Disc brake puzzle

[DaveP]

The last couple of times out on the tandem we've had a little problem with the BB7 disc brake on the rear wheel. it works ok, but after maybe 10 to 15 miles the inner pad starts to rub lightly on the disc, resulting in an irritating squeal. It starts with no obvious trigger event such as particularly hard braking. Applying the brake lightly only gives a few seconds relief, longer lasting peace can only be had by dismounting and winding back the pads. Especially the inner one.
Last Sunday I had to do this three times in about 8 miles. When I got home, careful inspection revealed that the disc was now almost touching the inboard edge of its slot in the calliper body. When we set out the disc divided this space in the ratio 2:1 inboard vs outboard as specified in the installation sheet.

I cant understand whats going on.
There is no play in the wheel bearings, the Q/R is tight amd the disc appears to be running true.
The mounting bracket bolts are tight and the calliper appears to be solidly mounted on its bracket.
I know that something has moved, but what?

Any ideas?

[spanner]

just a suggestion but it could be that the inner pad is sticking slightly you could try taking the caliper off and taking the pads out and cleaning the area behind the pad which is sticking and applying a light smear of coppergrease to the back of both pads then refit the caliper and reset the adjustment
you dont say whether it is a hydraulic or cable caliper and am assuming that it is a cable one another problem could be the cable is sticking in the outer which may be the cause of the problem try changing the cable first before checking the caliper(dampness may have caused the cable to oxidise causing it to stick)
also does the problem occur when two up but goes away when ridden solo?

[gilesjuk]

The tri mount on the Avid brakes (dished washers) is a mixed blessing. It allows for accurate alignment but I find that under heaving braking the caliper must move around causing the rubbing.

I've contemplated replacing them for flat washers, I reckon it would work so long as your frame is pretty straight.

[DaveP]

Errrm, I did actually say it was a BB7 - famously cable operated but thanks for the thoughts, Spanner.
I should also have said that it is a new unit, not much more than a month old and the cable was cleaned and lubed at the time of installation. I did wonder, at the time. if there could be a problem with the spring that holds the pads apart, but there has definitely been an overall movement of disc or calliper.
I dont actually enjoy riding it solo, so I dont know wether it would go away. I'm curious though - what would it mean if it did?

It's a relief to hear that someone else has cause to suspect the tri mount. I guess it has to be the prime suspect, although, as I said, I couldnt recall any heavy braking, particularly the third time - and I was watching pretty closely by then!
I don't know if plain washers would help though, as the system allows for sideways adjustment as well as angular...
I'm going to have to think about it
Oh the pain

[byegad]

You have checked the wheel bearings? Could be the wheel is able to shift so closing your 2:1 gap.

[spanner]

Errrm, I did actually say it was a BB7 - famously cable operated but thanks for the thoughts, Spanner.
I should also have said that it is a new unit, not much more than a month old and the cable was cleaned and lubed at the time of installation. I did wonder, at the time. if there could be a problem with the spring that holds the pads apart, but there has definitely been an overall movement of disc or calliper.
I dont actually enjoy riding it solo, so I dont know wether it would go away. I'm curious though - what would it mean if it did?

It's a relief to hear that someone else has cause to suspect the tri mount. I guess it has to be the prime suspect, although, as I said, I couldnt recall any heavy braking, particularly the third time - and I was watching pretty closely by then!
I don't know if plain washers would help though, as the system allows for sideways adjustment as well as angular...
I'm going to have to think about it
Oh the pain

the reason i asked whether the same problem arises when ridden solo is that the weight of the rider on the back is putting extra pressure on the rear wheel thereby causing it to shift slightly.
have you checked the wheel is running true as this may be an underlying cause of the problem as although the disc is running true if the wheel is slightly off true then this would cause the wheel to shift fractionally to cause the problem also a slack cone would also cause the same problem.
im still certain that the pad on the inside is sticking and causing the problem also was the cable inner just lubed and refitted or was it a brand new inner cable that was fitted? one final thought if the outer has a slight kink in it this would restrict the inner cable as well

[DaveP]

Ah no - the wheels fine! Its about the same age as the brake actually, and one of Spa's finest. It runs true and the bearings are just slack enough to allow the heavy point to settle.
And its not the cable - the inner pad on these is static.
No, I'm afraid the calliper has shifted somehow.
And I'm going to have to restrain it somehow...

[prm]

Not discounting Avids installation instructions, but several bikes/tandems may require a slightly different set-up due to fork, frame design and application.

A possible adjustment you may like to try. ??

Slacken cable, mounting bracket and calliper bolts.
Ensure both pad adjusters lock into their set positions and any calliper alignment spacers/washers are installed correctly.
Set both inner and out pads to centralise the rotor with the calliper body, locking the rotor.
Temporary secure cable. Apply the brake fully, force wheel forward. Tighten bracket bolts, and then calliper bolts.
Forcing the wheel forward could help with alignment and any overall movement.

Back-off both pads. Adjust inner pad to give the minimum amount of friction.
Set barrel adjuster fully in. Raise the operating arm approx 10-20mm from the stop, and secure the cable. This should ensure the inner cable is under constant tension with the additional tandem length.
Adjust the outer pad to the required amount of brake lever travel.

You may find moving the operating arm even closer to maximum travel, increases baking efficiency/leverage, also gives an indicator the outer pad requires resetting with brake fully applied.

Hope this may help.