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View Full Version : hydrolic disk brakes and wear of the pads...




RootsAndRocks
10-07-2009, 04:12 AM
I've had my new bike for a few weeks only, and I'm wondering how the brakes behave as the pads keep wearing off. Will travel of the lever increase to engage the brake? It's different than in a car, as in my Shimano brakes there is a pads-spreading spring, so I think a worn-out pad would have to travel further to touch the disk.




FullMonty
10-07-2009, 04:17 AM
no, the pistons self-adjust for pad wear. your lever travel should be fairly consistent until perhaps the very end of the pad life.

RootsAndRocks
10-07-2009, 05:26 AM
I hear you, but I don't fully understand...

schoenrock
10-07-2009, 05:42 AM
I hear you, but I don't fully understand...

:lol:
mAybE u sHooD cAll da hYdrOlic xperTz

FullMonty
10-07-2009, 06:06 AM
I hear you, but I don't fully understand...

as the pads wear out, the caliper pistons compensate by "starting" closer to the rotor. fluid will automatically pump from the reservoir in the lever and keep the pistons farther out.

skimtb1
10-07-2009, 06:06 AM
I hear you, but I don't fully understand...

as your brake pads wear, the pistons retract less and less back into the caliper.

when you replace the brake pads you will have to push the pistons back into the caliper in order to fit the new pads in.

boomforeal
10-07-2009, 06:20 AM
^this is my experience but i'll admit to not really understanding how its possible. in a closed hydraulic system, the fluid (optimally) takes up all available "space" between the master (lever) and slave (caliper) cylinders - applying pressure to the master displaces fluid within the system, actuates the slaves, pushing the pads towards the rim, and a spring pushes back to reset the brakes.

as your brake pads wear, the pistons retract less and less back into the caliper.

so where does this extra "space" come from (when your pads wear) and go (when you install new pads and pry the pistons apart)?

Lady Gravity
10-07-2009, 06:24 AM
good grief, are you an engineer? just ride the bike, and check your pads periodically.

RootsAndRocks
10-07-2009, 06:32 AM
as your brake pads wear, the pistons retract less and less back into the caliper.
makes sense, thanks. I think it's pretty cool how modern bikes are built...

FullMonty
10-07-2009, 06:35 AM
^this is my experience but i'll admit to not really understanding how its possible. in a closed hydraulic system, the fluid (optimally) takes up all available "space" between the master (lever) and slave (caliper) cylinders - applying pressure to the master displaces fluid within the system, actuates the slaves, pushing the pads towards the rim, and a spring pushes back to reset the brakes.



so where does this extra "space" come from (when your pads wear) and go (when you install new pads and pry the pistons apart)?

technically pretty much every modern disc brake isn't a fully closed system. basically there is a really tiny hole in the master cylinder between the bore itself and the reservoir in the lever. the reservoir is covered by a flexible bladder which allows expansion with heat. when you squeeze the lever, the master cylinder piston seal goes past this hole, seals that portion of fluid, and activates the caliper. you can actually see this happen if you bleed a shimano brake. my understanding of how the actual adjustment works is limited, but I'm gonna guess that when the master cylinder is activated and not enough pressure builds, more fluid is allowed to flow into the system. anyway, it works, and I am happy that we're not all running old Hope C2s where you had to play with a dial to adapt for pad wear and heat build-up.

boomforeal
10-07-2009, 07:03 AM
technically pretty much every modern disc brake isn't a fully closed system. basically there is a really tiny hole in the master cylinder between the bore itself and the reservoir in the lever. the reservoir is covered by a flexible bladder which allows expansion with heat. when you squeeze the lever, the master cylinder piston seal goes past this hole, seals that portion of fluid, and activates the caliper. you can actually see this happen if you bleed a shimano brake.

i've bled shimano hydraulic brakes and all of this makes sense.

my understanding of how the actual adjustment works is limited, but I'm gonna guess that when the master cylinder is activated and not enough pressure builds, more fluid is allowed to flow into the system.

this is what doesn't make sense to me. where does the "more fluid" come from? in a closed system you would need mo fluid to push mo cylinder to compensate for worn brake pads, but with the shimano's, somehow, you don't. this is what i'm wondering about.

anyway, it works, and I am happy that we're not all running old Hope C2s where you had to play with a dial to adapt for pad wear and heat build-up.

qft!

mattj
10-07-2009, 07:38 AM
this is what doesn't make sense to me. where does the "more fluid" come from? in a closed system you would need mo fluid to push mo cylinder to compensate for worn brake pads, but with the shimano's, somehow, you don't. this is what i'm wondering about.

