Redbullet

This is the first time I use disk brakes – new SRAM Rival road system, now about 200 km mild rides. I didn’t know about “bedding”, so I only cleaned the disks with 96% alcohol before the first ride. The rotors move free, they don’t touch the pads.
The rear brake seems to be fine (although I see no difference vs. Force 22 rim brakes).
The front brake has fluctuating power over the circumference of the disk. At higher force, it strongly vibrates and makes noise. I felt this 2-3 times, then I started to brake just below that level and relieve the lever before full stop, to avoid vibrations.

I am wondering whether the front brake will gradually start to work ok, or I should apply other special procedures to fix it.
Thanks.

fooferdoggie 

this could be because you did not bedtime right. its easy fix hit the disc with sandpaper clean it with alcohol and follow the many instructions on bedding. myself we have lots f hills so its easy to bed them in.

FBinNY 

The bad news

You're feeling the effects of resonance. No brake disc will ever have perfectly uniform friction properties through 360 degrees. So there'll always be some pulsing.

The front brake pulsing is noticeable because the fork is flexible and under the right conditions can set up a tuning fork type resonance.

This will be worst under specific conditions and, as you've learned, can be managed.

The good news.

This will pretty much resolve over time, and both the likelihood and severity of the problem will shrink.

maddog34

"The front brake pulsing is noticeable because the fork is flexible and under the right conditions can set up a tuning fork type resonance."


Please explain how two tubes, bridged at the bottom and top, can act like a tuning fork.

to the Original poster.. try re-centering the Brake caliper... loosen the two screws attaching the caliper to the mount.. then hold the brake lever tightly as you re-tighten the mount screws.

Iride01 

You've only ridden 200 kilometers on them. My discs didn't perform well till I had 300 miles on them. So you might have almost 283 more kilometers to go.

I didn't muck around with them either every time they squealed a little. Usually when they'd gotten wet or for other reasons. About all I ever did was clean them with alcohol if I was spraying some cleaner on the bike that might have gotten on the disc.

5000 miles later they stop way too well and are smooth and quiet as can be. Even when they get wet.

FBinNY 

I didn't say that. It was an analogy to help the OP understand resonance.

However, each brake pulse pushes the fork back a tiny bit. If the timing of the rebound matches the pulse frequency you get resonance, which amplifies the pulsing effect.

Redbullet

Thanks!
I know what resonance is, but I didn’t imagine I would find it so hard with disk brakes. It very rarely appears with rim brakes and it disappears in 1 minute by simply cleaning the rim.
I re-centered the caliper, cleaned the disk again and braked around 20 times from 25-30 km/h, plus 3 longer brakes of 30-40 seconds on descents. It improved. Still tend to vibrate, but at higher force. Overall, much more demanding than rim brakes...

maddog34

another source of "vibration" is the Holes or Slots in the disc itself.. some discs are quiet, some make a steady buzzing sound, AKA: vibration..... the only solution for this is replacing the discs with a different design... i've installed several SRAM discs with Shimano or Zoom brand calipers/pads... Personally, i've had no troubles with them.. yet.... (fingers crossed)

i put drilled/slotted discs on my old corolla.. i Hated them... they made a distinct buzzing when applied hard... and pedal pressure needed to be increased to get the car to stop too !' i trashed them and swapped to standard, non-drilled discs and the newer Ceramic pads......instant better brakes. the slotted discs i had on my '85 Rally Corolla worked fantastic... no holes in those...just angled slots.............

another thing i've ended Squealing brakes with is Coarse Emery Paper... Not standard Sandpaper, it can leave a glue residue that instantly re-glazes the pads.. Find a very flat, hard surface (i use my drill press table) and lay the emery paper on it.. then, using a circular motion, rub the pad on the paper.until it presents a clean surface free of glaze.. i turn the pad 90 deg. every few circlings to even out any angularity i might impart.... Reinstall the pads... i also lightly redress the discs at the same time, using the same coarse emery paper... 100 to 180 grit.. 220 works ok too... i then Re-center the caliper.

FBinNY 

Like you, (I suspect), I'm a long time roadie. I consider disc brakes on road bikes to be an overly complex solution to a problem that doesn't exist.

I believe the shift to road discs to be the outcome of mtb riders coming fresh to road bikes and bringing their mtb sensibilities with them. Of course, the industry was more than willing to accommodate them, since who doesn't prefer to sell more expensive stuff.

Given the short institutional memory, I wouldn't be surprised if some years from now, someone introduces a "integrated disc" concept that uses the wheel itself as the rotor, and markets its virtues of simplicity and lower weight.