Thommo,

I love Tunnel Motors as well.  However, running  on a 17.2" radius is too small.  I think their minimum is 22".  Where ever possible, go for the largest radius curve you can get a away with, especially engines like theses.  A good alternative would be to model a GP15 (some callthis engine the mini tunnel motor) which would be able to negotiate the smaller radius curves.  

The tunnel motors will look and operate much better of 24" and up radius curves.  30"  minimum would be ideal for these engines.

Good luck. . .

To make the tunnel motors run and track better, here's what I recommend:

1. Remove the middle wheelset on each truck and grind about 1/32" of an inch off each end, then replace it back into the truck. This will give you an extra 1/16" of play side-to-side for the middle wheelset and will help the loco track better around curves.

2. I recommend a decoder with Back EMF for the tunnel motors. The Athearn mechanism is stiff and less smooth than other locos like Atlas or Kato. The BEMF helps smooth out the loco at slow speeds and allows it to literally crawl through yard turnouts. You can watch the video included with my DCC article in MRH Issue 2 to see a demo of an Athearn tunnel motor with a TCS T1 decoder with BEMF.

Hi Joe,

Are you saying that your modification will allow the Athearn unit to

negotiate a 17.2 radius curve, or improve the overall performance

of the Athearn unit through all types of cuves?

Thanks, -Larry

I wouldn't claim miracles, just an improvement. An Athearn Tunnel motor is about 71 scale feet long over the couplers, which is 9.78 inches in HO. The curve radius guidelines would tell you that 2X is about the best you'll get on piece of equipment that long - and 2X of 9.78 inches is 19.56 inches. 18 inches is pushing it - and 17.5 inches certainly even worse.

To be safe, I wouldn't run these locos on anything under 20" and expect them to perform reliably - they're just to dang long. Using the curve guidelines in issue 1, we also get (rounding to nearest inch):

More reliable tracking - 2.5X = 24"

Extremely reliable tracking - 3X = 29"

Looks realistic on inside of curve - 3.5X = 34"

Looks realistic on outside of curve - 4X = 39"

Ultra-reliable hands off coupling - 5X = 49"

So even the 36" minimum radius I'm using on my HO Siskiyou Line is pushing it a bit in the looks department with these locos. And couplers may need aligned now and then (although we don't use these locos for switching much).

I wanted to judge my layout for six-axle compatibility so I borrowed one of Joe's SD45T-2's (same frame as the SD40T-2) to test with.

It definitely didn't like my only Walthers curved turnouts - derailed on the inner track side every time and there were a few places where I had a small vertical change combined with a sharp curve that resulted in the lead axle lifting off the track.. and then coming down derailed. Similar to what you experienced.

I got to rebuild a bunch of trackwork that pretty much needed it, although not all of it (and that blasted turnout) is six-axle ready still.

However, I do have an 18' radius level reverse loop on the layout and it had no difficulties with it.