From: js@hplvec.LVLD.HP.COM (Jon Sawyer)
Newsgroups: rec.models.railroad
Subject: Re: Helix Calculation
Date: 12 Oct 92 17:50:52 GMT
Organization: Hewlett-Packard Co., Loveland, CO

In rec.models.railroad, cwatters@magnus.acs.ohio-state.edu (Coyt D Watters) writes:

    Does anyone have a method for designing a helix?

I have recently completed building a helix for an HO layout.  Bear in
mind, though, that I haven't operated a train on it yet, so take the
following advice with a grain of salt.  :)

I had numerous constraints to work with:  the shape of the space I'm
using for the helix allows for no more than a 20" radius curve, and I
didn't want to exceed 2.5% grade (some of you may remember my posts of
several months ago seeking opinions about radius/grade tradeoffs).
I'll leave the math as an exercise for the reader, but a 20" radius
helix implies a 125" circumference of one cycle.  A 3" rise per cycle
keeps me under my prescribed 2.5% grade, but that doesn't allow much
headroom for trains!

I obviously had to use some technique that would keep the roadbed as
thin as possible.  I chose to use 1/8" hardboard (Masonite) for the
roadbed.  This gives me about 2 3/4" headroom for trains after you
subtract the rail height.  Not enough for all possible cars, but
enough for the rolling stock I plan on using.

Rather than cutting out complete circles and wasting most of several
sheets of hardboard, I carefully laid out 1/6 circle segments 4" wide
(inside diameter 18", outside diameter 22") so that I could get 42
segments from a single sheet of hardboard.  That's 7 complete cycles
for a total vertical rise of 21".  To get that yield, orient the 4x8'
sheet horizontally; lay out the 1/6 circle segments in 4 nested stacks
of 10, then get two more oriented vertically from the remainder at the
end of the sheet.  I won't even *try* to draw that in ascii.  :)  A
bandsaw is invaluable for this exercise.

In addition to the 42 segments, you also wind up with a zillion little
pieces of hardboard, more or less triangular in shape.  Cut these into
strips about 1" by 3", and use these to splice the circular segments
together into a continuous helix, as follows.

    1.  Butt two segments together, top side up.

    2.  Glue and clamp two strips across the seam, oriented so that
	their long axes are tangent to the inside and outside
	diameters of the segments, leaving enough room in the middle
	for track, kind of like this:


			_____-----------------_____
				  |_____|
				     |
				   __|__
				  |     |
			_____-----------------_____


    3.  Repeat 40 more times.  :)

Note that the strips are glued to the *top* of the roadbed, so as not
to further reduce the clearance for trains.  Gluing them underneath
would interfere with car midsections as they overhang on the inside of
a curve, and car ends as they overhang on the outside of the curve.
Gluing them on top of the roadbed doesn't interfere with the cars, as
the strips don't protrude up much farther than the rail itself (1/8").

As 1/8" hardboard isn't very rigid, I had to support it at relatively
short intervals.  Conveniently, the saw blade on my radial arm saw
leaves a 1/8" kerf.  I cut slots into the wide face of 24 1 by 2's at
3" intervals, angling them appropriately for a 2.5% grade.  I then
"sandwiched" the helix between 12 pairs of 1 by 2's spaced at
equidistant intervals around the helix.  Another tacky ascii sketch
follows; this is a cross-section showing the roadbed ("====")
supported by the 1 x 2's ("| |"):


			 |  |      |  |
			 |  |      |  |
			 | ========== |
			 |  |      |  |
			 |  |      |  |
			 |  |      |  |
			 | ========== |
			 |  |      |  |
			 |  |      |  |


Finally, you must secure the 1 by 2's to the roadbed somehow to
prevent a house-of-cards style collapse of your helix.  You can't
secure the 1 by 2's to their opposite member, as nearly any such
technique would cut down on the train's headroom.  I chose to drill
two holes in the roadbed on either side of each 1 by 2 and bend a
retaining clip out of stiff steel wire, shaped roughly as follows
(perspective view):

			   ________
			   \      |
			    \     |
			     \    |
			      \________
				  |   |
				      |
				      |
				      |
				      |


Put the clip around the 1 by 2 and stick the ends into the holes in
the roadbed, then bend the ends back to hold the clip in place.

This brief description leaves out many of the details, such as making
sure that the slots in the 1 by 2's are slanted one way for the 12
that fit on the inside of the helix and the other way for the 12
outside members, and stepping the slots on each 1 by 2 so account for
the vertical gain between each pair (3" rise per cycle divided by 12
equi-spaced supports = 1/4").  Make sure you think everything through
before you start cutting, especially any customizations you make.

Operation on the helix should be reliable, as others have mentioned.
The tight clearance is not a problem because you can reach through the
helix one level above or below the train to reach the other side of
the train.  This becomes more difficult if your train is longer than
the circumference of the helix; in my case, 10 feet.  This won't be a
problem on my smallish layout.  It would probably be difficult to pull
a train that long up a 2.5% grade around a 20" radius anyway.
Newsgroups: rec.models.railroad