The disc brake is a device for slowing or stopping the rotation of a wheel. A brake disc is connected to the wheel and/or the axle. To stop the wheel, friction material in the form of brake pads (mounted on a device called a brake caliper) is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop. Some newer mopeds come equipped with disc brakes, and older mopeds can be modified to use disc brakes.
Converting your moped to use a front disc brake
If you have the money, the easiest way to do this is purchase an EBR front fork w/ disc brake kit and Grimeca spoked wheel with disc from 1977 Mopeds, which provides a disc brake setup nearly identical to that used on the Tomos Streetmate and Revival TS (and similar to that used on the Arrow and MC50 Senior).
It is also possible to buy other small disc brake setups intended for scooters and small motorcycles, but procedures for adapting them for use on a moped vary.
One example of Disc Brake Conversion is this post, by Ed from the Dead Possums.
Puch Snowflake Disc Brake
"Start with GM 12557819 It is an idler pulley off some car. Center it up with the rotor, drill holes. Cut out the center to match rotor. Bolt together. Insert in hub. It will not just insert, you need to pound it in. It's a nice tight fit. I got it all the way in until it bottomed out.
Now put the wheel back on the forks. You will need to spin it to make sure it spins true. Theoretically, if the rotor assembly bottoms out on the inner hub, it should be straight. Make any adjustments needed before moving on.
Once the rotor is spinning true, drill and tap 4 holes in the side of the center hub to keep the rotor assembly from moving. Red loctite.
Fit the caliper on the disc to where you want it to sit. Pull the brake lever to lock the pads in place. Rubber band out tape the lever closed. You can then make a steel bracket to weld to the forks. Line everything up tack in place. Check to see wheel spins and brake works. If all is good, weld that shit like a boss."
Alternate Pulleys and Their Outer Diameters:
Mercedes Benz Pulley 1042001070 - 80mm O.D. GM Pulley 12557819 - 90mm O.D. Good Year Pulley 49026 - 100mm O.D.