Have Formula RX brakes and noticed the lever pull getting pretty long. Figured it was time to bleed, so I did but it made no difference. Examined the pads and they look about half done. Bled again with no improvement. Then eyeballing the pads while pulling the lever, I realized that it was simply taking up the gap between the pads and rotors before engaging. Ditched the return springs and now the lever is solid. Rode a 9 mile loop and there is no noise, dragging or other ill effects. Why springs? Cars dont have them. Motos dont have them.
brake return spring: necessary or just annoying
tyrebyter
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If I feel the springs in my Shimano brakes, they are pretty weak. If I push the pistons back with a tool, it feels like it takes considerably more force than that little spring can provide.
The spring adds some small assist to piston retraction, but the magnet type of brakes demonstrate that a spring is not needed for piston retraction.
With that, I would have to conclude that the springs, like magnets, are there primarily to keep the pads against the pistons so they don't rub, but clearly they are not needed for piston retraction.
The spring adds some small assist to piston retraction, but the magnet type of brakes demonstrate that a spring is not needed for piston retraction.
With that, I would have to conclude that the springs, like magnets, are there primarily to keep the pads against the pistons so they don't rub, but clearly they are not needed for piston retraction.
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exactly
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I think pressure throughout the system quickly equalizes especially for fluids with very low compressibility, and especially at the relatively slow speeds at which these fluids are moving.
So why does the brake 'want' to return to its original state? I haven't disassembled any brakes but I was reading this post the other day which shows a rubber seal deforms slightly when the brakes are compressed:
http://forums.mtbr.com/brake-time/ever-wondered-how-hydraulic-brake-calliper-works-848452.html
". . . but here's the clever bit - the rubber seal is a tight fit round the piston, but a loose fit in its recess in the calliper body. So, as the piston extends it drags the rubber seal with it, distorting its shape. When the lever is released, the 'squashed' rubber seal wants to spring back into shape, pulling the piston back in a millimetre or so with it (this is also the process that makes the callipers self adjusting as the pads and rotor wear - the piston will always be forced out until it touches the rotor, but will only spring back a fixed amount).. . "
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That's a good description. But I'm wondering if a spring in the brake lever also contributes to pulling the fluid back and the pistons back.
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Just got back from an 8 mile loop. Theories are great but sooner or later you must hit the trail. Started with an 800 foot climb followed by some chunky stuff and finally, flogged it down the canyon. It's a trail I've done many times. This time I PRed 7 of the 8 sections. The brakes didn't squeak, rattle, fade, creep or suddenly lock up at the realization that the pistons had not retracted. The lever was solid, modulation brilliant and power exceptioal. Conclusion: springs stay in the parts bin.
Ok this is exactly what is wrong with reading forums. If you do not know what you are talking about, don't type your opinion. I have been working on hydraulic disc brakes in cars, motorcycles, and bicycles for over 6 years. The master cylinder piston DOES NOT pull the caliper pistons back into the caliper body. That is the biggest misconception about hydraulic disc brakes. The caliper piston seal reflexes and the SEAL pulls the piston back into the caliper body. If a brake is an open system, this design allows air on the other side of the reservoir membrane to allow for expansion of the fluid through heat and the hygroscopic nature of DOT fluid.
Please get it out of your head that brake fluid somehow creates back pressure pulling the caliper pistons into the caliper body. It's nonsense.
Please get it out of your head that brake fluid somehow creates back pressure pulling the caliper pistons into the caliper body. It's nonsense.
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Uh oh. He pulled the "I'm a mechanic" card. I knew that was going to happen. I too was a mechanic for 20 years on heavy equipment, cars and motos, then engineer for 14 years. What you say is correct, but that's not going to prevent someone with extensive kekboard experience from explaining without empirical evidence why you are completely full of sh1t.
I think the difference between rider perspective and mfg perspective is worth noting here. From the perspective of the mfg, they need to fix the degree of freedom existing when the pads can sit anywhere in the gap between piston and disc when the springs are not present. During the mfg design and QA processes, they need to know exactly where the relevant parts (e.g. pads) are located during operation to ensure functionality in the real world meets design expectations. Issues relating to chatter, dirt buildup between pad/piston, turkey gobble, etc. were all presumably designed out of the system when the spring was present and the brakes may seem to work well without the springs. However, if you compare a large group of riders either with or without springs, more problems would likely occur for the group without springs. At this level of design, the springs are no doubt important even though they may seem unnecessary.
From the mfg perspective, it is cheaper to include the springs than to not include them, or they wouldn't be included.
