Unfortunately my instincts were correct regarding the Shimano rotors being glazed. This past Tuesday afternoon was my first ride on the 256SL following the harsh Miwok decent back in late November. Towards the end of that descent my brakes started to fade. Immediately I suspected they were glazed. On my first descent Tuesday I had zero braking power and the rotors were howling frighteningly loud.
My attempt to restore the Shimano rotor surfaces with sandpaper followed by a systematic break-in process was a disappointing failure. Brakes were oscillating and howling with no power.
I can only suspect the Shimano compound does not hold-up well to high heat. Even when operating at peak performance I was not impressed with the breaking power of the Shimano pads.
So this meant the new Ashima rotors went on along with the new SwissStop Disc "e" pads...
After following the bed-in process described below, I am extremely impressed with the Ashima rotor and SwissStop pad combo. Great power and modulation. The XTR brakes are beginning to live up to the high expectations I had for them. If this result holds up, these Ashima rotors are among the best value I have ever come across in road or MtB components, considering they can be bought right now on eBay for $28/pr!
The bed-in process:
No doubt there are many different methods by which one can "bed-in" a fresh set of brake rotor. Below I describe one such process that has worked for me in the past.
I start by riding on flat ground dragging the front or rear brake lightly to build up heat on the disk and pads while not allowing the rotors to make any noise (the dreaded squeal). After a minute or so of this when I feel the rotors begin to bite I do a couple of hard'ish braking runs from ~20MPH coming to a near stop, but not to a complete stop. It is critical you do not allow the brakes to make any noise or oscillate! This will cause uneven bed-in and that pattern that causing noise/oscillation will bed into the rotors surface and you may ruin your rotors. If the brakes make noise, brake harder and harder until it stops. I repeat the above process for the front and rear brakes independently. If your legs are burning and your breathing hard you've done it right. You need some heat to transfer resin and friction compound to the rotor surface.
After about 5 of these braking runs I stop and remove the pads from the calipers for inspection. As the pad compound goes through a thermal cycling it will look blackened, see photo below. Curiously, I noticed on the SwissStop pads that only the center of the pads were black and the perimeter looked like new compound. This is a problem since it will not promote even "bed-in" of the rotor surface. To correct this I lapped the surface of each pads with 120 grit sandpaper on a smooth workbench surface. Only a few seconds of a circular grinding pattern are required to remove the blacken layer and expose fresh compound and flatten the pad surface.
The pads then go back into the calipers, being careful not to contaminate them during this whole process and I repeat the same bed-in process of dragging the brakes followed by hard'ish braking runs. If you're doing this on asphalt/concrete make sure you use old tires because rapid acceleration/deceleration will quickly chew through your tires. I then head out on to a ride with a steep hill to complete the bed-in process. Yesterday I had a 2500 foot climb averaging about 10 percent grade. On the descent, I don't allow the bike speed to exceed much over 25 MPH and I brake both the front and rear as evenly as possible without allowing the wheels to lock, which the rear is prone to easily locking, especially if you have a heavily worn treadles bald rear like I do!
That should complete the bed in process. However, I am going to avoid any high heat descents (like Miwok) for a few rides to let the brake bed set-in (playing it cautious).
Pads and rotors following bed-in yesterday:
