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My wheels don't spin cleanly and I can see that the brake pads rub slightly when the levers aren't being pulled.
A while back, my LBS told me the rotors were worn slightly below minimum thickness.
Is this likely to be the cause of brake rub (e.g. pistons over-extended in the seals)?
Can I "reset" the piston position so they don't rub (assuming just pushing pistons back in will fix it until I brake again...)?
Trying to figure out whether I need to replace rotors or not.
No. You probably just need to centre the calliper again.
Slacken the bolts. Pull the brake hard. Hold it and nip the bolts.
Go for a ride and celebrate your success
Is it rubbing all the time, or just at one (or two) points on the rotation? If the former then recentring the caliper as above may sort it.. if the latter the rotor may just be slightly bent and you should be able to bend it back with a pair of pliers.
Both front and rear (& both inner and outer pads) are rubbing slightly around the whole disc. Which is why I was suspecting the pistons sitting too far out in the seals...
I can't see any warping of either disc though I guess there could be a very slight warp.
(Shimano Ultegra btw)
Sounds like your pistons don't want to retract properly, take wheel & pads out, clean around the pistons, silicone spray them and push back in, I put my fingers in and pull the lever to see and feel if the pistons are pushing out then back in properly, push them back again and re-centre.
Take the pads off first though 😆
Trying to figure out whether I need to replace rotors or not.
A while back, my LBS told me the rotors were worn slightly below minimum thickness.
Trying to figure out whether I need to replace rotors or not.
A while back, my LBS told me the rotors were worn slightly below minimum thickness.
Yeah, I know, but I'm asking are the two issues directly related! Concensus would seem to suggest not 😉
Sounds like your pistons don’t want to retract properly
That's not how disc brakes work.
Check that the rotors aren't warped. If they are, you will never get them to stop rubbing.
Remove the pads, replace if they're badly worn. Extend the pistons by carefully pulling on the brake lever. Clean the pistons with warm water and washing up detergent and a toothbrush, then rinse with warm water. Put a few drops of the appropriate brake fluid on each piston, then fit some old pads and use a screwdriver between the pads to push the pistons back in. Refit the wheel and loosen the caliper mounting bolts, then squeeze the brake lever to extend the pistons and center the caliper. Finally tighten the mounting bolts.
Sounds like your pistons don’t want to retract properly
That’s not how disc brakes work.
Eh? Of course the pistons retract - what are you referring to?!
But, anyway, some suggestions for me to work on...
That’s not how disc brakes work.
Eh? Of course the pistons retract – what are you referring to?!
The retraction is done by the seals. The pistons stick in the seals. When you apply the brakes, the seals deform as the pistons move outwards. When you release the brakes, the seals return to their original shape and the pistons retract. When the pads wear too much, the pistons slip through the seals a fraction and this resets the clearance. The pistons don't slide back and forward through the seals every time you apply the brakes so the "not retracting" idea doesn't describe the reason that brakes rub.
First, if the discs are worn, yes they should be replaced. There might be some safety margin, but even so it's not where you want to take chances.
Second, yes it can cause the disc to rub. There are parts of the disc that don't wear, like the spokes of the disc. On a worn disc these are fatter than the worn braking surface, which can reduce the clearance to the pads, and make rubbing my more likely. I have experienced this recently with Magura MT7, but the pad clearance on those is very small.
Personally, I'd try the pads out, pistons in, toothbrush/brake dust cleaning alcohol spray, piston lube thing. I reckon the pad dust does make them sticky and getting rid of this has helped mine in teh past.
While we are here, a quick question:
use a screwdriver between the pads
I use a range of medieval items from a massive steel tyre lever, tapered needle nosed pliers, screwdrivers...
Is there one tool to rule them all here for the pad pressing back job? Like a G-cramp or similar?
Is there one tool to rule them all here for the pad pressing back job?
Piston setting tool
I've also found worn discs to be far more prone to warping, and depending how well and where they measured, you could be significantly under min thickness on a large chunk of the disc.
(Are the Ultegra discs icetec? As they are even more sensitive to warping when getting thin, due to the aluminium core.)
A couple of unwarranted digs there imho, I thought pistons retracting was perfectly reasonable and well used terminology that only the pendantic would object to, seals pull them back but they're still retracting.
Sadly I was expecting some backlash but thought the silicone would have been the contentious issue, some may say mineral oil only on shimano.
Rubbing around the whole disc suggests the pistons, sorry, the seals aren't doing their job properly, so cleaning and lubing should solve the problem.