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OK. Let's think a little harder about what the springs do for the manufacturer while not re-inventing the brake. 1) The springs reduces the chance that the brake will chirp endlessly from an out-of-round rotor which reduces bogus warranty claims. 2) The springs prevent rattles which also reduces bogus warranty claims. 3) The spings create a condition where your once wonderful brakes get a really long pull at about half-wear which your LBS will correctly diagnose as worn pads. That's good for them and the manufacturer: pad sales.That's not a ding on them, it's just business. Springs are better for the manufacturer than they are for me. Your results may vary.
As far as 'data from a small pool', um, disc brakes have been commonly used on vehicles for a half-century. All I did was observe that indeed, bicycle brakes are not different in design and do not behave differently than any other brake I've owned since 1972.
I try to understand the physical world around me rather than live in fear.
As far as 'data from a small pool', um, disc brakes have been commonly used on vehicles for a half-century. All I did was observe that indeed, bicycle brakes are not different in design and do not behave differently than any other brake I've owned since 1972.
I try to understand the physical world around me rather than live in fear.
The springs also prevent buildup of dirt etc. between the pad and piston. Given the small pad clearance especially with F brakes, this can be important.
It is great to know you are interested in trying to understand the world rather than live in fear. This is becoming a growing concern in the US recently. The dark ages were a gruesome time, even foundational information such as the formula for cement was lost for years. This teaches a good lesson-- that nowadays we must be sure to share correct information and build and not dispose of understood knowledge. In this case, I remain curious what was wrong with your brakes as I never managed to tune my 2012 R1/carbon setup to satisfaction before giving up on them. I hope you figure out what was causing your soft lever as it certainly was not solved directly by removing the spring (per your original post). Removing the spring may have somehow inadvertently affected your lever, but it was not the direct cause. Please keep us informed if you crack the mystery.
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Ok, with all due respect, this is not the first snarky comment you have made about reinventing the brake. No one here has tried to reinvent the brake, and have put forth some good info that you would do well to read carefully.
You left out your original contention that the pad springs push back the pistons. Are you backing off of that idea now, as others have suggested?
Bicycle brakes are different because the pistons retract. Not necessary on cars, but drag and noise are much more important on a bike.
It seems you live in fear of anyone else's input. If you want to understand the world around you, you would do well to improve your listening skills.
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This problem was mentioned by yourself and another poster, so obviously it happens, but if the brakes are working correctly, it shouldn't. As the pads wear, the pistons should move in farther to compensate by sliding in on the piston seals. Self adjusting.
You must have dirt or lack of lube or something preventing that.
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Good info from SRAM:
The slave pistons in the brake caliper often use special seals that flex, or roll, slightly when the pistons/pads are pushed toward the rotor
during braking. when the brake is released, the piston seals relax and pull the pistons/pads away from the rotor. This is known as pad
rollback. The amount of rollback is an important factor in determining the distance the pistons must travel before the pads contact the
rotor. This distance reflects the amount of clearance between the pads and the rotor, as well as the amount of lever movement required
before the pads contact the rotor, known as deadband. The greater the rollback the greater the clearance and deadband.
Another function of these seals is to allow the pistons to self advance as the pads and rotor wear. As pads and rotor wear, the distance
between them increases, which affects piston travel and deadband. without the use of these special seals, pad and rotor wear would
require constant adjustment of pad clearance to maintain consistent deadband. but because the amount of roll in a piston seal is
limited, there is a point where the piston will slip through the seal. This means that as brake pads and rotor wear, the pistons will move
further than the seals can roll. As a result, the pistons constantly slip through the seal until the pads contact the rotor. Once the brake
is released, the pistons/pads return to a new resting position, closer to the rotor. This eliminates the need to adjust pad clearance or
deadband as components wear.
https://www.sram.com/sites/default/...fault/files/techdocs/gen.0000000004234_rev_a_hydraulic_disc_brakes_overview.pdf
The slave pistons in the brake caliper often use special seals that flex, or roll, slightly when the pistons/pads are pushed toward the rotor
during braking. when the brake is released, the piston seals relax and pull the pistons/pads away from the rotor. This is known as pad
rollback. The amount of rollback is an important factor in determining the distance the pistons must travel before the pads contact the
rotor. This distance reflects the amount of clearance between the pads and the rotor, as well as the amount of lever movement required
before the pads contact the rotor, known as deadband. The greater the rollback the greater the clearance and deadband.
Another function of these seals is to allow the pistons to self advance as the pads and rotor wear. As pads and rotor wear, the distance
between them increases, which affects piston travel and deadband. without the use of these special seals, pad and rotor wear would
require constant adjustment of pad clearance to maintain consistent deadband. but because the amount of roll in a piston seal is
limited, there is a point where the piston will slip through the seal. This means that as brake pads and rotor wear, the pistons will move
further than the seals can roll. As a result, the pistons constantly slip through the seal until the pads contact the rotor. Once the brake
is released, the pistons/pads return to a new resting position, closer to the rotor. This eliminates the need to adjust pad clearance or
deadband as components wear.
https://www.sram.com/sites/default/...fault/files/techdocs/gen.0000000004234_rev_a_hydraulic_disc_brakes_overview.pdf
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Well, I'm still hung up on this, even though the SRAM document I posted says the same thing, that the seals pull the piston back. And I'm sure they do, but I think vacuum might play a part as well.
