1/20 vs. 1/32 bracing

[SOCoach]

I question whether my kids can get the bonus and still keep the weight that low. Are there any design differences in going for the bonus aside from simply making the base wider?

[Balsa Man]

I question whether my kids can get the bonus and still keep the weight that low. Are there any design differences in going for the bonus aside from simply making the base wider?

No, just need to bring the base leg ends out a little. So base legs will be very slightly longer, and the force on them goes up just a little (from about 3.9kg to about 4.1kg under 15kg load). The greater width/leg separation low in the base means the lower X sets need to be significantly stronger (that inverse square relationship of strength to length being at work). But when you add up the factors that mean more wood weight, the gain of 5kg in load credit easily overcomes it. A 6gr non-bonus carrying full scores 2500; an 8gr circle bonus carrying full scores 2500. It takes significantly less than 2gr in extra wood weight to build circle bonus version.......

[SOCoach]

I am thinking I am going to try creating a jig like you mentioned in an earlier post. Do you have a picture of your edge piece that holds the legs at a 45 degree angle? The one that you glue on the edge of the tower jig? I can't seem to wrap my head around that.

[Balsa Man]

I am thinking I am going to try creating a jig like you mentioned in an earlier post. Do you have a picture of your edge piece that holds the legs at a 45 degree angle? The one that you glue on the edge of the tower jig? I can't seem to wrap my head around that.

I don’t have a pic that really shows anything. Not sure what you’re not getting. 5cm long each. Looking end-on, cross section is a capital “M” – the V in the M is the trough the leg rides in. The width – between the vertical legs of the M is the same thickness as the jig plates “3/16”, which is actually 0.177”. The height 0.118”. With these dimensions, the apex of the V is about 1.0mm above the flat side that goes onto the edge of jig plates.

[DarthBuilder]

It was a C tower and I found out it weighed about 13 grams

[SOCoach]

Thanks for the help. I understand now what you are talking about . . . . I can 3D print those. I assume you don't make one to go down the entire length of the jig right? Just at the ends, top and maybe in the middle?

[Balsa Man]

Thanks for the help. I understand now what you are talking about . . . . I can 3D print those. I assume you don't make one to go down the entire length of the jig right? Just at the ends, top and maybe in the middle?

Excellent; thanks.
As I'd mentioned, we print them at 5cm long, and we print them standing vertically on the printer bed. That way, the sides of the V notch come out nice and smooth. Printing horizontally, we got little ridges running along the V notch faces and were concerned they could damage/indent into low density chimney wood. Setting up the printer shapefile as a 5x5 square - 25 of them, separated by 1/16" or so, you can save significant printing time.

We set them up along the jig plate edges based on bracing interval used; with a gap between pieces of about 2cm at bracing points - about 0.5cm above the bracing points, 1.5cm below. You may have to trim piece length - just cut w/ razor blade. This minimizes the potential for leg getting glued to the jig. Also highly recommend putting little pieces of saran wrap or waxed paper on the pieces below bracing points to prevent glueing onto the jig from any glue run-down. The bottom end of the first piece at the bottom of the tower is set about 1/4" above the base plane, for clearance for 1/16" wide tension strip (set so bottom edge of the strip is 1/16" above base plane), and clearance for the lower end of bottom Xs. At the angle break point (where base meets chimney), top of the last piece below the angle break point about 1.5cm below the break, and bottom of first piece at the bottom of the chimney about 0.7cm above the break. At the top of tower, top of last piece about 0.7cm down to clear top ladders.

The ladders at top of base section need to go on after all Xs are on and the tower is lifted off the jig. They should be carefully cut/fitted while tower is on the jig, and marked as to which quadrant they go in. If you put the base top ladders in while the tower is on the jig, the leg holder pieces of the chimney will hit them, preventing you from being able to get the tower off the jig.