R8020 disc brake Cracked ceramic piston

[stevegreen]

Look out for possible customs duty, a handlig fee and VAT, if it arrives in UK after 01/01/21.

[bgnukem]

FWIW one of the causes of unevenly worn pads is that the friction coefficient between the pads and the disc is not what was intended by the brake's designer. The piston is offset so that it pushes on the pad off-centre, forwards of the pad centre. This offset produces uniform pad wear, provided the friction coefficient is within the design range.
.......
Needless to say one of the things that happens when the pads wear unevenly is that the pistons can tilt in the caliper bores, and this causes them to jam and/or crack more easily than normal. If you think your pads are wearing unevenly, with some (symmetrical) types you can swap left for right halfway through the pad life and even out the wear. However once the pads are badly worn to a taper, they are best off in the bin, before they cause caliper problems. Especially with an unfamiliar combination of pads and disc, or different usage conditions, it is worth examining the pads early on in their life to check for even wear.

cheers

That's interesting. I imagine ceramic pistons are rather less tolerant of misalignment loads than metallic pistons.

Have to admit I've always binned the Shimano pads supplied with the brakes I've bought. They make a horrible graunching noise on braking which sounds like it's destroying the disc rotors.

[bgnukem]

Shimano say:

The service interval on car and bicycle brake fluid is similar when they are used in the same conditions. A typical service interval for a car is to check the brake fluid every two years or 30,000 miles with normal road driving conditions, but it drops to an annual check for off road use.

Shimano similarly recommends an annual check of brake fluid but it may last longer if the bike is ridden in cleaner conditions. Bike brakes can sometimes be more fussy than car brakes because we are asking bicycle brakes to completely retract from the rotor surface so that there is no drag.

In order for that to work, we need our brake caliper seals to predictably flex the same amount at every piston and then break free at the same time and let the pistons auto-extract as the pads wear. If one seal lets its piston break free earlier than the others, you’ll end up with a piston migration issue.

The contributing factors are seal flexibility and stickiness, piston material and surface treatment, and piston scoring and contamination from them working in dirty conditions. That’s why many mechanics will say the first rule of working with disc brakes is to clean the pistons before pushing them back into the caliper.

Any time you change pads, bleed the brakes, or simply re-center the caliper, the pistons should be cleaned. One of the benefits of a mineral oil brake system is that we have some freedom to play with the piston and seal materials.

In the past we’ve used different surface treatments on alloy pistons to match seals that can behave more predictably, but our biggest breakthrough was the shift to ceramic pistons on our current performance brakes.

Since they are harder, resist scoring, and allow a better seal material, we saw a big drop off in the frequency of piston migration problems. Every part of the brake system has a give and take to it though and each brake manufacturer makes different decisions about what they think are the most important characteristics.

Interesting information! Since they used ceramic material for the pistons and not the piston bores, I assume the seals are fixed in the bores and slide along the pistons? I guess the ceramic pistons avoid the issue of corrosion in winter salt too.

They say check the fluid, but for cars the guidance used to be to renew it every 2 years, due to water absorption. I guess with mineral oil being used this is less of an issue for bike brakes, but do they say what the replacement interval should be?

[Brucey]

...Since they used ceramic material for the pistons and not the piston bores, I assume the seals are fixed in the bores and slide along the pistons? I guess the ceramic pistons avoid the issue of corrosion in winter salt too...

that is the way most hydraulic calipers are a made, and all the bike ones I have ever inspected. Shimano calipers commonly fail to a leak by corrosion in the caliper body, not the piston, because shimano don't use metal pistons (metal pistons are 'thermally inept').

cheers

[thelawnet]

Look out for possible customs duty, a handlig fee and VAT, if it arrives in UK after 01/01/21.

Incorrect.

Currently the item is liable for VAT and handling fee (above £15)

After 11pm 31 December, it would not be liable for any handling fee, because the VAT would be charged at sale (on everything - the £0-£15 exemption is lost).

