Master Resource Repository

Disc Brake Mount Tool

Buy the Disc Brake Tool

Green 2mm and 3mm Shims:

  • In most cases the shims are not used at all
  • For front 12mm Thru Axle applications you should use the 2mm shim
    • We use a shim here because these front hubs have a small diameter projecting into a pocket on the dropout. Without the shim the tool body will sit too far inboard.
  • For front 15mm Thru Axle applications you should use the 3mm shim
    • We use a shim here because these front hubs have a small diameter projecting into a pocket on the dropout. Without the shim the tool body will sit too far inboard.

Dumy Axle Collars:

  • The vast majority of dummy axles are either .75” or 1” outside diameter. We offer the DA Collar in both sizes. If you find cases where you need a custom size DA Collar, reach out to inquire about a custom offering.
  • .75” dummy axles work for most applications, but commonly for thru axle applications you might want to use a 1” dummy axle that includes larger diameter clocking features to keep the dropouts phased to each other.

Post Mount Bar:

  • The Post Mount Bar locates Post Mount Bosses with M6 threads. We include M6 x 20 socket head cap screws (2pcs) with the tool which will work in most cases. If you lose them or need custom length you can get more from any hardware store.
  • The bar is CNC engraved for use on Post Mount Front and Post Mount Rear. In either case you want to put the M6 screws through the holes closer to the text and you should be able to read the text from the side of the bar with the heads of the screws. I believe this is the more intuitive way to interpret things.
  • SRAM, Shimano, and others have similar but slightly different specs for the arrangement of these post mount bosses. This tool is designed around a linear 60º translation of the points. This averages the points in space such that each location is 10mm from the next along a 60º path. Rotors are offered in 20mm (diameter) increments which corresponds to the 10mm (radius) increments. This system is based on work done and shared by Peter VerDone.
  • Shimano’s 203mm rotor size is the biggest potential conflict. If you want to purpose-build a bike for a 203mm rotor with post mount it’s possible that this tool (as used in the 200mm position) will create issues. You can build the frame with an integral 180mm mount location and use a step-up adapter.
  • We might offer an alternate version of the “tool body” component with the true Shimano Post Mount locations. Please contact us if you’re interested in that.

ISO Front:

  • ALWAYS mount ISO adapter arms to the 200mm position on the Cobra Disc Brake Mount tool. 200mm doesn’t relate to the size of rotor you want to use — it’s just the position that the ISO arm was designed around.
  • In framebuilding, ISO Mounts are placed in the same location irrespective of the rotor size to be used. It’s the choice of ISO adapter (or combination of adapters) which puts your brake caliper in the correct position for the rotor that you will use.

ISO Rear:

  • ALWAYS mount ISO adapter arms to the 200mm position on the Cobra Disc Brake Mount tool. 200mm doesn’t relate to the size of rotor you want to use — it’s just the position that the ISO arm was designed around.
  • In framebuilding, ISO Mounts are placed in the same location irrespective of the rotor size to be used. It’s the choice of ISO adapter (or combination of adapters) which puts your brake caliper in the correct position for the rotor that you will use.

Flat Mount Front:

  • Generally this adapter will always be used in the 140mm position on the Cobra Disc Brake Mount Tool Body.
  • Flat Mount Front generally uses the same mount location for 140mm and 160mm rotors, and it’s the use of an adapter seated under the brake caliper which adapts the position of the caliper between the two positions.
  • If you’re intrepid and think you know better, this tool does allow one to translate the location out radially in 10mm (radius) increments. This is not advised typically, but technically possible and maybe useful in some cases such as cargo bikes etc.
  • As a reminder, you might need to use 2mm or 3mm green shims if you’re using 12mm or 15mm front thru axle. Quick Release front hubs will not require the use of these shims.
  • We include M5 x 22mm (2pcs) screws for mounting bosses to this adapter. If you lose them or require longer or shorter ones for your application you can pick some up from any hardware store.

Flat Mount Rear:

  • Generally this adapter will be used only at the 140mm ad 160mm locations.
  • Flat Mount 180-220mm aren’t generally accepted standards and if you use this tool to move the mount to those locations you might find it hard to connect the bosses to your chain stay with any elegance. It’s generally not advised, but if you think you know what you’re doing you can use this tool to create rogue FM positions.
  • The Cobra Disc Brake Mount Tool holds the FM Rear Bosses at a 28º translation from the 60º location system used for Post Mount, which is the basis of this tool.
  • We include M5 x 22mm (2pcs) screws for mounting bosses to this adapter. If you lose them or require longer or shorter ones for your application you can pick some up from any hardware store. Because Flat Mount Bosses come in 5mm length increments, you might find that you need to source different length screws. The provided 22mm screws work well for bosses with 15mm from screw head seat to caliper mounting surface.
  • The projected slot shape we machine into the Flat Mount Rear adapter is that of a 5.1mm slot with 2mm between centers. The Shimano spec for the bosses is a 5.2mm slot with 2mm between centers. If your bosses have a loose fit on the tool, seat them to the same side while tightening. The decision to make the projections with these proportions was to ensure the greatest compatibility across manufacturers with the least slop. We will likely offer our own bosses where we can ensure manufacturing tolerance and proper fit to this tool is optimal.