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  1. #1
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    Shimano four piston calipers, back from the dead.

    Shimano has added four piston calipers to it's SAINT gruppo this year. Other than superficial cosmetic upgrades, they look almost identical to their old XT four pistons. Apparently the only performance enhancing changes are the fact that the forward piston engages sooner, and has a slightly larger diameter.

    Shimano is claiming a fifty percent increase in stopping power over the two piston SAINT calipers, which are already very good I hear. Having original Shimano XT four pistons on all three of my bikes, I have always wondered why they ever discontinued this killer brake caliper.

    Have any XT four piston riders tried the new SAINT calipers? How do they compare to the originals?
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  2. #2
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    I'm also very interested in this comparison as I don't know what I'll be going for when my current XT 4 pots die. Hopefully the Saints also stand the test of time
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  3. #3
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    Quote Originally Posted by nnn
    I'm also very interested in this comparison as I don't know what I'll be going for when my current XT 4 pots die. Hopefully the Saints also stand the test of time
    Maybe that's why Shimano discontinued them. They were just too good. Who needed to upgrade?
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  4. #4
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    Quote Originally Posted by disease
    Shimano has added four piston calipers to it's SAINT gruppo this year. Other than superficial cosmetic upgrades, they look almost identical to their old XT four pistons. Apparently the only performance enhancing changes are the fact that the forward piston engages sooner, and has a slightly larger diameter.

    Shimano is claiming a fifty percent increase in stopping power over the two piston SAINT calipers, which are already very good I hear. Having original Shimano XT four pistons on all three of my bikes, I have always wondered why they ever discontinued this killer brake caliper.

    Have any XT four piston riders tried the new SAINT calipers? How do they compare to the originals?
    The leading piston would have to be smaller to hit the rotor first.

  5. #5
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    Quote Originally Posted by NZSpokes
    The leading piston would have to be smaller to hit the rotor first.
    I don't think so.

    The brake operates by pressure. There is even fluid pressure behind all the pistons. The piston force (pressure x area) needs to build to overcome the seals before the pistons will move.

    Piston area goes up as the square of diameter.

    Seal contact distance goes up linearly with diameter.

    This means that a larger piston will overcome seal force at lower fluid pressure.

  6. #6
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    Yes... a larger piston has more area on one face. Multiply this area by the distance moved and you have the volume of fluid required to move the piston. A larger piston requires a larger volume... more fluid. Therefore, a smaller piston would pump out first.

  7. #7
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    let's stay on topic, people.

    any real reason other than overall weight to upgrade from the old 4-pots?
    lets not make it a religion, it is recreation

  8. #8
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    Quote Originally Posted by XSL_WiLL
    Yes... a larger piston has more area on one face. Multiply this area by the distance moved and you have the volume of fluid required to move the piston. A larger piston requires a larger volume... more fluid. Therefore, a smaller piston would pump out first.
    This article features a photo of the caliper. It looks to me like the leading piston is larger than the trailing piston. If disc brake pads are like rim brake pads, you would want the contact to "toe in" with the front contacting first in order to avoid brake squeel and shudder. I am not an expert in brake design, but it seems to me like the piston at the front should contact first, and it looks to be the large one in this design.

    http://www.bikemagic.com/news/article.asp?UAN=6169&v=1

    The real question is why the hell did they wait ten years to do this?
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  9. #9
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    saint caliper

    There is a lot of improvements over the older 4 pot calipers. I owned a pair of the older ones when they first came out and i loved them too. i recently bought some new old stock old 4 pots on ebay and paired them with the new shimano xt 775 levers that have the increased leverage with the servo wave and the old calipers' piston seals could not handle the increased fluid pressure and the seals leaked. you could also see the caliper flex and seperate in the middle when the lever was pulled hard.

    The new saints are designed to handle the new lever design and also are stiffer in that the top part of the caliper is more fully closed eliminating the flex issue. Shimano also said that they have increased internal oil volume flow to make bleeding easier and help keep the temperature down when they get hot. the calipers are post mount so aligning and adjusting are a breeze compared to using shims to get the caliper aligned correctly.
    Last edited by Mr. Moco; 06-03-2008 at 12:05 PM.

  10. #10
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    I presume the Saint brakes are post mount only.

    Just tried the pistons thing and the large piston moves first on my Hope Mono M4s.

