I've been meaning to get onboard with this discussion for a while now, all sorts of thoughts swirling around in my head regarding modifying the trucks and drives on these units. I've been distracted though, so I'm very late to the game. Herewith, some delinquent ideas, since you're already essentially done
. While recognizing that you aren't trying to totally reinvent the wheel for this particular application, I also see that you
are actually reinventing a fair amount. So hopefully I'm not too out of line.
Firstly, just in a general way, I think you can draw some inspiration from the Atlas mp15dc Blomberg trucks, which are designed to fit under the cab floor. They share the same basic design premise as the Roco/Atlas trucks, meaning shaft to worm, worm to worm gear, idlers to axle gears. The big difference is in the size, number, and arrangement of gears within the box, and those gears affect both the form factor of the gearbox but also its height.
The Roco truck uses quite large gears, for the worm gear but especially for the idler gears. It also uses a single idler to each axle, versus two smaller idler gears per axle on the Atlas mp gearbox. The result is a long, boxy enclosure for the Roco design, versus a reduced profile for the newer Atlas design. Also, by reducing the diameter of the worm gear, the height of the drive shaft and worm can be lowered relative to the axle centerline. It all adds up to a lower, smaller gearbox.
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I presume you are trying to reuse the existing Roco gears, but you have also produced a new axle gear. It's not all that difficult to lay out the revised gear train in cad, and it's also fairly simple to generate 3d gear models from online sources that can be imported into cad for manipulation and printing. One interesting thing to note, is that the new Atlas gearbox has a compound gear for the worm gear, with a helical cut worm gear mated with a spur gear to mesh with the idlers. It would be a trick to machine such a custom gear, or to find a stock version available. But it's a relatively simple task to mate the two together in cad and 3d print the result. For point of reference, the axle and idler gears for the new Atlas gearbox measure at 11.89mm overall diameter, the pitch diameter would be a touch less than that. I don't have the worm gear diameter, as I would need to disassemble a truck to get that. It can be done if necessary.
If nothing else, the revised gearbox profile would accomplish the added clearance for the universal joint cups.
Next topic, the motor mount. I touched on this previously, but have done more thinking on the topic since. I really think the motor mount needs to be adjusted to lower the motor in the frame, and I don't think it would be all that difficult to achieve with a 3d printed bracket. The goal would be to drop the motor the same amount as the driveshaft, so something like 15mm.
A hole would need to be cut in the frame so the mounting bracket could drop down through the floor. A new bracket would need to be drawn and printed, with mounting flanges positioned just below the motor centerline. I got out the crayons again. The bracket would sit down in the hole in the frame, resting on the mounting flanges at either end.
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I've been tinkering with a Red Caboose frame and walkway, and it looks like the motor would remain hidden behind the fishbelly portion of the frame, even lowering it by 15mm. The top of the fuel tank would need modification to clear the motor, but the air reservoirs could be left as stock.
Lowering the motor simplifies the drive train, but more importantly it opens up the volume of the long hood for placement of the electronics and the use of a larger speaker.
The speaker is the last point. Many of us have discovered the Tang Band speaker modules, there are two, the 1931S and the 1815S, that would provide nice sound but are just barely too large to fit flat in most plastic diesel bodies. Both modules have the same dimensional and performance specs, the difference is that the 1931S features a convex driver cone while the 1815S features a concave cone.
I've had a lot of discussions about the Tang Band speakers with Bob Sobol from the forum associated with the Appalachian & Ohio O scale layout out in Colorado. One thing we discussed was the possibility of modifying the TB speaker enclosures to get them to fit in O scale locomotive shells. My purpose was for my sw1500 kitbash, but the A&O crew loves the Red Caboose gp9 models.
Bob cut apart a non-functioning 1931S module, which revealed that the wall thickness was something on the order of .075" to .080". I had previously worked on a module using a Dremel hand tool, the result of which was effective but very crude. I just recently collaborated with another modeler here to machine the speaker enclosure instead, and the results are very satisfactory. We removed .045" from both sides of the speaker, without breaching the enclosure wall, and also removed the mounting tabs from the ends. The result is a speaker that fits cleanly inside the Red Caboose shell, without modification to the shell, and without causing any bowing or other effect on the hood sides.
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The point of putting this speaker info here, is to support the idea of lowering the motor. Also it's food for thought of course. The same speaker mod allows for the fit of the TB modules in other Atlas locos like gp60's or gp35's. My opinion is that being able to maximize the sound quality, by allowing the use of a more capable speaker, makes the motor repositioning worthwhile.
I'll drop the speaker information into the Red Caboose specific thread as well. Happy to provide specs and methodology on the machine work if others are interested.