A couple of unwarranted digs there imho, I thought pistons retracting was perfectly reasonable and well used terminology that only the pendantic would object to, seals pull them back but they’re still retracting.
The rubbing pads problem is caused by one pad sticking (i.e. not extending), it's not caused by pads not retracting. What happens is the pistons can stick to the seals if they aren't serviced. If one piston sticks, it doesn't self-adjust, but the other one does. The brakes still work fine, but the rotor flexes a bit to take up the larger clearance on the side with the sticky piston. Eventually, the side without the sticky piston will self-adjust so much that there is no clearance and it will rub. Both pistons are extending and retracting, the problem is that the sticky one is so far out of adjustment. So, the problem is that a piston isn't retracting, it's exactly the opposite - one piston isn't adjusting itself out because it has bonded itself to the seal. The cleaning procedure I described above will usually sort it out.
So clean and lube then, roughly the same solution, jesus christ that was pointless and tedious.
Rubbing on both sides of a reasonably straight rotor made me think differently, excuse my ignorance and the use of the word pedantic, you've proven me so wrong.
It's generally easier to diagnose and fix things if you understand what's causing the problem.
Anyway.........
Pads removed, liberal spray of brake cleaner including Q-tip around exposed part of piston, let it dry off and then used a tiny dab of brake fluid on a clean Q-tip to re-lube around piston (luckily I had some Magura mineral fluid handy) followed by re-entering calipers.
Problem sorted (hopefully not just temporarily!)
Will keep an eye on rotors for warping and will replace if/when needed or when funds allow...
A while back, my LBS told me the rotors were worn slightly below minimum thickness.
Trying to figure out whether I need to replace rotors or not.
You've been told they're worn below minimum thickness and you've not replaced them - do you realise that we now just think you're an idiot?
A couple of unwarranted digs there imho, I thought pistons retracting was perfectly reasonable and well used terminology that only the pendantic would object to, seals pull them back but they’re still retracting.
You ok hon?
This place can be unfriendly, and i wish it was better
Will keep an eye on rotors for warping and will replace if/when needed
You'll know when it's the "needed" stage. About the same time you "need" an ambulance 😂
Not sure he is, the OP has been told to change a piece of safety critical equipment and is ignoring the warning. The same issue can also cause weird issues like he's seeing (as the wear isn't uniform across the braking surface). I've had 1 rotor run under thickness in the past. It resulted in a massive leak at the pistons, which went away as soon as the rotor was replaced.
I find his reluctance to change the rotors bizarre.
Bullying is about tone. There are ways of delivering a message (i.e. heed the safety warnings) without trying to mock someone.
Someone on here suggested a thickness measurement gauge a few years back, like this:
Ooh, I’ve never changed a rotor! My Devinci Troy is 6 years old and has the original rotors on it - didn’t even consider that they had a minimum thickness! Guess I better check…
Don’t think those cheapy gauges will work as the rotor will be at different thicknesses at different parts of the braking surface (so that could make it look less worn). Really needs a micrometer
Nope, not at all. Problem with a micrometer is that it measures over a considerable area. It cannot get into any grooves properly.
Are your thinking of vernier calipers? Micrometers are absolutely capable of measuring concave wear levels on a rotor
Edit: ignore, through bleary eyes I’ve just realised what your calipers are doing. Probably actually really good for a garage. Micrometers still work grand for the sizes and areas we’re talking about though
If you are using a micrometer with flat measuring faces they cannot fully penetrate the groove and thus will return a false reading when the groove may have made the disc too thin.
You’ve been told they’re worn below minimum thickness and you’ve not replaced them – do you realise that we now just think you’re an idiot?
And you do realize that we now just think you're a judgmental prick?
How this thread will end.
Judgemental, belittling and condescending replies say more about you than the person on the receiving end. scienceofficer, if that's all you can input to a thread then that's quite sad, glad you got that out of your system though and feel better hon. We all have our weaknesses, I should just let it lie sometimes.
Glad Vlad got it sorted in the end.
As others have said, replace the rotors. I've seen a large rotor dramatically bent out of shape when it was below spec. I wore some Hope rotors down to about 1.1mm at which point they were (unsurprisingly) overheating all the time...
rotors are a wear item.
I've just sorted the same issue. In my case, it was grubby pistons (oo er!). Followed thols2's advice and it worked a treat. Thanks @thols2!
Cheaper Ebay link for what Ambose posted, another additional tool to the collection 🙂
FWIW, the Ultegra discs appear to be IceTech so when you get down to min thickness, you're actually in a worse state than with a steel disc, as you're relying on aluminium for strength of the disc...