Elastic Performance & Tension

I’m often asked if elastic choice can affect a pattern. My answer is always a very simple yes. Strangely I’m never asked why. It seems we all know how elastic type and tension can affect a garment, but not how much or why.

Swimwear is manufactured almost exclusively with vulcanized flat rubber ribbon elastic. These come in varying thicknesses and widths. Generally, the thicker and wider the elastic, the more rebound tension it offers. Flat rubber also has a gentle increase in rebound tension as it’s stretched out, rather than the sudden onset of rebound you find with woven and knitted elastics. A problem all elastics have is that they deteriorate rather quickly when exposed to chemicals, heat or UV.

Which elastic is appropriate depends on its application. A simple fold over elastic in a swimsuit leg line, for example, is usually 8-11mm wide and around 0.6mm thick. If you were to choose narrower elastic it would not only lack the strength to stabilise the garment, but it would also dig into a soft part of the body while trying. Besides being uncomfortable, this would look awful.

Elastic wider than 13mm requires a great deal more force to stretch it and as such it doesn’t have a wide working range on the body. This makes it unsuitable for areas like the leg line (which change dimension significantly during movement) but very useful in bust support panels like shelf bras.

Wide flat elastics also have a problem going around small radius curves and cut outs. You’ll know your elastic is too wide for a small radius if the elastic rolls out rather than sits flat. What is happening is the outside edge of the elastic is stretching beyond its workable limit while the inside edge is unstretched. Because you can’t practically change elastic widths mid seam, you need to be aware of what minimum radius each elastic can tolerate.

Bound edges and straps typically utilize slightly smaller elastic about 5-7mm wide and 0.5mm thick. They can tolerate being thinner and narrower because they are essentially wrapped in an elastane fabric which increases the overall rebound tension of the binding or strap. Bound elastic while very stable, does not have the softness or stretch range of fold over elastic seams.

So now we’ve decided which elastic to use, how ‘tight’ do we need to make it? Well this goes back to the amount of negative ease already added to the garment. If you employ 12% negative ease and 3% elastic tension then the elastic is effectively stretched 15% on the body. If you were to use only 5% negative ease for dancewear, clearly you’d need to increase your elasticator tension significantly to attain the same final tension on the body.

Tip While dancewear clients are used to seeing garments looking ‘gathered’ because of the extra elastic tension, retail swimwear would be completely unacceptable on the sales shelf if it looked gathered. Swimwear on the sales shelf needs to sit relatively flat, forcing a very difficult balancing act between negative ease, elastic tension and comfortable fit.

“OK so prove that to me!” is most likely what you’re all thinking. People usually have two assumptions about elastic tension. The first is that you don’t need to add elastic tension if you have negative ease already added because, after all, if the fabric has to stretch then so too does the elastic and all the garments in the shops don’t have any gathers in them. The second assumption is that you need to add more tension to the area just under the cheek.

Let’s look at the first assumption. If I make a garment with 12% negative ease on the horizontal and 0% on the vertical to fit a stadard size 12 it will have a different leg hole and arm hole length to the exact same style made to fit exactly the same person just as well but if done with 8% and 3% respectively: ie; the amount of elastic used if no further ease was added to the elastic would be different and thus produce different tensions on the same person. The elastic should have the appropriate tension for the person wearing it … not for the garment on which it sits. This means a 12%/0% design will have less extra elastic reduction than an 8%/3% design …. hence why swimwear appears to have no reduction and dancewear appears significantly gathered (in commercially manufactured garments).

The second assumption is something I find a little disturbing. People put extra elastic tension in the area under the cheeks in the hope it’ll cup the cheek a little better and create a better fit, in a manner akin to elasticated bloomers. This theory was all well and good in the old days of non-stretch fabrics but it shows a complete misunderstanding of elastic. Elastic is like a spring … when you stretch it, all the coils seperate an equal amount. No matter how much you try to move a small section of coils closer together, if you let the spring control the tension as it was designed to do, then all the coils will even out. If you add extra tension to the cheek area then put on the garment and thereby stretch the elastic (or spring) it will not gather up under the cheek exactly, but it will even out around the whole legline and cause distortions either side of the area you tried to gather. Thechnically you havent gathered under the bottom but instead you’ve done the opposite and stretched it more. OK it may sit better because you’ve got overall more elastic tension, but its not fitting for the reason you think it is. In my opinion if you want a better fit under the cheek then you design the pattern right the first time so that the elastic isn’t being depended on to do the job. That way you end up with the same comfort and fit but without the unsightly gathers and pulls.

I can imagine all the dance mums out there currently calling me crazy and telling me I don’t have kids and I clearly don’t understand the problem. Let me assure them I do understand the problem all too well, I do have kids and I’ve made literally thousands of pieces of swim and dancewear myself and probably millions inderectly through manufacturers. This is the biggest bone of contention when it comes to elastics. Here’s my statement: If it looks and fits better when you add extra tension to the cheek area then you had a bad pattern to begin with. To clarify that, you need to remember that children grow very fast and most childrens’ dance patterns have much less negative ease in the garment (some are even zero or astoungly positive!). This means you need lots more elastic tension to make up for it. The elastic cheek reduction then will disappear in all those gathers and you won’t actually see any pulls because they’ll be neutralised by the gathers. What you also won’t see is that the elastic tension easily evening out around the whole legline … whammo you’re looking at bloomers aren’t you?! I’d rather make the garment to fit perfectly now and make another in six months time when the child has grown … of course that’s a time and financial consideration as well.

