Shimano uneven pistons / reset?

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Hexsense
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Location: USA

by Hexsense

Cleaner wrote:
Mon Oct 18, 2021 3:11 pm
With centerlock rotors it is not practical to shim the rotor offset (on DT Swiss and Shimano hubs).
centerlock shim is easier to live with than 6 bolts.
Once the centerlock ring shim with cut mounted on to the hub, it stay in place regardless if you have rotor and lock ring mounted or not. So, no extra precaution to change the rotor.
6 bolt rotor shim just fall right off when you detach all the 6 bolts thus require holding it in place when installing.
There are many centerlock shim kit available, 0.2mm each etc. for fine tuning.
Cleaner wrote:
Mon Oct 18, 2021 3:11 pm
The centerline of the hub can be shifted relative to the centerline of the rim by manipulating the tension of the spokes. If you move the hub relative to this datum and the rotor is mounted on the hub then it will move as well.
You got it backward. The centerline of the hub is SET by the frame dropout. Dishing the wheel only move rim left and right while the hub stay where it is in the frame (and relative to the caliper).

How did you just got it wrong on both topic...

Cleaner
Posts: 213
Joined: Wed Oct 28, 2009 7:13 pm

by Cleaner

You are correct, changing the dish on a wheel will not shift the position of the disc rotor with respect to the centerline of the hub. This relative position is determined by the dimensions of the hub shell and rotor spline interface.

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ms6073
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by ms6073

Cleaner wrote:The centerline of the hub can be shifted relative to the centerline of the rim by manipulating the tension of the spokes.
Sorry, what? Please help us all to understand how changing spoke tension is going to alter the position of the brake rotor in the brake caliper which is attached to the frame?

For the OP, if this is a frame with flat mounts, then I would suggest you find a shop that can face the mounts using either a Park Tool DT 5.2 disc brake mount facing tool, or Var CD-14500 disc brake mount facing tool.
- Michael
"People should stop expecting normal from me... seriously, we all know it's never going to happen"

Cleaner
Posts: 213
Joined: Wed Oct 28, 2009 7:13 pm

by Cleaner

Ignore everything I said about wheel dish relative to this issue. I already corrected what I posted about that and responded above that the rotor position on the hub is not affected by spoke tension and will do nothing to center the rotor in the caliper.

kode54
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by kode54

If the rotor is still off a bit, the pistons typically self center, meaning that the pistons are independent of each other. What makes noise is if the pads are off kilter (crooked) within the caliper. No wheel dishing or shims are required. Shims only required if swapping wheel sets with different hub manufacturers.
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Karvalo
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by Karvalo

Cleaner wrote:
Mon Oct 18, 2021 3:11 pm
Karvalo wrote:
Sun Oct 17, 2021 10:24 pm
Cleaner wrote:
Sun Oct 17, 2021 6:22 pm
If centering the caliper does not resolve the issue then look at the wheel. You may need to shim the rotor or adjust the dish on the wheel.
Explain to me how redishing a wheel changes brake rotor position? :wink:
The centerline of the hub can be shifted relative to the centerline of the rim by manipulating the tension of the spokes. If you move the hub relative to this datum and the rotor is mounted on the hub then it will move as well. With centerlock rotors it is not practical to shim the rotor offset (on DT Swiss and Shimano hubs).
You believe that when you dish a wheel you are moving the hub and not the rim relative to the bike? Interesting theory.

Why do you say the rim is a relevant datum for changing the position of a rotor in a caliper? Which one of those things is attached to the rim? :wink:

BigBoyND
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Location: Berlin, DE

by BigBoyND

Karvalo wrote:
Tue Oct 19, 2021 8:13 am

You believe that when you dish a wheel you are moving the hub and not the rim relative to the bike? Interesting theory.

