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Disc brake alignment - how long should back wheel spin for?

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Disc brake alignment - how long should back wheel spin for?

Old 03-12-17, 04:50 PM
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johngwheeler
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Disc brake alignment - how long should back wheel spin for?

I have a CX bike with disc brakes and am a bit concerned that I can't see a clear gap between the rotor and pad on one side of rear brake.

The clearance I can see on the other side seems to be tiny - much smaller than I imagined (< 1mm).

I sometimes hear a slight rubbing/friction noise, almost a squeak when slowly moving the wheel. I'm wondering whether I'm making contact with the brake pad on one side. The bike does feel s little bit slower than when I first rode it (only 2 days ago, bought new!).

Is there any way to actually test this? I was thinking there might be a "spin test" to see how long the rear wheel will freely rotate - but this is just conjecture!

BTW, the rotor doesn't seem to get hot after riding without braking, so I assume the friction can't be too bad.

Also, the brakes are now much noisier after about 50km of mixed road/off-road riding, and the front lever requires more pressure than it initially did to fully engage. I was braking heavily in my off-road section.

Should I just take it back to the LBS to have it checked over, or ride for a bit more to see if the squeak disappears? I'm having the bike checked up in 3-4 weeks in any case as part of the sales after-service.


Thanks,

John.
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Old 03-12-17, 05:02 PM
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Loosen the 2 allen head bolts that hold the caliper to the frame. Apply and hold the rear brake on ,with the brake on tighten the to bolts that you loosened . The disc should be centered at that point .
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Old 03-12-17, 05:26 PM
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If, after adjusting the brake alignment as suggested, you are unsure whether it is still rubbing or not or need a spindown test to be certain, it is not rubbing enough to matter. As to the lever being lower, you may need to adjust the pad clearance to compensate for wear.
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Old 03-12-17, 05:43 PM
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also check for any warpage/bend in the disc itself... this will cause momentary tightness, and uneven wearing of the disc.
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Old 03-12-17, 05:50 PM
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You should be able to see a sliver of light on both sides of the rotor for a full revolution. If it passes that test, the wheel likely won't move enough to cause any significant drag. The total gap between the pads is only ~3mm, so ~mm gaps on both sides is normal.
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Old 03-12-17, 06:17 PM
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Are the calipers mechanical or hydraulic? Mechanical require routine adjustment as the pads wear.
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Old 03-12-17, 06:40 PM
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Originally Posted by 1Coopgt View Post
Loosen the 2 allen head bolts that hold the caliper to the frame. Apply and hold the rear brake on ,with the brake on tighten the to bolts that you loosened . The disc should be centered at that point .
Thanks for the tip; this approach makes sense and doesn't sound hard to do.

John.
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Old 03-12-17, 06:43 PM
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Originally Posted by dsbrantjr View Post
If, after adjusting the brake alignment as suggested, you are unsure whether it is still rubbing or not or need a spindown test to be certain, it is not rubbing enough to matter. As to the lever being lower, you may need to adjust the pad clearance to compensate for wear.
Hopefully it's not brake pad wear - the bike is only 2 days old! However, I was on a really rough rocky trail and had the brakes on a lot - not going fast or very far (only 3-4km). Maybe the hydraulic pressure has dropped off?

John
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Old 03-12-17, 06:46 PM
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Originally Posted by maddog34 View Post
also check for any warpage/bend in the disc itself... this will cause momentary tightness, and uneven wearing of the disc.
The disk looks pretty true with a simple eye-ball check.

Originally Posted by gsa103 View Post
You should be able to see a sliver of light on both sides of the rotor for a full revolution. If it passes that test, the wheel likely won't move enough to cause any significant drag. The total gap between the pads is only ~3mm, so ~mm gaps on both sides is normal.
Good to know!

Originally Posted by DrIsotope View Post
Are the calipers mechanical or hydraulic? Mechanical require routine adjustment as the pads wear.
Hydraulic - but the bike is new, so maybe it needs a bit of adjustment after an initial factory set-up.

BTW, is the increase in brake noise normal after a bit of use? It's not much worse than another bike I have with mechanical disc brakes, but the sound is a lot rougher than when the bike was brand-new.

Thanks for all your responses!

John.
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Old 03-12-17, 06:51 PM
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also make sure the disc bolts are tight...had same issue.
did this:
Loosen the 2 allen head bolts that hold the caliper to the frame. Apply and hold the rear brake on ,with the brake on tighten the to bolts that you loosened . The disc should be centered at that point .


still an issue after rides. had a not so secure disc.
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Old 03-12-17, 06:52 PM
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The disc shouldn't touch at all, so any sound indicating contact is contra-indicated. OTOH - friction is a function of the material characteristics multiplied by the normal (perpendicular) force pushing the materials together. In the case of incidental rotor contact characterized by an occasional chirp, there's no significant contact force, so not much friction at all.

