Friday, February 19, 2016

Zip A Bag Chapter 15: Backpacks With Zippered Gussets



Today, we are deconstructing some ancient, miniscule backpacks from when my children were preschoolers. You can read my excessively-sentimental account of The Symbolism of Making Backpacks For Your Small Children here, but the essence of it is that my kids were starting preschool, and all the Target and Walmart backpacks were for hulking grade school kids (and had Teenage Ninja Mutant Turtles or Hello Kitty and/or flashing lights on them), so I made them each an Appropriately-Sized version.

It looks huge on Kate (age 3),

but here is Emily (age 5) with hers, and it already looks too small.

This post, therefore, is not a real tutorial. 

Mostly because I didn't want to make yet another backpack. Which means there are no in-progress photos. But also because the only part of the backpack that's relevant to Zip A Bag is the zippered gusset. And it's sufficiently similar to the other zippered gussets that it didn't deserve a tutorial of its own. So just a post-mortem today, with some very old brag shots and some drawings.

When I sewed these little preschooler backpacks for my girls, I made their gussets with off-set zippers i.e. the zippers were set into one of the gusset seams, sorta like couch cushions. 

The main reason was to utilize the full width of that gusset for storage (a centrally-located zipper would've divided the gusset opening into two shallower halves). 

But it was also to allow me to sew on a top handle, 

which a central zipper would've been in the way of.

You should totally feel free, however, to make backpacks with zippers that are installed in the middle of the gusset, like what we did with the utility case in that earlier Zip A Bag post. In fact, a symmetrical gusset is in some ways easier to sew. For comprehensiveness, we'll be dissecting both styles:

As we've done in the past with any bag, let's first analyze its structure to find familiar things and construction features.

A backpack is really an upright bag with two flat faces and a gusset between them to create thickness and three-dimensionality. It has straps sticking out of its back face and a zipper along some part of the gusset to access its contents.



Let's look at those two diagrams separately so we can talk about choices.

If you choose to install the zipper along one edge of the gusset, one side of the zipper tape is sewn to the gusset, and the other side to the front panel, where it has to be sewn along a curve. Some folks, particularly beginners who are more comfortable sewing zippers to straight openings, may not enjoy this. However, this is an elegant design because the zipper is concealed against a natural seam. Furthermore, if you want a handle, it can be easily topstitched to the top of the gusset.


If you install the zipper centrally, both sides of the zipper tape are sewn to the gusset, in straight seams. Beginners, especially, like installing zippers this way. However, should you want to attach a handle, it will need to be topstitched to the back panel, and might get in the way of the straps. Or it can be inserted into the seam joining the back panel to the gusset, but it will share the same insertion points as the back straps, and result in a very bulky seam, which beginners also dislike, particularly if it results in broken needles.

Bottomline: you get to pick what you like and weigh the pros and cons of your design choices. I mention this to assure you that backpacks, like any other bags, can be made in many, many ways, and no one design is superior to another, only preferable, okay?

Now, let's deconstruct the backpack design further - we'll take apart the off-set zipper design. The pink lines in subsequent diagrams are the zipper, or parts thereof.

In this first diagram, we're separating the gusset from the front and back panels.

Here is that gusset, all by its lonesome self. I've added letters to mark important points.

Now, let's imagine we can cut through that gusset and stretch it out in a long strip,

Let's do the same for the design with the central zipper:

Here, let's draw two parallels:

1 FABRIC STOPS
See those side pocket panels? They act like fabric stops to the zippered panel.

2 FULL GUSSET
You can add any number of external features to the gusset. For our backpacks, we added pockets to those pocket panels.

and a base:

Regardless of fancy features, when you reconnect that gusset into a closed loop once more, 

it is exactly like any other full zippered gusset. Like this
Chapter 14: Zippered-gusset utility pouch

and this.
Chapter 13: Collapsible Drum

Finally, we come to the most important part of this deconstruction: how to sew a fully-lined backpack without internally bound seams. In other words, we want the inside of the backpack to look like this: 

Specifically, we want the outer and lining layers to be sewn as two separate bags, connected at the zipper, with all the seam allowances sandwiched between those two layers and out of sight. The end result is the outside looking like this:

and the inside looking like this:

Q: Why don't we want to sew the two layers as a single composite layer and then bind the SA on the inside of the bag?

A: Because we don't like binding bulk. Backpacks, unlike pouches, have interfacing, and are often made of heavyweight fabrics themselves. For most of the seams in this bag, that extra bulk is a minor inconvenience. However, wherever the seams have insertions, you're going to feel the difference. 

Take for example, that stretch along the top of this backpack where the foam-padded straps are inserted into the seam. Which also has piping. Where the fabric is duckcloth and has a second layer of canvas interfacing. And that's just the outer layer; we haven't yet included the lining and its associated interfacing.

Trust me, you wouldn't enjoy binding all those layers in the seam allowance. 

Anyway, this two-layer hidden-SA method is easier than you think. Remember the "free seam allowance" technique we learned way back in this post?

We're going to do the same thing with our gusset. Remember that the two layers of the gusset (the outer fabric and the lining) are connected to each other only at the zipper. Along those two lines EF and GH, sew the stitches only up to the green SA region; do not extend the stitching line into the green region.

This leaves that green SA region free as two separate layers, so that they can be individually sewn to their respective layers.

Here are cutaway diagrams to show those two layers remaining separate in the outermost 1/2" (or whatever SA you use) edges of the gusset, for both configurations of the zipper.



May I anticipate one last question?

Q: Can you tell us how you made that front dome-shaped pocket?

A: Yes, I can, but I'm betting you already know how. Let's zoom in. That's the pocket, but doesn't it look like a mini version of the main backpack itself? 

It's made the same way except it doesn't have a back panel, just an opening at the back. So:
  1. Cut gusset pieces from outer fabric and duplicate those in lining fabric.
  2. Cut a dome-shaped panel from outer fabric and one from lining fabric.
  3. Install the zipper in the two-layered gusset, you did with the backpack's gusset, remembering to use the "free seam allowance technique" to keep the layers separate at the ends of the zipper.
  4. Separately sew of the gusset layers along one edge to the outer dome piece and the lining dome pieces, to get a fully lined pocket.
  5. Stitch the remaining edge of the gusset together (I just sewed the seam closed with RS together, left a gap and turned everything RS out through that gap) so there are no raw SA exposed.
  6. Topstitch that finished edge to the front panel of the main bag.


Voila! Backpack deconstructed.

Same old construction methods, just different shapes.

Now go forth and make your own!



3 comments:

  1. I remember looking at your post on those backpacks ages ago and thinking I would never be able to make them. Now they don't seem so impossible. You break things down into such clear, achievable steps.

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  2. Gracias por tanta claridad y generosidad en este post! Aunque no esté en mi idioma, y el traductor de Google hace que algunas cosas no tengan sentido, tus dibujos explican todo. Mi sincera admiración por tu costura tan profesional. Saludos desde Argentina

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  3. I'd love to know how you made the front pocket on the pink polka dotted backpack.

    ReplyDelete

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