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Part 30099 - Brick Arch 1x5x4 Inverted doesn’t seem to fit into the LEGO system. I cannot find a part that fits on the bottom stud and provides continuity to the concave slope of the arch. A tile is not high enough, and a cheese slope is too high. In other words, the height is 1.5 plates (1/2 a brick). Does anyone know why this is and/or have an example of a LEGO piece that fits this slope?

enter image description here

  • A Technic half-brick-thin piece such as piece no. 6632 would fit the arch if the slope really is 1/2 a brick higher than the stud. – technicguy1 Jan 26 at 22:00
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I would assume that this is to do with stability and flexibility of the element, combined with the rectangular nature of the system.

There are a number of uses of the part where it's not used to create a full circle, such as on the tailplane of 31011 or the fairing of a Ferarri and having a stud at the bottom allows them to secure the element on the top and bottom, rather than just from the bottom.

If the element was a true quarter circle, the end still wouldn't line up with the system height correctly, so I imagine that the decision was taken to finish it early with a stud, rather than complete it neatly.

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    Excellent answer. This makes a lot of sense. – Phil B. Jan 24 at 0:13
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This picture might explain the reason: enter image description here

As you can see, arches form a nice circle to fit round modified plate. All fits well, except the very bottom. Making the bottom edge fit the plate/tile nicely would result in gap with a round piece. The curve of this arch brick isn't full 1/4 circle to make it flush with a tile/plate.

However you may ask "Why didn't LEGO just extend the bottom curve to full 1/4 circle and cover the stud?". This seems to be an obvious solution, but that's a different topic and we may not know which decision caused the part to appear in existing form.

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    Thanks for your answer. Yes, raising that bottom stud by half a plate would make a slightly flattened circle. But my exact question is what you leave open at the end of your answer: why is this piece designed the way it is, and is there a piece that completes the circle ( or at least fit smoothly with the arch)? Why didn’t TLG make it the exact inverse of the usual arch? – Phil B. Jan 21 at 11:38
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    Probably stability and flexibility - Not every set that uses this element wants to create a full circle, and having a stud at the bottom allows them to secure the two together with a tile or plate, rather than butting up against each other. – Zhaph - Ben Duguid Jan 21 at 13:38
  • @Zhaph, to me your comment seems the only logical answer to this question. I think you should consider expanding your comment to an answer. – Michael Verschaeve Jan 23 at 11:09
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    @MichaelVerschaeve done. – Zhaph - Ben Duguid Jan 23 at 13:27
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Finding a piece that completes a flawless circle can't be done with this element's current design, but...

...If we stick only with Part #2339, we can build an outside frame that changes the direction of the studs themselves in a way that produces the desired effect.

enter image description here

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You could try and contact LEGO customer service directly for their official reason as to why this element is designed in such a fashion as to not produce a perfect circle in conjunction with its inverted twin, because a question this specific could only be verified officially by LEGO themselves, and anything else would seem to be speculation. enter image description here

  • Thanks for your answer! I like how you showed how to make a perfect circle using only the non-inverted part. I doubt Customer Service agents at LEGO will have a (canned) answer for this question but it might be something the folks at The New Elementary would want to take a look at. – Phil B. Jan 22 at 10:58

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