hamishm wrote:My new mountain bike has Avid Elixir R hydraulic discs which are rubbing on the pads. Both wheels.
Firstly, should they be completely clear of the pads when disengaged? I'd have guessed yes, but I'm told car brakes rub lightly on the pads always.
When the bike arrived (internet-sourced) the back brake pistons were very closed. I gently pried them open. I did the same to the front after I mistakenly operated the front brake without the wheel installed. After prying open, the pads are clear of the rotors, but as soon as I operate the levers they don't open enough and still rub. I think they are only rubbing on one side.
The Elixir R instructions say to align the calipers by loosening the mounting bolts, then tighten them while holding the brake closed. This hasn't helped either.
What am I missing?
Discs are supposed to run tight clearances. They self-adjust - if you push the pads back (ie push the pistons back) then apply the brake, the pistons will advance in their seals until they're running the correct clearance again.
So, given that the gap between the pads is pretty much set (to 1 rotor thickness + design clearance), the only variables you really have are to centre the caliper over the rotor (so there's the same clearance on each side), align the caliper (so the pad face and rotor face are parallel... if they're angled you have much less effective clearance), and ensure the rotor is straight.
The trick of loosening the bolts, applying the brake (several times then holding) then tightening the bolts takes care of centring and alignment. Unless your rotor is bent, you should have the same clearance - albeit fairly tight clearance - between the rotor and each point on each pad.
You might find it improves with a bit of use too, as the piston seals get used to what they're doing and the pad surface wears flat against the rotor.
who also got Avid Elixirs on his new toy, and they have improved somewhat after a ride