This is more of a thought or discussion than a question. Almost all non-brass articulated steam locomotives have the nonprototypical two swivel driver sets, instead of the accurate front only. The only exception I can think of in plastic is the Mantua 2-6-6-2. The reasons I can think of are the curve radius , ease of construction and cost. But I really like the full boiler swing and more of the steam lines and other details that can be in place. Any thoughts from you guys?
Curve radius is right. Very few model railroads run realistically broad curves. As a result, the front frame would swing to a cartoonish degree, and double track would have to have very wide spacing on curves. In addition, the front chassis drive gears would require a big hole in the bottom of the boiler. Big holes in boxy diesel cowls don't show. Boilers aren't boxy.
All good points . The mantua articulated seems to work very good , but it is very short. I remember back in my N scale days that A brass DM&IR 2-8-8-4 yellow went around a 15 in radius curve in a review. The hole in the boiler bottom would be the same for brass. Just for fun, do any brass owners with prototypical articulation loco want to do some testing on snap track.
I have both plastic and brass articulated locos. I was told that the reason plastic locos were doubly articulated was so they could get around tighter radii. Obviously if a mass manufacturer can increase the market for a product then they will do in order to get a return on investment. Given 90% of the market don't have the luxury of prototypical curves in basement empires. Also when you first start out, you may not be aware of the geometry etc but like the look of big articulated steam. What I would have liked to see is manufacturers of plastic locos, having a way of locking the rear set so they didn't swing and would look more realistic for those with the ability to run them on more prototypical radii. I keep planning on looking if there is a way to retrofit something to keep the rear set in line. Has anyone else looked in to this? or do most people just accept it?
Won't work. Mallets and other articulates had a hinge in the middle of the frame that allowed the front wheels to swing out from under the center of the boiler while the boiler stayed in line with the rear drivers. On most models, the front drivers stay centered. They only pivot, and drag the boiler out of alignment with the rear drivers on curves.
The drivers on real steam locomotives, including the articulated kinds, have 'lateral motion devices' on them to allow the forces inherent in rail travel and long heavy frames to work together. It would be far too complicated and expensive to build something like that even into the brass locomotives we pay for in scale. So, the manufacturers design the axles with side-play, sometimes with flangeless or 'blind' drivers, and with extra-long crank pinks that allow the rods to work at exaggerated angles from each other with even more side-play. As has been surmised already, this is strictly so that people will buy these very large gee-whiz models because: they're more affordable; they actually run on their layouts. The illusion has to be strong in the scale hobbies or we lose interest. I opted for the BLI Brass Hybrid UP 9000 because it has the true-to-type fixed frame, unlike the MTH version. BLI made the model with craft and care, and you can't tell that the driver sets have to swing as much as they do, and of course there's the blind middle axle or two...I forget now. This is the way they had to make their version of the Duplex 4-4-4-4. It also has the correct fixed frame and fixed axles, but if one expects it to work on 22" curves, something's gotta give! In BLI's case, the two central axles are blind. BTW, that beast is one of my three strongest pullers, probably because it's both heavy and long, but also because it bites the rails properly with only two axles.
I think the Mantua model is successful because of its drivetrain. The problem with using gear towers and a drivetrain through the boiler means the locomotive is articulated like a two truck diesel locomotive. If you could eliminate that and instead run a drive under the boiler with a universal joint at the pivot between the two engines, you would allow the boiler to stay rigid to the rear engine. My only foray into articulated steam was the chassis I built for ATSF 3322. It was more experimental than anything, but that locomotive has the benefit of an articulated boiler as well. Both engines could be rigid to their respective sections while still giving an accurate motion of the locomotive around curves. In certain high-end markets, an accurate Mallet motion may be possible. It would be more feasible for shorter locomotives, maybe USRA types? A UP Big Boy is probably still too long to have a rigid rear engine. A smaller 2-6-6-2 or 2-6-6-0 may work.
