Control Your Flat Spot: Learn how to direct strap tension to get the cup shaping you want!

I have spoken before about how there are regions of flatness in a bra cup, but I think the topic needs another visit, as I've found people sometimes underestimate how much flatness caused by the strap can impact the shaping of the cup - especially in large cups. By the end of this post you should know how to manage strange dents and intentional (or unintentional) regions of "flatness" to get the shaping and lift that you desire out of your bras.

An extended edition of this post is available on my ko-fi, which you can read with a one-time donation of $3/£2. You can subscribe to continue supporting my work and receive early access to/extended versions of my posts.

1. What is the flat spot in a bra, and what causes it?

In broad terms, flatness appears in a bra cup when a region of your fabric is under tension. The fabric wants to take the shortest route from point a to point b along this line of tension, so instead of rounding over your breast, creating a curve, it smooshes into it, creating a dent or flat spot.

The strap is the cause of the flat spots we're talking about today, as the strap pulls on the cup directly,  causing tension to be directed down into the cup. Usually, bra makers only really notice flat spots/strap tension issues in the cup when they present problematically, but in reality the strap tension always has some impact on the fit of the cups. When the strap tension is not properly managed, bra makers often find that there are dents down the side of the cup, which can be tricky to fix in some cases. Other times, a little upwards curve in a horizontal seam can be spotted below the strap.

Diagram showing a dent radiating down from the strap in a bra, and a horizontal seam being pulled upwards in the same area
Dents in the side cup and wavy seams are signs of poorly managed strap tension


2. How do you remove flatness?

I have bad news for you: flatness in a bra cup (that originates from the strap), is unavoidable at a conceptual level because the strap tension has to go somewhere*. However we can manage and minimise flatness to completely disguise it, or even use it to our benefit. 


*In a strapless bra, the neckline itself usually takes over as the flat spot, as the cup fabric is suspended from here.

Method 1: Reduce localised stretching of fabric

Flat spots occur when fabric stretches, so if we can stop the fabric stretching, we can reduce and sometimes avoid flatness! Problematic flat spots typically occur down the side of a cup, directly down from the strap, so here's what to do when you need a fix that doesn't involve changing the design of the cups.

1 - Use more rigid cup fabrics, including foam.

Less stretchy fabrics will stretch less, obviously, but foam is also less susceptible to forming these dents as it is more rigid in the third dimension, which helps to smooth out any potential dents.

2 - Increased strap width

Distributing the same force over a wider area will reduce the local tension in the fabric, which will reduce the dent.

3 - Reduced strap tension

It can sometimes be as simple as lengthening the strap that does it.  Strap overtightening is pretty common in the bra making world, due in part to continued issues with upper band tension in home sewn bras, and also in general due to people not really knowing what properly fitting bras are supposed to feel like. So you may find yourself tightening the straps when really you need to instead reduce the band size or adjust the fit of the band to increase upper band tension. Additionally, people often overtighten the straps when the cup is too large (which also can lead to extra space which exacerbates denting), so sizing down in volume may help here.

4 - Adjust the stretch direction in the outer cup

Sometimes all you need to do change how your pattern pieces are oriented on your fabric. Try making the region coming down from the strap out of a rigid direction on the fabric.

Limit stretch downwards from the strap to reduce denting

At a certain size, though, doing these things won't help you all the way, so you need to become a little more clever about how to manage it.

Sidenote: Upper Band Tension and Outer Cup Bulging

Sometimes the strap can actually be functioning correctly but there is still a dent in the side cup caused by a lack of tension in the outer cup. This is commonly due to a lack of upper band tension (more on this issue in the future), which allows the outer cup fabric to become loose and dent more easily when under tension. Consider if your bra needs more upper band tension (wires not sitting against the body, the lower band feeling tight, wrinkles in your band etc) before making other adjustments to the bra.

Method 2: Build the flatness into your design

There are two main ways you can run high strap tension through a bra without unwanted dents: across the neckline, or into a type of power bar (or some combination of the two).

Across the neckline

The straps in a full cup bra direct tension along the neckline and armpit
The strap tension is directed through the neckline and armpit instead of downwards

This works by splitting the tension in two - the inner neckline pulls the strap medially, the armpit region of the neckline pulls it laterally, balancing each other out. 

Using a taut neckline, you can bypass the tension flowing downwards into the cup. The issue is that, as the neckline is tight, the bra needs to enclose most of your tissue, otherwise it will cut in. 

Directing the strap tension entirely into the neckline can help get that slightly retro silhouette that features a rounded outer breast, if that is something you are looking for. If you would like to avoid this shape but still want the security of a tight neckline, use a combination of neckline tension and side support.

Into Side Support

Diagram showing how the power bar is pulled flat by the strap tensionDiagram showing how the power bar is pulled flat by the strap tension
The strap tension makes a power bar work, rather than causing unsightly dents

Most large cup ready-to-wear bras feature side support in some capacity and this is a big reason why.

Moving the strap to the side of the cup allows you to use the strap tension to pull the side tissue flat, without any denting. Different types of power bar have different levels of effectiveness, most successful is a dedicated power bar as the stitching joining it to the cup is very rigid and can stop any remaining stretch from the strap tension.

Diagram explaining different types of power bar

Simply moving the strap outwards can have this side-support, reduced denting effect too, without any dedicated power bar, if you don't wish to change the design of the cups too much.

This allows the tension to come off the neckline, and is why power bars are great for breasts with lots of upper fullness, as I wrote here in a previous post.

In the extended version of this post, I talk more about strap position (both height and width) and how this can have an unexpected impact on cup projection, with a particular eye towards designs with integrated power bars. Think patterns like the Harriet and Lusamine, or ready to wear bras like the Cleo Marcie. Read it now on my Ko-Fi!