Shimano R785 disc rub

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grnrcr
Posts: 105
Joined: Sat Nov 05, 2011 12:23 am

by grnrcr

The R785 hydraulic Di2 setup on my bike is driving me nuts. The front brake always rubs after hard braking coming down a steep descent. Not long descents. Not dragging the brakes. It seems as soon as I brake hard, the front starts rubbing and it takes a few seconds for it to go away. Worse still, after the rubbing goes away, I start getting brake rubs when I get out of the saddle and really torque the front end.

It just seems the pistons on the front caliper are not retracting properly and for some reason, they go out of alignment after hard braking so I am able to make them rub when I get out of the saddle.

Does this sound familiar to anyone? How did you get it fixed? I took it to a LBS but they couldn't find anything wrong. I am tempted to just buy a new front caliper and see if that fixes it.

Thoughts?

seanblurr
Posts: 297
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by seanblurr

Generally disc will start to rub when hot, this is normal. Things expand. The off/on "rubbing" (i.e not constant rub) is generally due to a slightly out of whack disc. Normally unnoticeable, but when things expand a fraction of an inch, you start to notice it.

How new is the bike? Sounds like you may be a fairly new convert to discs.
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11.4
Posts: 1095
Joined: Tue May 23, 2006 4:33 am

by 11.4

When you adjusted your calipers originally and bled them, did you have a proper spacer between the pistons? If not, you can end up setting them too close. They do need some slack in the tolerances to accommodate slight deformities in the rotor. Even if the rotor looks perfect when cold, when it gets really hot it can get a little bit of wave into it. Remember the metal doesn't have anywhere to expand when it gets hot except sideways, which makes a temporary wave in the disc.

grnrcr
Posts: 105
Joined: Sat Nov 05, 2011 12:23 am

by grnrcr

Yep, new to disc brakes and have really enjoyed them except for what I'm describing. I'll have the rotor looked at. Didn't realize quick and hard braking can generate that much heat to expand the rotor that much.


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CBJ
Posts: 1058
Joined: Tue Feb 02, 2010 4:22 pm
Location: Brooklyn

by CBJ

I have never had rub due to expansion and this is riding prolonged down hill mountain biking. I think you just need to make sure you rotors are centered between the pads. One trick is to loosen the caliper and stick in a paper business card on each side to make sure the calipers are not pressed in too far. Not sure if the shop has experience with dics brakes even though it's not that difficult. There are tons of good videos on Youtube that will tell you how to center your rotors. I highly doubt its the caliper that needs replacement.

TheKaiser
Posts: 653
Joined: Thu Sep 05, 2013 3:29 pm

by TheKaiser

grnrcr wrote:The R785 hydraulic Di2 setup on my bike is driving me nuts. The front brake always rubs after hard braking coming down a steep descent. Not long descents. Not dragging the brakes. It seems as soon as I brake hard, the front starts rubbing and it takes a few seconds for it to go away. Worse still, after the rubbing goes away, I start getting brake rubs when I get out of the saddle and really torque the front end.

It just seems the pistons on the front caliper are not retracting properly and for some reason, they go out of alignment after hard braking so I am able to make them rub when I get out of the saddle.

Does this sound familiar to anyone? How did you get it fixed? I took it to a LBS but they couldn't find anything wrong. I am tempted to just buy a new front caliper and see if that fixes it.

Thoughts?


Regarding the rubbing, I agree with the others that heat can slightly expand things, but there should be enough pad/rotor clearance to accommodate this without substantial rub. I second the business card trick that CBJ suggested, but if you want to be really thorough, before doing that, add a few other checks to the mix.

1. I am assuming you have sighted through the caliper to see that your pads and caliper slots are completely parallel to the rotor faces. I know the shop proclaimed it fine, but many mechanics just listen for rub on the workstand and then proclaim it fine, when really it is barely passable and there is substantial room for improvement. If your caliper is twisted or not centered that can really cut down on your clearance.
2. If the caliper seems set fine, then pull the wheel and pads to expose the pistons. Gently squeeze the lever a bit (not too much or you can pop a piston out!) and look to see if they are both moving evenly back and forth. Then push them both back into the caliper using a tire lever or some other appropriate tool. Hold one in place while squeezing the lever which forces only the other piston to move. Repeat this for both pistons, which will have the effect of ensuring they both move smoothly in and out of their seals.
3. Push them both back in, reinstall pads, wheel, but before squeezing the lever, do the business card trick that CBJ mentioned, with a card on both sides. This will have the effect of setting the piston extension to a point that is slightly wider than normal. It will increase lever throw, but will also offer you more pad/rotor clearance.

Regarding rub when throwing the bike side to side, increasing the clearance as above may do the trick but I should also suggest the obvious, which is ensure that your front wheel is really tight. Are you running open dropouts or through axle?

with open dropouts I have run into the situation with Shimano hubs where one of the axle locknuts is not perfectly square, so when you tighten the QR down it will twist the dropout. Depending on the orientation of the axle when you install it, the twist could happen in any direction, which is maddening when dealing with tight tolerances between pads/rotors.

seanblurr
Posts: 297
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by seanblurr

Might I suggest you ride the bike for a few hundred more miles and then come back and see if the issue still persist. Right now you are on brand new rotors and pads so your factory tolerances are very minimal. Any rotor warpage or heat expansion will cause a rub that might not show itself on a broken in set. My Caad12 Disc did exactly as you are describing after really hard braking, though now after many miles it doesn't happen anymore except in the most extreme situations.

My Enduro and Downhill mountain bikes go through brakes like you wouldn't believe, and what you are experiencing is fairly common. I certainly don't claim to be an expert with all things disc brakes, but in my opinion (however much that matters), I really wouldn't worry too much about this and just ride the bike. Disc's are not some miracle system that is going to cure the worlds problems, despite some in the road market claiming them to be. They make noises, they rub, sometimes they don't work as well, and sometimes you need to replace parts.
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grnrcr
Posts: 105
Joined: Sat Nov 05, 2011 12:23 am

by grnrcr

Super helpful, guys. Really appreciate it.

@TheKaiser I've read about steps similar to what you wrote. I assumed the shop must have done them already. I'll do it myself to be certain. Thanks for taking the time to write the reply.

@seanblurr it's reassuring to know what I'm experiencing isn't too out of the norm. Thanks for sharing your experience with this disc newb.


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grnrcr
Posts: 105
Joined: Sat Nov 05, 2011 12:23 am

by grnrcr

Just spent some time on the bike. Here is what I found:

The tolerance between pads and rotor is definitely a lot slimmer in the front than back. I adjusted the front by pushing the pistons back, inserting two business cards between rotor and pads, then squeezing the lever to set the "zero" position of the pistons. However, as soon as I completed the steps, removed the business cards, then squeeze the brake a few times, the added tolerance went away and I am no longer able to insert the business cards between the rotor and pads.

Do I have too much brake fluid in the front system and it's causing the pistons to sit too close to the rotor?

Ravnsnaes
Posts: 103
Joined: Thu Dec 22, 2005 2:48 pm
Location: Denmark

by Ravnsnaes

I've had similar problems, with the front brakepads rubbing the disc after hard or prolonged braking. Switching from organic pads to metal sintered ones seemed to help, because the organic material on my original pads was starting to crumble and disintegrate, eventhough I live in a totally flat area north of Copenhagen and don't get my brakes anywhere near hot enough, to cause any kind of overheating or damage to the discs or pads.

seanblurr
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Joined: Thu Sep 05, 2013 7:47 pm
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by seanblurr

Unfortunately, discs don't work that way. You can't just remove fluid.
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