Very frequently (at least once a week anyway) I get a question related to strap lengths ... usually it refers to the binding style straps for bikinis. People want to know how they can calculate how much the strap lengths change with each ready to wear size. The length obviously depends on things like the initial design, the type of strap, whether or not it has elastic, where it is on the body, and what size the wearer is. So how do you successfully predict what it should be for each size. Normally I say make it a bit longer than you need it and test it on your smallest and largest, then you can grade in between.
I say that because it's a lot easier than explaining things in terms of vector math and elastic modulus ... and that's mostly because our blocks have different negative ease between the horizontal and the vertical (12% and 0% for example).
Technically we have all the information we need so there should be a simpler way to do it ... well it seems there is ... but really only if you're doing a lot of styles because you need to set up another block.
Let me digress a little. I've been getting a large number of questions related to elastic harnesses, for going over clothing or for naughty behind closed door activities. While it's not really swim or dance, it is stretch pattern related and it is quite closely related to things like bikini straps ... so they ask me. As I said, I'd normally just say make your bikini straps a little longer and test them because it's going to be a one off thing for a home sewist .. but what if the entire garment is just straps? ... where's your start point? .... where do you tie off your first strap to test it? Obviously "a little longer" doesn't work anymore.
So I thought I'd have a go at solving the problem ... and it was way easier than I thought ... mostly because I'd never given it much thought until now. What I needed to do was to be able to draw the harness lines straight onto my block, measure them and reduce their length appropriately ... but that's hard because of the different ease in each direction. Hang on ... why do I need to use a swimsuit block or dance block? I have a zero ease catsuit block (https://www.patternschool.online/product-page/multi-ease-catsuit-blocks) ... 0% vertical and 0% horizontal ... really all I should need to do is find the right size (or select an existing RTW size) and create a custom zero ease block and then shrink everything a certain percentage evenly for drafting my elastic lines ... so this is what I tried.
I did some more accurate measurements on my wife, found the right zero ease one piece sections from the multi ease catsuit block set and pieced together a perfect zero each body block just for her. Ok step one done.
Next I selected various elastics and assessed them for tension and comfort, and listened to her feedback on when something felt too tight ... subjective yes but that's fine for this exercise. We decided that 17% reduction was needed for the first 1/2" elastic ... strangely the 3/4" and 3/8" were also 16% and 17% respectively (I didn't expect that). So ... next I simply reduced the zero ease block to 83% in both directions (negative ease of 17%). See the first image below. Technically all I'd have to do now is draw lines on the block of where I want the straps to be and then measure those lines ... and that should be it ... that simple.
So I did a quick design based on the many you already see on the Internet (Pinterest - last three photos) ... that's the second image below. I had a stack of 1" brass split rings (drawn as 21.6mm on the image because I wanted everything to scale) so I drew those where I wanted them first, then I used 12.5mm elastic (drawn as 10.6mm) to join them together. I'm just using what I had on hand to test the concept initially.
OK so that all seemed really simple too. Next I used Corel Draw's dimension tool to measure each length of elastic and draw it on the "pattern" (third image below). Now I can just print it and cut everything to length (plus fold over seam allowance for the elastic - I used 20mm at each end so I could hold it steady under the machine foot) and put it together ... that was the tricky and time consuming part.
Miraculously it was a perfect fit on my wife, first time, for tension and comfort (even through all the protests) but obviously I wasn't allowed to photograph her, so the photo is on her dress form we made a few months ago that I still haven't completed. So really it is that simple to predict strap length for ready to wear ... you just need a reduced block in both directions (17% in my case) for you smallest and largest sizes and draft your design lines straight on to the blocks like I just did, then grade between them. I'm going to try another one in a different width of elastic if I can find some larger rings.
As far away from swim and dance as this appears, the underlying concept of strap length still applies and if it works for this level of garment then it'll certainly be ok for those doing RTW swim ... it's really about how frequently you need to know strap length.
Or you could just use an adjuster :P