The Principle Stretch Direction
Where Does The Negative Ease Go Exactly?
The objective of stretch fit pattern making for stretch wear is to create a pattern which will result in a garment of even (not necessarily equal) tension on the body in all directions. This is critical. If tensions are not even then the garment may ripple and move on the body. If you think of a one piece swimsuit as a tube that encircles the body there are two questions to consider. How long is the tube and what is its circumference? These are the two principle stretch directions.
To make a swimsuit ‘shrink’ onto the body at least one of these directions requires what is referred to as negative ease. Negative ease is where a body measurement is reduced by a certain amount in the pattern. It is the opposite principle to adding ease to a measurement for non-stretch pattern making. Generally, we reduce only the measurements that go around the body. There are two main reasons. Firstly, when a person twists and bends they do not significantly change any of their girth measurements, however they do change their body length. Thus adding limited or no negative ease in this direction adds a certain amount of softness to the garment when the wearer moves. If the garment is unable to stretch vertically, because it has already been reduced, it will move over the body or distort rather than change shape with it.
Secondly, though more a consideration for ready to wear manufacturing, it is best to allow for more variation in girth than height. You’ll fit more customers within each size if you set your principle stretch direction around the body. Try to consider each size as a range of measurements. For example, while we define a size 10 as an 84cm bust line, we would hopefully design a garment that would comfortably fit a bust of between 82 and 86cm, with similar allowance made for waist and hips.
The more horizontal negative ease you include in your block, the smaller the number of people your garments will fit within each size. Can you answer why softer surf style swimwear brands fit just about anyone, while firmer high fashion swimwear usually does not, yet still sits a lot better?
There is also a subtle third reason which is complex and often overlooked. Put simply, when you apply tension to a stretch fabric it will naturally tend to shrink in a right angled direction. The fabric is trying to relieve the tension you’ve applied to it. How much it counteracts depends on the fabric. Recent fabrics with higher elastane content do not counteract as much, some remarkably not at all. You will need to test your fabric and make allowance for this amount.
Tip: Sometimes the fabric will decide the direction for you. A single direction stretch fabric might have a print running only one direction, for example. Another might stretch more in one direction than another. Usually the fabric’s greatest stretch direction should run around the body.
The table below illustrates the typical relationship between horizontal and vertical negative ease for 180gsm, 75% two-way stretch nylon elastane. Generally the more you reduce the horizontal, the less you need to reduce the vertical to apply the same even tension to the body. Which set is used depends on the fabric, designer and the garment’s intended purpose.
Understanding where the tension is being applied by the pattern is key to predicting where the garment will move to when on the body. If you tension varies across the body the garment will always try to move to the area of lowest tension. If it's higher in just one place then you'll get ripples appearing in the garment as the perpendicular tension collapses. An even tension across the entire garment is essential to a stable fit ... meaning traditional shaping techniques, such as darts, are even more important (contrary to popular belief).
Anyone designing swimwear needs to understand this principle almost second nature or they will forever be chasing themselves around in circles trying to work out why a particular garment keeps failing to properly fit or moves where it shouldn’t.
But how do you determine what total amount of negative ease is appropriate for any particular fabric the first time? You need to asses the fabric’s stretch and rebound properties. In the past many designers have simply wrapped a section of fabric around the bust until the tension ‘feels about right’, measured how much fabric was required at that tension and then compared it to the bust. For example, if it takes 74cm of fabric to comfortably wrap a bust of 84cm, you have a reduction of 74/84 x 100 = 88% or a negative ease of 12%.
These days manufacturers provide you with fabric performance statistics on request. They are required to perform national standards testing on all fabrics they sell. Which standard they use depends on the country and the measurement system used. This data should be able to tell you how much percentage reduction is required to create a certain tension. Some people still find this data difficult to interpret and resort to the old way of assessing a fabric, but I strongly recommend getting the manufacturer’s sales representative to explain how to interpret the system they’ve used as it will give you much more reliable and consistent results and save time in the long run.
For demonstration purposes this site will base all patterns on either 8% or 12% horizontal negative ease (it'll say in each case) and 0% vertical negative ease. These values suit the common 75% two way stretch spandex fabrics. Some people suggest that if you are making swimwear for the first time you might want to base your patterns on 8-9% horizontal negative ease and 3% vertical negative ease instead as this creates a more forgiving pattern ... it does, but then you'll need to change all your patterns again in the future once you've got the hang of it so I'd honestly go with no vertical negative ease.
Remember, every time you change your fabric the elastic modulus (how much it stretches) may change, so you may need to change your block, or the patterns you cut from them will not be the same. I have different blocks at different values of negative ease already prepared, so I choose which one I want based on the fabric and it's intended use.