    A larger piston requires a larger volume... more fluid. Therefore, a smaller piston would pump out firs
    This is abysmal reasoning because it requires the fluid to take the path of most resistance.

    Gets my vote for worst use of the word "Therefore" on MTBR this month.

  11. #11
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    so when are the new saints going to be available? august?

  12. #12
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    Quote Originally Posted by disease
    This article features a photo of the caliper. It looks to me like the leading piston is larger than the trailing piston. If disc brake pads are like rim brake pads, you would want the contact to "toe in" with the front contacting first in order to avoid brake squeel and shudder. I am not an expert in brake design, but it seems to me like the piston at the front should contact first, and it looks to be the large one in this design.

    http://www.bikemagic.com/news/article.asp?UAN=6169&v=1

    The real question is why the hell did they wait ten years to do this?
    Well taking a close look at the photo, unless your wheels spin backwards in the US the leading piston is smaller.

  13. #13
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    Quote Originally Posted by disease
    This article features a photo of the caliper. It looks to me like the leading piston is larger than the trailing piston. If disc brake pads are like rim brake pads, you would want the contact to "toe in" with the front contacting first in order to avoid brake squeel and shudder.
    Disc brakes are not like rim brakes. The braking thrust in a disc brake is reacted by the pad backing plate thrusting against the caliper. With rim brakes, the mounting point is necessarily at a distance from the rim and this results in a twisting torque on the brake pad. All "toeing in" with rim brakes is just countering the natural tendency to "toe out" after the pad has made contact.

    Because of this there is no reason to suppose the leading edge of the pad needs to contact the brake first. In fact by having the trailing edge contact first, the pad backing plate will be angled in a bit (call it toe out) at the point of contact. When the second set of pistons make contact and bring the pad back to being parallel to the rotor, the pad will effectively have pivoted on its trailing edge instead of trying to slide the backing plate past the face of the caliper slot under ever increasing force - this could well provide better modulation.

  14. #14
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    If thats the case why does my cars disk brakes have a shim to ensure the disk pad has toe in?

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    Car brakes aren't as sensitive to the modulation difference as bike brakes, being a much larger system.

  16. #16
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    Still think hes wrong.

  17. #17
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    They were...

    Quote Originally Posted by disease
    ...

    I have always wondered why they ever discontinued this killer brake caliper.
    ... heavy as flock. THis is fine for trail bikes or freeride bikes, but XT is more about lightweight XC bikes. They went the Hope Mono Mini route with two pistons and one piece mono block calipers for lightweight and stiffness. The 4 pot makes good sense to me for a heavier duty trailbike or FR bike, where weight is less of an issue.

    I had a set, and they had amazing stopping power while still having really good modulation. IIRC, they have dissimilar sized pistons as well for toe-ing in the pad when you applied them, which is probably why the modulation was so good on those. The power ramped up nicely.

    My 4 pot XTs used to rub all the time. The pad clearance was just too tight. Maybe mine needed new seals, tho... who knows. No matter how many 0.2mm shims I added or took out, they would always rub a tad on perfectly straight rotors. I love those brakes, but the rub issue made me take them off my bike. It just drove me too nutz to run them. If they fixed this issue with the Saints, I would totally run a set on my trailbike.

    *edit*

    Here is a larger pic of the new Saint caliper:


  18. #18
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    Quote Originally Posted by NZSpokes
    Still think hes wrong.
    Now that's really pissing me off. Talking about me in the third person. Just plain rude.

    I'm getting confused with this toe in/out talk and I think I even got it muddled up in my post.

    The *fact* that I have been able to explain and verify with an experiment is that large pistons come out first. Quite why this is used in braking systems is not something I know but I am hypothesising.

    FWIW, the Hope Mono M4 has the larger piston as the trailing piston as well.

  19. #19
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    Quote Originally Posted by petercarm
    I presume the Saint brakes are post mount only.

    Just tried the pistons thing and the large piston moves first on my Hope Mono M4s.



    This is abysmal reasoning because it requires the fluid to take the path of most resistance.

    Gets my vote for worst use of the word "Therefore" on MTBR this month.
    Or the smaller piston is sticky... The difference in resistance can be considered negligible... particularly in comparison to the difference in hydraulic ratio/advantage. And actually... the larger piston also has more area in contact with the seal... so there is lower pressure (as it requires more fluid/volume) behind the larger piston... AND more resistance.

    Technically 2-piston calipers have a dominant piston because the banjo doesn't feed the two halves quite evenly... but you see that in most (properly setup) systems the pistons move about the same amount.

    My XT setup has the smaller piston move first... This is how the old Clark 6pot brakes were described as well...

    "The system operates as follows: First to make contact with the rotor is the 9mm. This followed a millisecond later by 14mm and then the 18mm. The benefits of this is that the braking pressure is progressive and helps to avoid snatching or in-voluntary lock up. Some hydraulic systems 'lock up' very quickly and all too easily. The Clim8 is different, no less powerful but thanks to the variation in piston size and the progressive way in which they apply pressure to the rotor 'lock up' is kept under control. The 7000 series aluminium used to make the rotors is predominantly used in the aerospace industry where high tensile strength and lightness are critical."

    So yes... he is wrong.

    An experiment does not prove fact.
    Last edited by XSL_WiLL; 06-03-2008 at 07:23 PM.

  20. #20
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    Thank you, thats what I thought.

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    If you're going to accuse me of being wrong can you at least do it with some decent reasoning or critique my own reasoning.

    Force behind the piston of radius (r) arising from pressure (P) = P x A = P x pi x r ^2

    Force restraining the piston is some unit force per mm of seal length = K x 2 x pi x r

    When the force is equal the piston is free to start moving :

    ... P x pi x r ^ 2 = K x 2 x pi x r

    ... P x r = K x 2

    Solve for pressure as a function of radius:

    ... P = K x 2 / r

    i.e. the pressure required to move a piston is proportional to the inverse of its radius

    Now if you think hydraulic systems with incompressible fluid have large differentials in pressure across the caliper, you ought not to have come off your medication.

    If you've got small pistons moving first then I cannot refute your observations but it means that neither you nor I have a clue why that is the case.

  22. #22
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    I guess Shimano have got it wrong then..............

  23. #23
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    Uh? Where have Shimano got anything wrong?

    C'mon. Fault my reasoning. Step up.

  24. #24
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    Quote Originally Posted by pimpbot
    My 4 pot XTs used to rub all the time. The pad clearance was just too tight. Maybe mine needed new seals, tho... who knows. No matter how many 0.2mm shims I added or took out, they would always rub a tad on perfectly straight rotors. I love those brakes, but the rub issue made me take them off my bike. It just drove me too nutz to run them. If they fixed this issue with the Saints, I would totally run a set on my trailbike.
    That's exactly the same experience I had w/ three different sets. I _never_ had them run rub free. I have a set of Formula Oro24's now and these are a breeze to set up rub free. I prefer the feel of the old shimano 4-pots though.

    Dave

  25. #25
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    Quote Originally Posted by petercarm
    If you're going to accuse me of being wrong can you at least do it with some decent reasoning or critique my own reasoning.

    Force behind the piston of radius (r) arising from pressure (P) = P x A = P x pi x r ^2

    Force restraining the piston is some unit force per mm of seal length = K x 2 x pi x r

    When the force is equal the piston is free to start moving :

    ... P x pi x r ^ 2 = K x 2 x pi x r

    ... P x r = K x 2

    Solve for pressure as a function of radius:

    ... P = K x 2 / r

    i.e. the pressure required to move a piston is proportional to the inverse of its radius

    Now if you think hydraulic systems with incompressible fluid have large differentials in pressure across the caliper, you ought not to have come off your medication.

    If you've got small pistons moving first then I cannot refute your observations but it means that neither you nor I have a clue why that is the case.
    Your formula supports my reasoning that the larger piston has more resistance...

    Where did you get these formulas?

    Volume = pi*r^2*depth (or stroke). This is common knowledge V = SA*Height.

    So a smaller piston has less volume per unit of movement than a larger piston. The MC pushes a set amount of fluid. That means the smaller piston fills the volume, starts generating pressure, and actuates first.

    I did fault your reasoning... and I did it without being a jackass about it... Pompous know-it-all...

  26. #26
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    You still don't get it. The resistance to movement is greater. Exactly. The force to move is *much* greater. Is the penny about to drop?
    Last edited by petercarm; 06-04-2008 at 09:06 AM.

  27. #27
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    Quote Originally Posted by XSL_WiLL
    Your formula supports my reasoning that the larger piston has more resistance...

