Disk brakes have been one of the major innovations in the bike biz recently. Firstly introduced on mountain bikes, they are now available on every sort of bike (from road-racers to commuters). Unfortunately, as for any other bike components, the mounting standards are skyrocketing: 51mm International Standard, 74mm Post Mount, 70mm Flat Mount! Of these three different versions the easiest to retrofit on an existing bike frame is the International Standard.
In this standard the disc caliper is attached to the frame/fork with two bolts that are 51mm apart (center to center). For the rear caliper, the two mounting eyelets are positioned on two circumferences with radius 78.1mm and 39.9mm. The circumferences are centered on the wheel axel. For the front caliper the eyelets are instead positioned on two circumferences of radius 49.7 mm e 87.3 mm.
Step 1: Make the Brake Support
First of all the brake support must be realized. Choose a thick (at least 5mm) metal plate and shape it to fit your frame/fork geometry. The shape of the support can range from a simple rectangle to a more complex laser-cut shape. Once you have the support ready drill the two 6mm holes, 51mm apart.
Step 2: Weld!
Now it comes the welding! The most difficult part here is to hold the support in the right position, considering also all the deformations due to the heating of the part during welding. The punk solution is to use the brake itself: screw the support to the brake, mount the wheel, place the brake on the disc rotor and while braking to keep the brake stable make some small welding spot to keep the support in position. Then dismount everything and finish the welding. Unfortunately this really simple solution has many drawbacks..the bigger one being the alignment precision. You will in fact almost certainly end up with that bloody "zin-zin" noise between the rotor and the brake pad! If you want to reach the ultimate accuracy you will need something like this. The jig keeps the brake support in position and guarantees the alignment between the brake pad and the disc rotor. Make also sure your jig is strong enough, so to prevent movement of the brake support during the welding (heating/cooling).