Review on Adjustable MTB Bike Disc Brake Bracket Converter Frame Adapter Mounting Holder - Star-Art 20/31.8mm by Chad Young

All in the name of MTB. 31.8mm is probably a common mountain bike tube diameter, front and rear. However, keep in mind that if you have fenders/struts to upgrade to disc brakes, you will need to remove something. In addition, when braking, the caliper is pressed into the support tube. For the front, this is the fork tube. Any bending of the adapter while braking can cause the disc to twist. Not my idea of good design. A while ago Phil Wood made a disc brake that fits both front and rear without an adapter. I wish Phil would update their design as they are much easier to work with than any modern disc brakes.