Nerding out on Brakes shall we? Not another tech deraliment

First time hearing that, for any brake. But that sounds like a design rather than material issue. No doubt phenolics can get damaged by sharp edges.

Incorrect material choice is part of the design. 

It's not sharp edges. The piston is flat where it is pressing on the backplate but where the hole is, where the wear material is anchored in, it creates a bulge. To me it looks like it gets hot enough to become soft and thus deforms.

Curiously filing it off it looks looks to be very hard (it creates fine particle dust) and possibly glass filled. Why this happens is beyond my knowledge, but I can 100 percent confirm it happens through use. And is more severe in the back than the front. 

I believe phenolic resins are a thermoset not a thermoplastic.

That means they do not melt at high temperatures. You have to get them hot enough to actually damage the polymer chain for them to do anything.

Correct they are thermosets, most are very, very stiff in compressive loading situations, >30ksi. Just as stiff as the aluminum caliper under similar compressive loadings. The only reason there would be impressions and deformations in the phenolic piston blocks is due to poor design leading to stress raisers that allow for point loading putting the material into a plastic deformation condition above typical elastic conditions. I personally have never seen such issues with the phenolic inserts in my Hopes, and I have yet to ever hear of anyone mention it before. 

I don't have a stress-strain curve for Bakelite or any other phenolic resin, but I feel pretty confident that its going to be quite steep, as most thermoset resins are, that's why they crack so easily. I believe you guys are thinking that our brakes are capable of generating significantly more clamping force than they actually are. 

The enduro mtb mag brake shootout has what I would assume are line pressures measured during the brake dyno testing of the brake systems. If that is correct at ~800 psi of fluid pressure you are going to be hard pressed to start flexing anything in those pistons. Using the total caliper piston surface area of a maven for instance, ~1106mm^2 with a line pressure of 56bar you are generating about 1392lb/f total, or about 350 each thereabouts. 

If anyone has a spare insert I can pop it on our Instron and see how much deflection is actually happening at 350lb/f. I have a 5kN load cell for the short fame system. 

I have tested stainless castellated pistons with a 1.5mm wall thickness back to back with phenolic. If your goal is to insulate the calipers from heat, the phenolic is better. No contest. Pretty much instantly 10% lower caliper temp in comparison.

From a manufacturing standpoint, stainless is easier to deal with. All you need is a mill or lathe and you’re good to go. Phenolic pistons have additives to increase max temp and strength. They are compression molded and then ground to their final OD.

Brake pressures can get quite high. I’ve seen 3500psi from a code lever. That’s a fair bit of force through each piston, but when you look at stress levels, you could get away with a thinner wall than what you’d be comfortable with really. 

How much lever force was required to obtain that pressure? I could probably do the hand calcs but don’t really feel that motivated lol.

Don’t remember off the top of my head since that was a while ago. It was a fair but though. Only possible with a stoppie on pavement or just squeezing the levers hard just because. Although I do remember that 3500 psi was not as much as what was possible just squeezing the levers as hard as you could. If I remember right 0-1000psi was the real world range. 

I understand the basics behind it. On the other hand, if you have a new brake with flat (or as designed) pistons, that grow bulges on the surface after you go through a few sets of pads, bulges that prevent the bleed block from being inserted unless filed down, those have to come from somewhere.

I think my brakes were not the only ones I saw this on, but other than heat generated during use, I do not see a single reason for these on my brakes. It's not a special piston or something like that, the brakes were serviced, inspected, used by me and only me all the time.

I'll have a look if my current ones have the same problem and I don't think I have any pictures from when the issue was most pronounced, so typing it out is all I have at the moment.

EDIT: hot damn, I have pics. Not of my bike, but all the same.

Primoz, could that be a build up or dust, dirt, brake pad material ?

Nope. You can see the castleation in one of these bulges and nowhere else on the piston where they should be. Also this was very hard, I used a file to file them off on my brakes to be able to put the bleedblock in the caliper. The stuff was hard. 

The Base is fundamentally quite a different brake to the others. In addition to the swinglink delete:

Base also has 4 x 18mm pistons vs 2 x 18 + 2 x 19.5mm on the others.

Base doesn't not have a hose banjo at the caliper.

The Bronze, Silver, Ultimate designations are levels of premium, The oversimplified explanation is that Bronze has bushing pivots, Silver adds bearings, Ultimate has titanium

Bronze also lacks the contact adjuster, and no banjo fitting.  SLV/ULT both have that.

This isn't anything other than a "Hope Evo GR4s are the sexiest brakes out there" post with a little side of "Profile makes the best sounding hubs". There is some dry dirt and warm temps in southern Alberta and I've got the day off tomorrow, so will report back on how they perform. 

 

Welp. First ride on the GR4's and initial thoughts:

Pros: look amazing. Tons of power. Short throw (which I love). Light lever feel.

