Viewing 40 posts - 1 through 40 (of 52 total)
  • 4 pot brakes are better because…
  • Premier Icon ayjaydoubleyou
    Free Member

    One of the things on the “bikes are too heavy now” thread was 4 pot brakes becoming the norm.

    shimano are now offering their xc/trial line as 4 pots, and a common thought on the internet is that Hope X2 are only for very light duties, and the “enduro” E4 is needed for even a trail hardtail.
    (I’ve only got experience with these two brands, are others including big red similar?)

    The difference in pad contact size is significant on the hopes, but on shimano its barely any increase.
    Again, the armchair engineers don’t seem to go this far, they just state that 4 pistons are better. What is the actual science behind this? Aside from pad area why is splitting the force from the lever pull over 4 often smaller pistons more benficial?

    My entirely anecdotal experience being that SLX and XT felt better than Zee, and X2 felt better than E4 (and are the nicest feeling brakes I have ever used). Plus the 4 piston versions have been more maintainence and installation/alignment hassle, higher cost, and higher weight.

    Premier Icon thols2
    Free Member

    You can control the distribution of pressure on the pad better because the leading and trailing pistons don’t have to be the same size. The longer pads have more surface area for cooling, so do the calipers.

    Premier Icon steveh
    Full Member

    For me on shimanos it’s not about more power but much better modulation with the 4 pot versions. That’s enough to mean they’re all I run and weight disadvantage is small.

    Premier Icon DickBarton
    Full Member

    Larger braking surface but lower pressure…2-pots have massive pressure behind the pads compare to 4-pots…
    4-pots offer larger braking surfaces and should be quicker stoppers…
    Both work well, but 4 is more so tends to have people wanting them as they appear betterer.

    Premier Icon akkwlsk
    Free Member

    Overheating (lack of it actually) and modulation.
    Both noticeable mostly in steep, technical sections or very long descents.

    Premier Icon thegeneralist
    Full Member

    The longer pads have more surface area

    You’re shitting me right? The XT four pots on my new bike take a different pad from the other Shimano brakes I have…

    FFS

    Premier Icon binners
    Full Member

    I’m not so sure about bikes being heavier, but I’m a lot heavier than I used to be. Even more so since lockdown. For those of us classed as ‘big units’ we need all the stopping power we can get.

    I’ve got Magura MT-7 four-pots on my bouncy bike which are absolutely ferocious. When I get back on my hardtail the XT stoppers on it feel like a set of V brakes

    Premier Icon 5lab
    Free Member

    You can control the distribution of pressure on the pad better because the leading and trailing pistons don’t have to be the same size.

    I’ve seen that claimed elsewhere but I don’t belive it’s true. Both pistons run on a common feed from the master cylender, so both are applying the same pressure regardless of their size.

    Premier Icon ayjaydoubleyou
    Free Member

    On the fairly homogenous interface of pad to rotor, does F=μR not apply? And therefore independent of pad size?

    Premier Icon tjagain
    Full Member

    using a 4 pot you can havre a long thin pad rather than a square one. It improves braking because more of the pad is further from the axle even on the same size disc

    Bigger pads / lower pressures etc make no real differnce to stopping ability – just to feel

    Premier Icon tjagain
    Full Member

    so both are applying the same pressure regardless of their size.

    But that pressure acts over a different area so the force generated changes with the size of the piston

    Premier Icon argee
    Full Member

    The answer is they just are

    Premier Icon molgrips
    Full Member

    A-level physics tells you that for the same lever force, the same force is transmitted to the brake – even though the pad area is larger, the pressure is less. BUT a bit of engineering suggests that the higher the pressure at the pad the higher the temperature at the pad-disc interface (because the same energy is dissipated over a larger area*) which means lower friction**. Bigger pads are therefore better, and a 4 pot brake is a way to get a long thin pad without needing a really wide disc.

    * not exactly convinced of this yet though

    ** I think this is because at the pad-disc interface there’s a fine layer of pad material being vapourised or eroded and the hotter it gets the more this gets in the way of friction. Some good graphs of force vs temp in this paper: https://www.mdpi.com/2504-3900/49/1/100/pdf

    Premier Icon thols2
    Free Member

    Both pistons run on a common feed from the master cylender, so both are applying the same pressure regardless of their size.

    As TJ explained, a larger piston gives more force on the pad than a small piston, assuming the same fluid pressure. For example, at 100 pounds per square inch pressure, a 1 square inch piston would produce 100 pounds of force, while a 2 square inch piston would produce 200 pounds force.

