Rubbing disc brakes

Discuss light weight issues concerning road bikes & parts.
RocketRacing
Posts: 882
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

So my new tt bike has rubbing front disc brakes. Sram s900 aero hrd. I was able to subtlly bend the rotor to eliminate rub, and get good central running of the rotor between the pads. But after a ride. It is back. Small adjustment and all is good. A bit of braking, back. Fixed again.

Tolerances are tight, and i presume the issue is with small shifts in the brake pads.

I admit to not bedding the system, so i will do that tomorrow. I have about 150kms on the bike now, but mostly flat to hilly stuff, so no big stress on the brakes.

Thoughts? Is this a problematic system (sram...) or doe things just need bedding and more time. I wonder if a thinner/more worn rotor might have less clearance issues.

All my other disc brake bikes are fine (cx, xc, fatty). This is my first disc bike for road.

Tips/opinions appreciated.

by Weenie


Hexsense
Posts: 935
Joined: Wed Dec 30, 2015 12:41 am

by Hexsense

1. Is one piston move further than another when you pull the lever?
It is fine if it is a little bit more, but if one is clearly move a lot more then you have one sticky piston.
watch this video to clean brake piston. https://www.youtube.com/watch?v=vQXFFgRButo

2. Hydraulic brake can somewhat self adjust. That can backfire and make correct set-up harder too.
The correct procedure before adjusting the alignment include complete brake piston position reset (manually push the piston back into place all the way on both side). Park tool video above also show how it's done.
Then, unbolt both brake mounting bolt to let it move around. Insert disc brake alignment tool (get one from Amazon or anywhere else) onto disc rotor. Light squeze brake lever into place while also wiggle the caliper a bit to make sure it sit in alignment then lock the bolt. If your disc rotor is not completely straight, find a spot on disc rotor that would set caliper good for your set-up...

3. thru-axle recommended torque are generally around 15N.m My bike put disc in different place between 0-10 N.m. At 10-18N.m i don't have brake rub on my set-up. If you take wheel off and on, make sure to tight it to similar torque each every time.

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Lewn777
Posts: 833
Joined: Thu Mar 09, 2017 5:35 am

by Lewn777

Right so you are truing the disk and the warp keeps coming back?
The newness of the disk won't matter as the pads should be self centering.
I would check the tourque on the rotor bolts.
A good bedding-in heat cycle might help the situation, if the rotor is still going out of true it might be a manufacturing issue. A feeler guage is a good investment if you don't have one.

ChiZ01
Posts: 314
Joined: Wed Jan 01, 2014 6:20 pm

by ChiZ01

sounds like you need to reface the fork's disc flat mount. This is a very common problem when the manufacture cheap out on QC

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Alexbn921
Posts: 230
Joined: Wed Apr 10, 2019 6:39 pm

by Alexbn921

Disks pads take a good 100 miles to wear in and everything else to settle in to a stable state.
Even new disks don't come perfectly straight. You can eye ball it but getting a dial gauge on a truing stand to get them perfect is worth it.
Removing the pads and cycling the pistons will help with a sticky piston. Also make sure the mounting surface is perpendicular to the rotors. Sometimes it can be all wonky.
I use a flash light or white paper on the back side of the caliper to see the gap between the rotor and pads.

gaspard123
Posts: 12
Joined: Wed Nov 08, 2017 6:07 pm

by gaspard123

don't assume the caliper is correctly installed. it needs to be perfectly parallel to rotor and torqued to spec (i.e. pretty tight mounting bolts)

RocketRacing
Posts: 882
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

Hexsense wrote:
Fri Sep 13, 2019 12:36 am
1. Is one piston move further than another when you pull the lever?
It is fine if it is a little bit more, but if one is clearly move a lot more then you have one sticky piston.
watch this video to clean brake piston. https://www.youtube.com/watch?v=vQXFFgRButothe NDS

2. Hydraulic brake can somewhat self adjust. That can backfire and make correct set-up harder too.
The correct procedure before adjusting the alignment include complete brake piston position reset (manually push the piston back into place all the way on both side). Park tool video above also show how it's done.
Then, unbolt both brake mounting bolt to let it move around. Insert disc brake alignment tool (get one from Amazon or anywhere else) onto disc rotor. Light squeze brake lever into place while also wiggle the caliper a bit to make sure it sit in alignment then lock the bolt. If your disc rotor is not completely straight, find a spot on disc rotor that would set caliper good for your set-up...

3. thru-axle recommended torque are generally around 15N.m My bike put disc in different place between 0-10 N.m. At 10-18N.m i don't have brake rub on my set-up. If you take wheel off and on, make sure to tight it to similar torque each every time.
1. the NDS contacts the rotor first. THis is the side that is prone to the rub. However, when released, they both fall back the same distance. What is interesting is that the NDS pad moves BEFORE the DS. This makes me think there is stick, or much more likely, a bubble in the DS of the system. Would a sticking caliper piston be common for a brand new bike? In my mind, this assymetry could bend the rotor, but one would expect to have the bend go in the other direction from what I am correcting.

I should add that in a neutral position, there is even space between the rotor and pads on each side. This space is also about even along the length of the caliper (in my mind, suggesting the rotor is both centered, and the calliper is straight relative to the rotor).

2. I think you are on to something here. Based on the NDS pad coming into contact with the rotor first, it would lead me to think that the entire caliper needs to be shifted to the DS a touch (Assuming there is not an air bubble on the DS of the caliper). but that being said, in the neutral zero brakes situation, the spaces are uniform on both sides of the rotor. I suspect I have a lazy seal vs a truly stuck piston, given that the piston return is without issue.

3. My through axle is torqued to 10nm (factory spec). The rotor is centerlock and I did not check that. the bolts to mount the calipers are 5nm, and when tightened, I can't budge the calipers.

RocketRacing
Posts: 882
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

Lewn777 wrote:
Fri Sep 13, 2019 12:43 am
Right so you are truing the disk and the warp keeps coming back?
The newness of the disk won't matter as the pads should be self centering.
I would check the tourque on the rotor bolts.
A good bedding-in heat cycle might help the situation, if the rotor is still going out of true it might be a manufacturing issue. A feeler guage is a good investment if you don't have one.
So far, yup.
I think I disagree on the newness not being a factor, as things are self centering, but with wear, the clearance between the rotor and pads will increase in time. So intolerance to warping will be highest in a new system.

rotor is centerlock. Did not check that torque. The disc to pads clearance is even along the caliper (after I trued the rotor... again).

I will do a bedding procedure tomorrow for sure.

RocketRacing
Posts: 882
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

ChiZ01 wrote:
Fri Sep 13, 2019 12:51 am
sounds like you need to reface the fork's disc flat mount. This is a very common problem when the manufacture cheap out on QC
Have a look at my other two replies for more detail. Refacing seems like a big deal, but I see how a twisted caliper (due to a non 90 degree angle between the rotor and caliper mount surface) could do this. However, I do not see an overt twist when eyeballing the rotor to pad interface along the plane of the rotor). When trued, the rotor spacing stays quite uniform on all planes relative to the inside and outside pads.

Based on my other replies, do you still think this is the issue? My gut is to adress the NDS pad contacting the rotor before the DS pad.

RocketRacing
Posts: 882
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

Alexbn921 wrote:
Fri Sep 13, 2019 1:07 am
Disks pads take a good 100 miles to wear in and everything else to settle in to a stable state.
Even new disks don't come perfectly straight. You can eye ball it but getting a dial gauge on a truing stand to get them perfect is worth it.
Removing the pads and cycling the pistons will help with a sticky piston. Also make sure the mounting surface is perpendicular to the rotors. Sometimes it can be all wonky.
I use a flash light or white paper on the back side of the caliper to see the gap between the rotor and pads.
Yup, I think cycling the pistons is next, given the DS one seems lazy.

RocketRacing
Posts: 882
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

Thanks so far folks. A sticky piston (differential diagnosis - air in the DS of the caliper) resulting in assymetrical initial pad contact with the rotors seems like a good place to start. IF I can fix that, at least I have ruled that out as a possible cause.

ChiZ01
Posts: 314
Joined: Wed Jan 01, 2014 6:20 pm

by ChiZ01

RocketRacing wrote:
Fri Sep 13, 2019 4:33 am
ChiZ01 wrote:
Fri Sep 13, 2019 12:51 am
sounds like you need to reface the fork's disc flat mount. This is a very common problem when the manufacture cheap out on QC
Have a look at my other two replies for more detail. Refacing seems like a big deal, but I see how a twisted caliper (due to a non 90 degree angle between the rotor and caliper mount surface) could do this. However, I do not see an overt twist when eyeballing the rotor to pad interface along the plane of the rotor). When trued, the rotor spacing stays quite uniform on all planes relative to the inside and outside pads.

Based on my other replies, do you still think this is the issue? My gut is to adress the NDS pad contacting the rotor before the DS pad.
you can make the caliper nice and even on the bike stand, but once actual braking force is applied on the road, the caliper will move due to the uneven mounting face. The rubbing after 1 ride is a big giveaway.

