Avid BB7 disk brake problem

Ramanujan

I have a slight problem with my Avid BB7 road disk brakes.
This is the first time I’ve had disk brakes so I’m not that familiar with how they should feel compared to rim brakes. Before I start I should say that I’ve had these brakes a week now and the pads have been properly bedded in. I’ve also consulted several sources on how to set the brakes up (including Avid’s .pdf file) and I’m confident that I’ve set them up correctly.
The problem I’m experiencing is that when I’m applying the brakes, it feels like the pads are biting the rotor more positively in some places on the rotor than others. In other words, when I grab a handful of brake it feels slightly juddery and not smooth and consistent like my rim brakes used to. I often feather the brakes on long downhills to just scrub off some speed and it feels like the brakes bite strongly for a second and then less strongly for a second and so on. The same happens when you want to stop quickly and grab more brake: only it’s more pronounced.
I’ve tried cleaning the rotors with disk cleaner and I’m pretty confident that the rotors are true. I’m at a loss to figure out what it might be. I haven’t actually inspected the pads yet, so maybe this should be my next step.
Can anyone throw any light on this problem?
Many thanks,
R

ride_whenever

front or rear? If front then it is probably fork judder. Check your headset bearings and front cones.

Check the rotor is true, centered correctly and the bolts are tight.

Is the pad contacting the arms of the rotor or are they squarely on the braking track? If you have the caliper too close to the hub then as the arm comes though the pad you get more braking leading to a fluttery effect. This can be remedied by spacing the caliper out a little further.

Ramanujan

Hi,

It's on both front and rear brakes.
I've already checked headset and cones and both are fine. The bike is new.
As I said, the rotor looks true but it's hard to tell if it might be a tiny bit out.
Also the pads are square on the rotor.
So am I right in assuming that this fluttery, juddery braking is not normal for disk brakes?

thanks
R

nicklouse

sounds normal for new brakes.

as the pads move over the holes in the disc things change.

does it change with wheel speed?

also as the discs are ground there is a direction to the grind. this can also some times be felt.

Ramanujan

nicklouse wrote:

sounds normal for new brakes.

as the pads move over the holes in the disc things change.

does it change with wheel speed?

.

Yes.
At higher speeds it's less noticeable. But at low speeds it's rather pronounced.

cheers
R

thelonegroover

I had BB7s on an old Marin HT, never noticed anything like that. Check your cable inners are operating smoothly. What sort of bike are they on?

Ramanujan

It's an On One pompetamine.
As I said, it's a week old so it's unlikely to be the cables. They all seem smooth.

The Rookie

It wouldn't be cables anyway, interface between disc and pad, more bedding in, try and some good hard stops from circa 20mph, AFTER centering the caliper.

Ramanujan

The Beginner wrote:

It wouldn't be cables anyway, interface between disc and pad, more bedding in, try and some good hard stops from circa 20mph, AFTER centering the caliper.

Hi,
I've already done around 100 miles on the bike over the last weeks commuting, including several high speed stops. I would have thought the pads would have bedded in by now, but I could be wrong.

nicklouse

just use the bike it will sort its self out.

it is normal.