    Premier Icon SirHC
    Full Member

    4 pots are more faff to setup, twice as many pistons to balance and maintain.

    Never wanted any more power than my 2 pot formula curas.

    Premier Icon welshfarmer
    Full Member

    obvious answer is obvious

    Premier Icon ayjaydoubleyou
    Free Member

    As TJ explained, a larger piston gives more force on the pad than a small piston, assuming the same fluid pressure. For example, at 100 pounds per square inch pressure, a 1 square inch piston would produce 100 pounds of force, while a 2 square inch piston would produce 200 pounds force.

    but to acheive this wouldn’t you need to double the input (lever) pressure or area?

    Premier Icon martymac
    Full Member

    Anecdotal,
    But the 4 pots I’ve used (zee’s)didn’t seem to offer any more power compared to my 2 pot xt.
    I definitely did notice that they seemed easier to modulate though.
    Better modulation would make it easier to hold a brake at the limit of traction for a time period. That’s the theory anyway.
    That’s how it felt to me too.
    Ymmv.

    Premier Icon pdw
    Free Member

    but to acheive this wouldn’t you need to double the input (lever) pressure or area?

    No, but using the 2 square inch piston would result in twice the lever throw. If you doubled the lever piston area, you’d get back to the same lever throw, and the same leverage (i.e. producing 100 pounds of force).

    Premier Icon thols2
    Free Member

    Yes. The ratio of the master cylinder piston to the wheel cylinder piston is the leverage ratio. If you have a 1 square inch master cylinder piston and apply 100 pounds force to that, you will get fluid pressure of 100 psi. Then, if you have a 2 square inch wheel cylinder piston, you will get 200 pounds force, but the larger piston will only move half as far.

    All levers, winches, etc. work on the same basic geometric principle. If you double the force, you halve the distance moved.

    Premier Icon pdw
    Free Member

    Aside from pad area why is splitting the force from the lever pull over 4 often smaller pistons more benficial?

    I suspect that it results in the force and thus heat being more evenly distributed across the pad. If you push a pad in the middle with a single cylinder, the ends will bend away from the disc. If you push a pad at both ends, the middle bit of the pad will flex less than the ends. Spreading the heat over a larger area obviously increases the amount of braking you can do before any part of it gets too hot, and will also help dissipate the heat more quickly.

    Plus bigger pads and calipers mean more mass (temperature increases more slowly) and more surface area overall (lose heat quicker).

    Premier Icon VanHalen
    Full Member

    As TJ explained, a larger piston gives more force on the pad than a small piston, assuming the same fluid pressure. For example, at 100 pounds per square inch pressure, a 1 square inch piston would produce 100 pounds of force, while a 2 square inch piston would produce 200 pounds force.

    so on the abov reasoning single pots are better/more powerful brakes as the piston creates more force for a given lever input. unless the above is wrong? or i mis-understand it totally!

    Premier Icon thols2
    Free Member

    unless the above is wrong? or i mis-understand it totally!

    You misunderstand it. What matters is the total area of the pistons, combined. For example, if you compare two 10 mm pistons with one 13 mm piston, the bigger single piston will have 69% more area than one of the smaller pistons (13 * 13 = 169, 10 * 10 = 100, 169/100 = 1.69). But two of the smaller pistons will have a greater area than then single large piston.

    Premier Icon poah
    Free Member

    2 pot shimano callipers have more surface area then the 4 pots. The pad is bigger with the 4 pots.

    2 pots 25mm pistons = 490mm2
    4 pots 17+15mm pistons = 403mm

    Should take less fluid to move the 4 pots than the 2 pots.

    Premier Icon thegeneralist
    Full Member

    All levers, winches, etc. work on the same basic geometric principle. If you double the force, you halve the distance moved.

    This is one of the few sensible comments in amongst a thread of O level Physics fails.

    Also, to achieve this magical increase in force on the brake pad, instead of doubling the size of the piston size, they could have halved the size of the lever cylinder….

    Premier Icon thols2
    Free Member

    2 pot shimano callipers have more surface area then the 4 pots. The pad is bigger with the 4 pots.

    The levers also provide leverage, so that also needs to be taken into consideration. I have some Saint 4 pots on one bike and SLX two pots on another. They feel similar enough that I can’t really tell them apart without checking which bike I’m riding. I have some Deore two pots on another bike, they are absolutely not as powerful.

