slowster wrote: …. it's often recommended with new pads of the organic type to do several repeated hard brakes at speed, e.g. going down a hill, to roughen up the pads and improve their friction....
Thats not quite what is happening when the pads are being bedded in. There are several things going on during the bedding in process of which the two most important are
1) that the pads make full contact with the disc. With new pads of good quality, in a well-aligned caliper, on a new disc, this happens fairly quickly. However with an imperfect trifecta, this may take much longer to happen, if at all.
2) the working face of the friction material is taken through a few thermal cycles.
The latter thing is important with any brake pad which contains friction materials which are held together with a resin binder. This includes many pads which are described as
- organic
- resin
- kevlar
- semi-metallic
- ceramic
amongst other things. AFAICT it is only sintered pads that don't really need 2) above.
What happens is that during heavy braking, the working face of the friction material reaches a high temperature and that temperature profile penetrates into the thickness of the pad material. The resin reacts in thee main way when heated; it melts, outgasses and then finally burns (at higher temperatures). The bedding in process ensures that the face of the pad gets hot enough for burning to occur, which leaves a more porous/resin free 'skin' on the working face of the pads.
By contrast, if you do nothing but gentle braking you will often get melting only near the pads surface (which causes pad glazing to occur) and/or outgassing during normal braking. When the pads are glazed any outgassing is double-bad because the gas cannot escape through the (normally slightly porous) pad material. If the pads are glazed badly enough then it may become almost impossible to retify the situation by simply repeating the bedding-in process; I suspect the brakes simply don't work well enough for the pads to get sufficently hot for anything really useful to happen. 'Deglazing' the pads (eg by abrading their surface) renders the pad surface freer of resin which means there is less burning to do and the suraface of the pad is once again more porous and allows some outgassing to occur before braking is affected.
Once the pads are bedded in, small amounts of further outgassing will occur whenever the pads wear and/or the brakes are used harder than normal i.e. whenever fresh pad material below the surface sees high temperatures for the first time. Because the volume of material thus being affected for the first time is so much less than with a new pad, this fresh outgassing etc is usually tolerated without issues.
cheers
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