How Do I Work Out How Much To Use?

What you are trying to do is look at the position on the body where you think the elastic is going to sit, measure that, then place the elastic there under the tension you want it to be. The difference between the measured length and the length of the actual elastic is the amount of reduction you use. Sounds crazy but bear with me. This amount of reduction will almost always (at least in swimwear elastic widths) be greater than 12% hence meaning your elastic should be smaller than your cut pattern.

Elastic tension must be sized accordingly to the finished size of the person based on the rebound required and offered by the elastic. As all elastics are different this will change constantly. It should be obvious that no one amount is suitable for all elastics. Generally the 8-10mm flat elastic that is used by 90% of swimwear manufacturers is reduced by 2-3% as its applied to a 12%/0% garment (even if they don’t realise it most of the time!). When the fabric and thread weight is added the elastic doesn’t actually appear puckered because its already stretched out the 2-3% by the stitching process, but don’t be fooled because you’ve lost that % in rebound terms so the final tension will still be correct (I think this bit might confuse a few people). Just because you don’t see tension, doesnt mean its not there under the weight of four layers of fabric and thread.

Woven poly elastic has terrible rebound predictability and doesnt tend to stretch as well as flat rubber so people don’t need the 2-3% in that case or the final tension will be too tight. Some people confuse this as meaning all elastic should be zero but that is incorrect. Clear elastic needs about 6-8% extra reduction but please dont bother with clear as it deteriorates too quickly.

Again you need to measure the length of the leghole, for example, on the actual body, and not against the garment. It is the body measurement that needs the desired tension applied to it. Lets say you’re leghole on the body is 60cm. When you measure the leghole on your garment you find its only 54cm. The total reduction along the line of the seam on the pattern is thus 10%. Now take your elastic and stretch it, get a feeel for it, place it on the body etc. 8mm flat rubber at 0.6mm thickness tends to feel right when stretched 15% (ie; a 10cm length hits its rebound at 11.5cm). The reduction in elastic therefore is actually mathematically the opposite eg; 10/11.5 or 87% of its original lenth … or in our terms you need a total of 13% (100-87) reduction in elastic length for it to feel good on the body. Now if your garment already has 10% reduction along the seam line you only need to add a further 3% negative ease to the elastic relative to the garment.

Sounds complicated but you really need to be able to visualise the principle and get proficient at it if you hope to have consistency from garment to garment. After a while you do get to know when you need to add a little more or less and that’s just fine.

Rule : Don’t blindly apply the same reduction or lack thereof to all elastics. Base the reduction on the properties of the elastic and then compare its length to the finished length on the pattern … that is the reduction you will require.

How Do I Apply Elastic?

There are two ways to apply elastic. One technique is to apply it to the wrong side, fold it over and top stitch. The other is to apply it within a wrapped binding. Let’s have a look at the first technique.

Above is an example of fold over elastic on a leg hole. The elastic is first applied to the wrong side of the fabric/lining pair using an overlocker (serger) under whatever tension you have decided. There are many ways of judging tension: some do it by feel, others use pressure plates (see the attachment), others use special mechanical metering devices (only available for industrial machines) and of course some use carefully measured cut lengths (most domestic sewists). Whatever the method you decide, you need to apply it to the wrong side using a stretch stitch. I use an industrial overlocker exclusively. A zig zag or lingerie zig zag will work fine but try to avoid excessive needle punches as this does weaken the elastic and tempts breakage. Plain zig zag is better than the triple for this reason. You then fold the seam over so that the elastic is now encased and top stitch with either another zig zag or with a coverstitch machine. Top stitching babylock machines are also good at this but domestic coverstitch machines tend to bounce over side and crotch seams leaving missed stitches. Industrial coverstitch machines are without a doubt the ultimate top stitchers (see below).

The close up on the right shows what a binding attachment looks like. It works by feeding the elastic into the device under tension created by a pressure plate. A narrow strip of fabric is then wrapped around the elastic as it is applied to the garment in one swift operation. The devices can be used with both industrial coverstitchers and industrial zig zaggers. They do not work on domestic machines as domestics can’t produce the length of stretch stich required to support the possible stretch demanded. Binding attachments can also be used to create tie straps in the same operation.

Some people also utilise a third technique. That is to put the elastic into a hem casing. This is almost exclusively a domestic practice (I’ve only ever seen it done commercially in tutu panties). I don’t really think it’s an approprite swimwear technique for aesthetic reasons and is actually harder and more costly to do than the above. It does have a place in tutus because the elastics do break frequently and you don’t want to throw away the cost of all those ruffles … not to mention you don’t really see the hem of a properly made tutu … it also makes the elastic tension adjustable if you get it wrong the first time or the wearer’s size changes or even if the choreography requires more tension. Just something to think about!