Disc brake pad alignment - It doesn't have to be a pain

Discuss light weight issues concerning road bikes & parts.
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menkar
Posts: 16
Joined: Tue Aug 10, 2010 5:23 am

by menkar

SRAM's description of disc caliper function:
"The slave pistons in the brake caliper use special seals that flex, or roll, slightly when the pistons/pads are pushed toward the rotor during braking. When the brake is released, the piston seals relax and pull the pistons/pads away from the rotor. Another function of these seals is to allow the pistons to self advance as the pads and rotor wear. As pads and rotor wear, the distance between them increases, but because the amount of roll in a piston seal is limited, there is a point where the piston will slip through the seal. This means that as brake pads and rotor wear, the pistons will move further than the seals can roll. As a result, the pistons slip through the seal until the pads contact the rotor. Once the brake is released, the pistons/pads return to a new resting position, closer to the rotor. This eliminates the need to adjust pad clearance as components wear."

A key point to understand in this is that each piston is independent of the other one. If you push both pistons all the way back they don't have to advance equally. I can hold one back while advancing the other. I can thus align the pads to a rotor that isn't centered in the caliper. The only requirement is that the caliper be aligned parallel to the rotor. I loosen/tighten the caliper screws just once to make it parallel and I'm done with ever having to move the caliper. From this point on the pistons/pads can be individually advanced to the rotor without dealing with caliper screws. Whenever I replace a rotor or pads, or the pads rub because they didn't advance evenly I push them back and carefully re-advance each of them to be an equal distance from the rotor.

I'll also mention that business cards or special spacing tools have never worked for me because the lightweight road discs on my bike are too flexible to hold the caliper in a fixed position as I tighten the screws.

LiquidCooled
Posts: 108
Joined: Mon Oct 02, 2017 5:46 am

by LiquidCooled

menkar wrote:Whenever I replace a rotor or pads, or the pads rub because they didn't advance evenly I push them back and carefully re-advance each of them to be an equal distance from the rotor.
Could you describe in a bit more detail how you do this? Do you remove the wheel, re-advance a piston, put the wheel back, check clearance, repeat? Or do you somehow do it with the wheel in place? Etc?
2017 Giant TCR Advanced Pro 0 Disc
2003 Cannondale R1000 (CAAD7)

by Weenie


menkar
Posts: 16
Joined: Tue Aug 10, 2010 5:23 am

by menkar

I have 160 mm SRAM Centerline X rotors which are somewhat flexible. I can deflect them about 1 mm by pushing on them with my fingers and thumb. The gap to the pads on each side of the rotor is ¼ mm. Starting with the wheel installed and pistons/pads pushed back so there are large gaps, I start advancing the pads with the brake lever while carefully watching the gaps close. Usually one side will close faster. When the smaller of the gaps gets to about 1 mm I push the rotor with my fingers and thumb until it touches that pad to prevent it from advancing further. While holding it there I keep carefully advancing the other one until its gap is similar. I frequently let go of the rotor to check the centering and push it one way or the other depending on which pad I want to hold back until the pads are fully advanced. This took me a little while to get the hang of but I've changed the pads a number of times and the rotor once and I've never loosened the caliper screws.

I wear clean gloves when touching the rotor to avoid contaminating it and I clean the pistons with a q-tip and brake fluid before pushing them back into the caliper.

andreas
Posts: 164
Joined: Thu Jun 12, 2014 10:21 pm

by andreas

That is actually the same method that Hope is promoting in their manuals.
CENTRALISE THE PADS OVER THE DISC

This step is very important and mustn’t be ignored.
Gently pump the lever to bring the pads closer to the disc.One pad might enter in
contact with the disc before the other. If this happens, hold the disc against the pad
that is already in contact with the disc to allow the other one to move.
For an optimised lever feel, both pads must enter in contact with the disc at the
same time and allow the same clearance (see arrows) when retracted.The disc
should not be flexing at any time.

jencvo
Posts: 78
Joined: Thu Oct 18, 2018 12:01 pm

by jencvo

andreas wrote:
Sun Jul 07, 2019 9:09 pm
That is actually the same method that Hope is promoting in their manuals.
CENTRALISE THE PADS OVER THE DISC

This step is very important and mustn’t be ignored.
Gently pump the lever to bring the pads closer to the disc.One pad might enter in
contact with the disc before the other. If this happens, hold the disc against the pad
that is already in contact with the disc to allow the other one to move.
For an optimised lever feel, both pads must enter in contact with the disc at the
same time and allow the same clearance (see arrows) when retracted.The disc
should not be flexing at any time.
Do you have a link to the manual or a video where they say that?

TheDoctor
Posts: 44
Joined: Sun Dec 17, 2017 9:56 pm

by TheDoctor

jencvo wrote:
Thu Jul 11, 2019 12:32 am
andreas wrote:
Sun Jul 07, 2019 9:09 pm
That is actually the same method that Hope is promoting in their manuals.
CENTRALISE THE PADS OVER THE DISC

This step is very important and mustn’t be ignored.
Gently pump the lever to bring the pads closer to the disc.One pad might enter in
contact with the disc before the other. If this happens, hold the disc against the pad
that is already in contact with the disc to allow the other one to move.
For an optimised lever feel, both pads must enter in contact with the disc at the
same time and allow the same clearance (see arrows) when retracted.The disc
should not be flexing at any time.
Do you have a link to the manual or a video where they say that?

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by Weenie


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