The facts about floating motorcycle rotors or discs

Most articles written so far on this topic are simply “Wrong.”

OE Rotor

First let’s explain the basic types of motorcycle rotor:

Solid Rotor

These are as the words suggest, a simple piece of flat sheet or sometimes billet steel. Often heat treated and ground, these are found in various shapes and patterns, mainly done for cosmetic purposes, to make the rotor look less like a dinner plate, but these profiles or cut outs do nothing to change the fact that a solid rotor is exactly that. Solid rotors are of course much cheaper to make and when made in volume are even stamped from sheet steel for further cost reduction, a process which in itself loads the rotor with stresses that often lead to rotor distortion, buckling or warping under heat and brake vibration.

Being thin in construction at 4-6 mm in 99% of cases warping does occur, often under load, unlike with an automotive disc or rotor where warping is almost impossible and vibration is caused by other factors, discussion on which is not essential to this article.

OE Rotor

How The EBC SD System Works Perfectly

Photo: EBC Square Drive Button
EBC square drive system for perfect floating operation
Conventional round bobbins used in factory two piece rotors

Two Piece Rotor

90% of rotors branded as floating types are actually simply a two piece rotor where the outer rotor ring is riveted to a central hub by circular mass produced rivets. A rotor made with circular rivets does not float at all because the tolerances (differences in PCD – pitch circle diameter of the inner and outer components) would need to be so large that the rotor would rattle and shake from day one. At this point it is essential to differentiate between axial floating and radial floating. Any motorcycle rotor built with axial float often described by people as a self centering rotor is also doomed to failure. The pad and piston knock back effect of a rotor with axial float is simply a killer and causes sponginess and can render brake levers coming right back to the motorcycle handlebar when trying to re-connect the “knocked back” pad with the rotor under braking. Not good at all!

So zero axial float is needed or desirable in any type of rotor.

Under braking and heat generation the outer ring or rotor blade of every two piece motorcycle rotor suffers from band width growth, in other words the outside of the rotor grows outwards but the inside SHRINKS inwards. It is this shrinking inwards that defeats the myth that two piece rotors actually float.

Imagine how the outer ring and inner hub behave having a tiny tolerance over the circular rivet (to avoid rattle). The rotor ring shinks inwards, hits the circular button, locks up and then, unable to go anywhere, the rotor warps or distorts.

So there are two ways to go from here 1) increase the tolerances between the rotor ring hub and circular rivet or 2) change the rivet design.

That is where the EBC patented SD-System (standing for Square Drive) comes in.

As shown in the photographs top- left, the rivets connecting the outer rotor ring and the inner hub both have parallel sided slots in them and the rivet is also made with flat sides that connects the two parts of the rotor and generates LINE CONTACT as compared to POINT contact allowing the rotor ring to expand and contract without binding during the heat of braking and cooling.

Here we get a perfect line contact between all the components that allows the rotor to freely expand the few thousandths of an inch that are all critical without any “Locking up”.

No distortion, no vibration just silky smooth firm brakes for life.

If your bike is fitted with solid rotors, it’s time to change them out now. If you bought a two piece non-EBC rotor, only time will tell how wise that investment was and maybe if you never really do load up those brakes and get them hot, you will be OK.

Anyway we wish all biking friends happy days and safe riding.

EBC Brakes – World’s largest aftermarket supplier of motorcycle brakes