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Reply to "Sunset SD40-2's"

A tank drive means that there are drive shafts emerging from the central fuel tank, usually with universal joints, conveying power to the trucks fore and aft.

It was probably the most common arrangement for 2-rail O scale diesels made during the 20th Century, including Weaver, P&D Hobbies, and Sunset/3rd Rail.  Some object to this setup.  Because depending on the length of the fuel tank and style of truck, the protruding drive shaft can be visible when viewed at or near eye level, which ruins the illlusion for some.

Most newer low-priced road diesels from Atlas and MTH use a small can motor mounted vertically on each truck, that swivels with the truck.  This is sometimes known as a "China drive" because these locos are made in China.  As mentioned the gear ratios are usually too tall for best performance, and the motors are much smaller, developing less torque.  Also since the worm gears in each truck are self-locking (i.e., wheels cannot turn the motor), the motors can sometimes buck each other at very slow speeds, which is not a desirable characteristic.

@GG1 4877 The Atlas AEM-7 and SW's don't have a tank drive.  They have what I like to call a cassette drive.  This is what most locos in S and HO scale use, maybe N scale too.  A central motor in the body has two shafts with a universal joint on each end.  This drives a worm at the top of a clam shell "gear cassette" which is mounted on, or part of the truck frame.  So in these cases you don't see a protruding shaft.  The truck sides are ornamental because the axle bearings are inboard.  This design lends itself to mass production,  but it's difficult to retrofit a cassette drive to existing models.  First, the clam shell gear cassette has to be specific to the truck wheelbase.  Also, the frame has to have a gantry or pivot point for the gear cassette.  If the pivot point is too high, the trucks will tend to "wheelie" under load.  This was a problem with the 1970s Atlas F9s (and Red Caboose GPs that used the same drive.)  It can be mitigated through the fabrication and installation of "Armstrong brackets."  If the pivot point is low enough, the model can usually have a detailed cab interior.

I love the Midwestern Model Works drive, it might be the best of all.  But are they actually being produced?  What's the wait time, and how much do they cost!?

Last edited by Ted S

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