Full Monty very effectively explained that the brakes are NOT closed system. The "extra" fluid comes from the reservior. That leftover space in the reservoir is then taken up by the expansion of the bladder. When you "reset" the pistons you are pushing fuild back from the brake into the reservoir.


-m

boomforeal
10-07-2009, 08:22 AM
Full Monty very effectively explained that the brakes are NOT closed system.

still confused (i thought the expanding bladder just dealt with heat levels, and get this part) but thanks for your effort and patience.

slider32
10-07-2009, 12:54 PM
The "reservoir" on the brakes is NOT, the master cylinder. the master cylinder is the small tube that the piston is in that the lever pushes, This draws fluid from the reservoir. When the pistons move out further, they draw more fluid from the reservoir, into the master cylinder.

philly56
10-08-2009, 12:24 AM
^ DING DING! we have a winner!

Chunk
10-08-2009, 12:36 AM
It's different than in a car

No, it really isnt at all. Motorcycle brakes, car brakes, bicycle brakes, if you understand one you understand them all.

Typed in "open system hydrolic brake" in :google: and this was the first picture (google images)

The "bleed port" is the part that you arent understanding. Its what makes it an open system.
http://www.mbaction.com/Media/News/Hydraulic_lever.jpg

EDIT: Also, those springs on the caliper dont do too much to actually spread the pistons back out. They are mostly there to prevent vibration and rubbing noise.

boomforeal
10-08-2009, 05:28 AM
k got it now. missed the part about the reservoir being different from the master cylinder. . . thanks chunk and slider!

RootsAndRocks
10-08-2009, 06:28 AM
EDIT: Also, those springs on the caliper dont do too much to actually spread the pistons back out. They are mostly there to prevent vibration and rubbing noise.
I think this this is the key statement! I thought the spring would spread the pads apart as far as the caliper design allows, forming a gap between the pad and the disk, and consequently more lever travel would be required as the pads wear off. But if the pads are almost touching the disk when at rest, than the amount of travel for the pads remains unchanged no matter how thin they are. All clear now!

schoenrock
10-08-2009, 08:35 AM
i'm sooo glad i drank some sauce. if i was sober right now i'd have to go for a drink.
like oh my god

XXX_er
10-08-2009, 04:42 PM
good grief, are you an engineer? just ride the bike, and check your pads periodically.

as a person who has made a career of fixing stuff broken by people who "just used it" I can say buddies quest for knowledge is a very good thing

trivia time:turn of the century (and not the last one) dr oldsmoblie bought a car and did not drive it until he had taken it apart so he could understand it completely ... eventually buddy started a company building oldsmobiles

biggles604
10-08-2009, 04:55 PM
as a person who has made a career of fixing stuff broken by people who "just used it" I can say buddies quest for knowledge is a very good thing

I agree completely. I think there would be a lot less mechanicals if people took the time to understand how their bikes worked. They aren't complicated. I don't think everyone should do their own maintenance, that takes a bit of talent, but at least understanding why something is or isn't working, would make life easier.

