Easement curves explanation

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schul4

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#1
If any of you listen to the modelrailcast show podcast, you know Dave Emery. I was talking with him the other night and he explained easement curves very well. Here is his explanation.:cool:

Dave Emery: The idea of an easement is, as the name implies, to ease your way into a curve. That's true vertically (i.e. with the superelevation) and horizontally. Consider 18" radius snap track: You go from tangent right into that sharp curve. If you introduce a piece of maybe 22" radius at either end of the curve, the trains run -much- better, since they don't lurch right into a sharp turn. In both cases, the idea is to start out less sharp and build your way into the middle/sharpest point. A good rule of thum. The mathematics for railroad easements is a bit complicated and of course takes more space than we can give up on the layout. But anything you do here helps. For flextrack, if you connect the two tangents of the curve, the flextrack will tend to assume a natural easement. Charlie, a pragmatic way to tackle easements is to first set your minimum radius, e.g. 18". Buy a couple of pieces of snap track at that radius. Then when planning a significant curve, place the pieces of 18" snap track in the middle of the curve. Push them "out" a bit so that the 18" curve doesn't end right at the straight track. For 18", push the snap track out 3" or so. Now take some flex track, attach one end to the tangent, and flex it until it starts to line up with those 18" snap track pieces. If you were planning an 18" radius semicircle, you should add about 2" across the bottom of the curve and about 4" inches at the middle. So instead of having a perfect semicircle that is 36" wide and 18" deep, you want one that is about 38" wide and 22" deep. Of course, adding more is better :)What I actually do is use some of those between-the-rails track gauges. I have one that's 18" radius. I'll tack down some flextrack and then run this gauge through the curve to make sure the entire curve is at least 18".
 
#3
A while ago I wrote an Excel spreadsheet which calculates easements if you give it inputs of the required final radius and the easement's length. It outputs data in terms of offsets from the tangent line at intervals until the final radius is reached, and also gives the instantaneous radius at each point. It's here:

http://www.nyx.net/~jpurbric/easement.zip
 





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