[Closed] 4 pot brakes are better because...

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.

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.

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.

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

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

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

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.

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

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

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

The answer is they just are

https://thelostco.com/blogs/blog/2-piston-vs-4-piston-mtb-brakes#:~:text=4%20piston%20calipers%20are%20physically,of%20power%2C%20or%20less%20fade.

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

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.

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.

obvious answer is obvious

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?

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.

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).

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.

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).

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!

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.

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.

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....

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.

2 pots 25mm pistons = 490mm2

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

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.

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.

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.

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.

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.

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.

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.

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.

More piston area for a given size of pad...

So more piston area for heat disposal

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

tjagain showing off the size of his calipers... 😉😂

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.

But the 4 pots I’ve used (zee’s)didn’t seem to offer any more power compared to my 2 pot xt.

On steep stuff I found swapping to a 4 pot Zee on the front (from a 2 pot XT) enabled me to actually stop, rather than just slow down. I've still a 2 pot on the rear.

But if we're saying the only difference is between a 2pot caliper and a 4pot caliper (same levers, same fluid, same rotor, etc) the only difference will be in the effect on the pistons. The rest is identical. When there is a smaller piston area, the pressure acting on them and causing them to move is greater.

More piston area for a given size of pad…

So more piston area for heat disposal

Except that the total piston area is essentially the same, as 4-pots use 4 smaller pistons.

4-pots give you larger pads with the force more evenly distributed for the same piston area.

But if we’re saying the only difference is between a 2pot caliper and a 4pot caliper (same levers, same fluid, same rotor, etc) the only difference will be in the effect on the pistons. The rest is identical. When there is a smaller piston area, the pressure acting on them and causing them to move is greater.

No. For a given force on a given master cylinder, the *pressure* in the caliper piston is exactly the same: it's the pressure of the hydraulic fluid.

If you use a smaller caliper piston, you'll get less force (but the piston will move further).

4-pots have smaller caliper pistons, but with more-or-less the same total area. Each piston exerts less force, but the total force acting on the pad is the same just more evenly distributed over a larger pad.

When there is a smaller piston area, the pressure acting on them and causing them to move is greater.

No, unless you are talking about the master cylinder piston. The hydraulic pressure is determined by how hard you squeeze the lever, the mechanical leverage of the lever, and the size of the master cylinder piston. If the first two are constant, then a smaller master cylinder piston will give increased hydraulic pressure. If all those three are constant, then the hydraulic pressure will not change.

If you have the same hydraulic pressure, then a larger wheel cylinder will provide more force against the pads, so it will give more power. Two small pistons with the same total area as one large piston will provide the same force against the pad, but will let you run longer pads. The larger pad area should be less prone to overheating.

Yes, but that's what I'm saying - the piston area of 4 small pistons is smaller than that of 2 large pistons, it's 20-25% less in most 4pots. So force applied to the fluid is the same (tech 3 levers are identical for 2 or 4 pot) and the release area for the fluid is smaller, thus they move further or exert greater pressure on the pad/rotor. No?

Agreed that a a pad with the same area but a different aspect ratio will provide better contact to the rotor and thus make better use of that pressure and friction at the expense of slightly higher heat generation in the rotor.

I think almost everyone is missing the important points. There's two main scenarios to think about:

1. The start of the braking phase when the pad approaches and contacts the disc, under low force.
2. Peak braking force, when there are big loads on the pad from three directions.

At the start of the braking phase, a 2 pot caliper moves the pad perpendicular to the disc, so when it touches the disc, all of the pad hits at once. However, in the real world a 4 pot caliper never has both pistons on a side moving in perfect synchrony, so the pad always hits the disc at an angle. This gives a more gentle start to the braking, hence 4 pots have better modulation (all else being equal).

At peak braking force, the pad is under very high shear force from the disc. This shear force causes leverage of the pad surface vs where the piston exists the caliper. Worst case scenario the pad will lose partial contact as its rotated from being parallel to the disc and could even start juddering/skipping. It's unlikely to get that bad with a modern brake but it is clear that having two pistons supporting the pad will hugely reduce that problem, just as standing on two legs is more stable than standing on one.

So a 4 pot brake is better at both light braking and heavy braking, if executed as well as a 2 pot brake.

the piston area of 4 small pistons is smaller than that of 2 large pistons, it’s 20-25% less in most 4pots. So force applied to the fluid is the same (tech 3 levers are identical for 2 or 4 pot) and the release area for the fluid is smaller, thus they move further or exert greater pressure on the pad/rotor. No?

Technically, the pistons don't exert pressure on the rotor, they exert force. Pressure is measured in units like pounds per square inch or newtons per square meter (which is one Pascal pressure). Mathematically, that's written N/m^2. There's a unit of force being divided by a unit of pressure. One atmosphere is approximately 14.7 psi or 101 kPa

Force is measured in Newtons (N) in science, but we often use pounds or kilograms in everyday life. If you multiply pressure by area, you get something like this:
1 m^2 x 100 kPa = 1 m^2 x 100 kN/m^2
= 1 x 100 (kN x m^2/m^2)
= 100 kN

So, when you multiply pressure by area, you end up with force.

If you have smaller pistons, you will have less force applied to the rotor for the same hydraulic pressure. That's a brute scientific fact.

Assuming you have the same rotor/pad clearance, the smaller wheel cylinders will result in less travel at the lever.

However, Shimano levers and calipers are generally interchangeable, so any differences in piston area must be relatively minor. A 25% difference would make a fairly big difference in brake power and feel, so Shimano obviously keep them reasonably similar. However, you can't assume that all manufacturers have the same policy of levers and calipers being interchangeable.

FYI, the current Hope piston diameters are:

X2 - 22mm (380 sq.mm)
E4 - 16mm and 16mm (402 sq. mm)
V4 - 16mm and 18mm (456 sq. mm)

A lot of reviews claim the E4 and V4 pistons are 2mm smaller than they really are.

So, the Hope DH brake has a bit under 20% more piston area than their XC brake. Which means they will be noticeably more powerful with the same levers and rotors, but have a little bit more lever travel and probably be slightly spongier. That makes sense.

Okay real life example - Hope X2 (2*22mm) vs E4 (4*16mm) vs V4 (2*18mm, 2*16mm)

Okay so the input force (someone pulling the lever) is say 25N acting on the master cylinder. That force is then applied via the master cylinder to the fluid in the form of input pressure. For simplicity let's say it's a square piston with each side 5mm, so 25mm2, which gives us 1mPA (P=F/A) of pressure in the fluid. On the other end we have either 760mm2 (X2) 805mm2 (E4) or 910mm2 (V4) which means that assuming the same travel/work done to move the pistons, the V4s generate 20% more force than the X2s and the E4s generate less than 10% more than the X2a. This just relates to force applied from the calliper to the pad. But a given the brake track on a rotor is usually less than 22mm in depth, I'm guessing that not all of the force from the X2s actually make it to the rotor, whereas for the 4 piston callipers , the longer pad with less depth means that all of the generated force is applied to the rotor.

EDIT - damnit, beaten to it. Distracted by actual work again.

I’m guessing that not all of the force from the X2s actually make it to the rotor, whereas for the 4 piston callipers , the longer pad with less depth means that all of the generated force is applied to the rotor.

The rotor is the only thing stopping the pads. All the force is applied to the rotor. It doesn't just magically dissipate into the air.