# Marble Roller Coaster Create Start of Loop (Part 6 of many) [VIDEO]

Article by Adam Bridgman, CSWE updated July 30, 2013

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Create Loop Start

Many people believe that the shape of the loop should be circular, the problem with a circular loop is that it has an abrupt change in curvature and in the case of a loop causes a significant loss of speed at the top of the loop.  The shape that we should use for this loop is a clothoid.

This shape starts with 0 curvature and then spirals, which is the perfect shape for a loop because the marble will not hop, and it will not lose a significant amount of speed.  However this shape is very complex to model in SolidWorks, we can generate coordinates and create a curve.  But even getting the coordinates is a fairly complex task.  As a simple workaround I will demonstrate sketching by tracing a sketch picture.  This topic is covered in the surface modeling course.

Open the start part and save as loop start. Roll back before connector male by dragging the bar at the bottom of the feature tree up.

Insert a sketch on the right plane, draw a line over from the origin 5 inches, finally use command search for “picture”.

Find a picture online of clothoid spirals and insert it in here.

Next use the spline tool and click on points along the path provided, I will go up to 180 degrees and try to avoid creating too many spline points so that the spline is smooth.  Use the line to position and set the scale of the sketch picture, once the picture is accepted the line can be deleted.  Then draw a centerline in the opposite direction and make the start of the spline tangent to it.  Right click the spline and show curvature combs.

The better you draw your clothoid the more consistent the curvature comb will be.  The curvature should start at 0 and consistently increase along the path.  My clothoid is not perfect, but were building a marble roller coaster, not one that will carry people, so lower tolerances can be accepted.  For a perfect clothoid an equation driven spline might work, or a curve through XYZ points is more appropriate.  This approximation should be good enough for now.

Although this spline is a decent approximation of a clothoid it still has a major problem, our loop must translate sideways by at least one track width.  To do this a 3D sketch will be required.  To insert a 3D sketch click the sketch tab of the command manager, sketch flyout, 3D sketch.

Create a spline that imitates the original, it should have exactly the same number of spline points, and be sketched along the YZ (press tab to alternate between the planes) plane without any relations to the 2D sketch.

Ctrl + Click the spline point pairs and add an Along X relation to each of them

Create a centerline along Z and make the spline tangent to it

Press spacebar and select the top view.

Drag the points over to the side to make a smooth line from this perspective.  Also add dimensions if required.  Add a line along Z for the other end (to make the connection easier).

Roll back before the sweep, and create a line between the two rail connection points.

Accept the sketch, roll forward past the sweep, edit the sweep feature and turn off merge result, click OK.  Insert a new move/copy bodies, click translate/rotate, select the sweep body (1), check off copy (2),  click the selection box (3), select the end point of line (4), select the other end of the line (5), click OK.

Hide all sketches, and roll to end, edit the move/copy bodies under the male connector. Make the faces coincident and the edges concentric as required.

Add a combine feature and add all the bodies to make a single body part.

Finished loop start

Watch this video for a demonstration of this technique: