4 pot calipers
Discussion
This is all guesswork on my part but it seems logical enough..
1 You can fit lots of small diameter pistons nearer the edge of a disk and get more mechanical advantage than one whopper of a piston whose larger diameter puts its center further from the disk perimeter.
2 It's easier to make small pistons ridged, a big saucepan is likely to bend in the middle.
3 You can vary the size of multiple pistons to apply less force on the leading edge of the pad.
4 There's got to be some thermal advantage but I can't quite work out what it might be.
Edit to add... Oh and a single pot implies a sliding calliper which I happen to know eventually gets gummed up and sticks!
1 You can fit lots of small diameter pistons nearer the edge of a disk and get more mechanical advantage than one whopper of a piston whose larger diameter puts its center further from the disk perimeter.
2 It's easier to make small pistons ridged, a big saucepan is likely to bend in the middle.
3 You can vary the size of multiple pistons to apply less force on the leading edge of the pad.
4 There's got to be some thermal advantage but I can't quite work out what it might be.
Edit to add... Oh and a single pot implies a sliding calliper which I happen to know eventually gets gummed up and sticks!
Edited by leorest on Thursday 8th November 18:12
Brakes work by having a master cylinder under your the pedal, this has a small surface area but a reasonable amount of travel (stroke). The fluid is incompressable, so the pressure at the pistons of the brake caliper is the same as the single piston under the pedal. The area of the piston(s) in the caliper is larger and so (force = pressure x area) the force given out by the pistons in the caliper is higher, but the travel is much shorter. More pistons (4,6,8, 10 or even 12) in the caliper means more surface area and more stopping power, you can also use larger brake pads which give a greater contact area with the brake disc - more friction = even more stopping power.
Having pistons either side of the caliper/disk means that you can have a fixed caliper, without having to have a slide which could gum up.
Having additional pistons on a given side means that the pad can be longer for a given radial size. There's no advantage in having a piston/pad bigger than the radial size of the disk contact area, so if you want bigger pads you have to have more pistons, otherwise big bits of the pad aren't going to do anything.
Having additional pistons on a given side means that the pad can be longer for a given radial size. There's no advantage in having a piston/pad bigger than the radial size of the disk contact area, so if you want bigger pads you have to have more pistons, otherwise big bits of the pad aren't going to do anything.
Edited by dilbert on Saturday 10th November 18:34
Actually it is more to do with even pressure being applied to the area of the pad backing, allowing larger radially shaped - or even multiple pads.
There is one more sneaky reason; by having different diameter pistons, it is possible to bias the pad contact upon braking by creating a 'leading shoe' effect to allow smooth enngagement and reduce noise.
There is one more sneaky reason; by having different diameter pistons, it is possible to bias the pad contact upon braking by creating a 'leading shoe' effect to allow smooth enngagement and reduce noise.
leorest said:
My answer assumed the total calliper piston areas were the same e.g. Comparing a fictional 1 pot sliding calliper (76mm diameter piston) with the ubiquitous Princess 4 pot (38mm diameter pistons)
A 76mm piston will have twice the area of two 38mm pistons! For an equivalent area the single piston calliper would require a piston of approximately 54mm.Mr2Mike said:
leorest said:
My answer assumed the total calliper piston areas were the same e.g. Comparing a fictional 1 pot sliding calliper (76mm diameter piston) with the ubiquitous Princess 4 pot (38mm diameter pistons)
A 76mm piston will have twice the area of two 38mm pistons! For an equivalent area the single piston calliper would require a piston of approximately 54mm.
Agreed a 76mm piston will have the same area as FOUR 38mm pistons. Which was the point I was trying to illustrate.
You are also correct that two 54mm pistons would have the same area.
Here's the maths just to clarify...
(To get the formatting of columns I had to use dashes instead of spaces/tabs)
Diameter----Radius----Radius**2----(r**2)*pi
76mm-------38mm----1444--------4536mm**2----*1=4536mm**2
54mm-------27mm----729---------2290mm**2----*2=4580mm**2
38mm-------19mm----361---------1134mm**2----*4=4536mm**2
An interesting question comes to mind about two piston callipers now that you bring the subject up..
