Hi Matsuo, here is a more detailed explanation.

I started with your file, but deleted everything except for one tread piece in a flat area and the curve:



Next, I think it is a good idea to make the tread straddle midway through the curve instead of ride on top of it, I think the kind of alignment you want will be middle-to-middle alignment? So to get that it should be in the middle. To do that, I went to the front view and zoomed in a ways, then clicked down on this point near the middle of the vertical lines:



Then dragged it down to move the mid point of that line to align with the vertical-line intersection of the path:




Now you need to place the first point. To do this, start the Point command (under Draw curve / more) then drag a construction line between the centers of the 2 faces, like this - start by moving the mouse to one side:



Then instead of clicking and releasing the mouse, press hold down and drag and you will pull out a construction line - move your mouse over to the other center point:



Now you can grab the midpoint of that:



That is your kind of anchoring hinge point where you want each tread to touch each other (on one side at least).

Now you do the steps that I showed above to replicate that point at even distances along the curve, delete the curve and draw a polyline in its place instead. After you have done that you will see this situation:



Note the point indicated with the red arrow - the tread is not currently of the right size to align the other side with that hinge point. You need to scale the tread so that it is arranged so both hinge points of the tread are at ends of the line segment, currently only the start one is.

You can use the scale command to edit the tread so that it is of the correct size.

To do that, select the tread, and then run Transform / Scale. You'll now pick 3 points for the scale at the spots indicated here:



The first point is the origin point of the scale, pick it at the good hinge point, you want that to stay locked in relation to where it is. Then for the first reference point, drag out a construction line same as described above, and place the first reference point at the middle of that. Now for the third point click the end of the line segment.

The tread will be scaled by this relative amount, so now it is of the proper size running exactly along one of the line segments:



Now that it is the proper size to fit, you can select it, and run array curve again, pick the polyline as the path and you will get the fully aligned tread (I think this is the kind of arrangement you want?):



Let me know if any step is not clear, or if this wasn't what you wanted.

- Michael

If you have something slightly differently arranged than this, use the "Move" command to first move the tread to be lined up exactly with one line endpoint, then you can use Scale after that to size it to fit as shown above.

- Michael

Also manz was very much on the right "track" :) above with the idea to break the curve into equal sized markers.

But once you have done that you don't want to use the curve anymore, because you don't want to get aligned to just some tangent along the curve, you want to have pieces to be aligned only specifically in those line directions.

When the curve is only made up of lines, then when it gets a curve tangent to align each piece to, it will be along those line directions and not at some more smoothly changing curved piece.

- Michael

cool method!