Why a Sewn Bike Bag Is Not Waterproof and a Welded One Is

Almost every bicycle bag on the market is described as waterproof. Most of them are sewn. Those two statements cannot both be true, and the difference is visible if you know what to look at.

The problem with a sewn seam

A sewing machine makes a hole every few millimetres and pulls thread through it. Coating the fabric does nothing about the holes. A sewn bag in heavy rain leaks along the seam lines first, which is why water appears in the corners rather than through the middle of a panel.

What high frequency welding does

High frequency welding uses a radio frequency field to heat the TPU layer on two panels until it fuses, then presses them together. The result is a continuous bond with no thread and no needle holes. Our Bicycle Waterproof Tail Bag, Waterproof Frame Cross Bag and Waterproof Triangle Bag are all built this way, and the tail bag adds a hemmed seal at the opening.

Denier is the other half of the answer

Denier is the weight of the yarn. In our welded range you will see 420D, 600D and 840D, and the Waterproof Triangle Bag uses all three in one bag: 600D on the outside for abrasion, 420D on the inside where nothing rubs, and 840D on the bottom where road spray and the ground do the damage. The tail bag uses 840D throughout, because a bag under the saddle sits directly in the rear wheel spray.

Zips leak too

A welded seam is wasted if the closure is an ordinary zip. The welded bags use waterproof one-way zips, and the sewn PU and EVA bags use waterproof two-way zips. Either way, the zip has to be closed fully to the end stop. A zip left 10 mm short is an open hole.

When sewn is the right choice

Sewn construction is not a failure, it is a different trade. Our sewn PU and EVA bags carry things a welded bag cannot: the Bicycle Phone Holder Bag has a 360 degree rotating base, a headphone port and three card pockets plus a wallet and passport pocket. You cannot weld those features. If you need document storage and a rotating mount, take the sewn bag and accept that it is weather resistant.

How to check a supplier's claim

Ask two questions. Are the seams welded or sewn, and is the zip a waterproof zip. A supplier who cannot answer both is describing the fabric, not the bag.