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Old 02-20-2009, 05:17 PM   #1
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Backround on Iron Brakes...

Reading my new cycle world mag and thought I'd share some interesting info on iron motorcycle brakes and some of their development over the years.
Kinda take for granted how much tech goes into todays brake discs.
The following was either copied or summarized the best I could from the April 2009 issue of cycle world magazine. Enjoy.

A modern 310 mm stock kawisaki rotor weighs around 2.9 pounds while a 290 mm Brembo superbike race rotor weighs in at 3.7 pounds.

Why would they do that when the goal on a race bike is to be as light as possible?

Well, the reason is because in a racing application, the rotors must be able to hold in more heat due to the extreme conditions they must endure.
The more mass the disk has, the more heat it takes before the discs begin to melt and distort. Soo your stock rotors are actually lighter and more desireable assuming you were'nt subjecting them to extreme heat like in race conditions.

Did you know that when brake disks were first appreared in racing, each disk was 7mm thick and weighed around 7 pounds each??!

Now, your probably thinking those 7 pound rotors from the 70's could probably take some serious heat. Well, yes they could, the problem was that they were rigidly mounted to their disk carriers. Modern disk brakes use "floating" buttons between the disk and the carrier. This allows the disk to expand and contract freely as it heats and cools.

Another contributing problem was the actual pad area or "sweep area" was much wider compared to modern bikes. Early bikes used a 50 mm wide pad track. What this means is that on a 310 mm brake disk, the the outer edge of the disk is moving almost 50% faster across the pad surface compared to the inner edge of the disk. This caused the outer edge of the disk to get hotter and expand more than the inside of the disk. This would stretch the inside of the disk(because it was rigidly mounted). When the brakes would cool, the inner part of the disk would be a bit too big(stretched) and cause the disk to form a slight cone shape When the outside cooled down and contracted. This would push the pads back causing the lever to come all the way down to the bar at the next turn. Modern bikes use a more narrow pad track (around 30-32mm wide) through the use of longer curved pads and additional smaller pistons(usually 4 or even 6) rather than only 2 large ones.

Another feature of modern iron brake disks is the shape and holes/gaps that are machined into the disk. Most people think that these are there to give the pads more "bite" when actually their main purpose is to help disperse the heat. The wavey shape of some rotors in addition to creating more suface area, is said to create air turbulence around the disc and help reject heat from the disk to the air. The gaps in the disk are there to prevent the hot gases, evolved from brake pads, from reducing friction.

The weights of tires, wheels and brake disks are more important than any other part of the motorcycle because they must be accelerated not only in a straight line, but also around their axes of rotation. Lightening these parts has therefore a double effect on vehicle performance. Because these rotating parts resist the riders attempt to steer by virture of their gyroscopic stability, lighter parts equal lighter, more responsive steering.

Superbike racers are still required to use iron rotors soo the battle continues to produce disks that are both as light as possible while still maintaining their durability.

The article also goes into detail about the advantages and costs associated with newer carbon and carbon composite rotors.
Pretty interesting stuff. I'd suggest picking it up if you can.
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Old 02-20-2009, 05:56 PM   #2
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05 r6

good info man.
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