Brake Question
#1
05-27-2012, 06:02 AM
#2
05-27-2012, 06:49 AM
It shouldnt be excessive.. When the pressure that applies the caliper is released, any contact with the rotor should help to return the piston, basicaly "clearancing" itself. You'll always have some rub unless you have a return spring, cant recall any HD I've seen having one.. If its excessive, does it slow the bike? You may have a caliper sticking in the bore.
#3
05-27-2012, 10:19 AM
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#4
05-27-2012, 12:47 PM
Road Master
#5
05-27-2012, 02:27 PM
#7
05-28-2012, 07:21 AM
Club Member
When applying the brake the pistons obviously move and press the pads against the rotor. When releasing the brake the pistons should retract, but the pads will simply be pushed away by the rotating rotor.
When things drag two simple things may cause it. One is that a build-up of brake dust limits free movement of the pads in the caliper, the other is that dirt prevents the pistons fully retracting. One of my non-Harleys needs regular cleaning to free the pads.
So I suggest removing the pads, disconnecting the caliper and cleaning any dust and dirt you can see. An old toothbrush is useful!
When things drag two simple things may cause it. One is that a build-up of brake dust limits free movement of the pads in the caliper, the other is that dirt prevents the pistons fully retracting. One of my non-Harleys needs regular cleaning to free the pads.
So I suggest removing the pads, disconnecting the caliper and cleaning any dust and dirt you can see. An old toothbrush is useful!
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#9
05-28-2012, 05:53 PM
Road Master
For reassembly of the caliper halves, use a new rubber seal washer and apply thread lock adhesive to the bolts. In some cases the old seal washer can be re-used successfully, but you may not know if it works until it's back on the car under pressure.
There is a nice reason why return springs are not needed on disk brake calipers. The internal seal is a square section o-ring held in a groove in the caliper bore. Hydraulic pressure seal is affected against the smooth OD of the piston for the 1600, or against the cylinder bore for the Twin Cam. As the piston moves forward to apply the brakes, the rubber seal drags a bit on the mating surface because of friction. The rubber seal is a bit flexible and springs a little to deform slightly, being pulled along by the movement of the piston. When the hydraulic pressure is released, the rubber seal springs back and pulls the piston back a little bit along with it. This provides just enough return motion to give a thin air film space in front of the friction pad so it will not drag on the rotating disk.
Problem comes when the rubber seal loses the flexibility as it ages and goes hard. When the piston seal is no longer flexible it will not provide the proper return motion for the piston, and the brakes drag, and the pads wear faster. The cure for this is to repack the calipers to install new rubber seals. Notice that the only return force for the piston comes from the flexibility of the rubber seal. This is why it is very important that there should not be any restriction in the hydraulic line, such as a deteriorated hose. The piston also has to move freely in the bore (exclusive of the seal function) for this to work
There is a nice reason why return springs are not needed on disk brake calipers. The internal seal is a square section o-ring held in a groove in the caliper bore. Hydraulic pressure seal is affected against the smooth OD of the piston for the 1600, or against the cylinder bore for the Twin Cam. As the piston moves forward to apply the brakes, the rubber seal drags a bit on the mating surface because of friction. The rubber seal is a bit flexible and springs a little to deform slightly, being pulled along by the movement of the piston. When the hydraulic pressure is released, the rubber seal springs back and pulls the piston back a little bit along with it. This provides just enough return motion to give a thin air film space in front of the friction pad so it will not drag on the rotating disk.
Problem comes when the rubber seal loses the flexibility as it ages and goes hard. When the piston seal is no longer flexible it will not provide the proper return motion for the piston, and the brakes drag, and the pads wear faster. The cure for this is to repack the calipers to install new rubber seals. Notice that the only return force for the piston comes from the flexibility of the rubber seal. This is why it is very important that there should not be any restriction in the hydraulic line, such as a deteriorated hose. The piston also has to move freely in the bore (exclusive of the seal function) for this to work
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