    Also, to achieve this magical increase in force on the brake pad, instead of doubling the size of the piston size, they could have halved the size of the lever cylinder….

    There are practical limits to how small you can make it. I assume that they have already made the master cylinders as small as possible to save weight.

    Premier Icon pdw
    Free Member

    2 pots 25mm pistons = 490mm2

    I thought they were 22mm, so come out at a bit less (380mm^2) than the 4 pots.

    Premier Icon luket
    Full Member

    My layman’s understanding led me to question how for a given distance travelled the force could be any different and I haven’t seen a good answer. Plus we’re on the same levers either way, so the same finger feel is applying the same pressure increase to the fluid in the system. But I’ve got Shimano 2 and 4 pot brakes and the 4s have proved for me a worthwhile upgrade. They are quite different.

    So I think it’s all about the other factors. Spreading the load in the right place over a bigger and differently shaped contact area, heat dissipation etc. This stuff seems to add up to enough to make a real difference.

    It makes sense to me that it might only take effect when you’re applying significant power, just because at any less either brake has got plenty to spare anyway.

    I do, however, still have some question marks. I’ve got a gravel bike with a normal set up for that – small rotors, 2 pots. I ride it somewhere pretty flat. The brakes have never really felt nice and smooth. I just don’t think they ever get hot enough. So I theorise that sometimes over-braked makes things worse, rather than just “not making things better”. I could downsize the rotors on that but I don’t really want to throw money/resources at that minor problem.

    Premier Icon wobbliscott
    Full Member

    My understanding is that a 4 pot calliper allows you to have a larger surface area pad for the same piston surface area and therefore applied braking force. The force applied to the pad is might be the same in a 4 pot and 2 pot case (as this is a function of total piston surface area and pressure in the brake fluid) and we all know that pad surface area has no effect on friction, so no difference in the friction force generated – so 4 pot brakes are not more powerful necessarily…however a larger surface area pad does have an effect on temperature for the same friction force, so the benefit of multi-piston callipers is that they can accommodate larger pads which delivers the same braking force at a lower temperature as the applied force is spread over a larger pad surface area. So more temperature capacity of the braking system ultimately leads to more braking capacity, more braking can be done as you stave off temperature effects like brake fade, and better wear and tear of the consumable braking components.

    At any brake size 4 pots are better than 2 pots and 6 pots even betterer and so on and so forth. Like with everything its all about compromise between cost and performance.

    Premier Icon Superficial
    Free Member

    In addition to the above:

    1) By using elongated pads on 4 pots, for a given pad surface area the pads can be closer to the edge of the disc. This increases the mechanical advantage – presumably by about the same amount as increasing the size of the disc by a couple of mm.

    2) I wonder if there is a way that pads (on two pots) vibrate when they’re under load, reducing pad contact time. 4 pots don’t permit the pad to oscillate or hit the disc obliquely which may eliminate some of these inefficiencies.

    Premier Icon Northwind
    Full Member

    TBF, they just aren’t for most things. Pad size, power, fluid volume, none of them are dependent on piston size at all. The best 4-pots aren’t ime any better than the best 2-pots, and anyone that’s got the resources to make the best of either could do the other.

    There are clever things you can do with 4 pistons, like have different sizes so that they respond differently to the lever. And for bikes there possibly is some advantage in terms of packaging- you can make a longer thinner caliper and that’s kind of naturally the shape calipers want to be with the brake mounts and narrow rotors.

    They have some disadvantages too. More seals means more drag and more to go wrong, and more cost when it does. And also much more chance of unequal piston movement which can lead to pad drag etc.

    But let’s get real, the reason most bike companies use 4 pots is fashion and marketing.

    It’s sort of funny because this is one of those things where we’re literally just copying a motorbike trend. When the GSXR SRAD came out, the 600 came with 4-pots and the 750 came with 6-pots. People absolutely lined up to fit the 6-pots to their 600s, then discovered they were worse in pretty much every way. Heavier, worse feel, expensive pads, and much more unreliable with endless drag issues. And the same m/c was used on both and suited the 4 pots better. But a decade later it was still pretty much the law, if you had a SRAD 600 you fitted the 6-pot Tokicos because they MUST be better as they’re on the 750 and Busa.

    Premier Icon molgrips
    Full Member

    1 square inch piston would produce 100 pounds of force, while a 2 square inch piston would produce 200 pounds force.

