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Brake rotor selection for motorcycles is an interesting topic due to all the differing opinions, designs, and options. Overall, the OE rotor design is sufficient for OD and heat capacity under normal conditions. I have spent some time in the automotive field with rotor design and development with heat capacity, cooling, and fade testing. This article will try to take a common sense approach to rotor design, function, maintenance, and aftermarket choices based on what I have learned over the years and applying it to two wheels.

Normally for automotive, there are really two factors in OE rotor design – Effective Radius (rotor OD), Heat Capacity (rotor thickness), and Cooling (vent design). For motorcycles, the Effective Radius is the most important since modern motorcycle rotors are neither vented nor much different in thickness (4-6mm). In substitute of those, I’ll talk about the fastening of the brake disc to the carrier (fixed, semi-float, full-float).

Effective Radius (ER): To keep it simple, we’ll call the ER the midpoint of the pad on the swept area of the braking surface. In general terms, brake torque can be calculated by Brake Force at the pad/rotor interface * Distance (ER) away from center. The larger your rotor OD, the more output you can generate for a given brake, all else being equal. Growing the ER is like using a longer wrench for better leverage on a stuck bolt.

Increasing your ER from the OE size can get expensive pretty quick. If you are lucky, you will have available (to purchase) an adapter bracket to move the OE calipers outward to fit your larger rotors. Most brake kits do not offer just that option as, go figure, they have a caliper to sell you!

Rotor Attachment Methods Why float? The purpose for not using fixed or one-piece rotors is pretty simple. First, the floating rotor allows for better pad knockback (where the rotor “knocks back” the pad such that there is no contact and reducing rolling drag). Second, it will accommodate for a small misalignment of rotor to caliper / pad. This will allow for full pad contact even when parts are not perfectly aligned. Imagine a fixed rotor with a misaligned caliper – the actual contact (and pad/rotor wear) would only be a portion of what is available. As you might think, the full-floating allows for more of the above benefits over the semi-floating design.

Aside from rear rotors with no carrier that bolt directly to the rear wheel, there are a couple different methods to attach the rotor carrier that attaches directly to the wheel, and the rotor disc that the pads contact.

• Semi-floating Rotors: Most OE rotors, even on Italian bikes, are a semi-float design. That is, there is no direct attachment between the disc and the carrier, but they do not move freely of each other without moderate force. If you have semi-float rotors, you can move the disc independently of the carrier by pushing and pulling the disc perpendicular to the carrier. The actual attachment is a rivet-style with a spring sandwiched between the rivet surface and the rotor components on each side. The springs are always in contact with the rotor components creating the ‘semi-float’ effect….

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