Flexicoil Truck Sideframe Conversion – CP SW1200RS part 2

This post was inspired by a discussion through comments on my previous post about my CP SW1200RS project.  The Point 1 kit instructions indicate that it was designed around either an Athearn SW7 drive or a Proto 2000 SW9/1200 drive.  I made a somewhat arbitrary decision to go with the latter.

The P2K drive is either closely or directly derived from the architecture of a Kato switcher drive.  The trucks are designed so that electricity is transferred from the wheels to stamped copper/bronze wipers that attach to the axle ends protruding beyond the wheel face on each side of the truck.  The sideframes are basically U-shaped clamps that snap into place on opposite sides of the trucks, thereby adding to the entire assembly’s rigidity.  They whole works holds itself together with just enough play to allow the wheel pairs to move on all three axes, thereby providing some articulation as the truck traverses curves and slightly uneven trackage. Here’s a photo of the stock truck with one sideframe removed.


The P2K SW9/1200 model comes with AAR Type A switcher trucks, which are correct for their SW9/1200 model.  The Point 1 kit provides resin castings of an EMD Flexicoil truck, which are correct for the SW1200RS.  In the photo below, the resin part (grey) is intended to be a substitution for the stock sideframe (black).


Mounting these parts on the P2K trucks is challenging because, as I outlined above, the design of the P2K truck incorporates the U-shaped sideframe to reign in the slop of the assembly just enough to keep it from falling apart without being so rigid as to impede the trucks ability to flex through curves and across minor undulations in the rails.  The stock sideframe has a pin in the centre that also helps to keep everything aligned.  You can see this pin and the hole that receives it in the first photo.

The solution for mounting the cast resin sideframes, as per the kit’s instructions, is to drill out the hole designed to receive that pin and tap it for a machine screw that will later hold the copper/bronze wiper in place.  I initially followed those instructions.  The photo below illustrates the the results.


This approach didn’t inspire much confidence.  If that screw is snugged up tight, it locks the copper/bronze wiper, which then holds the two axles in permanent alignment with no ability to flex.  As I’ve noted, the entire truck is designed to have some sloppiness, within limits.  To permit this sloppiness, the instructions direct that this screw should be almost snug, or backed off just enough to allow for some play.  I wasn’t confident in judging “just enough,” so I experimented with a variety of very vague and minute increments of torque and tried to get the amount of play in a modified truck to match that of a stock truck.  That was a very vague and imprecise process.

The kit instructs that, once the appropriate torque of that screw is attained, the sideframes are to be glued onto the copper/bronze wiper.  I suppose the idea was that this secures the screw from backing itself off.  I didn’t like that this approach because it buries the screw’s head under the sideframe, and proceeds under the assumption that the buried screw will never back itself off.   The wiper really should be able to float, so that ruled out gluing the screw head to the wiper.  I suppose one could try to secure the threads of the screw.  That’ presents a whole new set of challenges.

I’m sure people have put this kit together per the instructions and had no problems, but I just wasn’t comfortable with it.  The model is intended to operate on the WRMRC, and this layout is so large that it puts extraordinary demands on all of our equipment and systems.  We address through very high mechanical standards, but things that might never wear out on a typical home layout are taxed to the extreme on our club layout.  I doubted that the kit’s recommended approach would be durable and stable enough for our applications.  At this point, the project was stalled while I sought the assistance of everyone in my circle of friends and online associates.

The second weakness of the Point 1 sideframe, in my opinion, is the aesthetic of the casting itself.  The resin part doesn’t compare to the sharp detail of the Athearn injection moulded rendition of the same truck.  I decided that I would use the Athearn part.  The photo below compares the Athearn part (black) to the Point 1 part (grey).


Using the Athearn part presented new challenges.  The Athearn drive, for which this part is intended, has an entirely different electrical pickup method where stamped metal frames sit inboard of the wheels, as opposed to the P2K/Kato approach where much smaller metal wipers is are mounted outboard of the wheels.  The plastic sideframe on the Athearn truck does not contribute to the truck’s mechanical structure, so the back side of this part entirely different from the P2K sideframe, which is also different from the Point 1 part.  The first photo shows the back of an unmodified Athearn Flexicoil sideframe.  The second shows the back of an unmodified P2K switcher sideframe for comparison.



Two possible approaches for attaching the Athearn part to the P2K trucks include: 1) sculpt material from the back side of the Athearn sideframe until it exactly matches that of the part in the Point 1 kit, then substitute the reworked part into the kit’s assembly instructions; 2) rebuild the Athearn sideframe to function exactly like the P2K part.  I had initially pursued the former, but I eventually abandoned that method for the latter because of my lack of confidence in the durability and stability of that approach, which I already mentioned above.

The idea for the second approach came to me from a post on the diesel detailers proboards forum. I basically followed the instructions outlined in that discussion thread, which describe how to change sculpt the back of Athearn sideframe and graft on the clips donated by the P2K sideframes. The author of the thread indicated that he did the conversion for one locomotive in 30 minutes.  I guess I work much slower than him.  It took me 90 minutes to do two trucks.

While I had the trucks completely disassembled, also decided to substitute NWSL nickel silver wheels for the stock plated wheels.  On a home layout, one might get a lifetime of use out of plated locomotive wheels, but we wear the plating off of such wheels used on our club layout.  If you opt to do this, be careful with the P2K axle/gear.  I broke one of the axles in the process, and replacement parts from P2K elude me.  Here’s a photo of the dead axle.


I ended up ordering Kato parts, hoping that, because they share the same DNA, they’ll be the same as the P2K parts. They have yet to arrive, so I’ll post an update when I get them.  Hence, the work on one of the models is stalled, but I’ve done the conversion to one pair of trucks. Here’s a before/after shot of a converted truck next to one in stock form.


With the conversion completed on two trucks, I can reflect back and say that I’m not sure this was the best approach.  I suspect it might have been an easier task to simply use the Athearn drive, swap out their AAR-A switcher sideframes for their Flexicoil sideframes, then replace the open frame Athearn motor (assuming that’s what you get with these models) with a better quality motor.  Even if converting the Athearn drive proved to be an equal amount of work as the P2K drive, the Athearn truck would retain all of its stock structural and mechanical components.  Also, if something breaks, replacement parts are easy to get from Athearn.

I would only know if the Athearn SW7 drive would have been a better approach if I get my hands on one and test-fit it onto one of my Point 1 shells.  Unless the Kato gear that I ordered doesn’t work, or other unforeseen problems with the P2K drive emerge, I’m committed to using the the P2K drive.