At last. Something I know about.
An easement spiral should be, at minimum, as long as the longest car you plan
to
run.
The radius of the curve together with the length of the spiral determines the
offset required.
The offset equals the length of the spiral squared divided by 24 times the
radius.
To go the other way, a curve of 30 inch radius with a 3/4 inch offset requires
a
spiral length of 23.25 inches.
Its very easy. 24 times the radius is 720. For the top of the right hand
fraction we need 3/4 of that which is 540. Then we need the cube root of that
which is 23.2379.
Since a 72 ft car is 13 1/2 inches in S gauge, the spiral is longer than we
need. A 13 1/2 inch spiral with a 30 inch radius curve would require on off set
of 1/4 inch.
But 30 inches corresponds to a 35 degree curve which is sharper than NMRA
standards for a 70 ft car with diaphragms.
The recommended curve is 30 degrees or 36 inch radius.
Therefore your dimensions would be 36 inch radius, 13 1/2 inch spiral with a
7/32 off set. If you don't have diaphragms, the 30 inch radius should do. Or
you
might get away with increasing the length of the spiral.
If you want 80 ft cars the recommended minimum radius is 25 degrees or 43 1/2
inches, for 85 ft cars it is 20 degrees and 54 inches.
But, in any case, I recommend setting up a trail and running coupled cars of
various lengths through it.
Curves can be laid out with bent sticks or lengths of rail, but a
mathematically
generated spiral is the most compact and the most esthetically pleasing.
A layout with spirals looks and operates better than one with a much larger
radius but no spirals.
And I very much recommend NMRA standards if you want a layout runs well.
The math is not that difficult. If you are interested I can explain it here or
you can message me off site. Or I can do the math for you.
Ab Baird(Canadiannorthernhistorian)
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