    Yes but as said the piston would move half as far BUT you need to preserve a minimum movement so there’s enough pad clearance to cope with dirt and the fact that rotors are never really true.

    So given that constraint, you need to adjust the bore of the pistons and the master cylinder, which means that you’ll end up with the same total force at the pad for the same force at the lever. But a larger pad gives less pressure for the same force.

    If you wanted to increase the force at the pad whilst reducing pad clearance, you’d just alter the leverage ratio at the lever end and not bother making a more complex 4 pot calliper.

    I have V2s and Mono Minis, which are basically the exact same brake except V2s are a lot larger. With light braking there’s not much difference but as soon as you need a bit of force the V2s are vastly better. Pad clearance is similar and the lever travel is similar.

    Premier Icon DezB
    Full Member

    the reason most bike companies use 4 pots is fashion and marketing.

    Too right. How else are the bike companies gonna get us mugs to part with our cash on new brakes when the existing ones are perfectly good enough?
    4 pot brakes are the new 35mm bars.

    Premier Icon pdw
    Free Member

    I agree with most of what Northwind says, except:

    Pad size, power, fluid volume, none of them are dependent on piston size at all.

    We’re not talking about changing piston size (much), we’re talking about more pistons with the same total surface area, and that does allow for bigger pads.

    I suspect that if your riding is close to the thermal limits of brakes then there may be advantages to 4-pots, but otherwise, you’re right: marketing and fashion.

    Premier Icon Daffy
    Full Member

    You want to stop faster and the force applied at the lever is the same. If you decrease the area of the pistons, you INCREASE the pressure applied at the caliper. P=F/A so smaller A = bigger P. If you do this with a single (pair) of pistons, the force on the pad will be unevenly distributed, so your piston size is limited. Multiple smaller pistons overcome this. Generally a smaller piston on the leading edge and a larger one on the trailing edge balances the applied pressure over the longer pad. In addition, you increase the amount of friction material in contact with the rotor at the point it’s most useful, thus getting a better feel. Other marginal gains will be that the application point of the material is slightly further from the hub and thus the brake torque radius will be higher and thus slightly more powerful. The downsides of this will be that all the force is applied in a radially similar position, so heat buildup will be potentially higher, bu the calliper is also larger (in area, not mass or fluid) so should dissipate heat to the air faster. A further downside (especially on MTB brakes) is in balancing the pistons. In a car the rotor will not bend as a result of the brake force, but this is quite easy to do on a MTB.

    Premier Icon duncancallum
    Full Member

    More piston area for a given size of pad…

    So more piston area for heat disposal

    Premier Icon tjagain
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    I have 6 pots on the tandem! Best tandem brakes eva!¬

    Premier Icon matt_outandabout
    Full Member

    tjagain showing off the size of his calipers… 😉😂

    Premier Icon thols2
    Free Member

    You want to stop faster and the force applied at the lever is the same. If you decrease the area of the pistons, you INCREASE the pressure applied at the caliper. P=F/A so smaller A = bigger P.

    Nope. The “P” in that equation is the pressure within the hydraulic fluid, not between the rotor and pads. What matters is the force you apply to the pads (“F” in the equation). To increase that, you need either a smaller master cylinder piston or a larger wheel cylinder piston.

    The full equation is actually this:

    FW= P x AW
    P = FL/AL

    Therefore:
    FW = (FL/AL) x AW
    FW = FL x (AW/AL)

    Where:
    FW = Force exerted at the wheel (against the pad)
    FL = Force exerted by the lever (against the master cylinder piston)
    AW = Area of the wheel cylinder piston
    AL = Area of the master cylinder piston

    The leverage ratio (R) is:
    R = AW/AL

    Therefore:
    FW = FL x R

    In order to increase R, you need a larger wheel cylinder or a smaller master cylinder piston.

    However, there is another use of pressure, that may have confused you. There is the pressure of the pad against the rotor. If AP = Area of the pad, and PP = pressure at the pad, then:
    PP = F/AP

    This pressure is not the same thing as the pressure within the hydraulic fluid. Up to a point, this will make no difference to the stopping power, because the force exerted by the pad against the rotor stays constant regardless of pad size. However, using a really small pad will mean that the pad material will become much hotter. Eventually this will be too much and the pads will fail. Therefore, you need to use a pad that is large enough to keep the pressure between the pad and rotor to an acceptable level. Using a four-piston caliper allows the pressure across the larger pad to be controlled because the pad won’t distort as much, as explained above.

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