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I would appreciate any links or explanations on how hydraulic bike brakes work please. I'm hoping that understanding the theory might help diagnose problems and pick which model to buy.
Things I'm looking for are how the pistons move back when the levers are released, where the air (and fluid) goes, how is the pressure is distributed over pistons to give toe-in, how does the lever movement move the fluid etc.
if only there was some kind of way of searching the internet with your exact query as some kind of search term
Would you care to suggest a search that might return the results I'm after please, because I have failed to craft one.
I'll take that as a "no" then 🙁
The basic idea is that the lever pushes a piston which pushes fluid down the hose which pushes the the pistons behind the brake pads.
The pistons in the caliper are sat in rubber seals which flex when pressure is applied, and then spring back to their original position when pressure is released. This is what retracts the pistons when you let go of the lever.
If the pads or disc have worn the pistons advance further than the seal can flex, and the cylinder will creep through the seal slightly. This is how they adjust for pad wear. Of course, when you do this, you need more fluid in the system. This comes from a reservoir in the lever. The reservoir has a rubber bladder which allows the volume of liquid to change without any air getting into or out of the system. The reservoir is connected to the rest of the system via a port which gets shut off by the first part of the lever stroke, which means fluid then only gets pushed into the caliper, rather than into the reservoir.
The diagram labelled "Open system hydraulic brake lever" on this page shows the lever quite well:
https://bikeadvice.in/disk-brake-master-cylinder-assembly-part-3/
My phone isn't letting me paste links for some reason but epicbleedsolutions.com have a good piece on how brakes work.
Brakes aren’t real.
They are a work of friction 😉
@ bigjim
"High Five"
how is the pressure is distributed over pistons to give toe-in,
The pressure is even across the back of the piston and the pistons should move perpendicular to the disc surface. If they are moving at an angle this is usually dirt getting built up on the piston create uneven friction. This is also what can cause one piston to travel more that the other.
OK. Thanks very much for the useful posts. That sort of makes sense. So if the pistons withdraw based on the return spring in the levers that doesn't really explain why I'm seeing one pair of pistons move less than the other pair (in a four piston calliper), and none of these diagrams have the spring that pushes the pads apart. It could be dirt, but an identical amount of dirt in two pistons, is that plausible?
Not entirely understanding the bit about adjusting for pad wear yet - if the reservoir is out of the circuit during the final phase of lever travel then won't the pistons will have moved back before more fluid is drawn from the reservoir (as they're shown doing on The flying Ox's animation)?
And still none the wiser about what makes Guide better than Elixir, other than some vague waffle about reservoir shape in the SRAM marketing. Or Saint Vs XT. And I'm still learning about pads laying down a coating on the disc, and what happens when a different pad compound is used on an old disc or heat transfer through the pad backing.
Still, nice to know all this is a trivial Google search away for some 😉 and that motorbike brakes are identical in every way to mountain bike ones!
The spring doesn't push the pads back, it's there to stop the pads rattling. The flex, or twist really, in the rubber seals retracts the pads once you let go of the lever.
In a four pot, the oil enters the caliper and should push the pistons out evenly, but in reality, because it comes from one end, the pistons with least resistance will move 1st. So if you've got dirty pistons, they tend to want to move less easily than the cleaner ones. Slightly sticky pistons are not uncommon.
The brake wear part, as someone had said above, happens when the pistons move further than the seals will twist and slides past the seal slightly, then when you let go of the lever the remaining seal twist will then retract the pistons, but let far than they used to.
MTB brakes are just like moto brakes, infact that's how Hope started, the owners used to ride motorbikes and wanted better brakes for their MTB's. MTB brakes are just much harder to make as they have to be very small and light too, which doesn't matter on a motorbike too much.
The wear adjust happens due to the piston sliding slightly through the seal rather than the seal just flexing so it doesn't return to its original position when released. In fact at the point that the lever passes the reservoir port when released the seals will be flexed very very slightly inwards, which will draw a tiny bit more fluid in when the port is uncovered.
As for what makes different brakes different, well fundamentally they're all the same, it's just the devil is in the detail of the implementation. There is another design which is a closed hydraulic system but I don't think anyone is making bike brakes that way anymore.
Try flossing the piston with some IPA soaked string. Love a tip from Calvin
https://www.pinkbike.com/news/tech-talks-fixing-sticky-pistons-park-tool--video.htm
I am by no means an expert but I think what is missing here is the simple explanation of what I am going to call hydraulic leverage. This is my sort of GCSE physics version that helps me understand it, so it likely has lots of simplification, and I am happy to be corrected.
1) The lever has a small diameter piston which gives a cross sectional area of say 6mm2 and that piston can move by say 10mm. Which means have 60mm3 of fluid volume that moves.
2) The caliper has a piston with a larger diameter of say 20mm2, so if we push the lever to full travel then 60mm3 volume pushed through the brake lines will give 3mm of movement of the piston in the caliper.
3) So to reiterate 10mm of lever piston movement, gives 3mm of caliper piston movement.
4) An important point in any hydraulic system is that the pressure is the same everywhere. Pressure is force per unit area. If at our lever we apply 6Newtons of force at the lever piston, then we have a pressure of 6Newtons divided by 6mm2 =1N/mm2. Convenient..
5) Remember at the caliper end we have a piston area of 20mm2, so the pressure in the system is 1N/mm2. That pressure across the 20mm2 of the caliper piston then yields 20N of force applied to the back of the piston.
6) And that's how you turn 6N into 20N.
7) You can make more force by considering the leverage of the actual brake lever blade on the piston too...