Teaching an Old Dog New Tricks: Seam Welding VX21

VX21 – or Dimension Polyant X-PAC VX21, to give it its full name – is a popular bikepacking bag material choice. It’s a lightweight, high strength, waterproof fabric with low stretch and good durability, and it’s easy to repair. But one area where VX21 has fallen behind other fabrics for bikepacking is in the fact it has to be stitched (rather than seam welded), which means the seams can allow water to enter under sustained rain. That is, until now…

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Jenny Tough riding her mountain bike with Backcountry Series packs in the backcountry


As part of the process of updating our Backcountry Series, we knew that one of the key criteria for the new packs was simply that they had to be as waterproof as our other series are. Not content with the already significant challenge of designing new packs to be entirely waterproof, we wanted to keep the VX21 fabric the Backcountry Series is known for. The new packs build on the rich legacy of the Backcountry Series, so maintaining that visual link and tying the new packs back to the origins of bikepacking, whilst also pushing the genre forward in a way that’s new and exciting, was important.

VX21 also has some exceptional properties that we wanted to make use of – it’s soft, lightweight and easily rolled, making it perfect for compressible areas. That said, we wanted to bring in another, stiffer material too, for increased abrasion resistance and additional strength – the 420D fabric we use in reinforced areas of the Expedition Series (such as the base of saddle packs) combined with VX21 creates a really nice mix of properties for mountain biking.

A close up view of the mix of fabrics on a Backcountry Saddle Pack for mountain bikes and dropper posts

Having decided on our materials the journey to waterproofing began with seam taping, which felt like the obvious solution. However, seam taping comes with significant challenges; VX21 has a DWR (Durable Water Repellent) coating which can interfere with the ability of seam tape to adhere to the fabric. We tested five different seam tapes but struggled to find anything that would reliably stick to the slippery seams.

There are a lot of variables that go into seam taping, from which side of the fabric is showing at the seam through to different types of tape and activation temperatures. We’ve successfully seam taped the Backcountry Accessory Pocket where it only has to adhere to the reverse side of the VX21, but we were unable to find a reliable seal across every seam on the bags in the series. ‘Creative pattern cutting’ can overcome this to some extent and is a significant part of our design process, but decreasing pattern complexity while creating advanced bags with a mix of materials is a significant hurdle to overcome.

One of the greatest challenges is simply heat management. With the inclusion of the Expedition 420D fabric, part of the bag design was seam welded, with the VX21 then stitched on. Attempting to add seam tape resulted in the heat used to apply the tape reactivating the seam welds and weakening them. It’s a challenge we’re used to facing and where our skill in simplifying patterns comes into its own, but with so much complexity in the new packs, the point where all the fabric meets would simply be too weak to meet our high standards.

Jenny Tough's Shand Bahookie with a full complement of Backcountry bikepacking bags

We are fortunate to work with suppliers who are experts in welding and heat management, and are able to experiment, test and work with us to bring solutions and ideas that complement our inhouse expertise and it was during a factory visit that the solution became apparent. An experiment resulting in a weld with suboptimal pull strength got us thinking and a few experiments later, with modified approaches and bonding materials, we had a weld with much better pull strength.

This was uncharted territory for us and the factory – to our knowledge, no one has ever successfully welded VX21. The result was visually fantastic, but we had to be certain that the welds were up to the quality we demand of our gear. There’s no point innovating for the sake of innovating and accepting a result that is worse than the existing approach!

The first stage of testing, in the factory, is pull strength. Two seams of fabric are welded and a machine pulls the fabrics apart, measuring the exact point at which the weld fails. We also put the welded fabrics in an ageing machine – essentially an oven – which replicates riding in hot conditions for an extended period of time to see how lasting the welds are.

Outside of the factory, we do our own tests, both in our sample room and in the real world. We created lots of swatches of fabric welded together and put them in tough conditions. Swatches were submerged underwater for extended periods and even joined our founders on a family holiday in Hawaii. Pre-production packs were sent on trips to Africa and Canada, tested by our ambassadors in temperature extremes and tough environments to back up our findings and confirm that the welds would live up to the challenges of bikepacking and backcountry riding.

A Surly Fatbike with a Backcountry Full Frame Pack being testing in the Canadian Rockies in freezing conditions

It’s been a long road, but the results speak for themselves; the Backcountry Series is as waterproof as the rest of our product range and durable enough for technical riding in challenging terrain – and we’ve even managed to retain the legacy look and feel of the range. We were the first to bring seam welded bags to the bikepacking market with our Expedition Series and are proud to once again be the first to find a way to weld VX21, continuing our long tradition of innovation and moving bikepacking forward with every iteration.