Slotted Bikini Bottom
Why did it Distort?
This article is based on a discussion that took place in our Facebook group. From time to time great discussion topics come up that really need to be preserved so we stick them in here for everyone's reference. If you want to read the full thread and contribute to the discussion then you'll need to join the group. This thread is located here:
I had intended to do this as a tutorial but I didn't want to lose all the great discussion that went with it from the group so I'm doing a kind of hybdrid.
This particular bikini was really popular in the Group polls exactly as it is in this photo for a reason ... it's sexy and has a lot of impact ... people like it, it sells well and their customers all see the impact. Indeed I've seen a very similar PDF pattern offered by several different Indie designers ... and have two of them to examine.
Most people won't notice or realize that it's not 100% as the designer intended and that there are a few bugs in the pattern that if corrected could make this style even better ... to read about those you'll need to follow the link above where they're discussed at great length indeed.
The conclusion of that it discussion is that the garment cannot be stable if the slot goes past the center front line. So we're going to show you how to bring it back correctly.
We begin with a high waisted bikini bottom that's fairly square cut on the sides ... we can't really use a narrow band pattern or there'd be nowhere to put the slot ... one designer,Bikini Design Club, also offers this in two styles (this one and an even higher waisted version) ... so we need to maintain that possibility as the designer intended (this is our job as pattern makers). Keeping in mind what the designer intended is very important ... what we're doing here is really going against that because we're trying to fix the garments instability for the sake of bringing conclusion to the discussion. Some may well fairly say they prefer the original design because of it's sexiness and not care at all about the instability or deformation ... and that's just fine too!
The side on this pattern I've chosen for the demonstration is 7cm high and we're going to use 2cm wide straps on the right hand side. so I create two long rectangles 2cm wide that go from the side seam to cross the center front and rotate them until they sit where I think they look right tangential to the waist and leg hole (dotted red rectangles). exactly between these two I've drawn a center line for my slot. Now you'll note that because the waist and leg lines curve away from the rectangles, our strap will increase in width as we get closer to center front ... this is what we want for stability ... what we don't want to do is create a 2cm wide strip that parallels the waist and the leg line along the curve or just replaces the curve altogether with rectangles!
Now we want to stick a circle on the end. As we mentioned in the discussion, a circle at 12% reduction when stretched out, becomes an oval .... and we're after a nice round end when its on the body ... this means we need to reduce the width of the oval along the line of stretch. We also need to pull the circle back from center front so that it's no longer being pulled on by anything except the left to right tension of the body! As a general rule of thumb I don't like to get any closer than the radius of the circle ... ie; the bigger the circle, the further away I locate it ... this is arbitrary and up to the designer, but the closer we get the more distorted and unstable the garment becomes ... if you place the center of the circle beyond the center front then you get what you see in the photo above!
So how much do we reduce the circle to make it into an oval. We need to consider two things: the negative ease of the block is 12% and those two small straps will stretch about an extra 10% under load because of their narrow width (we'll take that bit off after). so we're going to reduce the diameter of the circle to 79.2% ( 88 - (88x0.1) ) and rotate it parallel to the line of tension (in this case the guide line we drew earlier). We then duplicate that across so it's half a full circle width away from center front when on the body.
I've also drawn a blue line from the top of that second oval to the side seam ... this is the length we need to reduce by a further 10% to compensate for the narrow width of the strap.
So let's measure that blue guide line and shrink it 10% and mark off where to reduce the straps.
That's pretty much all there is to it. Yes it's that simple. But I'd like to expand on why. Take a look at the illustration below. The dotted blue lines represent the anchored tension in the pattern piece when on the body. Because we still have the center front line intact we have a the garment stable through the crotch ... there's nothing allowing the tension in crotch to drop the garment further that the waistline elastic will allow ... clearly we still have direct tension in the top strap from left to right. Now the tension line in the lower part of any bikini bottom is dipped through the crotch whether we have a slot cut out or not because the crotch is pulling the fabric against the waist line elastic ... it is not simply left to right as you might think ... if you drew a rectangular grid on your pattern piece before sewing it would look like the lower line when on the body .... and this line is now running parallel to the lower strap (almost) so all the tension will be pulling only on the oval to open it out into a circle! That's it!