Other than the problems with gumming up and sticking, are there any other differences between a theoretical 2 piston sliding calliper (both pistons on same side) and a more conventional (piston on each side of the disk) calliper?
dilbert said:
Having pistons either side of the caliper/disk means that you can have a fixed caliper, without having to have a slide which could gum up.
Having additional pistons on a given side means that the pad can be longer for a given radial size. There's no advantage in having a piston/pad bigger than the radial size of the disk contact area, so if you want bigger pads you have to have more pistons, otherwise big bits of the pad aren't going to do anything.
To quote myself, I probably should have said that there is no advantage to having a piston/pad bigger than the radial size of the disk if the piston is parallel.Having additional pistons on a given side means that the pad can be longer for a given radial size. There's no advantage in having a piston/pad bigger than the radial size of the disk contact area, so if you want bigger pads you have to have more pistons, otherwise big bits of the pad aren't going to do anything.
Edited by dilbert on Saturday 10th November 18:34
One could improve the mechanical advantage by having a big piston, that is engineered to bear on a smaller contact area. The thing is that the radial size of the disk is usually constarained by the internal dimensions of the wheel. Since the disk size is often maximally large, there is little possibility of having a piston greater in size than the radial size of the contact area. Fortunately it is possible to develop all of the necassary mechanical advantage at the master cylinder end of the system, where space is not at as much of a premium.
It could be possible to devise an alternate scheme to develop greater mechanical advantage at the hub end of the system, but it would involve having a big piston and a small contact area. This in turn would force a smaller radial size of disk, and although similar or even better performance might be achieved, it would compromise the lifespan of the disk and pads.
Overall, it is easier to increase the annular friction area, and use more pistons to apply the same pressure over a larger surface, to get better braking performance. That approach can improve lifespan, because one does not have to use the brakes in quite such an agressive way to achieve a given level of braking perfomance.
It can be seen that optimal braking performance is achieved where the disk/pad contact area is as large as it could possibly be, with a given pressure applied as evenly as possible to that area.
With these things in mind, I imagine that it is possible to maximize that contact area as much as is possible in a system where the pistons are on both sides of the disk. My reasoning is that the caliper its self can be quite thin where it passes between the inside of the wheel and edge of the disk, and still maintain sufficient strength.
Where there are pistons on both sides of the disk, only the hydraulic fluid need pass around the disk, so the size of the caliper is minimal and the contact area maximal.
Where the piston is on only one side of the disk, the whole slide mechanism must pass through this space, and the size contact area of the disk cannot be as great.
In practice the clearances necassary in that area, to achieve a reliable design, would dwarf the potential gain of one system over the other. In an extreme design requirement, placing pistons either side is the more efficient solution in use. It is also more complex to manufacture, and the sliding solution is more normally seen.
Edited by dilbert on Saturday 10th November 19:00
leorest said:
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Agreed a 76mm piston will have the same area as FOUR 38mm pistons. Which was the point I was trying to illustrate.
The point you are missing is that a four pot caliper has two pots opposing each other so the effective area is only that of two pots. For example the total force on the pads would be the same with a single piston sliding caliper, or a twin opposed piston caliper with the same diameter pistons.Agreed a 76mm piston will have the same area as FOUR 38mm pistons. Which was the point I was trying to illustrate.
Mr2Mike said:
leorest said:
Agreed a 76mm piston will have the same area as FOUR 38mm pistons. Which was the point I was trying to illustrate.
The point you are missing is that a four pot caliper has two pots opposing each other so the effective area is only that of two pots. For example the total force on the pads would be the same with a single piston sliding caliper, or a twin opposed piston caliper with the same diameter pistons.leorest said:
I can't quite visualise how it works like that? I'm not doubting you I just don't get it yet. Think I need to sleep on this one...
Each piston in a floating calliper has twice the effect it would have in a fixed calliper because the reaction from the piston is used to apply the same force to the 'other' side of the disc.The more pistons there are the less mechanical advantage there is. Where there is less mechanical advantage at the hub end, it can be made up at the pedal end. Overall is is quite easy to come out with a fixed level of mechanical advantage with an increasing number of pistons.
