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Why Hoodie Side Seams Twist After Washing

Why hoodie side seams twist after washing: yarn torque, knit density and finishing, with what the textile research shows and what it doesn't.

Side seams that started straight and now run forward toward the belly. A cuff that sits slightly rotated. A hem that looks like it was cut on a slant. The garment isn't badly sewn. It's doing something knitted cotton has always done, and it has a name. Several, in fact. Cotton Incorporated's technical bulletin on knit torque calls it skew, and notes that torque, spirality, bias and shear distortion are all used for the same thing.

What's actually happening

Yarn is spun with twist. Fibres get wrapped around each other in one direction, and that twist is what holds the yarn together. It is also stored energy. The yarn spends its whole life trying to untwist, and the trade has a word for how hard it tries: twist liveliness. The Cotton Incorporated bulletin found that the more twist a yarn carries, the livelier it is and the more the fabric skews.

Loopback terry and brushed fleece are knitted on a circular machine, in a tube. Every loop in that tube carries a share of the yarn's twist. The machine adds its own lean too. Each feeder lays down a course per revolution, so the fabric is built as a spiral, and the bulletin says machines with a lot of feeders produce substantial course skew straight off the needles. Depending on the direction of the yarn twist and the direction the cylinder turns, those two effects either stack or partly cancel.

Straight off the machine, under tension, it looks flat and square. Then it relaxes. The bulletin is plain about the sequence: some relaxation happens when the roll is first unwound, more happens in wet processing, and drying without tension brings out the most. A home wash and a tumble dry are exactly that kind of wet, untensioned relaxation, which is why the fabric finally does what it always wanted to do.

The seam doesn't move. The fabric moves underneath it.

One caveat. Most of the published research, including the bulletin and a 2009 Textile Research Journal paper by Murrells, Tao, Xu and Cheng, measures single jersey, the plain knit of a T-shirt. The face of loopback and fleece is a jersey knit and the mechanism is the same, but we haven't found comparable published figures for heavy fleece itself. The numbers below are for jersey.

Why it shows on some hoodies and not others

The Murrells paper modelled spirality from yarn residual torque, yarn type, yarn count, fabric tightness, number of feeders, machine rotation, machine gauge and dyeing method. Most of those come down to a few things you can see in a finished garment.

  • Yarn twist. In the Cotton Incorporated trials, 18/1 ring-spun cotton knitted into jersey went from 10.5% skew at a twist multiple of 3.0 to 18.5% at 4.0, measured after five wash and dry cycles. Open-end yarn of the same counts skewed less in that trial, 3.5% to 8.7%.
  • Ply. Twisting two yarns together against each other can cancel the torque, but only when the plying twist is balanced. In the bulletin's air-jet trial, two Z-twist ends plied at 2.5 turns per inch still skewed 17%. At 12.5 turns per inch they measured zero. At 14.5 the fabric skewed the other way. So "two-ply" on a spec sheet is a good sign, not a guarantee.
  • Stitch density. Tighter knitting skews less. On the same machine and yarn, the bulletin recorded 8.6% skew at the tightest course length tested and 16.8% at the loosest.
  • The knitting machine. Feeding alternate courses of S-twist and Z-twist yarn cut skew to 1.4% in one trial, against 15.3% with Z-twist alone on the same machine. That's a mill decision, and you can't see it on the tag.
  • Finishing. This is where the received wisdom needs correcting. In the bulletin's finishing trial, compacted jersey that was left to skew before finishing measured 3.5% after five washes, and resin finished jersey 2.0%. Fabric that was pulled straight during finishing did worse, 8.1% and 6.0%. Forcing the fabric square at the mill doesn't remove the twist. It stores it for later. Heat setting, which is often mentioned in the same breath, is a process for synthetic fibres and doesn't feature in the cotton data.
  • Heat. The common line is that a hot wash brings twist out faster. We couldn't find a test showing that hot water produces more total skew than cool. What the sources document is that wetting and tension-free drying release it. Heat does matter for shrinkage, which is a separate problem covered in washing without shrinking.
  • The cut. A tubular body has no side seam, but it still twists. The bulletin describes the wales spiralling around the tube. There's just no seam line to make it obvious. Open-width fabric is slit along a gap left by missing needles and finished flat, which straightens the wales, and the skew shows up instead in the courses, and so at the side seams. Real side seams also let a garment be shaped properly through the body, which is a fair trade.

Can it be fixed

Not really, and it's better to say so. Once the fabric has relaxed, the twist is in the knit. The bulletin's dyed fabric that had been squared up during drying skewed as much after laundering as fabric that hadn't been touched. Squaring a garment by hand at home is the same idea on a smaller scale. It makes that dry look right. It doesn't stop the next wash relaxing it again.

What a careful routine does is keep the garment from picking up extra distortion that has nothing to do with twist. These steps are practical sense rather than tested methods.

  1. Wash cold or at 30, gentle cycle, inside out. That protects the pile and the print, and it keeps shrinkage down.
  2. Turn the spin down. A hard spin leaves heavy fleece creased into whatever shape the drum left it in.
  3. While it's damp, lay it flat and square it up. Line one side seam against the edge of the table, pull the hem straight, smooth the sleeves out from the shoulder. Cosmetic, but worth doing.
  4. Dry it flat. Wet 400gsm fleece is heavy enough to stretch itself on a hanger, and the shoulders take the worst of it. Drying flat and free of tension also shows you the garment's true skew instead of hiding it.
  5. Expect most of it early. AATCC TM179 measures skew after two laundering cycles, the bulletin used five and the Murrells paper three. We haven't seen published data on what happens over fifty washes, so we won't guess.

Leave the iron out of it. Heat and pressure flatten the pile on brushed fleece and leave a shine that doesn't brush back out. The rest of the routine is in the care notes.

How to check a hoodie for it

The lab version is AATCC TM179, the Test Method for Skew Change in Fabrics After Home Laundering. Marks are applied before washing, their alignment is measured after two standard laundering cycles, and the result is reported as a percentage with a direction. It's a test method, not a pass mark. The acceptable limit is whatever the buyer and the mill agree.

The home version is simpler. Lay it flat on a table, front up, sleeves straight out, hem square to the edge. Sight down the side seam from the armhole to the hem. A slight lean is normal for knitted cotton. A seam that has wandered well round to the front is a fabric with a lot of twist in it, and it won't wander back.

Weight on its own doesn't settle it. A heavy fabric knitted loosely from thick, lively yarn can twist more than a lighter one knitted tight from balanced yarn. Twist, ply, stitch density and finishing are the variables the research points at, and none of them is on the swing tag. That's the same argument as in what actually counts as a heavyweight hoodie. The number is a start, not the answer.

Twist isn't a defect in the way a dropped stitch is. It's the fabric telling on how it was spun, knitted and finished. Wash it cool, dry it flat, square it up while it's damp, and you'll at least be looking at the fabric's honest shape.

Sources

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