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wheelbuilder
Posts: 705
Joined: Wed Feb 08, 2017 2:10 am

by wheelbuilder

It is going to take riders a long time to come to the realization that the current iteration of disc flat-mount road bikes are not going to be silent all the time. Most of the experiences in these types of threads are coming from individual bike owners and their saga of what the symptoms are and how they remedied it. I know there are several pro mechanics on this site and I can guarantee that our experiences probably entail and endless line of complaining riders with unrealistic expectations.
When I do a long descent and then release the brake..........ting,ting,ting
When I do a panic stop then release the brake....................ting,ting,ting,
When I remove and re-install my front wheel......................ting, ting, ting,
When I climb out of the saddle............... ting, ting, ting
When I sprint......................... ting, ting, ting
When I am JRA...............................................................ting, ting, ting
After I replace pads.........................................................ting, ting, ting
After I bleed the system...................................................ting, ting, ting.

These brakes are going to make noise at some point or another. That I can guarantee you.

Pistons seals expand with heat after a descent or hard stop and the pads rub for a while before the seals contract again. A microscopic bit of road dust gets between the piston and the bore and the piston will not retract fully and the pads will rub. Caliper mounting surface not perfectly surfaced and on an even plane, pads will rub, Didn't torque your T/A to the exact spot as before, pads will rub. It goes on and on. Riders need to lower their expectations in regards to silent road disc operation.

You should go jack up one of the corners of your car and spin the wheel freely......report back on findings.
Lift the front wheel of your motorcycle off the ground and spin the wheel freely.....report back on findings. These systems use comparatively massive fluid systems with huge tolerances and still will sometimes barely turn on their own. A road bikes system is miniscule with basically a couple of drops of fluid in the entire system with very tight tolerances. I'm surprised they even work at all let alone as good as they do.
Never cheer before you know who is winning

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Lewn777
Posts: 833
Joined: Thu Mar 09, 2017 5:35 am

by Lewn777

wheelbuilder wrote:
Fri Sep 13, 2019 5:00 am
It is going to take riders a long time to come to the realization that the current iteration of disc flat-mount road bikes are not going to be silent all the time. Most of the experiences in these types of threads are coming from individual bike owners and their saga of what the symptoms are and how they remedied it. I know there are several pro mechanics on this site and I can guarantee that our experiences probably entail and endless line of complaining riders with unrealistic expectations.
When I do a long descent and then release the brake..........ting,ting,ting
When I do a panic stop then release the brake....................ting,ting,ting,
When I remove and re-install my front wheel......................ting, ting, ting,
When I climb out of the saddle............... ting, ting, ting
When I sprint......................... ting, ting, ting
When I am JRA...............................................................ting, ting, ting
After I replace pads.........................................................ting, ting, ting
After I bleed the system...................................................ting, ting, ting.

These brakes are going to make noise at some point or another. That I can guarantee you.

Pistons seals expand with heat after a descent or hard stop and the pads rub for a while before the seals contract again. A microscopic bit of road dust gets between the piston and the bore and the piston will not retract fully and the pads will rub. Caliper mounting surface not perfectly surfaced and on an even plane, pads will rub, Didn't torque your T/A to the exact spot as before, pads will rub. It goes on and on. Riders need to lower their expectations in regards to silent road disc operation.

You should go jack up one of the corners of your car and spin the wheel freely......report back on findings.
Lift the front wheel of your motorcycle off the ground and spin the wheel freely.....report back on findings. These systems use comparatively massive fluid systems with huge tolerances and still will sometimes barely turn on their own. A road bikes system is miniscule with basically a couple of drops of fluid in the entire system with very tight tolerances. I'm surprised they even work at all let alone as good as they do.
Oh my god!
So you're saying that all disk brake systems are noisy and drag? So in an environment where every watt counts and weight is important, disk brakes might not be best suited to the application of road bicycles? :shock:

by Weenie


Jugi
Posts: 540
Joined: Sun Jun 24, 2018 8:10 am

by Jugi


RocketRacing wrote:
I admit to not bedding the system, so i will do that tomorrow. I have about 150kms on the bike now, but mostly flat to hilly stuff, so no big stress on the brakes.
As a very broad generalization, in my experience current disc brakes need to be used to work well. Sometimes they need considerably hard use (as in enduro/freeride/DH type use) to properly bed in, not just as bedding in the pads but bedding in the whole system.

As has been mentioned, the pistons are (or should be) self-centering. New calipers and seals can be sticky just because they are new, so they won't necessarily self-center every time, every condition and every temperature.

Achieving DH like braking energy on a TT bike will be a challenge, but that is what I would do. Go through the suggested brake pad bedding in process and continue with some high energy braking efforts.


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