No customs duty would be charged, because it's below £135.

Also it's not the date of arrival, it's the payment date that's the relevant criterion.

[brucelee]

Look out for possible customs duty, a handlig fee and VAT, if it arrives in UK after 01/01/21.

Incorrect.

Currently the item is liable for VAT and handling fee (above £15)

After 11pm 31 December, it would not be liable for any handling fee, because the VAT would be charged at sale (on everything - the £0-£15 exemption is lost).

No customs duty would be charged, because it's below £135.

Also it's not the date of arrival, it's the payment date that's the relevant criterion.

Which item are you talking about the caliper or the piston, p robikekit seem to be UK based (Manc) and the pistons are only a fiver....

[thelawnet]

Look out for possible customs duty, a handlig fee and VAT, if it arrives in UK after 01/01/21.

Incorrect.

Currently the item is liable for VAT and handling fee (above £15)

After 11pm 31 December, it would not be liable for any handling fee, because the VAT would be charged at sale (on everything - the £0-£15 exemption is lost).

No customs duty would be charged, because it's below £135.

Also it's not the date of arrival, it's the payment date that's the relevant criterion.

Which item are you talking about the caliper or the piston, p robikekit seem to be UK based (Manc) and the pistons are only a fiver....

I was referring to the most recent post, which was for a caliper.

For the pistons only then the relevant date is still the payment date, so it still doesn't matter if it arrives after 31 december, as long as you paid before that

[MikeDee]

I wonder how good these pistons and seals are? Probably allow more heat transfer to the brake fluid. What material is in the center of the piston? https://www.ebay.com/itm/Bicycle-Hydrau ... 4453444527

[Brucey]

I think the inner material is a ceramic. It is less likely to crack if it is encased in aluminium, and provided the aluminium piston walls are not too thick, the net heat transfer into the fluid is unlikely to be disastrous. FWIW the link above is for pistons that fit four-pot calipers (~17mm and ~15mm dia respectively).

For standard (twin opposed piston) calipers you need ~21.9mm dia pistons like these;

https://www.ebay.com/itm/Road-Bicycle-Hydraulic-Brake-Caliper-Piston-Bike-Disc-Brake-Replace-Parts-Kits/233804852772?

listing above shows a blowtorch being played on the ceramic inner material.

cheers

[brucelee]

EDITED IGNORE FIRST PARAGRAPH OK, I got a new 105 caliper and have fitted it and primed the system as described in the shimano workshop manual. <strike>The brakes are a bit soft.</strike> One issue I had was that I was using a bottle of mineral oil like the small one here https://www.ebay.co.uk/i/383769003657 with a small nozzle. When squirted into the syringe, very small bubbles (almost a foam) appeared that I was unable to get rid of by shaking. This problem doesn't seem to appear when extracting fluid from a large container through the tube. I was reasonably careful to maintain the calipers orientation to that indicated in the documentation during the priming, Do the cylinder bores have two holes - one coming from the bleed nipple, the other from the hose so the fluid can progress from one hole to the other without forming a void ? I saw a video of a guy just pumping fluid back and forth from the bleed nipple to the funnel, anyone tried this ? I've left the bike overnight in a vertical position and will try 'burping' it again later. If that doesn't work I'm going to try running the back of the head of an ultrasonic toothbrush around the various components...

OK, I got a new 105 caliper and have fitted it and primed the system as described in the shimano workshop manual.
The brakes have a lot of travel (about half way to the bars) before they engage, then they are firm. I've tried pumping them loads to get them to self adjust.

