General Discussion

[Balsa Man]

Good points, all, Jeff.
Couple quick thoughts back. 1) what is level, and 2) using the right tools & techniques.

#1 is , as you note a subjective thing; how precisely can you measure and adjust, and how much time and money can you throw at it. 1/8th inch is ok, depending where. 1/8th across the 20cm opening is (imho) atrocious- even a short, crappy bubble level with semi-caring use can get you better than that. 1/8th across the testing platform (=/>32cm ) is significantly better- that would get you around 1/24th at the opening; not good, not bad; maybe a good degree of level to make competition contingency design decisions from. That brings us to 2)

If you have a bubble level w/ the bubble at the center, regardless of how long it is, you have a poor tool. If you have a bubble level with bubbles at the ends, the longer it is, the closer you can see.
But there is a simple way to improve precision; 2…..sticks, one of which is straight. Like 2 decent meter sticks- lay the straight one across a diagonal, centered on the opening. Use the other (looking at mm if it’s a meter stick, marking w/ a pencil if its just a stick- measure the height of the ends of the diagonal stick. Get them reasonably even- a couple mm is pretty easy, certainly 1/8th inch. Switch the straight stick to the opposite diagonal. Same measurement process, so the ends are equally high, and at the end heights for the other diagonal. Using this, if you’re off 1/8th at the ends, now you’re talking ~1/40th around the edges of the opening.
This same technique, carefully and precisely used is a good way to get the top of towers really level, too. In my earlier post, wondering about competitors checking level, I was thinking doing what I just described.

[jander14indoor]

Agree, some standard is appropriate. But, as a tournament director, would you throw out the event if the supervisor couldn't deliver level, but was consistent? The strong bias is to avoid throwing out events given how hard students worked to prepare.

On your second para, what are you measuring to? The floor? If so, you are going to be disappointed, floors are notoriously bad if you have an older building and not great in a new one. That's on a hard floor, don't get me started on carpets!!

Here's a more reliable method if you have time. Get a good true straight edge, expensive to buy, cheap and relatively easy to make if you have time. Be carful with a wooden meter stick, they tend to be floppy and conform to gently warped surfaces, even on edge. Check the surface for flatness along several directions. Diagonal, across, along the edges. Use a feeler gauge to make sure Gaps aren't bigger than say 1 mm or so. Then use any long level you've checked for accuracy (the nice thing about levels is you can self check them) or the new electronic levels (like one by Wixey) in two directions. That should leave you as good to go as is reasonable.

Jeff Anderson
Livonia, MI

[SLM]

In light of the recent posts and discussion on the importance of a level testing platform, it would be instructive to see how the behavior of a tower changes when its support surface is inclined.

This summary analysis is for a tower with a rectangular base and chimney, and focuses on the inner legs of the tower only. Consider the tower shown below. The distance between the legs is 20 cm, the overall height of the tower is 70 cm, and the chimney has dimensions 5.5cm x 5.5 cm x 55 cm.


If we place the tower on an inclined surface, then the sand bucket no longer remains along the centerline of the tower. Consequently, when the inclination angle exceeds 2.86 degrees, the chain holding the bucket comes in contact with the inner side of the tower at point A (see figure below).


Since the chain carries a tension force, it exerts a direct pressure on the tower at A. As a result, the inner leg(s) of the tower have to carry significantly more compressive force compared to the ideal case where the support surface is completely horizontal. For example, if the surface is inclined by just 4 degrees, the force in the inner legs increases by about 40%. So, if you want the tower not to fail prematurely on such an inclined surface, you need to design the legs for a total load of 21 kg, not 15 kg. If the inclination angle is only 2 degrees, causing a force increase of about 24%, then the legs should be design for a total load of 18.6 kg instead of 15 kg.

It is also important to note that a measurable support surface inclination could significantly increase the amount of force in the bracings. Stronger wood may have to be used for these secondary members to compensate for such support surface imperfections.

Let me know if you are interested in seeing the analysis calculations.

[iYOA]

what are some general jig building designs?

[Faustina]

SLM, thank you for the calculations. But suppose the tower is 40 cm tall, not 70. How does this change your calculations? In other words, how many kilograms would a 40 cm tower have to be designed to hold? I'm wondering how significant the difference is.

[SLM]

SLM, thank you for the calculations. But suppose the tower is 40 cm tall, not 70. How does this change your calculations? In other words, how many kilograms would a 40 cm tower have to be designed to hold? I'm wondering how significant the difference is.

If the tower is 40 cm in height, then the chain would not physically come in contact with the inner side of the tower (at point A) unless the inclination angle exceeds 12.4 degrees. Nevertheless, even for inclination angles smaller than 12.4 degrees, compression in the inner legs increases. If the angle is 4 degrees, the force increases by about 28% (the legs need to be designed for a total weight of 1.28 x 15 = 19 kg), If the angle is 2 degrees, the force increases by 14% (the legs have to be designed for a weight of 17 kg). If the inclination angle is only 1 degree, the increase in the force is about 7%, the legs need to be designed for 16 kg of weight.

[iYOA]

has anyone started testing?
we have built one that is 10.6g, 70cm and have yet to test it.

[Balsa Man]

If it holds a full 15kg, you'll be doing very well for this point in the season.

[nwang10]

Is epoxy allowed on Division B towers events?

[jpmacguy]

what about zap a gap? is it better than epoxy?