Track Radius on a Hidden Helix

Model Railroader Layouts and layout building
1

I am building a new layout and using 25" radius as my minimum due to space limitations. there are two helixes (one at each enad of a dogbone-type pattern). My question is do I compromise my standards by going to a tighter (22") radius on the helixes which are hidden? Longest piece of rolling stock is 65 ft with locos all RSD-4s.

Looking for opinions.

RicZ

2

Unless you are in N scale, that’s an idea that’s been proven bad multiple times by good modelers for medium-to-long trains.

If you are in HO, besides the steep grade of the helix itself (likely near 3%), the friction of the curve will add about 1.25% additional effective grade (for 25"R) and about 1.5% for the 22" radius curves.

This additional friction also increases the tendency to “string-line”, that is, for cars to derail across the center of the curve.

Have you built and operated a helix before?

Folks often assume that helixes abrogate the laws of physics, sadly such is not the case.

3

Therefore, keeping the 25"r is the way to go, and with my 2% grade, the effective grade is really about 3.25%. My “long trains” will be 10-15 cars normally. Hoping that the " stringing out" problem will not be a factor.

RicZ

4

HO?

Actually, expanding the 25’ to 26" or 28" or 30" will be the better way to go. Your 15-car train will wrap well around the circumference of each 25" R lap (with power and caboose), which puts it well into the danger zone for string-lining.

“Hoping” in the face of contrary experience doesn’t seem like the best strategy, but good luck.

I hope that you at least mock it up first before committing. The results may be quite instructive.

I’m not sure how you are computing the nominal 2% grade at 25" R. To achieve even the minimum HO clearance (3 inches, per NMRA S-7) plus the thickness of sub-roadbed and track would put you above 2%, even before allowing for hand room (which you need).

2% grade at 25" radius seems to only yield about 3 1/8" railhead-to-railhead clearance,. even before allowing for sub-roadbed (if my math is correct). That seems very unlikely to be satisfactory…