But when that sewing project is your very own perfectly fitting bra pattern - it's even worse than frustrating! It's like you have been betrayed by your underwear! <wails and weeping here>
What happened? Did you ever think it is the elastic to blame? It might be. Let me explain...
We all know that elastic varies in width among manufacturers, but did you know that it also varies greatly in strength too? Some elastics have a lot of stretch which means if I pull on 10" (25 cm) I can pull it out double its original length to 20" (50 cm). That means it stretches 100% over its original length (20" over 10" x 100% = 200%). Elastic that stretches to 15" over its original 10 has 150% stretch (15/10 x 100%).
Generally speaking you want elastic with between 140-180% stretch for the bottom band of a bra. There's a huge difference between 140 and 180% stretch. What happens if your original pattern was drafted and sewn using an elastic much stretchier than the one you are using now? In other words your first bra was sewn using elastic that was 180% stretch and the second bra was only 140% stretch. Hmmm - that means the bra band on the second bra will feel tighter than the first. 40% tighter in fact! Guaranteed. Sometimes too tight to wear, even.
At Bra-makers Supply, we have two types of bottom band elastics, which we call the OLD and the NEW elastic (because I am SO creative with names...lol). The OLD elastics EB-37, EB-47 and EB-67 have a stretch that varies between 170-180% which is definitely on the weaker side of bra band elastics. That's the elastic I used to create all the Pin-up Girls patterns with. You can see that this 10" stretches to 17"
The NEW elastics EB-372, EB-472 and EB-672 are much firmer as we wanted elastics that would stand the test of time during the washing and wearing process. This NEW elastic is much firmer, with about 140-150% stretch, which means it won't ride up and will last for years! It also means that if you use 3/4" elastic now, you will get the same holding power with 1/2" elastic!
You can see that 10" here stretches only to 15"
Most bra pattern designers use a "standard" elastic stretch percentage of 170%. That means you should always check your elastic against the norm. If it is less than 170% chances are good that you need to increase the length of the back band so it will have the same comfort level on your body.
Here's the important part:
You need to increase the length of the band by the difference in the percentage of the two elastics. In this case, one elastic is 170% and the other is 150%. That is 20% difference. You need to increase your back band pattern piece by 20% in length. Here is how to do that.
Measure the back band pattern piece. Draw a vertical line in the centre of the back band as shown above. Calculate 20% of the length (or whatever the difference is between the two elastics in question). Spread the two pieces apart by that distance. True up the lines at the top and bottom edge of the band.
Your new back band is ready to use. Remember to label the piece as "band for elastic with 150% stretch!"
Wasn't that easy? Now you can buy whatever elastics you like and know they will fit every time!