rplst8 posted:I don't know if it helps you 2R guys, but put in a reservation for 2 W&LE 3R SD40-2 in 3R. I found this thread very helpful in learning about the other SD40-2 models out there. I'm glad I didn't buy any of them. One question though, what exactly is a tank drive?
Thanks for your reservations. Every bit helps.
2-rail vs 3-rail irrelevent. It used to be the majority were 3-rail, they are now a decreasing minority.
It's the total reservations that matter.
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!?
Thanks for the expanded clarification Ted. I had not heard the term cassette drive before. Coming out of HO and N for so many years I'm quite familiar with the gear towers coming up from the truck to a drive shaft connected inside the body. There were kits designed for Athearn and others that added gears to improve the gear ratios for very slow speed performance pre command. I think it was Rail Power Products, but that goes back to the 80's and my teens which I kind of remember?
The vertical can motors in HO and N do predate the common term of "China drive" as Rivarossi, AHM, and early Atlas used a vertical motor powering a single truck going back to the late 60's. Most were produced in Italy or Austria at the time. They were know for their speed more than anything else 
but I still have several as all the Rivarossi GG1s used this drive.
W&LE recently started renumbering their SD40-2's in the 7000 series, I would want mine with the 4K gallon fuel tank. The ex-MILW RD SD40-2's have 3,200 gallon tanks.
On the tank drive - it may well be the most common, but it probably doesn't work well on things like F7s, GP7s, or RS-3s. The problem is geometry - the angle of the u-joints becomes extreme with smaller wheelbase models. An F7 would simply lock up on a 36" radius curve. U joints have a critical angle, beyond which they just don't work at all well.
Thanks for all the info on the tank drive! I was aware of that style (central motor with universal joints) but never heard that term.
It would seem to me, that with the way it's implemented in the 3rd Rail models, the drive shaft would need to be able to lengthen when going around turns since the pivot point is further forward. Do they use a slip yoke of some sort?
The Athearn HO models I had in my youth used a similar design, but the driveshafts went to a worm gear with a universal joint above the pivot point.
Bob knows a lot more about O scale locos than I do. But I have a 2nd generation Weaver GP38 that has U-joints. Strictly speaking it's not a tank drive, because the high-low transfer tower is mounted on one of the trucks. But it does have an underfloor drive shaft that runs the length of the body through the fuel tank, with two intermediate U-joints. It's a 3-rail unit. I've operated it through un-eased "O42" (21" radius) circular curves with no issues. The driveline angles get pretty extreme, but it works without hesitation or binding.
The U-joints used by Weaver / P&D have a telescoping yoke. I like this design better than the slotted hollow tubes originally used by All-Nation, etc. Without some kind of telescoping shaft and a provision to retain the horned ball / swivel, I can definitely see the ball sliding out of the cup on a sharp curve. [Now let the debate begin about whether the slots on opposite ends of the tube should be in the same plane, or offset 90 degrees! ]
Sunset 3rd Rail has recently offered F3s and GP7s which are a central tank drive with U-joints. The 3-rail versions are rated to negotiate un-eased O54 (27" radius) curves. With a telescoping shaft or sliding yoke the geometry can be made to work. But I would agree that a truck-mounted system is probably a better choice for very sharp curves.
The 3rd Rail drive system does operate on 054 minimum curves in 3 rail O scale and is more of a function of the swing of the coupler with a fixed pilot versus the tank drive. There are a lot of universals that allow for this accommodation. The 2 rail versions are 54" radius recommended but the F units have negotiated 48" radius without issues. The FL9, FT, FP7, F7 models are all able to take the 054 curves barely. The longer models don't do quite as well.
There is no doubt that these are never going to be O42 or O36 models. That is where the vertical truck motors make the most sense.
The Lionel/American Flyer U33C's (also labeled as U36C's) use "tank drive." They can negotiate traditional Flyer 20" radius.
Rusty
My GGD F7s, FP7s with single motor drive will go around 48 inch radius with no problem. Even my E7s will do this. I just got a pair of Alco PAs and it looks like they will too. I set them on the curve and there was still play in the trucks. I don't have them programmed to run yet.
You guys have a different definition of tank drive. The reason it is called "tank drive" is because the drive shafts come out of the fuel tank area, straight in to the closest gearbox.
The smaller wheelbase locomotives use a higher drive shaft connecting to a tower. U joint angles are much, much lower. Same with the shaft that goes all the way from one truck to the other - much, much lower U joint angles.
With a good size motor, a tank drive on an F7 just won't work except on straight track.
Maybe the pics in this thread will clear things up
The original Weaver RS3s had a sprocket tower in the tank with shafts out to both trucks. The motor was in the hood. It would go around 24 inch radius although the pilot over hang was so much it could not pull anything. It could work with 40 ft cars on 36 inch radius.
While the motor was on in the tank, the drive train was.
And by the way E7s are not particularly small locos.
Yes- it will work on the E7. I doubt seriously that it would work on the SD7. The central drop-down gives you slightly more room, since the drive shafts can be longer. The original CLW tank drives were only on the very long SD40 series, and even then they were not great. The longer the wheelbase, the more the truck swivels.
Any of these systems require high quality U joints. Don’t get me started on early Overland.
bob2 posted:Yes- it will work on the E7. I doubt seriously that it would work on the SD7. The central drop-down gives you slightly more room, since the drive shafts can be longer. The original CLW tank drives were only on the very long SD40 series, and even then they were not great. The longer the wheelbase, the more the truck swivels.
Any of these systems require high quality U joints. Don’t get me started on early Overland.
Works fine on Sunsets SD7s. It is more visible in the single tank vs dual fuels tanks but was designed and tested for it's minimum rated radius.
I assume that is with a center drop-down, not a motor in the tanks. That helps the angles.
I just ordered a couple 2-rail Chessie SD40-2 from Sunset. These are my very first true 2-rail engines. I hope they get enough orders. I've got time though, I'm planning to tear down the Black Diamond Railway and rebuild a new 2-rail layout with one 3-rail upper level. I'm working on the layout design now.
Rich, Are you going to make any videos of the construction like you did for the black diamond?
Mike DeBerg posted:Rich, Are you going to make any videos of the construction like you did for the black diamond?
Mike,
Yes! That's part of the fun. I'm excited because it has been a long time since I got to work on and build a layout. I will definitely video as I go and try to document the whole thing from the beginning unlike my last video series where I got the idea to make the videos after the platform was done. I'll probably even have a time lapse of the whole thing from tear down to rebuild. This will be years in the making though.
Rich
Daniel Raible posted:Out of curiosity, do you know if the Sunset offering would be an accurate representation of the W&LE prototype?
I see those units periodically in Medina. If I were to place a reservation it would likely be for one of those.
Since all 51 W&LE SD40-2 units are hand me downs from various Class I Railroads, it's not really feasible to make an "accurate" version as there would be 51 variances.
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