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R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 3:20pm
by brucelee
I know there's another disc brake slagging thread going on at the mo but my rage ego requires a separate sounding board. Previous Post
https://forum.cyclinguk.org/viewtopic.php?f=5&t=140982&p=1531580#p1531580. So after the above I got some new discs and a set of mineral pads and yesterday I went to fit them. The wear on the the removed pads was uneven. I'm speculating that the discs shown in the previous post allowed movement of the pads into the cutouts as the disc rotates causing the leading edge of the end of the cutout to scrape material from the pad and this material may be responsible for the damage to the caliper finish. I was unable to retract one of the pistons - It was making a gritty noise. Decided to strip the caliper and found the piston had disintegrated :
There was significant granular debris remaining in the caliper. I can't imagine what could possibly cause this kind of damage. Unfortunately the bike's from ribble and I am currently preparing a legal case against them. I've refit the remains of the piston (I'm one of those weird people who actually likes to ride their bikes, which requires them to be operational....), But I'm concerned that when I refill, any debris I didn't manage to get might migrate to the master cylinder and do further damage there.
Can I get a piston as a spare ?
Is this likely to be a materials fault ? Or an assembly fault ?
Can I get a different compatible caliper as a drop in replacement ?
Is there a quick way of getting a replacement caliper under shimano's warranty without having to go via ribble?
Should I refill the system given my concerns about the debris above ?
Cheers,
Bruce.
Re: R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 3:43pm
by slowster
From your other thread:
brucelee wrote:Ordered a new set of pads and went to refit the old ones. Couldn't get them in. Tried pushing the pistons back in with a plastic tyre lever and they go so far and stop.
Did you use the tyre lever
as a lever and press the end of it against the pistons? From Park Tool's
website:
If you have have a flat mount Shimano hydraulic brake, take extra care to apply even pressure on the pistons so as not to damage the ceramic pistons..
Re: R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 4:14pm
by brucelee
Yes, I did exactly that. And even if I hadn't, it would not explain the extent of the damage or the fact that they would not retract easily in the first place.
Re: R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 4:47pm
by ElCani
I also contributed your earlier thread. Assuming it’s the same brake, I would not be attempting to replace the piston (not sure if they’re available, probably not), I would replace the whole caliper.
Obviously I don’t know what’s happening with you and Ribble, but Shimano parts have a two year warranty in the UK.
Re: R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 5:10pm
by Brucey
in the photos, the fracture surfaces on the piston all look clean and fresh. It is not unknown for pistons to seize, or to twist in their bores so that they then jam. The result is then that attempts to move them can easily break them. If the fractures were 'old' then after a time, they would be as grubby as the outside of the pistons, and these are not.
Shimano ceramic pistons do occasionally crack in otherwise healthy calipers (which has caused much wailing and gnashing of teeth in some corners of the internet) but they are also easily broken through poor handling or should they jam/seize.
New pistons are not listed as spare parts in shimano's ev techdoc. Needless to say shattered remains of piston are not a suitable substitute for an intact piston. No-one in their right mind ought to be riding a bike like that.
Unevenly worn brake pads can have various causes; seized or draggy pistons included. It is quite possible that a small fault has cascaded into a major one here.
Pragmatically you need to buy a replacement caliper. Since many shimano hydro calipers are basically the same thing but with different pistons and different paint on the outside, there is no need to spring for another ultegra one. Composite pistons (as found in the less expensive calipers) are thermally less good but they are also less likely to crack or jam in the caliper bores, so may suit you better.
[edit; there are aftermarket pistons for shimano MTB calipers and some of these are the same size as 'road' ones. You could investigate that option further.]
cheers
Re: R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 5:35pm
by thelawnet
I'm pretty sure that, excluding the 4 piston brakes, all Shimano's ceramic pistons are the same.
There are lots available from China
E.g.
https://a.aliexpress.com/_mr1rV4t
Re: R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 5:38pm
by fastpedaller
I'd have never thought of ceramic being a material of choice for a calliper piston..... and wonder why not a metal alternative?
Re: R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 6:58pm
by bgnukem
Thermal insulation I assume, stopping too much heat being conducted from the pad to the brake fluid, boiling the fluid?
Can't help the pad temperatures/fade though?
Re: R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 8:05pm
by DevonDamo
I'd echo Bruce's suggestion of buying a whole new caliper. The reason I say this is that they're not ridiculously expensive and you can clean up the old one and put it in your spares box. I've done this and have now got a ready-to-go spare caliper. (Most shimano calipers are supposedly non serviceable, with no pistons or seals available, but I think a thorough clean and reassembly will fix most problems.) It was slightly different for me as my old calipers had merely seized through years of dirt build-up rather than having a shattered piston - you'll need to either get a new piston or reuse one of your existing pistons should either of your calipers seize in future. Another difference is that on road bikes, my experience has been that brakes are trouble free and require almost zero maintenance - just the very occasional replacement pads and bleeding once in a blue moon. However, since I've started regularly riding mountain bikes up to their axles in sludge, I've had a few problems with sticking pistons and leaking seals.
Re: R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 9:25pm
by thelawnet
fastpedaller wrote:I'd have never thought of ceramic being a material of choice for a calliper piston..... and wonder why not a metal alternative?
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.
The non-ceramic piston calipers are in fact cheaper.
Re: R8020 disc brake Cracked ceramic piston
Posted: 14 Dec 2020, 11:13pm
by Redvee
That calliper is only fit for the bin, not the spares box. I'd go with a R7000 hydraulic calliper, it might be a few grams heavier but will also be a few pounds lighter. It doesn't matter if you buy a front or rear calliper, they are both the same it's just the mounting bracket on the front that alters things.
Re: R8020 disc brake Cracked ceramic piston
Posted: 15 Dec 2020, 12:33am
by Brucey
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.
If the friction coefficient is too low then the pad will wear more at the toe. if it is too high the pads will wear at the heel instead.
Different pads, different discs, contamination etc can shift the friction coefficient in either direction. If you use OEM pads and discs, contamination-free and in largely dry conditions, pads should normally wear uniformly. Using pads which 'give more power' may do so at the expense of uniformity of wear.
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
Re: R8020 disc brake Cracked ceramic piston
Posted: 15 Dec 2020, 12:39pm
by brucelee
Cheers, nabbed some for spares.
I'm looking at
(REMOVE SPACE)https://www.p robikekit.co.uk/bicycle-brakes-and-pads/shimano-105-br-r7070-hydraulic-brake-caliper-flat-mount-without-rotor-or-adapters/11746875.html Is this a suitable drop in replacement ? I'm assuming it's rear but doesn't say so. I also assume it comes with pads. Anyone know if there's a better price for this anywhere or a cheaper alternative ?
I going to try to warranty the ultegra one, anyone know of a way of doing this whilst avoiding the original retailer ?
DANGER WILL ROBINSON **** when I try to insert a URL to p r o b i k e k i t, the forum changes it to
http://www.getnastycomputervirushere.co.uk can someone forward this to the mod ? - Somebody's not playing fair. ****
Cheers,
Bruce.
Re: R8020 disc brake Cracked ceramic piston
Posted: 15 Dec 2020, 12:46pm
by brucelee
anyone know if this will work temporarily ?
https://www.ebay.co.uk/i/273771316831?chn=ps
Re: R8020 disc brake Cracked ceramic piston
Posted: 15 Dec 2020, 12:50pm
by tincaman
The calipers are identical front and rear, what changes is the mounting plate, which you already have. The 7070 is virtually identical to the Ultegra one, you won't notice the difference