Cons: the rear brake line pulled out of the calliper, so I lost my rear brake for about 3/4 of my ride. Chunky, loose, dh trails are sketchy with only a front brake. Lol. Looks like with the way the brake line runs on my bike and the angle of the banjo on the calliper is a little funky and they didn't leave quite enough line. No one was hurt, everyone learned something and the shop is taking care of me, so all is well in the world. 

Might try to go for round too tomorrow. Weather is still ridiculously above seasonal around here and want to take advantage of it!

That's terrifying

Oof! Did the line pull out at bottom out or top out? Hard to decide which is more scary, especially on a jump or drop or something 

I think it pulled out when I was compressed. I was taking things pretty easy (on my own and first ride since, like, November? We're supposed to be skiing over here), so I wasn't at speed or after any kind of big jumps/drops, so it wasn't nearly as bad as it could have been. 

The shop I go to was going to pull the shock so they've got a good picture of the what things are happening all the way through the travel and make sure there's enough extra line to make sure there's no chance of it happening again. 

That sucks! I lost a front brake with my radics last year in the most muddy, greased owl poop conditions, the kind where the rear brake did nothing due to how steep the trails were... I thought I was on the verge of death all the way down... I bought T4V4'S the following Monday lol. Granted the Radics lost all brake pressure not from a line pulling out but because they hate me lol. I tried to bait Taylor into looking back into his emails after some conversation on PB a month or so ago to no avail. Those beauties will continue to live in the parts bin... 

Happier with the T4V4's anyway. Can't wait to get some GR4's and move the T4V4's to the bike with Dominions.  

Weren't carbon rotors and pads all the rage about 12 years ago?

What happened? Confused

I always assumed carbon or carbon/ceramic mix brake rotor need higher operating temperature to achieve optimal braking performance. Road cycling heat requirement are relatively small and environmental cooling is high so you are always trying to heat them up. Steel works better at cold temperature and the upper temperature limit in bike brake system would most likely be our pad size and small oil volume.

Nope. 

Carbon carbon (F1 stuff) needs high temps. Carbon ceramic (fancy road car stuff) doesn't. And turns out roadies regularly overheat their brakes. They have small rotors, braking events are savage (from over 60 kph to below 20 for hairpin turns) and the cooling you mention is not that substantial. 

Roadies bombing down steep mountain passes will easily surpass 60kph. We're talking 100+ kph on the extreme end, and the speed will more quickly build up again compared to mountainbiking because of obvious reasons.

It can be a very different ballgame, but again, this is on the extreme end of things not like your everyday commute to work...

Yikes. That's just how the Madonna is designed unfortunately. You have to remove the shock and cycle the linkage to make sure it doesn't tug on the rear brake line under hard compression. RAAW build video shows this pretty well. I measured 3 times before finalizing the position and also used electrical tape to make sure the hose doesn't creep back and forth as the linkage cycles over time.

Good call on the tape to keep an eye on things. The boys at the shop got a little creative and I think we managed to figure something out. Ultimately, the zip ties along the chainstays are a little looser and we've got a bit of extra line routed by the BB to allow a bit of movement. I'll have to keep an eye out to see if rubbing becomes an issue. 

All that being said, made it out for another ride yesterday afternoon. The GR4's feel amazing. I love the adjustability in the levers. The trail I rode is short (1.2km with a touch over 200m of elevation) and starts fairly steep and technical. Conditions were SUPER loose (almost like it hasn't rained in months. lol), so wasn't necessarily a good test for the capability of the GR4's, but so far I'm stoked with them. 

I think it's supposed to drop back down to mid -20c temps by next weekend, so not sure there'll be any more riding on them before spring hits. 

Well, after 18 months, 2 warrenties, 2 seal kits, 6 sets of pads, 1 new lever body, 1 diaphram, and far too many bleeds, I've finally called it quits on my T4V4's. That being said, I'm reverting back to Codes for the time being (I'm 140 LB's on a trail bike w 203 Freeza's) and I'm wondering if anyone knows if there's any difference between the older RSC and new Ultimate/Silver Stealth models other than the body angle change and carbon lever blade on the Ultimates? I heard somewhere a while back that they may have changed the Swinglink cam ratio slightly on the Stealth versions but can't seem to find anything else that backs that up.

If you're asking this because you're looking to buy new, this might be interesting: https://www.hibike.com/bike-parts/brakes-and-accessories/disc-brakes/sram-code-rsc-disc-brake-black-bulk-pbd2103d40e8c02d0c5edd36d09788733#var_BAS00123 

I know it's European, but 120 euros for a pair of code Rscs might be worth to handle the import if they're more expensive locally... I'm all good with brakes (have RSCs on both bikes), but if I had a lesser set on any of my bikes (particularly GFs), I'd be scooping these up immediately. At this point I'd be more interested to try something stronger if I felt spendy. 

quick before king donnie the fat increases the fascist kingdom tax once more