    Where did you get these formulas?
    First principles of hydraulics. You'd be an idiot to design hydraulic systems without knowing these... and I design hydraulic systems for race cars in my spare time.

    Volume = pi*r^2*depth (or stroke). This is common knowledge V = SA*Height.
    The volume argument is wrong. The fluid will not flow behind the smaller piston in preference to flowing behind the larger piston where the ratio between piston force (hydraulic) and piston seal force means the pistons moves more readily.

    So a smaller piston has less volume per unit of movement than a larger piston. The MC pushes a set amount of fluid. That means the smaller piston fills the volume, starts generating pressure, and actuates first.
    As mentioned above this is wrong.

    I did fault your reasoning...
    No. You've repeated the exact same "smaller volume" thing that got me het up in the first place. I critiqued that the fluid would not flow to the piston of greatest resistance. You have not yet critique a single element of my argument. All you have done is repeat the same wrong assertions about the volume of the pistons. While your equation for volume is right, it is completely irrelevant to the question of which piston moves first.

    and I did it without being a jackass about it... Pompous know-it-all...
    I can't pretend that that criticism isn't valid. I don't expect any apology for being a pompous know-it-all. I just expect you to eat a bit of humble pie when you realise I am right and you are wrong.

  28. #28
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    Still wrong.

  29. #29
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    Likewise all you've done is repeat the same thing over and over again while using that as a reason for why I am wrong.

    I still don't see how there is LESS resistance behind a piston with MORE contact with the seal...

    I don't design hydraulic systems... calling me an idiot is uncalled for. You're the one b!tching about people being rude, but you're even more rude. You also contradict some statements you have made earlier.

  30. #30
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    Quote Originally Posted by petercarm
    I can't pretend that that criticism isn't valid. I don't expect any apology for being a pompous know-it-all. I just expect you to eat a bit of humble pie when you realise I am right and you are wrong.

    Guys.... the discussion is good and interesting, please keep it polite (it has been borderline so far, but seems headed the wrong direction)

  31. #31
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    Consulted Hope... they said the smaller piston should move first as there is the least resistance.

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    Quote Originally Posted by crisillo
    Guys.... the discussion is good and interesting, please keep it polite (it has been borderline so far, but seems headed the wrong direction)
    Fair call. Lets keep it nice.

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    Quote Originally Posted by XSL_WiLL
    Consulted Hope... they said the smaller piston should move first as there is the least resistance.
    Didnt think my old brain had it wrong.

  34. #34
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    Well, whoever you talked to at Hope evidently had their head shoved up their fundament. Here's a video I took today of a brand new Hope SE Mono M4:

    Hope Mono M4 big piston moves first video
    Last edited by petercarm; 06-04-2008 at 04:14 PM.

  35. #35
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    Quote Originally Posted by XSL_WiLL
    You also contradict some statements you have made earlier.
    Not one contradictory statement. Point it out. It may help us get to the bottom of this.

  36. #36
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    ...
    Last edited by petercarm; 06-04-2008 at 04:39 PM.

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    Sorry vid does not open for me.

  38. #38
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    What format vid will work for you? I can convert. The vid is genuine and I would like you to see it.

  39. #39
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    Apple Quicktime video format added...

    http://hotair.fastmail.co.uk/bike/mt...oves_first.mov

    This is a screen grab, so not the best quality.

  40. #40
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    My small pistons move first, on all three of my M4's. I think there could also be a variable in that when you make small lever inputs, the pads are forced against the rotor by all (as I have seen in automotive applications as well), but then further inputs will yield more output from the small piston first, then the large.

    Anyhow, my small pistons move first. You can go by a brand new Hope. The seals are very tight and need to be broken in before the brake comes to its full potential.

  41. #41
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    But why?

  42. #42
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    Next big deal: We are attempting to answer the wrong question. The force on the piston is minimal until it comes into contact with the rotor. There is barely any control over whether just one or multiple pistons move until the piston contacts the rotor. The first piston to move is likely to carry on moving until something stops it.

  43. #43
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    Quote Originally Posted by petercarm
    The first piston to move is likely to carry on moving until something stops it.
    This part I agree with. But what will fry your noodle is do both pistons apply the same force once both have made contact?

