I came up with a large helix which can be made to be stacked on top of itself. It is made for K'nex ball machines, and only works with the original K'nex balls. This element requires quite a few pieces however, but the finished product is rather cool.

When complete, the ring measures about 27 inches in diameter and a circumference of just over 7 feet.

If you make it, send me pictures and I'll post them on here. The picture below features this helix in my current machine, however, there is a smaller helix (with blue track) in the center of it. This tutorial is not including that helix.

Step 1: Piece Count

Before you start, here is a piece count for a 1 foot section(measured by the outer track) of the helix. These are the pieces you will need: Rods :
-Green : 10
-White : 9
-Blue : 9
-Gold Notched : 5

Connectors:
- Orange : 5
- Yellow : 11
-Gold Tab : 10
- Y connector : 5
- Grey one slot connector : 5

Other:
Spacers : 5
Track : 2 (It takes about 14 feet of track to complete a full circle, 7 feet on each side)

Total : 76

Step 2: The Inner Ring

Here you will create the inner ring.

Picture 1 : Create this

Picture 2 : Add the green pieces

Picture 3: Add the top

Picture 4: Add gold rods on the bottom connectors

Picture 5: Another view

Step 3: The Inner Track

Here you will be creating the inner track and spacing.

Picture 1: Add the Y connectors. Note the placement
Picture 2: Another view
Picture 3: Add the inner half tabs
Picture 4: Another view
Picture 5: Make one of these for each inner gold tab.
Picture 6: Put the parts on like this
Picture 7 and 8: Other views

Step 4: Outer Ring and Outer Track

Now it’s time to create the outer ring and give the circle its contour.

Picture 1 : Place the orange tab connectors on the Y connectors
Picture 2: Add blue rods between the connectors
Picture 3, 4, 5: Other views

Step 5: Add Track and Layer Supports

Now you will add the track, and layer supports. The layer supports are optional, and are not included in the part count for that reason. You would use the layer supports if you make more than a full circle.

Pictures 1 and 2: Track added

Pictures 3 and 4: Layer supports Stagger them as shown, and connect them to the layer above on the same spot.

Step 6: You're Done!

Your done! Below are some pictures of a finished product. It gives examples of the usage of the layer supports, and a support technique I use to keep the layer supports square.