done by inserting the arm in the set-up with no spacers. See how many are needed, and try again. Add spacers so that you use as thick spacers as you can. Only the last or two last should be thin if no thick ones fit in. This way you can keep the number of washers as small as possible. The more washers there are, the more they wear. This causes the endplay to increase and that is bad for performance. There should be minimal endplay, so that you can barely feel it. You can often create some play by gently tapping the arm in the set-up, if you need to create some. Suitable endplay is around .001-.002 , the thin spacer is .003. You can also leave it sit tight, but only if you know there is exactly .000 play. In the breaking in the play increases a tad. Do not use too little play, this will destroy your bearings in no time. See that the comm extends the whole length of the brush hoods (look through the hoods) so the brushes have good contact.

Tighten the screws. Now, check the endplay again. If unsatisfactory, readjust. Insert the brushes. See that they do not bind. Insulate the short end of the spring. Put just any shunt wire in place and lock with the spring. See taht the short end of the spring does not touch the opposite side of the brush hood. Same for the other side. Oil the bearings lightly. You are ready for the primary break-in of the motor. Break in with low voltage. I put my motor in the power supply with 0 volts, and then gently turn up the power. This eliminates the risk of burning the comm. I let the motor un around two volts for a minute, and then turn it up to 3.5 volts. There I let it run for about 30 minutes. Then, I open the motor and clean the parts. I put the screws in with Loctite. I coat the brushes with heat transfer compound and put them in. I then insert the shunt wires. I use threefold Alpha shunts. For springs, RL are the choice. Here, you should check that the short ends of the brushes and the shunt in the brush slot are aligned so that the spring pushes the brush straight. If not, bend the short end of the spring a little. Add a phenolic to the can end to protect the bearing from acid flux. Solder in the pinion. Give the motor a few minutes' break-in on 3.5- 5 volts. The motor is ready for testing. My experience is that you should never race untested motors. About ten laps with race speed will tell you the naked truth of the motor. If the comm turns brown or starts smelling, it's a no-go. If still in good shape, you have a race motor.

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