XXX_er
10-08-2009, 05:20 PM
Just as there are some people who should not be allowed to pro-create there are also some people who should not be allowed to own tools

but if you understand a little of whats going on it can save you money or wrecking stuff or consider that brakes are probably one of the more important systems on a big bike

I always found brakes easy to understand ,in auto shop I was the only student to get a perfect exam score and I wrote the test slightly drunk

Br

Lady Gravity
10-08-2009, 05:30 PM
Just as there are some people who should not be allowed to pro-create there are also some people who should not be allowed to own tools

hey i own tools! :P
yeah i understand what you're saying, and i agree to some extent. i'm happy to be able to do minor repairs on my bike, and understand how things work, but i'm also VERY happy when i can either bribe or beg somebody else to do the work for me.

understanding the nitty gritty of brakes doesn't appeal to me, but i can get how it would to others.

my comment was kind of borne out of a frustrating day at work (at this point, make that the whole week) so it was a little knee-jerk.

Oldfart
10-08-2009, 08:48 PM
hey i own tools!
yeah i understand what you're saying, and i agree to some extent. i'm happy to be able to do minor repairs on my bike, and understand how things work, but i'm also VERY happy when i can either bribe or beg somebody else to do the work for me.

understanding the nitty gritty of brakes doesn't appeal to me, but i can get how it would to others.

my comment was kind of borne out of a frustrating day at work (at this point, make that the whole week) so it was a little knee-jerk.


No worries Lady G, we forgive you. Now get your ass back in the kitchen and bake us some cookies!:P

Lady Gravity
10-08-2009, 10:40 PM
:lol: i haven't made cookies in awhile, but i bake a mean brownie! :P

iWish
10-08-2009, 11:39 PM
Don't mean to open this back up... but now I am curious (and a little drunk)... I understand most of the system, but how do the pads (or really the pistons in the caliper) know what distance to stay away from the pad? The pads don't constantly sit against the rotor, so is there some sort of mechanical do-hicky that tells (makes) the pads stop a certain distance from the rotor?

I understand that more fluid enters the system from the reservoir as that systems becomes larger caused by brake wear and the pistons are then allowed to travel further towards the rotor, but then why doesn't the system just fill up with fluid until the pads are pushed up against the rotor? What stops them?

Jeckyll
10-09-2009, 12:02 AM
... What stops them?

The little hole labeled "bleed port" in the diagram allows the pressure in the system to return to ambient once you let the lever go back to the rest position.

Most pads do drag a tiny amount (car brakes, motorcycles etc).

Oldfart
10-09-2009, 12:08 AM
Don't mean to open this back up... but now I am curious (and a little drunk)... I understand most of the system, but how do the pads (or really the pistons in the caliper) know what distance to stay away from the pad? The pads don't constantly sit against the rotor, so is there some sort of mechanical do-hicky that tells (makes) the pads stop a certain distance from the rotor?

I understand that more fluid enters the system from the reservoir as that systems becomes larger caused by brake wear and the pistons are then allowed to travel further towards the rotor, but then why doesn't the system just fill up with fluid until the pads are pushed up against the rotor? What stops them?


When you release the brake lever, the piston "pulls" the fluid back via suction. The pistons at the caliper follow suit. A little tiny bit of brake fluid goes from the reservoir after each application of the brake in response to the wear of the pad. As the lever piston comes past the bleed port, the "make up fluid" enters then, thus increasing the oil volume and keeping the pads at about the same distance from the rotor. See how the piston covers the bleed port when the lever is pulled, that means it pushes the same volume each time which means the pistons move the same amount each time, but as the pads wear, there is a greater vloume of fluid in the system on the other side of the bleed port so the pistons can't retract as far back. I'll bet after that poor explanation your really confused.

brgbdg9nanana
10-09-2009, 12:14 AM
Your spelling is good. :google:

XXX_er
10-09-2009, 12:38 AM
I understand most of the system, but how do the pads (or really the pistons in the caliper) know what distance to stay away from the pad? The pads don't constantly sit against the rotor, so is there some sort of mechanical do-hicky that tells (makes) the pads stop a certain distance from the rotor?



I am a bit sketchy on this part but the way I understand it with at least hayes mtnbike brakes is the rubber seals move to a certain point in the piston to get the piston/pads close enough to provide braking and then the rubber seals flex the piston away from the pads when you let off the brake lever

Timer
10-09-2009, 02:26 AM
I understand that more fluid enters the system from the reservoir as that systems becomes larger caused by brake wear and the pistons are then allowed to travel further towards the rotor, but then why doesn't the system just fill up with fluid until the pads are pushed up against the rotor? What stops them?


