@shanr: you don’t need a curve. For your treads, take an Empty and rotate it 36 degrees around the global x-axis (which is our wheel’s rotation axis). The object origin of your tread must be in the same place as your Empty (that is where the axes of your Empty are crossing). Add an Array Modifier to your tread and only select “Object Offset” as offset in your Array Modifier). Set the “Fixed Count” to 10 (since 10 * 36 degrees is a full circle of 360 degrees). Rotations and scaling should be applied (with CTRL + R and CTRL + S) because otherwise these transformations would be applied for every duplicate in relation to its predecessor in the duplication order. You can analyze it here in this file:
https://www.dropbox.com/s/5z9nvdcyaegl3kh/ArrayModifier_Demonstration.blend?dl=0
Select the original monkey head (top middle; this also selects the duplicates) and scale it, rotate it or move it in object mode and see how it influences the duplication. Doing the same transformations in edit mode also applies to every duplicate but the duplicates now change identically in an absolute manner and not relatively to the preceding object instance.
