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Old 12-29-18, 10:04 PM
Andrew R Stewart 
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First if the courts come calling I didn't say you should do this...

There's no real difference between a factory produced hole and a consumer one if both are aware of basic fabrication issues. For both the fork and the rear it's about placement WRT the amount of material around the hole to still deal with the stresses.

The rear is usually pretty straight forward. Does the frame have a brake bridge between the seat stays? If so and if it's distance from the rim allows an available caliper to reach then drilling a hole in the bridge's middle is the route. Check the caliper's mounting bolt diameter for the proper final dhole's size. It should be at least half, or less, of the bridge's diameter for safe strength of the remaining bridge material. One will want to deburr the hole best possible and dab some paint of the raw metal for corrosion protection. One problem with mounting a caliper to this cross tube (the bridge) is keeping the bridge from crushing with the tightening of the mounting bolt/nut. There are special washers that have a curved and a flat face to cup the tube better with, sometimes these come with the brake caliper.

For the fork the idea is the same but the fork does see much higher stresses so more thought is needed. Generally on a BMX bike the fork is a unicrown one with about a bottom to crown race seat, of the steer/blade welded area, of around 1.25" or more. You want the brake bolt hole below the weld bead best possible but still about 3/8 to 1/2" above the very bottom of the steerer. If your fork is different is design of dimension you better give us photos and numbers... Again checking that the hole's placement will allow an available caliper to reach the rim. Again deburring the hole and dabbing paint of raw metal.

When drilling either hole it's best to create a dimple in the surface with what's called a center punch (look it up). This will help keep the drill bit from "walking" away from the point you want the hole to be at. Start with a small bit, like a 1/8". Then work up to the final diameter in a couple of steps. If the initial small hole is slightly off you can drift the next step drilling over with a bit of care. For the rear you can really only drill from one side so keeping the drill perpendicular to the stays, or at a tangent to the rim, is important to keep the caliper well placed. For the fork you can drill the small pilot hole on each side first then drill through both holes from the front after.

Before you do anything that's not reversible do your homework and measure a few times. You might want to look at bikes with caliper brakes already installed to get the big picture first. Good luck. Andy
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