Observations by Fred Wright

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I've studied the LDISG Rule curve radius rule of thumb and the NMRA's RP on curve radius. Both are valid to a point. The rule of thumb tends to be more useful, because it deals with mechanics of cars on curves instead of just appearance. Appearance alone is just too subjective when you are trying to make the most use of your space!

Remember, the 3x rule of thumb is used for reliable tracking with body-mounted couplers on car lengths up to 1/3 the radius. The 3X rule seems to work for reasonably weighted cars in long trains on helii. In another words, if you stick with the 3X minimum radius, curve radius will not be the cause of your derailments.

Lionel and other 3 rail O often operate on curves that are only 1.3X the length of the longest car, and pretty reliably as well. What is different?

  1. truck mounted couplers
  2. cars are coupled at a considerable distance apart
  3. loose tolerance on couplers so they won't bind if not meeting straight on
  4. little-to-no underbody detail to hinder rotation of trucks
  5. bodies mounted higher than prototypical to give additional clearance
  6. trucks spaced closer than prototypical to ends of cars, especially on longer cars.

A long train is 10 cars, and string-lining is not unusual on the sharpest curves with long trains if any of the cars are particularly light.

I have kept note of what has worked and not worked on my own and my father's layouts, and track the reports of others on the various forums with respect to radius because it interests the engineer in me. I have concluded that most of the time reliable operation at 2.5X without any modifications is achievable as long as length of cars and locomotives is reasonably uniform. I would not want to push this hard in a difficult-to-access helix though.

To push curve radius to 2X the longest car length does generally require truck mounted couplers on the longer rolling stock. If not using truck mounted couplers, extensive testing is in order to verify that a given piece will work on your track reliably, and what modifications are needed to obtain that reliable operation. Note that the presence of truck-mounted couplers implies cutting away of underframe structure such as sill details or locomotive pilots in the area where the coupler or trucks must swing. [However, backing up a long train with truck mounted couplers is not very reliable because the couplers tend to push laterally against the trucks, causing derailments. - Joe Fugate]

I have never tried to modify my rolling stock to consistently achieve less than 2X, so I can't offer any advice except to look at the 3 rail O models (I buy those RTR).

Narrow gauge cannot generally achieve as proportionately as small a radius as standard gauge. The reason for this is the low-slung nature of the prototype where wheels are often between underframe structural members. The 3X rule seems to be about the best one can expect.

Fred Wright

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