A sports bra moves with the body in repeated, high-friction motion. Chafing usually comes from three things: a seam or edge that is thick, hard or in the wrong place; a band or elastic that is too narrow or too stiff; and a fit that lets the garment move against the skin. Fabric test results can be fine while the construction still rubs.

Seam type and position

Flatlock and bonded seams are flatter than a standard overlock seam, but they still need to be placed away from pressure points. A seam that runs along the underarm, where the arm moves, rubs more than one set slightly inward or to the back. Compare seam positions on the pattern with where the marks appear on returns.

Edge finish

Armhole and band edges can be finished with a folded hem, binding, elastic or a bonded edge. Raw or stiff edges rub. Check that elastic is soft and faces away from the skin, that cut ends are covered and that any bonding tape is flexible. Run a finger around the inside edge and feel for ridges.

Elastic and band

A narrow band concentrates pressure, and one that is too loose lets the bra ride and rub. Check the band width and the elastic’s stretch and recovery on finished garments before and after washing, with a method such as ASTM D2594 or the one your lab suggests, and wash with a procedure such as ISO 6330.

Fit across sizes

A fit that works on the sample size may not on the smallest and largest. Wear-test each size group for a workout, ideally running or jumping, and look for marks afterwards. Ask wearers where they felt rubbing, and compare with the pattern.

Fabric surface

Pilling or fabric roughness after washing can add to rubbing. Check pilling with ASTM D4970 on washed fabric.

What to specify

Write seam types and positions, edge finish, elastic specification and a wear-trial requirement for each size group, and keep an approved sample. At inspection, turn the garment inside out and check that no cut thread ends, bonding edges or elastic ends are exposed. If marks still appear on returns, photograph them and map them onto the pattern, because the location usually points straight to the seam or edge responsible.