Another brake related thread has caused me to become quite curious about the ‘self energizing’ effect, so I’ve been giving it some thought...
The diagrams below are representative of GTA pads, i.e. the pad length is 138mm and the thickness is 16mm (including backing plate). This is just an example, the fundamental principles apply to all brake pads.
The following is only my personal understanding based on my own analysis, FWIW, starting with some diagrams:
Apologies for diagram quality, something makes them a bit fuzzy when exported from the native program...
All disc brakes generate a relatively weak but not insignificant self energizing effect which acts to increase the force with which the pads are pressed against the disc faces. This effect is inherent and ‘automatic’, and could also be described as being an ‘auto-servo’ effect because to whatever degree it reduces the pedal effort required to achieve a given braking effect. The effect is geometrically generated at the pads and creates force that is manifested unevenly over the pad area, more strongly at and near the pad leading edge and less strongly farther from the leading edge, i.e. the effect is stronger at / near the leading edge but very much weaker at / near the trailing edge.
The pad force created by this effect is additional to the piston force. The effect is broadly similar to the self energizing effect generated by ‘leading shoes’ in drum brakes. The effect in leading shoe drum brakes is far stronger than it is with any disc brake (the geometry which creates the effect is a lot more extreme with drum brakes), being the reason why drum brakes typically do not require artificial force amplification, whereas disc brakes typically do (other than in extremely lightweight cars).
With disc brakes the effect is created by friction induced drag (between disc and pad) being resisted at a point which is physically offset from the source of drag (i.e. offset from the disc face). This creates an effective lever arm which redirects some of the ‘drag force’ more or less laterally (this effective ‘lever arm’ is represented by dimension ‘C’ in the diagrams)..
The self energizing effect may be relatively weak with disc brakes (compared to drum brakes), but still strong enough that the pads are not evenly pressed against the disc faces, even when ‘staggered’ size multiple pistons are used (i.e. smaller pistons near the pad leading edge and larger pistons near the trailing edge). This causes uneven temperatures over the pad area, and in turn this causes uneven wear, manifested as longitudinal taper wear (i.e. greater nearer to the leading edge, and lesser nearer to the trailing edge). This tapered
wear in turn creates free play and sponginess within the caliper, manifest to the driver as free play and sponginess at the brake pedal.
One end of each very rigid pad backing plate bears against the caliper body (represented in the diagrams as points ‘A’), being where rotational force created by the disc / pad friction is resisted (i.e. where the pad is rotationally constrained in the caliper). Points ‘A’ are in effect points of articulation which act similarly to a hinge, allowing a pivoting action, or would do if the pad were not wedged between the pistons and disc face.
The disc face is laterally offset from the point of resistance (A). Friction occurs at the pad / disc interface creating force that is laterally offset from point A, and so creates a force vector (blue arrow) which acts tangentially to the disc face, ‘pulling’ the leading edge of the pad inward against the disc. This is the self energizing effect, caused by friction and drag acting via an effective lever arm causing the pad leading edge to ‘push’ harder against the disc. This is somewhat simplified, because the self energizing effect acts on the entire pad face, but while it will be relatively
strong near the leading edge, it will be very much weaker toward the trailing edge.
Note the angle of the blue arrows relative to the disc faces, representing force vectors. The more parallel this vector is with the face of the rotating disc the weaker the self energizing affect will be (Figure 4), and so conversely the more angled to the disc face the stronger the self energizing effect (as in Figure 3). This is entirely a function of the dimension ‘C’ (backing plate to disc face offset), and the pad length, i.e. the distance between the pad leading edge (point B) and the where the end of the backing plate abuts and ‘pivots’ on the caliper body (point C).
Note that hypothetically, if the force vector (blue arrow) were to be truly parallel with the disc face then the self energizing effect would be zero, but this is not possible, unless all of the pad material were to wear away completely.
It can be seen in diagrams 3 and 4 that the vector angles decrease as the pads wear thinner, so newer / thicker pads must have a stronger self energizing effect than worn / thinner pads. All else being equal
, this might help explain why new full thickness pads seem to have significantly more ‘bite’ and generally a noticeably stronger braking action compared to substantially worn pads...
The braking effect
is generated by total hydraulic clamping force added to the self energizing force (or vice versa...), so if self energizing force is stronger then the braking effect (actual retardation) will be the same for a lesser pedal effort, assuming all else to be equal. So, for a given pedal effort the braking effect will be stronger with new pads, because they will generate a stronger ‘auto servo’ effect than pads which have worn significantly thinner...
Looking at the diagrams, it appears obvious that pads of shorter length (distance between leading and trailing edges) must also generate a more angled force vector, and so tend to self energize more than pads of greater length (therefore be more prone to worse taper wear). Pad chamfers in effect shorten pad length (as seen in diagrams 3 and 4), so will increase the self energizing effect, but decreasingly so as the pads wear thinner, causing the chamfers to gradually become smaller, until they disappear altogether, and the pad reaches its' maximum effective length.
A stronger self energizing effect may well reduce pedal effort, so in at least this aspect might be considered desirable. However I don’t think it is necessarily an unalloyed ‘good thing’ in all respects. The effect is ‘automatic’ and so not under the drivers’ direct control, and therefore cannot be directly modulated (only the hydraulic clamping force can be). So while it will tend to make the brakes feel more powerful and decrease the pedal effort, the downsides may be increased taper wear and for the brakes to possibly be somewhat more ‘grabby’ in their action.
This is a fault shared by most drum brakes, which typically have a very strong self energizing effect created by the shoe leading edges and a much stronger self energizing geometry than exists with disc brakes. ‘Grabbiness’ is a real issue for drum brakes, one of the reasons why disc brakes are superior.
With disc brakes the degree of grabbiness created by the self energizing effect is probably no big deal most of the time (far less than for drum brakes), but what if the car were braking at the extreme limit (either emergency or racing), and a brake locks up, especially on a slippery surface? Perhaps it would not have if the self energizing effect had been a bit weaker...?
My gut feeling is that it is probably better to have less rather than more self energizing effect, so while pad chamfers may be helpful (supposedly..) to reduce pad squeal around town, they may be significantly counter-productive for pad longevity, for pedal travel and feel (after some early taper wear), and perhaps for predictability in more extreme use...