Setting up a disc brake?

A.Lee wrote on 10/01/2007 19:39 +0100:

Quoted message said:

Should I ignore the surfaces on the caliper, and centralise it with the
pads as a guide point?

Center the disc between the pads. Also if you have replaced the o-rings
make sure you put two, not one, new ones in as having two different age
ones can lead to one piston moving more than the other when you pull the
levers.

--
Tony

"...has many omissions and contains much that is apocryphal, or at least
wildly inaccurate..."
Douglas Adams; The Hitchhiker's Guide to the Galaxy

On 10/01/2007 19:39, A.Lee said,

Quoted message said:

In effect, the caliper is off-centre slightly, even though the mating
surfaces on the caliper are central. The inner pad has around .5 - 1mm
gap when the brake is off, the outer is almost touching, and it can be
heard slightly scraping the disc when the wheel is spun.

I don't have experience of Hope discs, but similar problems with Shimano
can be caused be uneven movement of the pistons. To check for that,
remove the wheel and pads, and push the pistons back into the bore.
Then *carefully and slowly* press the lever and watch to see if both
pistons move together. It sounds like they're not. When I've had that,
I've pushed the pistons out as far as I dare, then lubricated with brake
fluid, followed by pushing in and out a few times. This just re-lubes
the seals. For obvious reasons, don't go mad on the brake fluid!

Once the pads and wheels are refitted, slowly press the brake lever
until both pads are touching the disc, then squeeze hard and hold for a
second or two. That does it for me every time - both pads move off to
maybe 0.5mm from the disc.

--
Paul Boyd
http://www.paul-boyd.co.uk/

in message <1hrqoz8.1fnl6uxget1kwN%alan@darkroom.+.com>, A.Lee

('alan@darkroom.+.com') said:

I'm just refitting my Hope 2 piston disc brakes, the front is no
problem, but the rear, although the caliper is set central to the disc,
the outer piston/pad touches the disc before the inner pad touches the
disc.
In effect, the caliper is off-centre slightly, even though the mating
surfaces on the caliper are central. The inner pad has around .5 - 1mm
gap when the brake is off, the outer is almost touching, and it can be
heard slightly scraping the disc when the wheel is spun.

Should I ignore the surfaces on the caliper, and centralise it with the
pads as a guide point?
I'm sure that must be right, but thought I'd ask for more opinion.

Hope sell a kit to grind the mating surfaces on your frame/fork so
they're /exactly/ square with the axle. That's a place to start. Once
you're sure it's /exactly/ square, use shims to get the adjustment. Check
that both pistons retract properly.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; It's dangerous to be right when the government is wrong.
;; Voltaire RIP Dr David Kelly 1945-2004

On 01/10/2007 19:39:54 alan@darkroom.+.com (A.Lee) said:

I'm just refitting my Hope 2 piston disc brakes, the front is no problem,
but the rear, although the caliper is set central to the disc, the outer
piston/pad touches the disc before the inner pad touches the disc. In
effect, the caliper is off-centre slightly, even though the mating
surfaces on the caliper are central. The inner pad has around .5 - 1mm
gap when the brake is off, the outer is almost touching, and it can be
heard slightly scraping the disc when the wheel is spun.

Quoted message said:

Should I ignore the surfaces on the caliper, and centralise it with the
pads as a guide point? I'm sure that must be right, but thought I'd ask
for more opinion. Thanks Alan.

There are a few possibles on this, one is motivation of the pistons, as
has been suggested push both pistons back in completely then pump the brake lever to adjust the caliper, if that does not cure it then check to see if the disc is catching one end of the pad and not the other, or the to or bottom of the pad, if this is so you may need the brake mount machining to
ensure correct caliper alignment.

Failing that, try fitting some very thin shims to push the caliper out a little, if you have trouble getting them, email your address to me via the
website in my sig and I will send you a few.

--

Buck

Give a little person a little power and create a big problem.

http://www.catrike.co.uk