Unfortunately the NMRA recommendations for curve spacing are
nebulous and outdated. With a little experimentation and
measurements with the equipment you intend to run, optimum
spacing can be determined. When on the outside track, the
inside overhang of long cars is critical as determined by the
truck pivot point distance. With steam locos, the outside
overhang at the pilot and cab ends are critical as determined
by the driver wheelbase. Other cases require different
determination. Curves and couplers interact also.
These are graphically illustrated on my site.
For more details with methods and extensive discussion of
problems and solutions, see first site below in curves.
Hope this helps.
MODELRAILROAD TECHNICAL INFORMATION
PROTOTYPE TECHNICAL INFO FOR MODELRAILROADERS
(Revised. New address)
MR TECHNICAL HELP GROUP
COUPLER HELP GROUP