  44. #44
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    Quote Originally Posted by petercarm
    Next big deal: We are attempting to answer the wrong question. The force on the piston is minimal until it comes into contact with the rotor. There is barely any control over whether just one or multiple pistons move until the piston contacts the rotor. The first piston to move is likely to carry on moving until something stops it.
    Funny how it's the wrong question once you find out that you're wrong.

  45. #45
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    Its gone quiet around here.........


    Maybe hes gone to bed.

  46. #46
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    the larger piston moves the lesser distance, but applies more force. it has nothing to do with stiction of any seals or fluid flow between calipers. your just being silly when you try spouting math or defy simple reasoning founded soundly in the laws of nature.

    the pully system above should allow you to wrap your mind around this concept and allow you to appreciate shimanos decision to switch to a four pot caliper for its saint line. adequate pad clearance without sacrificing modulation and excessive lever travel.
    Last edited by Panzerknacker; 06-04-2008 at 10:25 PM.

  47. #47
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    Right... it's about the hydraulic advantage/ratio.

  48. #48
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    You're all still wrong. Bike tech in the LBS was asked the straightforward question: "which moves first, big or small"

    "Big". He goes on: "When we get bikes come in with corroded brakes I have to clean them up. Once I've got everything moving nicely it is always the big pistons that move out first."

    He reckoned the reason was that the fluid was fed in directly behind the large pistons. I don't mind his reasoning, but I'm just reporting his observation: The big piston moves first.

  49. #49
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    Quote Originally Posted by petercarm
    You're all still wrong. Bike tech in the LBS was asked the straightforward question: "which moves first, big or small"

    "Big". He goes on: "When we get bikes come in with corroded brakes I have to clean them up. Once I've got everything moving nicely it is always the big pistons that move out first."

    He reckoned the reason was that the fluid was fed in directly behind the large pistons. I don't mind his reasoning, but I'm just reporting his observation: The big piston moves first.
    So now the math does not work for you?

  50. #50
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    Quote Originally Posted by Panzerknacker
    it has nothing to do with stiction of any seals or fluid flow between calipers. your just being silly when you try spouting math
    I'm not spouting maths for the sake of it. I have been presenting the right equations for the analysis of the question in hand. The pulleys picture doesn't help because pulleys work with a critical difference to hydraulics. If you have not taken account of this, you do not appreciate how the pulleys model does not explain hydraulics.

    You are probably working on the idea that the amount of string you pull on the right hand pulley system is the same as the amount of string you pull on the left hand one. The right hand one moves less, with more force. Agreed. The difference with a hydraulic system is that the fluid does not go in equal volumes behind each piston. The pistons are linked by open flow paths. Far from an equal amount of string being pulled in each system, it is that an equal pressure is developed behind each piston, regardless of how much fluid is delivered to each piston.

    There is no controlling of the ratio of movement between two slave pistons based on piston size. The pulley analogy breaks down and is built on a wrong assumption.

    or defy simple reasoning founded soundly in the laws of nature.
    All I'm describing is the most basic of 1.01 mechanics. Newtons law. If something starts off stationary and then moves, there is a force imbalance. Simple as that. F=ma. Try and find me a simpler law of nature, or one so widely applicable.

    In this instance, the force due to hydraulic pressure is readily determined. The pressure is the same behind all the pistons. (<<<< ---*** please if you disagree with this specific point, tell me because my argument hinges on it ***). The force is the pressure multiplied by the surface area.

    The force that is holding the pistons in place is supplied by the seal. When there is no hydraulic pressure behind the seal, the seal is relaxed and it is not providing much axial force. As pressure builds, the seal stiction needs to be overcome. Until you exceed the seal stiction there is no force imbalance and no acceleration.

    Again, my argument is that the seal stiction (of a clean and uncontaminated seal) is to do with the length of the seal material in contact with the piston. i.e. the piston circumference.

    Anyway, to get your head around it I have modified your pulleys diagram to be an a better model of the hydraulic system we're talking about:


  51. #51
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    Quote Originally Posted by NZSpokes
    So now the math does not work for you?
    Another antagonistic ad hominem contribution from a zero content poster. Congratulations.

  52. #52
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    Quote Originally Posted by petercarm
    Another antagonistic ad hominem contribution from a zero content poster. Congratulations.
    (a+b)^2=a^2 +b^2

    log(a+b)^2=loga^2 +logb^2

    a^2+b^2=c^5

    a^2 + b^2=x

    X=c^5

    c=x

    0=5

    Therefore, the amount of force on the pad is inversely proportional to the size of your penis.
    I wish I didn't know now what I didn't know then.

