Nerding out on Brakes shall we? Not another tech deraliment

I finally wrapped up my XT vs. XT brake test, here for your viewing pleasure - https://www.vitalmtb.com/features/comparing-shimano-xt-brake-performance-m8100-vs-m8200

The TLDR:

- New brakes worked much more consistently. Never had any wandering bite point issues and the lever feel stayed solid in varying conditions. When I tested the new XTR brakes last year, I had a similar experience. I think the new oil/seals play a huge part in this, and the 'stiffer caliper' probably has some effect too. Don't @ me about 

- New reach adjust dial is a pain to turn IMO. I like the grip of the old one and the ability to barely turn it and change the lever position. The new adjuster has preset detents. 

- You can do the bleed with just hex wrenches now thanks to the new bleeding port. Not a huge deal, but I always misplace my 7mm wrench so saved me some time. 

- The new lever pivot point/lever blade path: could not tell a difference (good or bad). I like the tiny lever feel of Shimano brakes, and you keep that with the new XTs. But all the marketing around the new ErgoFlow pivot location didn't amount to a difference in feel to me. 

- The new finned pad design is quieter than the old ones. It's not absolute silent, and you still get some classic Shimano pad knock under heavy braking or rolling over really chattery sections. I did a day testing with new pads, old pads, velcro under the fins, flexing the pad spring out, and fin-less pads. The best setup was finless pads and flexed pad spring. 

- Shimano says the new caliper is stiffer, and power transfer at the lever is more efficient, which = a bit more power. Riding back to back with the brakes, and one of each at the same time, I was struggling to say one felt more powerful. I did some testing with BrakeAce sensors, and those showed that I did brake for less time with the new brakes, and on average was riding faster. I also was able to generate some higher peak power out of the new rear brake. All of that to say—I'd say the new brakes are a touch more powerful, and I felt this most in the ability to modulate that power more precisely. They didn't feel so on-off like the old models.

I'll add that the new brakes still didn't give me the immediate stopping power of other brakes. I went back to Mavens last week and it was instant how much harder I could brake, and how much easier it was to generate that power. Not to say XTs don't work great for like 95% of trails I ride. I just think Shimano is holding out until that new Saint brake is released to compete power-wise with other brands. 

You mention in the review that the new M8200 aren't as powerful as the TRP Evo Pros - how do they compare to the DHR-Evo 9non Pro)?

I'd say similar story—the DHR Evo's still offer more power readily available at your finger tips.

It's crazy to me that Shimano didn't update the XT and XTR with larger caliper pistons. They've really fallen behind the standard for braking power. 

I'm pretty sure every other manufacturer offers more power at this point: maven, maven base, TRP DHR Evo/pro, Lewis LHP/LHT, Trickstuff, Hayes dominion, Hope, Magura, Intend, Radic. 

I agree with you that XT is good enough for 95% of situations, but that other 5% is pretty important. 

I just hope they release the new saint brakes this year with larger caliper pistons. Until then, Shaven is the move. 

Nice review btw - always interesting to hear your thoughts. 

Limiting power on a higher end 4 piston brake so that the DH brake can be even more powerful is such a stupid idea. Cheap/light/powerful, pick none of those things and buy some XTs... Now with added cable rattle and handlebar scuffing.

But it’s not really that fair to compare XT vs maven (or other big ones) anyway, XT is basically a trail brake and maven is full on gravity. Lets see what they are doing with that new Saint model when it comes, my guess is that pistons are bigger but not maven size. 

Anyone run their hope brakes without the phenolic insert? Seems like they could have the potential to deform and be less uniform than just the steel pistons 

Melting? They are hollow and could deform easier than the steel pistons for sure 

What is the up side to taking the inserts out, wasn't clear on this the last time it was discussed. Is it just because, or is there a functional reason for wanting to try it? 

The signature Hope squeal existed on the tech3 brakes as well, correct? If so, and given these brakes used strictly phenolic pistons, with a similar design to the inserts on the tech4 brakes, it seems possible that they are the common denominator causing the squealing. Now what the compounding factors could be that cause that in these brakes is beyond my knowledge base.

Will the Hopes work without the inserts? Probably. Given that the TS maxima’s use the same pad shape and no inserts in their SS hollow pistons. Do the inserts improve heat resistance? Probably.  But I’d say a good majority of riders are out here regularly fading these brakes, so that’s just something that would need to be tested individually. I’m planning on doing some messing around with brakes these year (have some HEL Advocates on route, and might swap calipers and levers with the Hopes for fun), so might give it a try myself, as in the summer the squeal makes POV video content hard to watch/listen to.

Its worth noting the HEL Advocates also use hollow SS pistons, but with more wall thickness. Although, the pistons are castellated which they note improves heat dissipations (allowing air to move from the centre of the pistons or less contact area with the pads?)

There's an idea for boutique brake manufacturers. Ducted ventilation for the back of the pads. 

Radic has near identical pistons with no insert. Trick stuff as well. If there’s flex with the phenolic insert, might get crisper action without 

Water cooling! 

Wait this is a real thing! 

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