Let’s get clear about it!
In FreeSpirits’ kit for Harley Davidson XG Street Rod there are two floating type brake discs, with hardened brake band in full-thickness. The hub and buttons are in ergal, in order to reduce their weight and still have a considerable resistance to stress.
What are floating discs? These discs consist of a braking band connected by means of the driving bushes to the hub which is connected to the front rim by means of bolts.
The presence of the buttons allows a small possibility of movement in the transversal direction. In this way the braking band can expand and adapt itself to the positioning of the pads.
This brake discs have a diameter of 320 mm instead of the standard 300. This difference is intended to move the point where the pressure between pads and disc is exerted more distant from the wheel center and help to dissipate the heat.
To explain this behaviour, we have to think of the wheel as a large disk that advances with a certain inertia.
To slow down the wheel you need a braking torque.
The torque capacity of a brake rotor can be expressed as 𝑇 = 𝜇 ∗ 𝐹 ∗ 𝑟
where μ is the coefficient of friction, F is force on each pad and r is the mean radius that is the distance between the center of the wheel and the center of the pad.
The direct consequence of this change is to be able to slow down our bike more efficiently by using less pressure on the lever, with a more progressive and powerful braking.
A further advantage of mounting a larger disc is the lower heating of the disc, but to clarify this point we first focus on a key concept usually defined as “fatigue” of the braking system.
Have you ever tried to measure the braking distance of your bike?
Let’s do an experiment that you can also try at home: suppose you have drawn a line on the ground, take a straight stretch with the bike and get to brake at a certain speed above the line marked on the ground until you stop completely. Now measure the distance between where the bike was stopped and the position of the line. Then repeat the experiment with the same speed and braking with the same power on the same line, then measure the distance travelled by the bike at each test. Repeating the experiment several times you will notice that as the braking system heats up this distance will increase. This factor is defined as fatigue of the braking system. For example, if you are on a mountain road with lots of hairpin bends and if the bike will take more space to stop after some braking, you will risk getting into the corners too quickly to stop.
This problem can be reduced by using quality braking systems, such as the kit in question, with oversized brake discs that require less effort to slow down the bike than the original ones.
A larger diameter disc allows to dissipate better the heat, having a greater surface in contact with the air.
Why limit the diameter to 320 mm?
Now that we understood the importance of a larger diameter, so why we have not increase further this diameter? The increase in the diameter of the rotor also has a negative consequence. When we increase the size of the rotor we also increase the inertia of the rotor itself. The rotors are connected directly to the front rim, so we increase the inertia of the front wheel. As seen before, increasing the inertia of the front wheel also increase the braking torque required to slow it down. Moreover, increasing the inertia of the rotor decreases the agility of the bike, because it increases the gyroscopic effect of the wheel, which try to maintain the motorcycle perpendicular to the ground.
For this reason, we decided to limit the diameter of the disc to 320 mm, so as to obtain the advantages described above and limit the loss of agility.
To overcome this problem, the discs included in the kit have been lightened as much as possible, using ergal on all the parts that are not subject to frictional stresses and working the discs with lightening to limit the rotating mass.
The FreeSpirits’ kit consists of anodized gold Brembo calipers, brake pads, cnc machined mounting brackets black anodized, in order to install the braking caliper in the standard forks and oversized discs.
The kit is compatible with the original brake line and brake master cylinder. It can be assembled quickly and the assembly is completely reversible, leaving always the possibility to return to the initial configuration as FreeSpirits tradition.
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