  53. #53
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    I see the playground bullies have really homed in on this one. Does it feel good to be in a gang and pick on the nerdy kid who likes maths?

    Any comment from anyone about the modified pulley drawing that properly models a two piston hydraulic system? Perhaps it is all going to go quiet when the consequences become clear.

  54. #54
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    I'm going from memory here, but folks used to (and still do I'm sure) install larger master cylinders in track cars to sacrifice some ultimate braking force in the name of modulation. In other words you'd have to push the pedal harder to lock the tires up.

    That tells me how the relationship between master and slave cylinder size impacts clamping force. In the 4 pot caliper scenario the MC size is fixed, but the SC size varies, changing the ratio. With a given size MC the larger SC will provide more clamping force, but will move a smaller distance for any given volume of fluid moved. It makes sense then that the small cylinder would move first, but it will not generate as much clamping force for any given pressure applied to the lever.

  55. #55
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    Quote Originally Posted by wreckedrex
    I'm going from memory here, but folks used to (and still do I'm sure) install larger master cylinders in track cars to sacrifice some ultimate braking force in the name of modulation. In other words you'd have to push the pedal harder to lock the tires up.

    That tells me how the relationship between master and slave cylinder size impacts clamping force. In the 4 pot caliper scenario the MC size is fixed, but the SC size varies, changing the ratio. With a given size MC the larger SC will provide more clamping force, but will move a smaller distance for any given volume of fluid moved.
    Right down to here, I agree with you.

    It makes sense then that the small cylinder would move first
    This I disagree with and is the nub of the matter. The volume pushed into the hydraulic lines by the master cylinder will take the path of least resistance. The question then is "where is the path of least resistance?"

  56. #56
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    Quote Originally Posted by petercarm
    I see the playground bullies have really homed in on this one. Does it feel good to be in a gang and pick on the nerdy kid who likes maths?
    Who, me? I'm not making fun of you. I'm jealous that you can mathematically explain this issue.

    Not trying to pick on you.

    Oh, I see....
    "Therefore, the amount of force on the pad is inversely proportional to the size of your penis."

    However much that might have seemed directed toward you, it wasn't meant to be. I don't even know why I quoted you...sorry. It was just a stupid joke not meant to be offensive.
    I wish I didn't know now what I didn't know then.

  57. #57
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    Quote Originally Posted by petercarm
    This I disagree with and is the nub of the matter. The volume pushed into the hydraulic lines by the master cylinder will take the path of least resistance. The question then is "where is the path of least resistance?"
    I'm going to assume that you're thinking it's the larger piston. I'd argue that it's not the path of the least resistance at all, but possibly the path of the least resistance relative to the force being applied. Makes sense I guess...

  58. #58
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    stop shitting math equations all over the tread, if your gona put that up do something with it.

    i finaly took thirty two seconds out of my life to google hydraulic press and voala:




    read the bottom right, here ill reiterate

    YOU HAVE TO PAY FOR TEH MULTIPLIED OUTPUT FORCE BY EXERTING THE SMALLER INPUT FORCE THROUGH A LARGER DISTANCE

    see? simple!

    have fun https://hyperphysics.phy-astr.gsu.ed...sc.html#hpress

    and finaly WTF is this garbage?

    its the same damn thing as the system on the right in teh original, you did nothing to change it other than delete a separate set of pulleys
    opinions were like kittens i was giving them away

  59. #59
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    Quote Originally Posted by Panzerknacker
    YOU HAVE TO PAY FOR TEH MULTIPLIED OUTPUT FORCE BY EXERTING THE SMALLER INPUT FORCE THROUGH A LARGER DISTANCE

    see? simple!
    And already mentioned (probably a few times before this even):

    Quote Originally Posted by wreckedrex
    With a given size MC the larger SC will provide more clamping force, but will move a smaller distance for any given volume of fluid moved.
    and already agreed upon:

    Quote Originally Posted by petercarm
    Right down to here, I agree with you.
    So can we stop with the hollering? Or at the very least start screaming at one another about things we *don't* agree on?

  60. #60
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    I asked: "Any comment from anyone about the modified pulley drawing that properly models a two piston hydraulic system?"

    And you replied: "WTF is this garbage?".

