This year Carbon Drive/Gates is the major sponsor of the Nahbs. I'm glad to see them step up and take a risk laying down the dollars to generate more exposure for their product, as the small handbuilt industry has been a progressive force in getting these systems in the media, on the show floors, and out on the trail.
I've had the opportunity to do a number of belt drive bikes, both single speed and IGH, and have been quite pleased with the results. What frustrates the dickins out of me is the bad wrap the product gets on many of the forums. I've found this to be mostly because of uneducated users or new builders who have not designed the bike for the system, instead attempting to retrofit an unacceptable frame or utilize standard frame stays/tubing for the build. Their frustration, directed at the belt drive parts, should be refocused on understanding mechanical forces and how to design to control them.
Minor rant aside, one of the few choices a builder has in the system design is to determine the best application for passing the belt through the frame. There have been many approaches, all successful in achieving the goal of breaking the frame so the belt may pass through; the split dropout, the lap joint, the coupler, machined male cone and receiver, bolt on stay/dropout interface, and the gate joint. I've used a few, some to experiment, some by customer request, but the best system that I've found is the removable gate.
I feel that the gate is the most efficient system as it allows the user to pass the belt without spreading or stressing the frame members, resists torsional forces through the rear end, adds strength to the stay, is easy to use, and visually blends with the lines of the frame.
So lets take a look at how you make one of these little guys...
We are going to start with some solid round stock that sits just above the od of the largest section of the seat stay. This stock will make it's way into the lathe and be turned down to match the stays od. After we have the od where we want it, it's off to the mill. I laid this gate out to utilize .375 overlaps, sufficient surface area to accommodate a proportional overlap as well as room for the connecting bolt. Here's our .600" stock milled .300" deep and wide enough to allow our .375 overlap plus the thickness of our slitting saw, used in the next step.
Now that our stock is milled, we'll reposition on the mill, center a slitting saw and part the piece.
Parted, it's back to the lathe to face the cut end to exactly .375" from the inside face.
Next, we'll take another section of .600" lathe turned round stock and mill it to create our .300" deep, .375" long overlaps of the center gate. Notice the left milled section is over sized to allow for the slitting saw and lathe facing.
We'll then begin to place the female bolt holes in. The mill is re centered on the piece using an edge finder, centered and then brought in .1875" (center of our overlapping sections). The hole is created with a center drill, then a first pass drill bit, a drill bit sized for our M5 threads, and then machine tapped.
And with the piece removed...
With the center section removed, it's time to measure the id, turn the plugs down in the lathe and fit it all together. Precision is important...