Why do you say the rim is a relevant datum for changing the position of a rotor in a caliper? Which one of those things is attached to the rim? :wink:
You must have missed the last page. He was corrected and agrees with the correction

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ms6073
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by ms6073

Cleaner wrote:
Mon Oct 18, 2021 10:52 pm
Ignore everything I said about wheel dish relative to this issue. I already corrected what I posted about that
Thanks and no worries.
- Michael
"People should stop expecting normal from me... seriously, we all know it's never going to happen"

andreas
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by andreas

I've had some calipers that just didn't want to improve by exercising the stuck piston/bleeding, which I ended up just replacing.

If the piston can't be fully pushed back, it's an indication that it has cracked internally. Otherwise it's probably just a dirty seal.

Note that if the brake system is overfilled, you could end up blowing the reservoir seal when pushing back the piston. One of my mechanics always installs a bleeding funnel on the reservoir before pushing back the pistons, as a security measure. I punctured the seal of an overfilled reservoir on a brand new bike and had to get a complete new brake lever, when adjusting the rubbing pads for the initial ride on a brand new "mail order" bike (badly prepared from the bike shop/manufacturer).

PoorCyclist
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by PoorCyclist

Here is update guys.. about 220 miles on the caliper and pads.

I took off the wheel and push the piston back. Cannot push all the way flush but not need to. I held down the overactive piston through the brake pad and press the lever and try to let the other "lazy" piston come out more. Q-tip around piston and alcohol as needed. Took a while until it is even.

Don't think it is frame for now but more like lazy piston vs overactive piston. lazy piston pad side had more wear. Maybe air bubble or careless bleeding.. as there was a shimano quick disconnect on the rear hose they said it's for shipping I assume they had to reconnect and bleed. Or careless at the factory. Since I only ever adjusted a fraction of mm on this twice and it was probably uneven at 0 miles. (this stuff is new to me and I didn't know to check)

Overall not too scary, everything seems to be made of sturdy material.

I used hayes tool as the spreader / prying and final shimming.

I think I will need new pads in another 200 miles. But now caliper slot is centered and I should have the range until pad replacement, which I will reset everything again.

First time I noticed the piston was overactive and rubbing was after I pulsed the rear brake down a steep downhill (like ABS). Maybe don't do that.

PoorCyclist
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Location: California's country side

by PoorCyclist

andreas wrote:
Wed Oct 20, 2021 6:28 am
I've had some calipers that just didn't want to improve by exercising the stuck piston/bleeding, which I ended up just replacing.

If the piston can't be fully pushed back, it's an indication that it has cracked internally. Otherwise it's probably just a dirty seal.

Note that if the brake system is overfilled, you could end up blowing the reservoir seal when pushing back the piston. One of my mechanics always installs a bleeding funnel on the reservoir before pushing back the pistons, as a security measure. I punctured the seal of an overfilled reservoir on a brand new bike and had to get a complete new brake lever, when adjusting the rubbing pads for the initial ride on a brand new "mail order" bike (badly prepared from the bike shop/manufacturer).

What is the symptom of a blown reservoir seal?

andreas
Posts: 296
Joined: Thu Jun 12, 2014 10:21 pm

by andreas

PoorCyclist wrote:
Fri Oct 22, 2021 11:56 pm
What is the symptom of a blown reservoir seal?
The power of the brake was weak and fading, and when I peeled back the cover from the hood it would be full of mineral oil. One could also visually see oil and air bubbles coming out from the side of the brake lever, around the joints between main lever body and the reservoir plastic cover that is attached with 3 screws. Shimano doesn't sell spares for this, so the shop replaced the whole $200 lever on warranty.

Towards the bottom of this thread you can see a video with an example https://www.bikeforums.net/bicycle-mech ... -leak.html

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musiclover
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by musiclover

In relation to a lazy piston:

If bleeding does not help, is it worth it replacing a piston before replacing caliper?

I am guessing since ceramics of the piston is harder than the aluminium body of the caliper it is more likely that the body will be worn out rather than the piston?..
I have retired from this forum, not wasting any more time here.

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