    Your own pulley analogy relies on the idea that tension in the rope is the equivalent of pressure in a hydraulic system. By joining the two pulley systems with the further pulley both of the initial systems receive the same tension (pressure), thus mimicking a hydraulic system.

    That is WTF that garbage was.

  61. #61
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    Quote Originally Posted by XSL_WiLL
    Consulted Hope... they said the smaller piston should move first as there is the least resistance.
    The small piston might have the least resistance, but the larger piston will exert more pressure on the rotor once all of the piston movement has stopped, i.e. the pads are tight against the rotor.

    I doubt the reason for the different sized pistons is to move one first, it's probably to place the pressure through the pads to the rotor in unequal amounts on each end of the pad. The bigger piston will have greater pressure on the pad/rotor contact area than the smaller one, that's hydraulics. Probably helps modulation, that would be the only thing I can think of that it would make a difference on. Though my Gustav's have 2 pistons, equal size, lots of pad area, they modulate very well, as well as stop very well. So, who knows?

    Monte

  62. #62
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    Quote Originally Posted by Monte
    The small piston might have the least resistance, but the larger piston will exert more force on the rotor once all of the piston movement has stopped, i.e. the pads are tight against the rotor.
    Just my edit above to add to my general agreement with what you have written.

    I doubt the reason for the different sized pistons is to move one first,
    Without backing down from this highly stimulating debate, I agree that a hydraulic designer would not rely on a piston moving first to achieve any particular design objective.

    IMO, the larger piston exists to overcome the friction force of the pad backplate being thrust against the end of the caliper and to balance the minor moment arising from the backplate contact point being spaced away from the rotor face.
    Last edited by petercarm; 06-06-2008 at 02:22 PM.

  63. #63
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    Quote Originally Posted by petercarm
    IMO, the larger piston exists to overcome the friction force of the pad backplate being thrust against the end of the caliper
    That makes some sense, depending on the caliper design. The Gustav pads "hang" from a pin, so to speak, so it wouldn't make sense in that case.

    Monte

  64. #64
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    Quote Originally Posted by Monte
    That makes some sense, depending on the caliper design. The Gustav pads "hang" from a pin, so to speak, so it wouldn't make sense in that case.

    Monte
    A lot of pads use pins for location, but most of the braking force is reacted by the backplate against the caliper. I haven't seen Gustavs close to, so I can't comment.

  65. #65
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    Eeeeeeeer,

    anyone have information on the NEW Saints, actual riding time or whatever!

    I got a set of 4pots with Straitline levers. Nice brakes, but upgradeitis brakus is here. So I could get a CODE, Code 5, Hope Moto V2, a Formula the One, or wait for the new Saints.
    Just to see if there´s anything more powerful and with a nice lever feel out there.

    Magura Gustavs are powerful, but have an awkward lever feel, so I sold them.

    Greetings Znarf

  66. #66
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    Hope Moto V2 isn't 4-pot. It's 2 HUGE pistons. The Mono M4 is 4-pot, the Mono M6 is 6-pot.

  67. #67
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    Yeah,
    I know, the ONES and the Gustavs ar not 4pot either.


    But I think there can be good brakes with eiter 2 or 4pots.

    Greetings ZNarf

  68. #68
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    At the risk of being reamed a new one:

    I'm no engineer and I'm allergic to numbers, so please be kind.

    It doesn't really matter which piston moves first. Sure, a larger piston should move before a smaller one does because it has greater leverage over the rubber seal (which flexes and doesn't overcome friction during most cycles).

    Once the large pistons push into contact with the rotor, the resistance of the large pistons increases exponentially and instead of pushing the large pistons with more force, the fluid takes the path of least resistance, which is now the small pistons. They press outward and push the small pistons into the back of the pads.

    Now, both sets of pistons are in place against the back of the pads. Increased fluid PSI should cause the larger pistons to apply more force because those pistons will move less for a given PSI increase. Greater leverage. Simple, right?

    From what I can tell, the old XTs toe the pads just like you toe a rim brake, with the trailing edge receiving the larger pistons and greater power.

    Anyway, I'll probably be buying a Saint 4-pot to replace my lame-as s '08 XT front caliper. Anything for more power and less vibration.

  69. #69
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    Best answer here yet.

  70. #70
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    Quote Originally Posted by NZSpokes
    Best answer here yet.
    Yup, pretty good.

