I found this interesting item from Elevn BMX Racing. It is a brake adaptor that can reposition cantilever brake bosses on a BMX frame. It is necessary that the existing bosses be the type that thread into the frame and are removable, a feature more common among aluminium and composite frames/forks than steel. After all, this is a product designed for the BMX racing demographic, which hasn’t been the realm of steel design in decades. However, the adaptor’s potential for 700C touring and cyclocross bikes is what caught my imagination.
The Elevn brake adaptor was designed to allow BMX frames built for the 20 x 1-1/8” wheel standard (ISO 451) to accept the 20 x 1.5” (ISO 406) standard. These two wheel sizes, though both often referred to as “twenty-inch” are actually different enough that neither tubes nor tyres are interchangeable. The ISO 451 usually takes a narrow, knobby tyre for the youngest age-group racers who will eventually grow into frames that take ISO 406, the same rim size that adults use. The Elevn adaptor allows the cantilever brake arms on a 451 frame to be repositioned to work with a 406 rim, thereby stretching the useful life of an existing frame for a growing child.
What do 10yr olds and BMX bikes have to do with cyclocross and touring bikes? Nothing, but the ability to easily reposition a cantilever brake boss is normally an insurmountable obstacle to changing the wheel size on those bikes. If the bikes have caliper brakes, there are a various reach brake calipers that would allow a mechanic to convert a 27” (ISO 630) to 700C (ISO 622), or perhaps 700C to 650B (ISO 584). With the Elevn adaptor, one can shift the brake arms towards the dropout from a position that works with a 451 rim to a 406 rim, a difference in radii of 22.5mm. The radius of a 650B rim is 19mm smaller than a 700C rim’s. Since most cantilever brakes have at least 15mm of pad adjustment, this would most likely allow one to convert a 700C frame/fork with cantilever brakes to fit a nice fat 650B wheel/tyre, assuming that the brake bosses are removable.
It is necessary that the brake bosses be removable, since if the original boss where still in place it would be in the way of the repositioned cantilever arm. Also, the Elevn adaptor cannot be oriented differently to fit a larger diameter rim because it relies on the spring anchor holes of the original boss to maintain position.
On one hand, this isn’t really relevant to most of us, but it’d be fun to experiment. I wonder if an older Redline Conquest frameset would work. Before the more recent versions, the older aluminium bikes had sorta high-ish bottom brackets and decent tyre clearance. That’d make an interesting setup.