4.2.4.2 Laying out a Compound Curve

Modeling Information

Project Process

Laying out a Compound Curve. 

The technique for laying out a compound curve is similar to the technique for laying out a simple curve. I use a curved radius template to keep the tie line straight (pun intended) and use the ribbon rail radius curve at the points of tangency.

Assuming that you have satisfactorily laid down the layout lines for the track, here is the the track laying process I use.

1. Pre-Installation Process  - Track Preparation. See 4.2.3 Straight Track for the standard track preparation process.

 Rail end checks. No changes or additions.

Tie spacing. I do not change tie spacing on curves. There is no significant visual difference between WWSL Standard 1 and Standard 2 on curved track, and I do not have any curves on Standard 3, 4 or 5 track.

Track cleaning. No change or additions.

Individual spare ties. No change or additions.

2. Installation Process

Easement. The easement in a WWSL compound curve is based on the curve radius. A 24 inch radius curve has a 5 inch 26 inch radius easement at each end of the curve. A 26 inch radius curve has a 5 inch 28 inch radius easement at each end of the curve.

 

Super-elevation. If you are going to model super-elevation of the track and/or modify the roadbed with vertical curves, now is the time to make those engineering modifications. See 4.2.4.6 Superelevation for Curves for additional information.

 

Join Track Sections.  There are several changes or additions when laying out curve track. Getting a smoothly flowing connection between tangent (straight) track and the curved track is difficult. The outward pressure of the rail at the joint often creates a kink.

 

To minimize this difficulty I measure the curve circumference and, at the workbench, I take the required lengths of flex track (plus 6 inches or so for the tangent), insert the rail joiners and solder each section together (without a gap) forming a single long, straight section.  This is the only instance where I recommend soldering rail joiners. Be sure that its the best soldering job - blobs will interfere with the inward moving through the tie plates

 

I place the moveable rail on the inside of the curve.

 

I start at the center of the tangent (if any). I place a Ribbon Rail straight template at the edge of the tangent and then add the appropriate Ribbon Rail curve radius templates as necessary to complete the curve. I then take this piece to the layout to install on the curve. I do an initial fit to see what works and doesn't work. This usually involves shifting the full curve template outward to meet the point of tangentcy.

 

Glue Down. I layout the curve using the foam board curved track template instead of a straight edge. The curved template is fastened by a screw into the subroadbed. (another reason why I don't use a center line for roadbed layout lines).

 

Tie Gaps.  Fill those gaps at the rail joiner with the replacement ties. 

 

Rail Gaps. There are NO rail gaps in the compound curve. The rail gaps are located on the straight sections several inches on either side of the curve. 

 Curves at Section edges. Cut the track and roadbed after the track has been installed and the adhesive is cured. See 4.2.3.1 Rail joints at section ends for additional information.