Hello again group. another question from soneone who is just starting out.
I want to understand spacing as it relates to curve easements. Hypathetical example:
HO scale with several sections of flextrac connected together to form a curve, easements and trackage to and from curve. Curve is 180 degree, say at the end of a peninsula. Track to and from curve is parallel, 48 inches apart. Would this result in a 24 inch radius curve, including easements, or would the easements to and from the 48 inch straight spacing result in a smaller radius curve ? Also, for HO scale, what is the formula or "rule of thumb" for length of easements ?
Thank you to those who can assist me with this.
Dave J.
I'm going to go metric on you, since it's easier to write in whole millimeters than in odd fractions of an inch.
If you want to connect two tracks 1220mm (48 inches) apart with a 180 degree turnback curve, the trick is to make the curve 600mm radius. Then the tangent lines at the theoretical point of curvature are 10mm outside the curve radius. Mark a point half way between the radius line and the tangent line. This will be the center of your easement. The ends of that 10mm connecting line are the theoretical point of tangency and the theoretical point of curvature. You will use both.
Now, measure 250mm (25 times the offset) back along the tangent line and mark that spot. This is your actual point of tangency. Next, measure a 250mm chord along the curve from the theoretical point of curvature, and mark the spot. You have just found your actual point of curvature. Using a flexible trammel (or a piece of easily bent flex track) you can connect the three points and establish the centerline of the spiral easement. If you lay it out on a piece of cardstock or thin plywood, you can simply drop that template into place and lay out your easements - as long as the curve radius and offset are the same.
My own work varies slightly. I actually draw my curve 624mm (24 inches plus 14mm) and offset outward from there. Then I lay the outer ends of my flex track ties on the drawn line - lots easier than trying to work with a centerline, especially since I anchor my flex with grey latex caulk.
Where I have concentric curves, I go wider (I consider 610mm to be my absolute minimum Class 1 curve radius) to 670mm radius, which works for my comparatively short rolling stock.. Then I use a 10mm offset on the outer track and a 20mm offset on the inner track. The 20mm offset results in a spiral a full meter long!
Hope this has been helpful.
Chuck (modeling Central Japan in September, 1964)
Easements use up some length and a little width (but are usually worth it). In the example you gave, two parallel tracks 48" apart, the resulting curve is 23 3/4" radius with 12" long easements when figured in my CAD program. Dending on how you figure your easments (computing vs. bent-stick vs. CAD), it could vary a little.
Easements at least as long as your longest piece of equipment are helpful, 150% the length of your longest equipment seems to work even better.
If this is your first design, I would strongly recommend that you hold off a bit until after you have had a chance to read John Armstrong's book Track Planning for Realistic Operation (Kalmbach). You will be a more-informed designer and will be happier with the end result. The book explains easements in some detail.
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Hello Chuck,
Thank you so much for an excellent reply to my enquiry ! I am used to working in metric, but posted in imperial, as that's mostly what I see here. I need to draw your examples to fully grap what you are saying ( just how my brain works), but I know already your information will be most helpful.
Thanks again,