WhyBike Motorcycle Blog

Perimeter Rotors

By James - 4/5/2007

Cyril Huze had a post on his blog called “Perimeter Rotors. Beautiful And Powerful.” about perimeter rotors and I think there is some misinformation being presented to the general public, most likely from the people using perimeter rotors on their custom bikes. Cyril states that perimeter rotors give you . . .

An evident powerful braking due to the diameter of your rotor matching the size of your wheels.

I am not saying in any way that they are bad or should not be used, but once you look at the physics you see why they are less efficient when compared to rotors located close to the hub. Perimeter rotors place the rotor, the piece of metal that the brake pads press against, along the outside of the wheel. Traditional rotors are mounted at the hub. Moving that mass towards the rim will increase your rotational inertia, so you have to use more torque to get it moving and need extra HP to keep it moving. Leaning the bike will take a little more effort too. And with all things motorcycle you should be worried about getting on the brakes when you really want to stop. With more rotational inertia, your brakes have to work harder to get you to stop.

The real advantage to a perimeter rotor is the heat dissipation. There is more surface area and there is more airflow around the rotor allowing it to cool faster. This allows the rotors to be thinner although no weight is shed since the the rotor has a longer circumference. The rotor is also moving slower, being further from the rotational axis, which leads to less heat buildup in the first place. Because of this, many only use one rotor, saving weight on a system that uses two hub-mounted rotors. This is where drum brakes really take a beating and one of the reasons why perimeter rotors are still more efficient than drum brakes.

Most rim-mounted rotor proponents bring up the notion of leverage. That you gain leverage as you move the rotor out from the hub. It is a logical and tangible notion, since it is easier to get leverage when you try to turn a bolt with a long wrench rather than a short one. Unfortunately leverage only comes into play when talking about static friction, or when the wheel is stopped. While the rotor is spinning, the brake pads do not care about leverage since they are just “squeezing” the rotor laterally, not applying a directional force to the rotation. Think about that long wrench you I mentioned a second ago. Will it help you turn that bolt from the side?

This is a controversial subject judging from the comments on a post by the Kneeslider. For most bikers, this won’t matter that much. If your bike is a show piece, I encourage you to make it unique and perimeter rotors are a good way of doing that. If you ride your bike on the occasional weekend, to bike night at the bar, and to the shop for it’s service, you won’t notice a difference. Now if you are commuting on your motorcycle or riding track days, where inches really matter, use a traditional rotor, close to the hub. It will give you the best performance.

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