How To: Building a Polymer Modified | Viper Scale Racing

How To: Building A Polymer Modified

CMPM, PolyMod, Compression Molded, all terms to describe the same beast. Known officially as the Compression Molded Polymer Modified, these cars are near the top of the HO Slot Car food chain. Second to only Neo Modified and Unlimited, CMPM cars feature Pro 10 Motor and Traction Magnets, full ball bearings front and rear, and Armatures well into the 1 ohm range. They are animals to drive, usually slammed on the rail with big power to drag them to the finish line. But don’t be mistaken, of all of the Modified classes, CMPM may be the toughest to wheel around any layout.

Viper V1 Polymer Modified
The first step in our Polymer Modified build is to drill the holes for the axle-retainer pins. For the chassis I went with our new “Firm” V1 Chassis (Part # 10013) . This chassis is between a Medium and a Stiff, perfect for the high down force inflicted by the Pro 10 magnet package we will be running. To drill the holes I used our Viper Pin Vise (Part #14102) with the ultra sharp drill bits included. When drilling you want to maintain firm but light pressure and “just keep spinning” until you make it all the way through. If you go too hard too fast you run the risk of snapping the bit, or even worse, drilling a crooked hole.
Next up let’s lay out our parts in front of us. For the front axle set up I went with an Aluminum Independent axle set with .335″ front tires (Part #12654). I used our new Gold Plated parts for the electrical parts, and seen on the left and right, our Pro 10 modified magnets. Notice on the left are our new HDF (High Down Force) magnets for modified racing. These Magnets feature a full back, and flat top, for a stronger Gauss reading at the business end. (HDF Part #12008) Also pictured here at this step is our Armature loaded up in the balancer, ready to balance. For the Armature I chose our 1.5 ohm Wide Gap Modified Armature. I epoxied the comm and windings, trued the comm surface flat, ground the lamination’s smooth, and now balance the heavy spots out of the armature.
Presto! Our now “Custom” Armature is ready to start fitting the bearings to the front and rear. When using Ball Bearings with your HO Slot car it is imperative to polish the shaft on the armature where the bearing will be spinning on. This will reduce heat and binding when cornering and the chassis and magnet package experience “Flexing” . So to do this I chuck the arm up in a rotary tool and lightly polish the front and rear only where the bearing will ride. ***Notice: the rear portion of the armature shaft is only polished towards the windings. You want to leave the shaft alone where the pinion gear will eventually be pressed on to.
The next step in this endeavor is not necessary but if you have the right tools can help to give your car the performance edge, and extend the life of that armature I just worked so hard on. With a diamond hone we carefully spin and pre-break in the motor brushes. This will reduce any Arc caused by gaps in the contacts, and also give you a head start on matching these brushes to the comm on the armature.
Once the endbell is prepped the Power Plant can be assembled. When assembling a Ball Bearing Power Plant there are a few extra things you need to do over a standard Bushing set up. Ball Bearings are narrower than the stock Bushings so this means you have to add spacer where needed. From Comm end to tail shaft bearing the calipers should read around .770″ . I came in at .767″ and called it (Lacking a .003″ spacer). So here is what I did: With 1 x .010″ (Thicker) White shim on the front of the armature, I added 1 x .005″ Steel spacer, and then 1 x .020″ Steel spacer, and loaded the arm into the endbell. Once loaded, I snap the Magnet Package on to the Armature. Then the last spacer I had to add was 1 x .020″ Steel Spacer onto the Tail Shaft of the Armature. ** Pro Tip: To keep your bearing, and spacer on the tail shaft during installation, grab a white plastic spacer off of any old armature to act as a retainer and keep them from falling off. This will be useful in the next step as well.
In this step we see our Polymer Modified starting to take shape. Note the white plastic spacer on the tail shaft. I am going to leave this on the car and what it is going to do is act as a shield when I put Locking Compound on to hold the pinion on. This will keep any of the bad locking compound away from the bearing, and keep the bad oil from contaminating the locking compound.
Press the pinon on to the tail shaft. I went with an HCS 7T Pinion (Part # 12476) and used our Viper Pinion Install Tool( Part # 13890) to complete the job with ease.
For the rear axle assembly I went with the 24T Orange crown gear from RPM’s. This gear needed spacing and I ended up happy with a .010″ Steel Spacer and a .014″ Brass Spacer to give me a mesh you can only dream of.
Before I press the rims on I wanted to double check my wet dream of a gear mesh so I went over to the run stand. On a Polymer Modified I would want around .25 amp draw at 5 volts, and what do you know, that’s what I got.
Now we are ready to press the rear rims on. Be aware of any width restrictions in your current racing league. I went with the ever common 1.312″ legal width and came in a little under just to be safe. The front was around 1.309″ I believe as well.
Boom Baby, let’s go Racing!!!