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Weird Disc Brake Behavior

by mikepulsifer
Tue, Apr 02, 2019 13:27
13

I recently replaced my disc rotors, replacing the ones that came with my Domane (Shimano) for a set of Ultegra rotors.  When I put the wheels back on, everything spun just great.  No rubbing whatsoever, even when I tested the brakes on the stand.

Here's where it gets a bit weird and frustrating.  When out on the road, when I come to a stop, there will be rubbing afterwords for about 30 seconds and then it disappears.  It's more pronounced if it's a fairly vigorous stop - not an emergency stop, just vigorous.

I put the bike on the stand and like before, there's no issues.

 

Any idea what's going on and how I can fix this?

 

Thanks

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13 comments

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Jack Osbourne snr | 5 years ago
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Bloody hell! That was a lucky escape!

Well done for testing the brakes properly in the stand. Much better to happen there than on the road.

Glad the Q tip/cotton bud and brake fluid method worked for you... You can tick off another box in the list of DIY maintenance tasks!

 

Good luck with the Duathlons 

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mikepulsifer | 5 years ago
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Well, this was an adventure (that I hope is over). 

I started off by taking the front wheel and pads off and giving the brakes a few good presses and sure enough, the pistons had a good deal of black gunk.  What really stunned me was when I got in there with the mineral oil and Q-Tip.  Chunks and flakes of crap where coming out.  Anyway, I got things seemingly working well, got the pads and wheel back on and started testing the brakes on the stand.  At one point, gave the brake lever a good hard squeeze and (POP!) the brake line broke into two at the splice/joint/whatever in the middle of the line.  Brake fluid spread everywhere.  

With my commute, I'm in no state to visit my LBSes mid-week.  They're closed Sundays.  I have my first duathlon in two weeks.  I scrambled and took my bike to my nearest LBS - the one that built the bike.  He'd have to order parts and the earliest he could look at it would be Wednesday.  I called the LBS that's a half-hour away, but sponsors my team.  They'd see what they could do, depending on how busy they were (they've been busy the last couple weekends).  I got there, he looked at it and thankfully, they were quite at the moment.  It appears as though that joint/whatever wasn't tightened properly when built and it was just an accident waiting to happen.  Back line wasn't much better.  He commended me on my work with the pistons (a relief) and I went home to finish up with the back.

Next morning:  40 mile hard (for me) ride with my teammate and at about mile 30, my first really HARD braking (from speed down a hill to a stop sign).  For the next 3 miles, audible rubbing.  Cussing ensued.  Later that night, I got in there again, and was able to remove a little more junk.

I hope this is it.

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Pilot Pete | 5 years ago
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Potentially a sticky piston, I had exactly this issue with a 2 month old 105 caliper on my winter bike this week. 

To fix it I removed the wheel and pads (then operated the brake which confirmed the diagnosis - one piston moving much more than the other).

I then got a flat metal bar and used a quick release clamp to clamp the bar holding the working piston in its fully retracted position. I then operated the brake again which forced the sticking piston out. I pushed it back and did this several times to free the movement. I then cleaned with isopropyl alcohol and forced it back in again, operated a few times and then replaced the pads and refitted the wheel. All good.

The other issue that could cause it is some trapped air in the system. I need to re-bleed the summer bike as I found today after hard braking downhill the heat generated was transferring into the caliper and the pads were scraping for the next 30 seconds on the flat with the brake released. The heat expands the air and the pads don’t retract fully until it cools again. Re-bleeding to remove the air, which will be at the caliper end should cure the issue.

 

PP

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Jack Osbourne snr | 5 years ago
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Let us know how you get on.

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mikepulsifer | 5 years ago
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Thanks.  Will give it a go.

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Jack Osbourne snr | 5 years ago
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That points towards sticky pistons.

 

Beacuse of the forces at play when you are actually riding, sticky pistons might not be obvious in the workstand, and even testing as I described above won't diagnose every case.

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mikepulsifer | 5 years ago
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What I've done on the road to try to head it off is pumping the brakes and braking more slowly (like an old man driving his car).  It doesn't solve it, but the rubbing isn't as noisy.  It's after more "normal" braking that I have this more pronounced rubbing for a bit.

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Jack Osbourne snr | 5 years ago
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Put the bike in the stand.

Wind up the back wheel to a good rate of knots and pull the brake on. Hard. Not just enough to stop the wheel... Really squeeze the lever.

If it rubs immediately after doing this, it's almost definitely sticky pistons.

 

If it doesn't, it could be something else although I'd be surprised if it was rotor expansion unless you are dragging the brakes before applying fully.

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mikepulsifer | 5 years ago
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I followed instructions from a Park Tool video when I swapped the rotors.  I should have thought there were other Park Tool videos that addressed this.

Interesting thing, though, which I hope doesn't matter.  I put my old rotors on some older wheels (where they were originally before I upgraded my wheels) and I didn't have this issue.

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Jack Osbourne snr | 5 years ago
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Using a bit of mineral oil on a cotton bud to help free and clean sticking pistons followed by a cleanup is not a schoolboy error. It's a widely used and accepted practice in professional workshops.

Dripping oil onto the pistons and then not cleaning it up would be.

Here's the very same Calvin from Park Tool explaining how to do it in the way I do it...

https://youtu.be/vQXFFgRButo

 

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kevvjj | 5 years ago
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Don't lube the pistons - schoolboy error.

Watch this - the best way to fix sticky pistons.

https://www.pinkbike.com/news/tech-talks-fixing-sticky-pistons-park-tool...

 

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the infamous grouse | 5 years ago
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friction -> heat -> rotor expansion.

you may have to eyeball the rotor centering between the pads, rather than using the tension-centering method.

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Jack Osbourne snr | 5 years ago
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Are you using organic or sintered pads?

Did you bed them in properly with the new rotors?

Did you torque the bolts/Centrelock properly?

Its probably sticky pistons though. Take the wheel and pads out and free them by working them back and forward aided by a piston press and give them a very light lube with some mineral oil on a cotton bud. Make sure you clean off any excess and give the rest of the caliper a wipe down with isopropyl alcohol or a proprietary brake cleaner

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