One experiment you might do is spin the wheel slowly and let it come to rest. mark the position, and repeat a number of times, marking where it stops each time. If there's enough friction to worry about, you'll find the wheel tends to stop in the same area a large percentage of the time. Otherwise, if there's no pattern, then there's no need to fret.

Of course, nothing should stop you from trying to eliminate ALL incidental contact, so I'm only talking about those situations where you can't seem to, ie. a slightly bent rotor, or a flexy frame.
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Old 03-12-17, 07:05 PM
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If all else fails, pull the pads out so you can see the caliper better, center the slot with respect to the disc, and put the pads back in. If they don't go in, then you need to push the piston(s) back out. In my experience, if the caliper isn't centered, you will never get it quite right.
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Old 03-12-17, 07:36 PM
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Originally Posted by johngwheeler View Post
Hopefully it's not brake pad wear - the bike is only 2 days old!
John
Brakes do not wear as a function of time but of use. Do a little searching and you will find instances of pads wearing during a single muddy race, and of mid-race pad changes. There is a tradeoff between long wear and good stopping ability.
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Old 03-12-17, 07:41 PM
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Originally Posted by johngwheeler View Post
Hopefully it's not brake pad wear - the bike is only 2 days old!
John
Brakes do not wear as a function of time but of use. Do a little searching and you will find instances of pads wearing during a single muddy race, and of mid-race pad changes. There is a tradeoff between long wear and good stopping ability. Very hard pads will last a long time but need more hand effort. The pads on your bike may have been chosen to show great stopping power at the expense of longevity.
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Old 03-12-17, 08:06 PM
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More refinement to this poster's situation in this second thread about the same issue. Andy
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Old 03-12-17, 09:02 PM
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Originally Posted by dsbrantjr View Post
Brakes do not wear as a function of time but of use. Do a little searching and you will find instances of pads wearing during a single muddy race, and of mid-race pad changes. There is a tradeoff between long wear and good stopping ability. Very hard pads will last a long time but need more hand effort. The pads on your bike may have been chosen to show great stopping power at the expense of longevity.
This is a very good point! I wonder if I have cause significant wear with my single ride? It retrospect I was ill-advised to attempt the trail, but without a recce, it's hard to know whether a bad patch is only for a few tens of metres or for kilometres! I got off the trail as soon as I could, once I knew I made the wrong choice. By the time I'd realized it was definitely beyond my ability (or too much for the bike), it was quicker to continue that go back, and I carried my bike over the first of it - definitely a good workaround up the rocks climbs!

John.
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Old 03-12-17, 09:06 PM
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Originally Posted by Andrew R Stewart View Post
More refinement to this poster's situation in this second thread about the same issue. Andy
Andy, thanks for your answer in my other thread.

I think the brakes are likely cause of the noise, so I'll try to re-center the calipers and see how I go.

If I have been foolish enough to cause significant wear to the brakes or rotor, then I'll just have to have them replaced and learn for the lesson :-)
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Old 03-12-17, 10:44 PM
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Your brake pads and rotors are fine. The other guy is talking about in the most insanely extreme situations. You aren't in that category almost nobody is.
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Old 03-13-17, 12:07 AM
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It would be helpful if you mentioned the exact make and model of the brake. You mentioned that the clearance to the pad that was not rubbing seemed very close. This may or may not be wrong--pads on disc brakes are spaced very close because the system can operate with higher mechanical advantage with the pads closer, particularly if the pads travel at a linear rate as the lever is pulled, which varies by brand/model.

However, not infrequently on new bikes, particularly on brakes that are actuated by DOT fluid there will be too much fluid in the system which will space the pads too close to the rotor.

If your bike was in my stand, I'd first center the caliper so that the disc was centered between the pads Loosening the brake mounting bolts, squeezing the lever, and tightening them down SHOULD center the caliper correctly, but often due to some kind of friction this doesn't work, and I always double check and frequently have to slightly loosen the bolts to align them by hand. Be careful to never do this with the wheel moving--I ended up in the ER with a bisected fingernail trying to adjust a caliper with the wheel moving this year.

Then, if I determined that the pads were too close to the rotor, I'd remove the pads and open up the lever bleed port, cover it with a rag, and them press the pistons out to force out any excessive fluid, then close the bleed port, clean up the lever, and reinstall the pads, and recheck. I've built a handful of new bikes that needed this with SRAM brakes.

