I’m in the process of building a helix for my HOn3 layout and hoping to go four tiers on it. Question is–How much verticle clearance should I maintain? Right now, the only loco I have is the blackstone K-27, but there will be others possibly as big as the Sumpter Valley mallets—even something with the ‘bear claw’ spark arrester. The radius on the helix is 20" and my rough calcs indicate that a 2% will yield about 2" net clearance and 2.5% will be 2.5" clearance (using 1/2 inch plywood and no ties).
Can someone advise me on the heights of the bigger narrow gauge equipment? Thanks, Steve
I just happened across a durango press rotary snowplow for the narrow gauge. I never thought about it before but as is turns out, it is the same body that is used on standard gauge lines. That being said, I would recommend keeping standard HO clearances. I guess the same could be said for any other MOW eequipment, such as cranes and the like.
NMRA Standards, which pretty much allow for the larger equipment, call for a vertical clearance (from the rail head) of 2-11/32 inches. Remember when calculating clearances for a helix to include the thickness of the roadbed and the height of the track, and if possible, room for your hand to deal with derailments…
That seems like a challenge in a couple of dimensions. You’ll probably need closer to HO standard gauge clearances for the largest HOn3 equipment. The NMRA standards others have linked will provide the information, and note that they suggest 2 11/32" as the HOn3 clearance. That probably won’t accommodate the outlier equipment. Standard gauge (1960s and earlier) vertical clearance is 3".
Also, you’ll need finger room for maintenance and retrieval of derailed equipment (nobody’s perfect).
Remember also that tight curves like 20" radius may add to the effective grade of the helix. This is especially true of the stiffer narrow gauge engines such as the larger 2-8-2s. Others have found it to be a problem when the ruling grade occurs in the helix.
Finally, I wouldn’t recommend going without ties to save a few sixteenths of an inch. Other have tried and discovered that bits of dirt and gunk that would otherwise fall harnlessly between the ties can derail trains when rails are laid directly on plywood. Dave Adams had this problem when he laid rails without ties in a staging area of his On3 layout. Not to mention the challenges of alignment, spiking etc. Many find that pre-fab flex track is an easier-to-maintain choice in a helix, particularly one with as little finger room as you are contemplating.
The best path might be to re-do your design with a bit more space for a less demanding helix.</