Using pistons on both sides of the disk is no significant penalty in terms of mechanical advantage, although you can achieve greater mechanical advantage at the hub end if you have pistons on one side of the disk and a floating/sliding caliper.
More pistons can equate to greater pad area which is a distinct benefit in terms of lifespan.
Perhaps the thing you are missing is that the overall level of friction for a given pressure is totally independent of pad area. In a practical sense, this will never be true in extremes, but in theory it is.
The difference is, pads twice the size last much longer, and consequently you can use them with greater mechanical advantage.
If you take a stock system, and somehow put bigger pads into it without changing the pistons, the pads will last longer, but the braking power will remain unchanged. If you then modify the pedal end to give greater mechanical advantage, you'll get better stopping power, and return the life of the big pads back to the same level as the smaller pads.
Using pistons on both sides of the disk is no significant penalty in terms of mechanical advantage, although you can achieve greater mechanical advantage at the hub end if you have pistons on one side of the disk and a floating/sliding caliper.
More pistons can equate to greater pad area which is a distinct benefit in terms of lifespan.
Perhaps the thing you are missing is that the overall level of friction for a given pressure is totally independent of pad area. In a practical sense, this will never be true in extremes, but in theory it is.
The difference is, pads twice the size last much longer, and consequently you can use them with greater mechanical advantage.
If you take a stock system, and somehow put bigger pads into it without changing the pistons, the pads will last longer, but the braking power will remain unchanged. If you then modify the pedal end to give greater mechanical advantage, you'll get better stopping power, and return the life of the big pads back to the same level as the smaller pads.
Edited by dilbert on Monday 12th November 00:17
dilbert said:
The more pistons there are the less mechanical advantage there is.
What is your definition of mechanical advantage in this instance? If a multi-pot calliper has the same effective piston area as a single pot one then the clamping force vs. hydraulic pressure should be the same (apart from negligible losses caused by seal friction etc.).leorest said:
Which has unity hydraulic mechanical advantage?
A) Single piston sliding calliper with the same area as the master cylinder.
B) Multi (opposing) piston calliper with total mathematical area equal to the master cylinder.
A?
F!A) Single piston sliding calliper with the same area as the master cylinder.
B) Multi (opposing) piston calliper with total mathematical area equal to the master cylinder.
A?
Mechanical Advantage
Edited by dilbert on Monday 12th November 12:53
leorest said:
Which has unity hydraulic mechanical advantage?
A) Single piston sliding calliper with the same area as the master cylinder.
B) Multi (opposing) piston calliper with total mathematical area equal to the master cylinder.
A?
If the multi piston setup has the same total piston area as the master cylinder then the (hydraulic) mechanical advantage is 1.A) Single piston sliding calliper with the same area as the master cylinder.
B) Multi (opposing) piston calliper with total mathematical area equal to the master cylinder.
A?
If the sliding calliper has half the total piston area of the master cylinder then the (hydraulic) mechanical advantage is 1.
GreenV8S said:
leorest said:
Which has unity hydraulic mechanical advantage?
A) Single piston sliding calliper with the same area as the master cylinder.
B) Multi (opposing) piston calliper with total mathematical area equal to the master cylinder.
A?
If the multi piston setup has the same total piston area as the master cylinder then the (hydraulic) mechanical advantage is 1.A) Single piston sliding calliper with the same area as the master cylinder.
B) Multi (opposing) piston calliper with total mathematical area equal to the master cylinder.
A?
If the sliding calliper has half the total piston area of the master cylinder then the (hydraulic) mechanical advantage is 1.
Thanks for the explanation. That makes it a lot clearer now!Edited by leorest on Monday 12th November 18:18
dilbert said:
leorest said:
Which has unity hydraulic mechanical advantage?
A) Single piston sliding calliper with the same area as the master cylinder.
B) Multi (opposing) piston calliper with total mathematical area equal to the master cylinder.
A?
F!A) Single piston sliding calliper with the same area as the master cylinder.
B) Multi (opposing) piston calliper with total mathematical area equal to the master cylinder.
A?
Mechanical Advantage
Not exactly hydraulic is it.
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