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i am thinking of adding a helix to my ho layout to free up space on the layout that my yard would have taken up and move it under my layout.

how hard are these things to buld and any good threads or web pages to give me pointers on it

my layout is HO
A helix is not terribly hard to construct but it takes up a lot of space. I have a large double track helix on my layout. The key thing to think about is you maximum grade and your minimum clearance. You have to include the thickness of the sub-roadbed and roadbed in your calculations. For example:

If you are using 1/2" plywood plus 1/2" of homsote for roadbed plus 3" minimum clearance you have a requirement to climb 4" before you crossover on top of the track again. Assuming a 2.25% maximum grade we can then calculate the minimum radius of the helix. First, with a 2.25% grade we can climb 4" in approximately 178" (4" / .0225). Note: This does not allow for any transition to the new grade. Plugging 178" into our circle formula gives us a minimum radius of 28.33" ((178/3.1415)/2). You will need to increase this minimum radius a little to address the transitions and also to allow for scenery and clearances. But basicly, you can see that you are going to eat up 5 feet of space.

Here is a picture of my helix. To gain extra space and reduce the grade, my helix isn't circular. It has varying diameters and some straight track.