Nerding out on Brakes shall we? Not another tech deraliment

DOT can be left unattended for longer. I think it would be cool to have brakes that are as reliable and low maintenance as car brakes and that would require DOT.

In regards to heat, ceramic and phenolic pistons are superior. Stainless with an insert is decent too if the insert material insulates well. Straight metal pistons have a noticeable negative impact on brake fluid temp even on trails that aren’t too crazy. Stainless pistons result in the brake fluid getting 10% hotter pretty much instantly. 

One possible reason is they are cheaper and easier to achieve tolerances with.

Since there is only a small edge of the stainless piston that is touching the brake pad that mean heat transfer is mostly from the air pocket between the brake pad and the bottom of the stainless piston ? A solid stainless piston would fare much worse in terms of heat capacity and transfer ?

Even with a thin piston wall in contact with the pad there is still significantly more heat transfer than other materials. Hard to get convection to out weigh conduction when it comes to heat transfer. When we were developing brake calipers, we really wanted to do stainless pistons because it's much easier to make stainless pistons, but the numbers just don't agree. This was testing comparing the temperature of the caliper body with stainless pistons similar to what you've described and phenolic pistons. The track was up on Chuckanut so nothing crazy.

I would avoid aluminum pistons like the plague. The thermal conductivity of aluminum is very high. I want to say like 4x that of stainless at least. Not what you want in a brake caliper.

Stainless with a phenolic insert works well too. Stainless pistons are just easier to make or source. Phenolic pistons are a very specific resin that is molded at rather high pressures and then ground to the right dimension. Requires specific machinery whereas a stainless piston can be made on any mill or lathe.

What about a double walled piston where an internal cylinder interfaces with the pad, the outer with the seal and the only link between them is the back face of the piston. So you do not heat the oil via the cylinder sleeve, only via transfer along the inner wall into the inner face. 

It'd be a PITA to make though. 

How about filling stainless steel pistons with a high temp epoxy and inset a neodymium magnet before it cures? That way you can have ease of manufacturing, good thermals and magnetic pistons

I see what you mean if we are actually comparing brake maintenance requirements of a DOT car vs. a mineral oil bike but that is an apples and oranges comparison because...

The car is an open system, and the air space at the top of the reservoir can self adjust to accommodate for fluid volume growth due to moisture absorption. 

In our bike brakes, which have a membrane sealed reservoir, with very little room to accommodate for fluid volume growth, my experience is that a DOT bike left unattended in garage or shed in a humid environment for a year or 2 will absorb enough moisture to exhibit signs of overfilling, and will either leak out the reservoir cap (causing corrosion of the controls) or not leak but auto-apply the brake. Either way, it is a real PITA if you have a bunch of bikes that see little use, and you want to grab one on short notice as a loaner, or because your primary ride is out of commission, as you basically need to do a mini bleed before each time you use the bike. In my experience, the equivalent mineral oil brake would still be working fine after sitting in that same environment for the same period of time.

You really know your stuff though, and it sounds like you are attuned to some issues associated with long term use of mineral oil that I am not, possibly related to more intense use cases, so what's the deal? What specific problems occur to mineral oil brakes that a switch to DOT would remedy?

With the assumption that brakes have to be bled often, which is the case for 90% of MTB brakes, the cons of mineral oil go away. That said, I've found mineral oils break down with heat much more readily. This is obviously tied to more extreme use cases like bike park of course. The other issue is actually moisture related. If you do get water in your brake lines somehow, DOT absorbs it and the boiling point drops a little. Meanwhile mineral oil does not absorb it and, with water being denser, it settles in the caliper and the boiling point is essentially converted to that of water. But the brakes should be sealed well enough that water doesn't really get in there. I've seen a Shimano brake where the caliper was entirely full of water due to a hole in the membrane, though. 

There are a lot of possible ways to add a layer of insulation to brake pistons. In theory you could come up with an insert with low thermal conductivity and high contact resistance between it and the piston body. The latter being a little more difficult since pressure on an interface decreases contact resistance and brake pressures are fairly extreme.

Couldn’t you build the heat break into your pad backing?

A thin G10 strip or something.

Or do you need the airflow on the back of the pad?

Adding a strip would certainly help, however the heat flux is inversely proportional to the thickness of the material so thinner buys you less. Finned pads really are a good idea cost aside. 

Maven bronze “tune” update:

2.3 TRP rotors and MTX pads feel great and are a quiet combo. Shortened the deadband while making the bite point and power management feel more refined.

I have used galfer greens in the past to make brakes more powerful and have a stronger bite point, I wouldn’t want those for mavens. 

I’ve always wondered if they have a patent on it, but haven’t dug around enough to find one.

Did a quick search on patent under Magura name and didn't find any about the magnetic caliper piston but found the one for the quick connect system of the gustav pro.

https://worldwide.espacenet.com/patent/search/family/086606047/publication/DE202023102367U1?q=DE202023102367U1

What temperature do the pads get up to? Magnets get very unhappy above 200°C. You have to glue them to the piston. You have to have the surface for it, which means more contact area for the pad and more heat transfer. You have to have steel back plates for the pads (finned pads are thus a no go). There are probably some more reasons why magnets aren't the bees knees. 

Seen mechanical calipers using magnetic pad retention lose their magnetism under heavy, heavy use. Think Pike's Peak or Highway to the Sun in HI. It requires deep and heavy heat cycling, but it can happen.

people in this thread want rube goldberg pistons with magnets and insulation but still say brakes are too expensive

Well Magura also uses plastic master cylinders with wood screws... 

My MT7s had a 66% failure rate in less than two rides and Magura USA were a royal pain in the ass about it.

Awful brakes and customer service, wouldn’t even run them if they were free.

The calipers are solid. The levers - not so much. My two pairs of Saintguras are well past their expected shelf life and they just keep on trucking.

I bought 4 brakes because they were on sale.

Installed the first two, bled up fine, got to the trailhead. Lever main seal failed while riding around to bed in the pads. It would slowly lose pressure and pull to the bar.

Took it to the one local shop I trust, he did a quick lever bleed and then agreed it was a failed lever.

Called Magura, and they told me the brake couldn’t have failed the way it did and that if the main seal was bad it would be leaking out of the brake. Which is wrong. The warranty guy said “It’s a bleed issue, but if you want to throw parts at it I guess I’ll send you a brake.”

Took another one of the brakes I bought to replace the failed one and mounted it. Half way into the first ride on that brake it failed in the exact same way. Called up Magura again. Told the guy I had the exact same problem. He told me “It may need to be bled properly”. Before I said something I’d regret, I told him he’d be hearing from my LBS. I took it to my LBS and had him check my work again and then process the warranty. They gave him a horrible time with it too.

Took the warranty brake they sent me and returned the two unused ones.

In summary, my 66% is accurate. I installed three brakes on my bike and two failed. I Had the LBS check my work every time. After the second one failed on a steep trail I decided they weren’t worth trusting with my safety. If they’d been super apologetic and helpful, maybe I’d feel different about it. But they were dicks and basically insisted that if there was a problem, I must be doing something wrong.