1991 R100GS - puzzling brake problem - Any front brake/wheel experts out there?

My 1991 GS is a 150,000 mile machine. It recently developed a pulsing front brake when it was applied. So, I took the following steps.
- mic'd the rotor and found it was below thickness spec, but thickness was consistent throughout rotation.
- used a dial indicator (mounted to fork) to check for run-out. It was .002" total, or .001" in each direction.
- I noticed that for about 90 degrees, the brake pad(s) would drag. For the remainder of rotation, it rotated freely.
Neither run out or thickness seemed too far off, but not believing my readings, and having a preconceived notion that the rotor was causing the problem, (it had 90,000 miles on it) I ordered the appropriate EBC rotor, and mounted it. Also installed a set of aftermarket 'all balls' front wheel bearings to eliminate them as a source of the problem. I snugged the axle nut and caliper bolts to a high enough torque reading that they were well in place. New pads haven't arrived yet, so I tested with the old pads by slowly turning the wheel by hand with it elevated off the floor.

The front brake continued to drag same as before when wheel was turned through approx. 90 degrees of rotation. For the remaining 270 degrees or so, no dragging. Same problem, but now with new rotor and wheel bearings. I marked the rotor with chalk to confirm that the dragging was taking place at the same place each wheel revolution. It was.

I'm at a loss. Your thoughts would be appreciated.

Ohiorider

How true is the hub to which the rotor is bolted? Can you check the runout there?

I would scribble a bunch of magic marker on the rotor surfaces and see how evenly it all wipes off when you apply the brake gently.

Thoughts

Thanks 20774 and Anton ....

I'm about through fooling with the problem for the day, but I did try loosening the caliper, and taking two feeler gages blades (10mm) and blocked the rotor from butting up totally to the fork. I shimmed between the caliper and fork, which moved the caliper just slightly away from the fork mount. Most of the friction vanished. However, if this is only because I've moved the rotor slightly away from the inside pad, which appears to be the source of the dragging, shimming the caliper probably won't cure the problem. However, I'm thinking that if the hub where the rotor mounts is slightly off, if it might be possible to add very thin shim(s) under one or two bolts, perhaps getting the disc in alignment. First things first ...... I need to make sure I mount a dial indicator securely, and try to get a better reading. It will be easier with a brand new rotor, since it is perfectly smooth all the way to the outer perimeter.

What about loosening one or more of the fasteners that hold the rotor on? Take runout readings with several different fasteners loose...see if the rotor is being distorted which would indicate something wrong with the hub/attachment.

I can give that a try also

Thanks for that suggestion, too. I'll let you guys know what I encounter.

I'd wait until the new pads are in place before doing much else.

Pads

ccolwell said:

I'd wait until the new pads are in place before doing much else.

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That's the current plan. Ordered EBC 77 pads from on line supplier. They shipped EBC 171. For different model airhead. They UPS'd correct pads late Fri afternoon, so I'll probably receive them today or tomorrow.

What about taking the rotor off and remount it moving it in a different spot ie: one bolt hole over then two bolt holes over. To see if it makes a difference?

In a different perspective. Maybe the issue is the pistons in the caliper are not retracting enough due to grime on the edges of the piston. You were running a narrow (worn rotor) and pads that were thin, that would have the piston out to it's extended position. Your 002" run out does not seem too excessive.

jagarra said:

In a different perspective. Maybe the issue is the pistons in the caliper are not retracting enough due to grime on the edges of the piston. You were running a narrow (worn rotor) and pads that were thin, that would have the piston out to it's extended position. Your 002" run out does not seem too excessive.

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Given that there are no springs to retract the pistons, slight runout in the rotor is what pushes the pads away when the brake is released.

PGlaves said:

Given that there are no springs to retract the pistons, slight runout in the rotor is what pushes the pads away when the brake is released.

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This not my understanding, especially when it comes to the single piston ATE disc breaks...could be for other systems like Brembo. On the ATE, it's the distortion of the black square o-ring in the bore of the caliper that pulls the piston back. Runout might help, but I don't think it's the primary reason.

20774 said:

This not my understanding, especially when it comes to the single piston ATE disc breaks...could be for other systems like Brembo. On the ATE, it's the distortion of the black square o-ring in the bore of the caliper that pulls the piston back. Runout might help, but I don't think it's the primary reason.

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OK - I won't disagree. But I will then next ask, how new and pliable is that square section piston seal? Or is it old, hard, and sliding? Or not sliding?

20774 said:

This not my understanding, especially when it comes to the single piston ATE disc breaks...could be for other systems like Brembo. On the ATE, it's the distortion of the black square o-ring in the bore of the caliper that pulls the piston back. Runout might help, but I don't think it's the primary reason.

Click to expand...

+2. It's my understanding that's true for disc brake calipers in general. It would be worth replacing the seals on a 25 yo bike.

I think the solution was right in front of me

Kind of embarrassed to post this, but what the hey ..... you guys were nice enough to offer several good solutions.

At 128,000 miles, I had the dealer rebuild my brake caliper and master cylinder. Two sets of pads arrived. Dealer installed one set, I decided to keep the other for a spare. At 148,000 miles (4000 miles ago, the front brake began making stops erratically. I pulled the caliper and noted that the pads had a strange wear pattern. They had a narrow semi-circle where the outer edge of the rotor wasn't making contact with the pad. No problem .... I have an old set of spares in the drawer. Put them on and things were good for 3000 miles or so. Then the front end began pulsing when I applied the brake.

Short story long, I just last week purchased and installed an EBC rotor and set of pads, but before installing the new pads, I jacked the wheel off the ground and spun it by hand and noted the brake was still binding noticeably over 90 degrees of rotation. That's when I began to think that something on the mounting hub was out of line.

Today my FA77 pads arrived. I pulled the current pads from the caliper, and damn, the same wear pattern was present. Placed one of the new pads back to back with the one I'd just removed ..... and no wonder! the pad material on the FA77 was in a different position from the pad I'd just removed.

Installed the FA77 pads, and with the brakes pumped up, again rotated wheel by hand and found there was no single spot where the brake dragged. It was apparent the pads were lightly touching the rotor.

Took her for a short ride, and found no pulsing of the front end. The new brake feels a little weak, but it got better near the end of the 20 mile ride, so I guess they need to bed in.

Sorry for running you guys through the hoops!

Bob (Ohiorider)