I've previously had a problem with the alignment of the caliper. If I used what I perceive to be the standard method of : slackening the bolts enough to allow the calliper to move, depressing the lever hard and re tightening; One pad ends up against the disc ((Worth bearing in mind that this bike is from ribble so maybe wonky in some way see https://forum.cyclinguk.org/viewtopic.php?f=5&t=133131 - hope this isn't libelous LOL, I can get my ruler out if necessary)). So I slackened the front bolt (rear brake) and pivoted it so that the clearances appear equidistant. But now I've read a post

people effectively using the rotor as a chisel on the pad rather than just taking half a second to make sure things line up

from the "yet another brake thread; not all about discs" thread, I'm starting to think that doing it this way isn't such a good idea. I'm wary about applying lubricant to the frame adaptor interface because the adaptor appears to bear directly onto the carbon frame. It also occurs to me that this is a precision fit process and inaccuracies in the frame surface or adaptor could cause this. It seems to me that the piston, pad and disc need to be precisely aligned and that the first process I described should achieve this. How should I proceed ?

Shimano dictate that the olive should be replaced but none was supplied with the caliper. How critical is this ?

Trying to get a big (1L) bottle of shimano fluid but everywhere seems out of stock, has everyone switched to baby oil ?, can anyone suggest a source ?

Cheers,
Bruce.

[Brucey]

flat mount calipers were announced with a fanfare of 'much more accurate mountings so fewer alignment issues are expected'. Cynical old gits like me were not taken in, not for a second. There is no design which someone won't find a way of making cheaper and/or worse, and it has been the same with all the disc caliper mounting standards that have been used to date. Of them all, arguably the ISO standard made the most sense, in that the mount could be accurately refaced in situ in a bike shop if required, using simple tooling. And it often is/was required.

Anyway if your frame has a badly aligned caliper mount and it is bad enough, you are going to struggle until you get this sorted out; no amount of 'clamping the disc in the caliper' will sort it out, if it is bad enough. If ribble won't sort it out, tell them that you will get a competent shop to sort it out and be sending them the bill, and that small claims action will follow.

Anyway if you have given up with ribble and fancy having a go yourself, (aggro always costs your more than just money) it might be a bit of a bodge but the caliper adaptor can usually be filed to improve the angle of the caliper if necessary. If this doesn't work for some reason, a replacement adaptor gets you to back to where you were, no loss.

If the brakes are spongy there is only one reason for this; they have not been bled/filled properly. You will know if there is a leak or a new olive is required, because fluid will come weeing out under sustained pressure.

A brucey top tip; get a flat spanner (or similar) which is the same thickness as the disc + two pad backings (about 5.5mm). With the bike angled in the workstand such that the hose runs 'uphill all the way' to the MC, the wheel out and the pads out, pump the brake until the flat spanner is gripped by the pistons. Then push the pistons back (which should force air up the hose) and try the brake again . If the brake is always spongy (even gripping a bleed block) then there is nowhere near enough fluid in the brake and/or there is air still in the caliper/hose somewhere. Repeat the pushback process and/ or rebleed the caliper using the manufacturer's procedure. If the brake is firm when clamping a bleed block, but spongy when the pistons are extended, the bleed is OK but there isn't enough fluid in the system; add more.

FWIW many Shimano brake users fill their systems with castrol LHM oil (or LHM+, or similar) and report no problems in use, even though it has slightly different viscosity etc. Don't feel tempted to use just any old oil though; most are not compatible with the seals, but LHM is.

cheers

[brucelee]

Up and running again, thanks Brucey ! I replaced the rear caliper and now have mineral pads front and back. I've done a complete bleed front and rear and have very firm brake levers. The front caliper does not seem to retract as far as the new one. The spring from the old pads was re-used on the front, is this OK ? It has been cleaned with isopropyl alcohol and greased with silicon grease. When I had the piston out of the rear one It didn't seem very smooth, sort of a mat finish. I've also done the alignment of the rear caliper by eye and with the pistons retracting more, this seems OK to me.
Looking more closely at the corrosion pattern on the rear caliper, I'm satisfied this was as a result of material being ejected from between the two pads and blowing back onto the caliper body. I'm hoping this is a metal pad problem that will go away with mineral pads. Went out for a wet and mucky ride Christmas eve (We've had flooding down here), and noticed that even though the bike got good and mucky, there was very little on the rear caliper and none at all on the front.
Cheers,
Bruce.