Can anyone tell me the reason for the single button that is embedded in the front brake rotor of my 2008 Road King?

At the 3:00 position.

 

I believe that is for heat expansion/contraction to minimize warping. The button is very slightly loose on there, allowing movement.

 

i am more inclined it to be a balance weight for the disk.
next time you have the wheels off your car, look into the webbing of the rotor and some times you will see a piece of steel wedged in there as a balance weight.

 

Highly unlikely ... The "button" is in the same spot on all of that type of rotor ...

 

That's the only plausible explanation I recall seeing. Makes that style of rotor a budget floater!

 

then it is a good ?.
my line of thinking is look at the rotor. there is no way that that can control expansion, the rotor would expand across the entire disk and not to one area and if it did, then it would distort. if the button is exactly the same across all rotors then maybe it has something to do with controlling induce harmonics. the ventilator holes on some rotors (not necessarily these, but good ?) can contribute to increase noise levels. if the rotor has a detached arm, it would disrupt any induced ringing or frequencies. that said, why the button, hide the open hole?

The judder phenomenon
judder is usually produced as a result of longer, more moderate braking from high speed where the vehicle does not come to a complete stop.[54] It commonly occurs when a motorist decelerates from speeds of around 120 km/h (74.6 mph) to about 60 km/h (37.3 mph), which results in severe vibrations being transmitted to the driver. These vibrations are the result of uneven thermal distributions, or hot spots. Hot spots are classified as concentrated thermal regions that alternate between both sides of a disc that distort it in such a way that produces a sinusoidal waviness around its edges. Once the brake pads (friction material/brake lining) come in contact with the sinusoidal surface during braking, severe vibrations are induced, and can produce hazardous conditions for the person driving the vehicle

also, vibration induced into the rotor have a tendency to force the pads away from the rotor so when the brake is applied, it has to travel some distance before contact is made making the rider to apply more pressure and the braking becomes aggressive.

 

I believe these types of rotors are called "semi-floating" rotors.

https://www.hdforums.com/forum/touri...these-new.html

https://www.hdforums.com/forum/frame...o-warping.html

 

Nice job LQQK OUT.... here I was all in on the judder phenomenon and you go and blow it out of the water with the actual answer...lol

 

well, i do not think so.
solid rotor: disk is attached to the hub and has to expand across the whole area and is constrained by the mounting to the hub.
floating rotor: the disk is allowed to move laterally and the caliper is fixed unlike the solid rotor, great for performance brakes
semi floating rotor: the rotor is made from TWO pieces and is joined in a way to allow restricted movement. great for heavy machines and moderate performance.

the above rotor is not a two piece rotor> once again, a good ? but below is a semi rotor
https://images.esellerpro.com/2375/I/657/742/rotor.jpg

 

That is what I know as a fully floating rotor, which I've been riding with for around 20 years now, on various brands of bikes. The mounting 'hub' is separate from the braking surface, the two being attached by 'buttons' which minimise heat transfer from the braking surface to the wheel hub and also allow for expansion of the braking surface.