Also, the cases of pads wearing out involve racing in extremely muddy conditions, and were mostly problematic with cable-actuated disc brakes (because they don't automatically advance the pads as they wear). It would be extraordinary if you did excessive wear with the riding you've described. My advice for your cross bike is to ride it wherever you feel you want to. My experience is that the relative lack of traction and the brutalness of riding a rigid bike with not-particularly-wide tires results in a pretty cautious approach to riding semi-technical singletrack, and I've felt pretty safe about the bike, even when not feeling safe about myself.
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Old 03-13-17, 12:59 AM
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Originally Posted by cpach View Post
It would be helpful if you mentioned the exact make and model of the brake. You mentioned that the clearance to the pad that was not rubbing seemed very close. This may or may not be wrong--pads on disc brakes are spaced very close because the system can operate with higher mechanical advantage with the pads closer, particularly if the pads travel at a linear rate as the lever is pulled, which varies by brand/model.

However, not infrequently on new bikes, particularly on brakes that are actuated by DOT fluid there will be too much fluid in the system which will space the pads too close to the rotor.

If your bike was in my stand, I'd first center the caliper so that the disc was centered between the pads Loosening the brake mounting bolts, squeezing the lever, and tightening them down SHOULD center the caliper correctly, but often due to some kind of friction this doesn't work, and I always double check and frequently have to slightly loosen the bolts to align them by hand. Be careful to never do this with the wheel moving--I ended up in the ER with a bisected fingernail trying to adjust a caliper with the wheel moving this year.

Then, if I determined that the pads were too close to the rotor, I'd remove the pads and open up the lever bleed port, cover it with a rag, and them press the pistons out to force out any excessive fluid, then close the bleed port, clean up the lever, and reinstall the pads, and recheck. I've built a handful of new bikes that needed this with SRAM brakes.

Also, the cases of pads wearing out involve racing in extremely muddy conditions, and were mostly problematic with cable-actuated disc brakes (because they don't automatically advance the pads as they wear). It would be extraordinary if you did excessive wear with the riding you've described. My advice for your cross bike is to ride it wherever you feel you want to. My experience is that the relative lack of traction and the brutalness of riding a rigid bike with not-particularly-wide tires results in a pretty cautious approach to riding semi-technical singletrack, and I've felt pretty safe about the bike, even when not feeling safe about myself.
The brakes are SRAM Rival 1 (as is the whole group-set). The bike is a 2017 Giant TCX Advanced Pro 2.

Thanks for the advice on pad alignment and on likely pad wear; as you say I would very surprised if I had worn out the pads in 45 minutes of cautious riding, even if I was hanging on the brakes to avoid going too fast over the rocks.

I'll try the caliper re-centering myself, but for anything else, the shop can do this in the 4-week post break-in service.

John.
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Old 03-13-17, 02:36 AM
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Have you actually gone through the bed-in process? This gives shoes how to do it. How to bed in new disc brake pads - BikeRadar
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Old 03-13-17, 05:42 AM
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Originally Posted by jimc101 View Post
Have you actually gone through the bed-in process? This gives shoes how to do it. How to bed in new disc brake pads - BikeRadar
No, I haven't done this. Is it too late to do it once the bike's been ridden a bit (c. 50km)? Could the brake noise I hear be a result of having not properly conditioned the disc brakes?
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Old 03-13-17, 06:57 AM
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Originally Posted by 1Coopgt View Post
Loosen the 2 allen head bolts that hold the caliper to the frame. Apply and hold the rear brake on ,with the brake on tighten the to bolts that you loosened . The disc should be centered at that point .
I've never had much luck with that approach. Tightening the bolts, even in stages, tend to shift the caliper out of alignment again.

I prefer to loosen the bolts just enough so that firm pressure or tapping with a screwdriver handle (or comparable) is enough to push the caliper into position, and then to tighten in stages while alternating between the two.
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Old 03-13-17, 07:44 PM
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Originally Posted by johngwheeler View Post
No, I haven't done this. Is it too late to do it once the bike's been ridden a bit (c. 50km)? Could the brake noise I hear be a result of having not properly conditioned the disc brakes?

Your Brakes bedded in while you were riding it . You would need to lightly sand the disc's to remove the brake pad material in order to start over. I wouldn't worry about it now. Depending on what the pad material is they could be very noisy. My Motorcycle has sintered metallic pads and they can be very noisy ( the compound is very hard). Same thing can happen with your cars brakes depending on the type of pads you buy.
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Old 03-13-17, 07:49 PM
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Originally Posted by dabac View Post
I've never had much luck with that approach. Tightening the bolts, even in stages, tend to shift the caliper out of alignment again.

I prefer to loosen the bolts just enough so that firm pressure or tapping with a screwdriver handle (or comparable) is enough to push the caliper into position, and then to tighten in stages while alternating between the two.
Maybe I should have put a disclaimer in there .... Your results may very . With that said it worked fine on my Goose.
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