Thats something i'm wondering about as well.

If the pressure exerted by the rubber bladder was strong enough to move the pistons, the system would fill up with fluid, right?
So that can't be the case. The drag of the piston seals probably prevents the system from filling up with fluid.

So the drag of the piston seals requires a certain pressure for them to move, right?
Now i pull the lever, lever piston has not yet passed the bleed port, but pressure starts building. This pressure is not yet strong enough to move the caliper pistons. Why doesn't this pressure just push all the fluid on the right side (see picture) of the bleed port back into the reservoir?

It would seem obvious if there was a valve between reservoir and lever body, but there isn't.(and shouldn't be, for temperature reasons)

wishiwasridin
10-09-2009, 05:09 PM
Untill it passis that port your right its not pushing the pistons at all.

It presses the pistons in stops the bike,,, when you release the lever the pads are only pushed back as far as the rotor pushes them as it turns. This is why if you squeeze them with the rotor out they stay squeezed in.

As the pads wear the rotor doest push them out as far so when the lever opens right up and exposes that port it sucks in more fluid to make up for the increase volume needed. The oposite can happen when they get hot, lever opens and some fluid goes back into the res...

There may be some action of the lever pulling the pads apart im not positive this has no effect...

edit: this can also be seen when you push the pads out to change them or after a rotor less squeeze,,, nothing stops you from pushing those pads apart when that port is open.

I would think that rubber bladder only keeps crap out and fluid in..

The little springs between the pads just stop them from rubbing once the rotor has pushed them out and I think reduces vibration.

Oldfart
10-09-2009, 05:23 PM
Thats something i'm wondering about as well.

If the pressure exerted by the rubber bladder was strong enough to move the pistons, the system would fill up with fluid, right?
So that can't be the case. The drag of the piston seals probably prevents the system from filling up with fluid.

So the drag of the piston seals requires a certain pressure for them to move, right?
Now i pull the lever, lever piston has not yet passed the bleed port, but pressure starts building. This pressure is not yet strong enough to move the caliper pistons. Why doesn't this pressure just push all the fluid on the right side (see picture) of the bleed port back into the reservoir?

It would seem obvious if there was a valve between reservoir and lever body, but there isn't.(and shouldn't be, for temperature reasons)


I would guess that the pressure in the system is low until the pads contact the rotor at which point the pressure increases, but the master cyclinder piston is past the bleed port so fluid is not forced into the reservoir. Until the pads contact the rotor, the only pressure idf from overcoming seal friction at the pistons.

mattj
10-09-2009, 05:37 PM
Untill it passis that port your right its not pushing the pistons at all.

It presses the pistons in stops the bike,,, when you release the lever the pads are only pushed back as far as the rotor pushes them as it turns. This is why if you squeeze them with the rotor out they stay squeezed in.

As the pads wear the rotor doest push them out as far so when the lever opens right up and exposes that port it sucks in more fluid to make up for the increase volume needed. The oposite can happen when they get hot, lever opens and some fluid goes back into the res...

There may be some action of the lever pulling the pads apart im not positive this has no effect...

edit: this can also be seen when you push the pads out to change them or after a rotor less squeeze,,, nothing stops you from pushing those pads apart when that port is open.

I would think that rubber bladder only keeps crap out and fluid in..

The little springs between the pads just stop them from rubbing once the rotor has pushed them out and I think reduces vibration.

This is so many kinds of wrong I am baffled. Have you even read the posts in this thread or did you just come up with this by looking at a brake with a bent rotor while riding?


-m

boomforeal
10-09-2009, 05:44 PM
This is so many kinds of wrong I am baffled. Have you even read the posts in this thread or did you just come up with this by looking at a brake with a bent rotor while riding?


