August 3, 2025 · Richard Palmer

Why Carbon Repairs Wrinkle (and How External-Pressure Molding Avoids It)

If you've ever seen a carbon repair with visible wrinkling in the laminate — a rippled, uneven texture under the clearcoat, sometimes with a slightly different sheen than the rest of the frame — you've seen the signature of vacuum-bag lamination applied to a repair it isn't well suited for. It's not a coincidence and it's not bad luck. It's a predictable result of how that method applies pressure, and it's the main reason we don't use it for structural work.

How vacuum bagging works

Vacuum bagging cures a laminate by sealing it under a single sheet of plastic film and pulling a vacuum, so atmospheric pressure — about 14.7 psi at sea level — pushes the film down onto the wet layup. It's a legitimate, widely used technique in composite manufacturing, especially for large, relatively flat panels where a single sheet of film can lie down smoothly. The problem is bicycle tubes aren't flat panels. They're small-diameter, curved, often tapered shapes, frequently with internal features like cable ports nearby.

Where the wrinkles come from

When a single flat sheet of vacuum bag film is pulled down over a curved tube, the film can't conform smoothly the way it would on a flat panel — it has to fold and bunch to follow the curve, especially around any complex geometry near the repair. Those folds transfer directly into the wet laminate underneath as wrinkles. Air can also get trapped in the low spots between folds, since a single vacuum port doesn't always pull a full, even vacuum across every square inch of a curved surface, especially near tight radii.

The result isn't only cosmetic. A wrinkled fiber isn't a straight fiber, and a straight fiber is what carries load efficiently in a composite laminate. Wrinkles create local stress concentrations — points where load doesn't transfer smoothly through the fiber and instead has to work around a kink. Under repeated flex cycling, exactly the kind of loading a bike frame sees every ride, those concentration points are where a repair is most likely to eventually crack again.

How external-pressure molding is different

Instead of a single sheet of film pulled down by vacuum, we cure repairs inside a silicone mold built to the actual shape of the tube being repaired. The mold applies even, positive pressure to the entire repair area from all sides at once — not by pulling a film taut over a curve, but by matching the curve directly. There's no folding, no bunching, and no low spots for air to hide in.

Fiber stays straight and properly oriented through the cure, instead of kinking around film folds.
Pressure is applied evenly from all sides, not from one direction pulled down by atmospheric pressure alone.
Resin consolidates uniformly, without the trapped air pockets that vacuum bagging can leave in tight curves.
The cured surface comes out smooth, which also means less finishing work before a refinish goes on.

This is slower and more labor-intensive than vacuum bagging a patch over a crack — a fitted silicone mold takes real setup — but it's the difference between a repair that's cosmetically fine today and one that's structurally sound for years of riding. It's also why we're comfortable guaranteeing every structural repair we do.

A wrinkle you can see under the clearcoat is a stress riser you can't — and it's exactly where flex-fatigue cracks like to start.

Tagged: carbon basics, process