Here's a thought experiment - connect two syringes with a piece of tubing. The system is full of water and no air, with the syringe plungers in the half way position. Push on one plunger and the other plunger moves back. Now pull the plunger backward, and the other one moves forward.
The spring in the brake lever moves the master cylinder back, so doesn't the piston have to move back as well?
Nice series of posts which seek to be correct-- a nice thing to document here as well as to diminish occasional misinformation.
One consideration with hydraulic lever assemblies is whether the piston is /directly/ coupled to the lever or if a relief exists that lets the lever pull back out without pulling the piston. I haven't taken apart my m985 levers and cannot determine from the techdoc whether this coupling is direct or not. What I notice is when I squeeze the lever quickly I can hear the fluid flow in the line, but when I push it back out at nearly the same rate, I do not hear the fluid flow. This is the best indication I have that the piston is not coupled directly on the m985. This could be tested by disconnecting the caliper and submerging the end of the disconnected hose in a cup of fluid and watching the flow, but my 985s work so perfectly I have absolutely no intention of messing with them until they need it for some reason. I wonder if anybody has tried this test or knows definitively which levers are or are not directly coupled. I know you can push out the ?m596? levers on my girlfriend's bike and they are obviously not directly coupled to the pistons.
Also important is if an internal spring drives out the master piston which would drive a vacuum to withdraw the caliper pistons.
The main rationale why only the seals should drive withdrawal is that this is the mechanism that causes the 'auto-adjust' feature of disc brakes. If some sort of vacuum mechanism also existed, it would interfere with the auto-adjust to some degree. Obviously the properties of the material, angles, and other dimensions of the seal can be designed to cause a range of piston withdrawal distances.
At home I do have my R1 levers which have been worked on so much I can do it with my eyes closed, I'll play with them to see if the coupling is direct.
One consideration with hydraulic lever assemblies is whether the piston is /directly/ coupled to the lever or if a relief exists that lets the lever pull back out without pulling the piston. I haven't taken apart my m985 levers and cannot determine from the techdoc whether this coupling is direct or not. What I notice is when I squeeze the lever quickly I can hear the fluid flow in the line, but when I push it back out at nearly the same rate, I do not hear the fluid flow. This is the best indication I have that the piston is not coupled directly on the m985. This could be tested by disconnecting the caliper and submerging the end of the disconnected hose in a cup of fluid and watching the flow, but my 985s work so perfectly I have absolutely no intention of messing with them until they need it for some reason. I wonder if anybody has tried this test or knows definitively which levers are or are not directly coupled. I know you can push out the ?m596? levers on my girlfriend's bike and they are obviously not directly coupled to the pistons.
Also important is if an internal spring drives out the master piston which would drive a vacuum to withdraw the caliper pistons.
The main rationale why only the seals should drive withdrawal is that this is the mechanism that causes the 'auto-adjust' feature of disc brakes. If some sort of vacuum mechanism also existed, it would interfere with the auto-adjust to some degree. Obviously the properties of the material, angles, and other dimensions of the seal can be designed to cause a range of piston withdrawal distances.
At home I do have my R1 levers which have been worked on so much I can do it with my eyes closed, I'll play with them to see if the coupling is direct.
Which levers do you have that do this?
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Yes. That exactly describes the behavior of all the disc brakes I've ever encountered. The devil is in the details though. What is the pad clearance of which they speak? I measure between 1 and 2 thousandths on my Formulas without the springs. That seems to work in my application. What clearance are you measuring on yours? And I appologize if it seemed that I am not listening to all the input. I am, but more weight is given to observed, experienced, measured results and industry input as you provided than theory based on emotion.
Also, I should have been more clear in the original post. The lever wasn't soft, it just had a lot of travel. The phenomenon with springs and clearance is repeatable. I can return it to the long lever travel condition by reinstalling the springs. Further, I have had numerous brake systems appart in the last 42 years and this one is not a mystery. I can observe and measure the behavior and it in no way contradict the design.
Also, I should have been more clear in the original post. The lever wasn't soft, it just had a lot of travel. The phenomenon with springs and clearance is repeatable. I can return it to the long lever travel condition by reinstalling the springs. Further, I have had numerous brake systems appart in the last 42 years and this one is not a mystery. I can observe and measure the behavior and it in no way contradict the design.
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