    Quote Originally Posted by Monte
    The small piston might have the least resistance, but the larger piston will exert more pressure on the rotor once all of the piston movement has stopped, i.e. the pads are tight against the rotor.

    I doubt the reason for the different sized pistons is to move one first, it's probably to place the pressure through the pads to the rotor in unequal amounts on each end of the pad. The bigger piston will have greater pressure on the pad/rotor contact area than the smaller one, that's hydraulics. Probably helps modulation, that would be the only thing I can think of that it would make a difference on. Though my Gustav's have 2 pistons, equal size, lots of pad area, they modulate very well, as well as stop very well. So, who knows?

    Monte

  71. #71
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    Good general description of the brake operation

    Quote Originally Posted by NZSpokes
    Best answer here yet.
    What? Even with this bit?

    Quote Originally Posted by D.F.L.
    It doesn't really matter which piston moves first. Sure, a larger piston should move before a smaller one does because it has greater leverage over the rubber seal
    Only bit of nitpicking (and I'm sure unintended by the poster):
    Now, both sets of pistons are in place against the back of the pads. Increased fluid PSI should cause the larger pistons to apply more force because those pistons will move less for a given PSI increase.
    The nitpick is that the force will be a given value based on "cross-section area x pressure". For any given "PSI increase" the volume flow and the piston displacement is determined by the elasticity of the system components, compared to the forces involved. I'd hazard that for any given "PSI increase" the larger pistons will move more (although the movement will be miniscule).

    Quote Originally Posted by NZSpokes
    But what will fry your noodle is do both pistons apply the same force once both have made contact?

  72. #72
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    No, he answered the question with out trying to be superior and wrote well.

  73. #73
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    Quote Originally Posted by NZSpokes
    No, he answered the question with out trying to be superior and wrote well.
    You have some serious double standards issues to deal with.

    A quick recap of my rude, superior, poorly written contribution:

    Quote Originally Posted by NZSpokes
    The leading piston would have to be smaller to hit the rotor first.
    I don't think so.

    The brake operates by pressure. There is even fluid pressure behind all the pistons. The piston force (pressure x area) needs to build to overcome the seals before the pistons will move.

    Piston area goes up as the square of diameter.

    Seal contact distance goes up linearly with diameter.

    This means that a larger piston will overcome seal force at lower fluid pressure.

  74. #74
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    I could quite well be wrong, often am. But I think DFL just put his point across well in a simplistic way that works.




    But I still think your wrong, I may e-mail Shimano tomorrow to get thier view. Got some friends there.

  75. #75
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    Any chance you could email my response (as above) to your friends at Shimano for them to criticise. It probably won't help to have another "my friend says" addition to the thread but if they can poke holes in my argument I'll take it.

  76. #76
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    Why I keep posting, I don't know.

    Equations put normal people to sleep, so I was trying another way.

    Yes, 'PSI' wasn't the best term. Squeezing the brake lever will move small pistons further than it would large pistons. Greater piston travel means less force, and vice-versa.

    Again, it really doesn't make much difference which pistons hit the rotor first. Let's say that the large pistons apply first and bring the pads to the rotor. As soon as they touch, the resistance (to further movement) of those pistons goes way up and the fluid, which is lazy, seeks the path of least resistance, which is the still not-fully-applied small pistons. Do you think the large pistons are creating much braking force prior to the small pistons applying? I don't think so. Does it 'soften' the initial braking force? Sure. Enough to make a real-world difference? I don't know.

    The only thing I'd really care about is if they can keep a wet brake quiet. If they did that, I'd buy them.

  77. #77
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    Quote Originally Posted by D.F.L.
    Why I keep posting, I don't know.
    Again, it really doesn't make much difference which pistons hit the rotor first.
    Absolutely agreed.

    As soon as they touch, the resistance (to further movement) of those pistons goes way up and the fluid, which is lazy, seeks the path of least resistance, which is the still not-fully-applied small pistons.
    Question for you (rhetorical, I'm afraid): does the fluid ever not take the path of least resistance?

    Do you think the large pistons are creating much braking force prior to the small pistons applying? I don't think so. Does it 'soften' the initial braking force? Sure. Enough to make a real-world difference? I don't know.
    All pistons moving an equal distance and arriving at very close to the same time is the ideal for me. If it weren't ideal, I wouldn't shim/align my calipers.

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