Back in February, we heard about a prototype Schwalbe mountain bike tire system that used a dual-chamber setup to both increase traction and minimize flats. The design has since been refined, and the commercial version of the product was officially unveiled last month. It’s called Procore, and we had a chance to see how it works at Interbike 2014.
First of all, with regular mountain bike tires, there’s a bit of a compromise.
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If the air pressure is too high, the tire doesn’t have as wide or compliant of a gripping footprint on the ground, plus it bounces wildly off of things like roots and rocks. If the pressure is too low, however, pinch flats can occur when riding over those same obstacles, and tubeless tires can even get "burped" right off the rim. As a result, most cyclists keep their tires at a somewhere-in-between pressure that isn’t ideal for either grip or protection.
Procore addresses this situation. Essentially, it allows cyclists to install a high-pressure inner tube (the blue thing in the photo), inside their existing tubeless tire. That tube doesn't take up all of the space inside, however – there's still an air space between it and the inside wall of the tire. For simplicity, Schwalbe refers to the tube as the inner chamber, and the space as the outer chamber.
The tube can be inflated up to 87 psi (6 bar), allowing it to serve as a firm pinch-flat-protection cushion, and to provide enough pressure to keep the tire seated securely on the rim. This means that the outer chamber – which is inflated separately – can be run as low as 12 psi (0.8 bar). With conventional tires, most riders don’t go much below 20 psi (1.4 bar).
As an added benefit, if the outer chamber is punctured and goes flat, the air in the tube will still allow the user to ride home. Flats in the outer chamber shouldn't be too common, though, as the Procore kit includes sealant that will plug smaller punctures.
On the prototype, two valve stems were required – one for each chamber. This presumably meant that users would either have to get a second stem hole drilled in their existing rims, or buy wheels with Procore-specific rims.
On the commercial version, however, both chambers can be inflated through one adjustable "dual valve" Presta stem developed specifically for the system. When the stem's selector is threaded right down, air passes through it and into the tube only. By extending the selector, however, the air only goes into the outer chamber. The process by which it works is pretty difficult to describe in words, but you can see it illustrated at the beginning of the video following this article.
The Schwalbe rep we spoke to told us that the Procore system should be available starting next year, in all three mountain bike wheel sizes. A kit containing two tubes, the sealant, and all the necessary mounting accessories will be priced at €179 (about US$230).
Procore adds a claimed 200 grams to each wheel, and can be used with all tubeless-compatible rims.