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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 three 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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• Full-floating Rotors: Most aftermarket rotors, are full-floating. Like the semi-floating rotors, the carrier and disc are separate parts, but that attachment mechanism between the two are different. Where the semi-float uses a rivet design, the fill floating design uses a “button” without any spring force applied to the rotor components. While the button is a mechanical attachment, the back side is secured by a snap ring, e-clip, or stacked spring that can be removed. The degree of free float is in the range of 0.5-1.0mm. These are completely free float and you can move them with no effort with your fingers.
• Comparisons: So why not just use full-floating rotors on everything if they are better? They are better for pure performance, but there is a trade-off for the benefits. The main disadvantage to the full-floating design is the noise factor. First, they rattle…all the time…high speed, low speed, they always rattle. Second, they are much more likely to make noise (probably low speed, low frequency howl or groan noise) because the mechanical attachment has been ‘loosened’. I guarantee that while you and I may be willing to live with the noise given a performance benefit, the OE will not. They do not want the complaints and warranty costs associated with noisy pads.

SIDEBAR: I purchased billet full-floating buttons to convert my OE Mille ‘R’ Brembo rotors from semi-float to full float. Guess what – since then, every time I approach a stop light or other lower speed brake apply, I get an awful groan noise where I can actually feel the vibration in the bars. Yuck. There are reasons for my trade-off and I’ll spare you the long story. END SIDEBAR

I think a discussion regarding rotor disc metallurgy would be helpful at this point. Two basic metals are offered for the rotor discs – cast iron and steel. Now there are many versions of each with respect to processing, but lets stick to the basics for this discussion. Its also worth noting that almost 100% of automotive rotors brake surface are cast iron. First and foremost, its cheap. Second, it can be poured very easily. I think the main reason for stainless steel being used for motorcycle rotors is corrosion. Most motorcycles are seasonal and get stored for a good part of the year where they would rust. Not good. Cars that are driven every day may get surface rust, but no accumulation.

Steel is available in both the OE & aftermarket sectors. It is the most popular material and usually priced cheaper than the cast iron versions. The main advantage is that they will not corrode as easy as well as the selection for brake pads is much greater. Cast iron also has its advantages, with the main being a higher friction level. There are also claims as to a better feel or modulation capability, but I have not experienced that. Given the choice where corrosion from high humidity is not a concern, the general concensus is to go with cast iron.

SIDEBAR #2: Have I mentioned that an excellent way to address fade is to go with the CRG Roll-a-Click adjustable levers on the fly? I have. They aren’t cheap at $100 each, but at the very least with the brake lever only, you can improve the feel and lever stroke of faded brakes on immediately as it occurs. Heh – you can ask for the clutch lever for your next birthday END SIDEBAR.

SIDEBAR #3: Wave Rotors I really have no experience with wave rotors. I do know some racers that say they work well, but for the most part in the circle that I know, they are purchased for the bling-factor. Great. From a design side though, it seems to take away the advantage of adding heat capacity with all the cut-outs. I would also be surprised if the pad wear was not compromised due to the interrupted cut that the pads see during braking. But, I have not heard any complaints first-hand. END SIDEBAR

SIDEBAR #4: Slots vs. Holes and De-Gassing The purpose for the holes and/0r slots is not weight reduction or cosmetics, but to allow pads to properly de-gas especially when they are new(er). As a pad is bedded in (especially the first time you fade them), it is burning off the binders, fillers, and other materials. This gas needs an avenue to escape at the surface of the pad and the holes / slots allow for just that path. Without them, there is actually a force generated by the escaping gas in the opposite direction of the apply that works against braking force. This gives a feel like the brakes have faded when they haven’t. So you need them.

As for a preference for slots versus holes, its really not so critical on motorcycle rotors as it is in the automotive field. For autos, I would stick with slots as they do have the surface cracking propensity as rotors with cross-drilled holes. As for motorcycles, the braking capability to weight ratio is so much better that I have never cracked one myself with either design. I know, I know…your friend of a friend’s cousin cracked a rotor riding through the mountains… Is it possible? Sure. Is it likely for a mere mortal, no way without a quality issue. END SIDEBAR

Rotor Maintenance: What? Its true. Some rotors require routine maintenance. Stock Brembo’s that are on Aprilia and Ducati sport bikes are semi-float. They are a common rotor (interchangeable) and have a common problem without maintenance – brake vibration. Almost everyone who owns an Aprilia or Ducati recognizes the bulletin board topic “Brake Vibration”. They take it to the dealership under warranty and get the rotors replaced. Unfortunately, the problem is in the rivets and it could return. They collect brake dust and other crap that binds up the movement of the disc. If the cannot move freely (as much as semi-floaters can), you will get brake vibration. Bottom line is that you have to clean the rivets with brake cleaner periodically to keep them clean and functional. Be sure to clean any braking surface you touch with your fingers with brake cleaner. Without a dissertation, just do it but don’t get any on the pads. Other than keeping buttons and rivets clean, its best not to mess with the braking surface. This will avoid contaminants in the pads and hopefully prolong good performance.

Brake Pad / Rotor InterfaceJust be aware that specific pads are for specific rotor materials. If you purchase aftermarket rotors, please ask the manufacturer (not the zit-faced kid at the dealer or on the phone that is selling you the product) which pads work well with there rotors. If the rotor manufacturer makes a specific pad for their rotor, consider using it. If there is a problem down the road, you will have a reasonable argument in hand that the system is completely ‘theirs’. If you get a BrakeTech kit, use the BrakeTech recommended Ferodo pads. It is that simple.

WWABD: I have a good deal of experience with stock and aftermarket rotors for motorcycles as well as background in automotive rotor design and performance. I have owned Brembo, PM, AP, and BrakeTech rotor kits including slotted and cross-drilled as well as aluminum and magnesium carriers. This also includes all of the above listed button designs. In general, the OE rotors are fine for every day riding and most mountain twisties. Aftermarket rotor kits that either increase the ER or increase the friction level are a good idea for those who do the regular track day or race at any level. I would also recommend them for those who partake in the tight mountain twisties where a lot of braking is needed like the famous Deal’s Gap. 318 turns in 11 miles is pretty hard on the brakes. Finally, don’t try to address fade with rotors alone. Since cast iron and steel have similar heat capacity and density, unless you go up in rotor OD (adding total disc weight), you will not improve fade. For motorcycles, fade is probably best addressed with pad selection unless you have loads of money.