Why should I bed in my disc brake pads and what's the best way to do it?
Bedding in brakes removes any glaze from your new pads, and any contamination from your disc rotors. It also transfers material from the pads onto the rotors, which helps them work together. It's essential to get the optimum performance from your brakes when you change pads. Follow these steps:
Clean your rotors - you don't want them to transfer any grease and grime to your new pads. You can buy dedicated brake cleaner.
Fit the new pads and find a place where you can build up a bit of speed. A hill is helpful for this.
Build up some speed and then brake to a gradual halt. Repeat a few times.
That's probably all you need to do. However some people, myself included, like to pour water over the hot brakes. I'm not sure this actually improves anything but even if it doesn't it sizzles and steams, which is pretty cool
Bedding in is a term applied to most metallic braking devices. Pads and rotors/discs wear at different rates and can end up slightly grooved depending on the natural variations in the pad/disc materials. when replacing one or the other or both items you need to bed them in to ensure that both surfaces conform to each other.
The best way to bed brakes in is NOT by slamming them on, but rather by applying firm, even pressure to the brakes which will help ensure they bed in correctly. Too light pressure and you could "glaze" the pad - i.e. turn it too hard and polished to effectively brake, too hard and you could damage the rotor or pad by scoring its surface or worse.
The reason you need to bed in your pads is so that they contact the disc rotor evenly. Even if you think you've set them up perfectly, the rotor and the pad are very hard and when you apply the brakes initially there is not going to be completely even contact between pad and rotor. By bedding them in, you're effectively wearing away the top layer of the disc rotor so you get completely even contact.
To do this you simply need to brake hard, generating heat and wear. I find sprinting down the road and put the front brakes on hard, whilst keeping my weight well back to prevent an embarrassing endo, works well. For the rear I again sprint off but then apply it steadily and firmly, also with weight well back and just enough pressure to brake hard without skidding. It helps if you're on a really grippy surface like tarmac.
The how has been described already: repeated brakes to near stop from some speed to put heat into pads. Try to avoid holding the brake after you've come to a full stop to avoid uneven transfer of pad material.
I think the why hasn't been fully covered yet, aside from the fact bedding in transfers some pad material to the rotors/discs. But the reason this is important is that most of the stopping power comes from adhesion rather than friction. Brand new pads have only friction between hard pad and steel rotor. Friction alone - as you'll know if you've ridden on new pads - doesn't slow you very effectively.
Adhesive friction between the pads and rotor, particularly with sintered pads, is much more powerful. Imagine the pad sticking like glue to the pad material on the rotor. As the rotor passes through the pads it smears material from the pads, rather than grinding the rotor down.
Adhesive friction is also used in tyre technology, particularly for performance tyres. I have a picture from a recent track day where I was able to stick a screwdriver to the side of the tyre on my motorbike. The tyres were literally sticky to the touch when they hit their operating temperature.
The same thing happens to your brake pads when they hit operating temperature. So when bedding in you're just getting them hot enough to smear a coating on the rotor, ready for you to take advantage of adhesive friction the next time you get them to operating temperature.
Yes, you will need to do a series of brake applications to ensure a nice even coat of pad material is layered on the brake rotor or disk. What your essentially doing here is cooking your pad so that the rotor will have pad material transferred to it. Also, your also making sure the pad is mature enough to be used for those big hills. If you want to know what a rotor looks like when you do this wrong, then check this car website out: http://www.cquence.net/blog/brake_pads_and_install_guidelines/