-m

well, no one has explained this aspect on this thread matt, so why don't you have a stab at it? though i have to say i didn't find your first post all that helpful.

mattj
10-09-2009, 05:45 PM
When you release the brake lever, the piston "pulls" the fluid back via suction. The pistons at the caliper follow suit. A little tiny bit of brake fluid goes from the reservoir after each application of the brake in response to the wear of the pad. As the lever piston comes past the bleed port, the "make up fluid" enters then, thus increasing the oil volume and keeping the pads at about the same distance from the rotor. See how the piston covers the bleed port when the lever is pulled, that means it pushes the same volume each time which means the pistons move the same amount each time, but as the pads wear, there is a greater vloume of fluid in the system on the other side of the bleed port so the pistons can't retract as far back. I'll bet after that poor explanation your really confused.


This explained it. Hence why I asked if wishiwasriding even read the thread posts.


-m

wishiwasridin
10-09-2009, 06:19 PM
When you release the brake lever, the piston "pulls" the fluid back via suction. The pistons at the caliper follow suit. A little tiny bit of brake fluid goes from the reservoir after each application of the brake in response to the wear of the pad. As the lever piston comes past the bleed port, the "make up fluid" enters then, thus increasing the oil volume and keeping the pads at about the same distance from the rotor. See how the piston covers the bleed port when the lever is pulled, that means it pushes the same volume each time which means the pistons move the same amount each time, but as the pads wear, there is a greater vloume of fluid in the system on the other side of the bleed port so the pistons can't retract as far back. I'll bet after that poor explanation your really confused.

This explained it. Hence why I asked if wishiwasriding even read the thread posts.


-m

Ha,
It may have been a confusing post but....

If ", the piston "pulls" the fluid back via suction. The pistons at the caliper follow suit" was true there would be no need for more fluid, the pads would come back to where they started and they would not compensate for wear.

The key point here is that the pistons are not brought back by the lever,,, this is how the compensation happens..

From how stuff works
"Self-Adjusting Brakes
*
*The single-piston floating-caliper disc brake is self-centering and self-adjusting. The caliper is able to slide from side to side so it will move to the center each time the brakes are applied. Also, since there is no spring to pull the pads away from the disc, the pads always stay in light contact with the rotor (the rubber piston seal and any wobble in the rotor may actually pull the pads a small distance away from the rotor). This is important because the pistons in the brakes are much larger in diameter than the ones in the master cylinder. If the brake pistons retracted into their cylinders, it might take several applications of the brake pedal to pump enough fluid into the brake cylinder to engage the brake pads."


http://auto.howstuffworks.com/auto-parts/brakes/brake-types/disc-brake2.htm

A.J
10-09-2009, 06:20 PM
I am a bit sketchy on this part but the way I understand it with at least hayes mtnbike brakes is the rubber seals move to a certain point in the piston to get the piston/pads close enough to provide braking and then the rubber seals flex the piston away from the pads when you let off the brake lever
This.

It is the slave cylinders' seals that handle the return duty of the pistons. This coupled with the master cylinders lip seal (one way seal) means that no "suction" is intoduced to the brake fluid.
Generally, negative pressures in hyraulic systems are to be avoided/minimized in design.

tazzmenn
10-09-2009, 06:44 PM
Ya hydros do lic so get mechanical disks.

XXX_er
10-09-2009, 06:55 PM
Ya hydros do lic so get mechanical disks.

for some people ya ...the BB7's are a great cheap /powerful/easy &cheap to maintain brake but they don't modulate all that well


I forget what brake but it was a poorley designed early "off brand"

on a long descent (So Long in williams lake ) during the 45 minutes of dh the brakes heated up so bad they stayed applied & buddy had to get off and push on some flats spots ...the design of this system did not allow for expansion

Lady Gravity
10-09-2009, 07:48 PM
for some people ya ...the BB7's are a great cheap /powerful/easy &cheap to maintain brake but they don't modulate all that well


agreed. considering that i almost did an OTB after grabbing the front brake like i normally do. pretty powerful, but definitely on/off

XXX_er
10-09-2009, 08:06 PM
I had a relationship like that ,on/off , on /off ...it was like so digital man