Trondelond
Active Member
Of course. It's a sliding one. I'm a bit closer to understanding it now.
Good old "two pump brake" problem.
- Thread starter Trondelond
- Start date
This sounds like a lesson in hydraulics to me...
So the volume of the system is constant (i.e. you don't put any more fluid in each time you push the pedal; it's a static volume that moves through the system), and the master cylinder only has a finite volume for a given cylinder size... so one pump moves a set volume of fluid through the system.
It makes perfect sense that even if the piston sizes are the same as standard, having twice as many means you need to move twice as much fluid to shift them the same ammount. - The physics of it means that you get twice the pressure (because it's two pistons) for the same input pressure.
Is this sounding right, I'm not a hydraulic engineer obviously?
Therefore you reach a point where the master cylinder can't move enough fluid to fill all of the pistons simulateously... and you need a bigger one (or smaller pistons)? - Pressure can be ignored because it's a function of the servo assist rather than how much foot you put on the pedal, and hydraulic fluid is by definition incompressable?
So the volume of the system is constant (i.e. you don't put any more fluid in each time you push the pedal; it's a static volume that moves through the system), and the master cylinder only has a finite volume for a given cylinder size... so one pump moves a set volume of fluid through the system.
It makes perfect sense that even if the piston sizes are the same as standard, having twice as many means you need to move twice as much fluid to shift them the same ammount. - The physics of it means that you get twice the pressure (because it's two pistons) for the same input pressure.
Is this sounding right, I'm not a hydraulic engineer obviously?
Therefore you reach a point where the master cylinder can't move enough fluid to fill all of the pistons simulateously... and you need a bigger one (or smaller pistons)? - Pressure can be ignored because it's a function of the servo assist rather than how much foot you put on the pedal, and hydraulic fluid is by definition incompressable?
watoga
Member
So, if you double the OE caliper area (since its 2 pistons), then its actually bigger than the AP area? 4580mm2 vs 4560mm2. Would this minimal decrease not help, rather than hinder, pedal travel?
Dave
stumo
Active Member
George, you've almost got it right.
Just because the "new" caliper has more pistons doesn't mean there's any more fluid needed to be moved IF the area of the pistons is the same (this all relates to the volumes involved with the master cyl and caliper pistons).
ok, if you're going to be pedantic then yes the amount of fluid will change but not enough to cause any problems and can be considered the same.
To get into pressures involved/needed then that's sorted out at a later date and all related to master cyl size, caliper piston size, no of pistons, swept areas, friction coefficient of the pads, pedal ratios etc etc etc but in the context of this discussuion we don't need to go there.......
stumo
Active Member
yes, if the oe caliper area is bigger then you could run into other problems but you shouldn't have any long pedal/ double press problems.
watoga
Member
Good, good, so hopefully for Trondeland it isn't a brake master cylinder problem that's causing the long-pedal/double-press problems. Possibly other issues such as fluid leak? Or air in the system?
Dave
Trondelond
Active Member
watoga
Member
stumo
Active Member
NOOBS....FFS :roll::roll::roll:
Trondelond
Active Member
But you still love us, right?
Ah - I get what you're saying... if the piston is the same size, then the same volume of fluid moved will act on both of them and move them the same ammount without a noticible increase, but if they are larger the master cylinder has to move further to achieve the same movement on a piston of a larger diameter? - I'm too caught-up thinking about them in terms of volume whereas it's more about area... because volume is a function of the movement anyway?
This is sounding suspiciously like calculus now... So the piston is described by x^2+y^2 (as it's a circle) and you integrate the equation to calculate volume due to movement!?
This is sounding suspiciously like calculus now... So the piston is described by x^2+y^2 (as it's a circle) and you integrate the equation to calculate volume due to movement!?
watoga
Member
Correct PobodY,
The volume will be Lx^2 + Ly^2 or L(x^2 + y^2), where L is the distance the piston physically moves perpendicular to the brake disc. If you integrate your above equation with respect to dL, then you get the same:
INT (x^2 + y^2) dL = Lx^2 + Ly^2 = L(x^2 + y^2)
Cheers,
Dave
The volume will be Lx^2 + Ly^2 or L(x^2 + y^2), where L is the distance the piston physically moves perpendicular to the brake disc. If you integrate your above equation with respect to dL, then you get the same:
INT (x^2 + y^2) dL = Lx^2 + Ly^2 = L(x^2 + y^2)
Cheers,
Dave
Makaveli_se7en
Member
Not questioning brake manufacturers product ranges, like everyone in manufacturing they make things that they can make money on. If that means using the same caliper castings with slightly different machining to fit different market niches then thats up to them.
But if you have small pistons pushing the middle of the pad, the pressure ditribution on the pad back face is uneven and poorly controlled. This can lead to many undesirable effects including:
Uneven pad heating, potentially leading localistised heating and warping of the disc
Uneven pad wear, shortening pad life
Pad edge flutter, noise generation mechanism, predominately higher frequencies
Pad spragging, leads to less efficent braking, poor pedal feel, pad sticking (tends to effect sliding fist more than opposed piston calipers)
Like I said it depends on pad size and piston size (also relative size in multi-piston calipers) and position.
If you want some more reading on the subject then try these:
Fieldhouse JD, Ashraf N “Observations of the disc/pad interface pressure distribution during variable braking conditions and its influence on brake noise”. XXVIIth μ Symposiumon 26 October 2007 in Bad Neuenahr, Germany
Published as a VDI-Fortschritt-Bericht (VDI-Report)
Fieldhouse JD, Ashraf N, Talbot CJ "Dynamic Centre of Pressure Measurements", 25th Annual Brake Colloquium & Engineering Display, SAE Ref 2007-01-3960 October 7-10, 2007, Orlando, FL, USA.
Also very interesting whilst on the subject of brake noise:
Fieldhouse JD “Noise Study Comparing Phenolic Pistons To Steel Pistons” 5th Sumitomo Bakelite Europe Friction Colloquium. Held every 2 years June 2007
My background is more in thermal judder and brake disc aerodynamics but John Fieldhouse was one of my PhD (The optimisation of Aerodynamic Properties of Ventilated Brake Discs) supervisor and is a bit of an expert in brake. He's done consultancy work with Alcon, Brembo, AP, Bentley Motors, BMW, Brabus, Nissan and Toyota to name a few and has published close to 100 papers on the subject
Sorry for the off topic.
Trip
New Member
Trond, I am using 35mm 4pot calipers and today was the day to test them. I am getting a longish travel compared to standard. I can break hard with one pump and lock all 4 wheels (which are 225 Toyo R888 on sticky gokart tarmac), but i noticed that if i pump it twice, i can lock the wheels with less travel.
Trondelond
Active Member
Had a go at the garage yesterday, and I think I'm more or less where you are now. I haven't tried locking them, but I'm getting better braking on the first pump now. Heeltoe will be a bit of a contortionist-act though, as pedal travel is still too long for my liking.
Jacked the car up and checked front wheels for play - there was none, so the bearings are ruled out. Figured I'd try bleeding the master cylinder (by yanking off the pipes and squirting brake fluid all over the engine bay), and then rebleed the whole system, but to my surprise the master cylinder has a bleed valve... I was not aware of this.
Got a little bubble out when I bled the cylinder, and proceeded to bleed the brakes in the order stated by the manual.
The brakes are better now - not too scary to drive with, but still nowhere near what I think they should be.
I'm left with two options as far as I can tell; overhaul the master cylinder (which seems to be fine according to the procedure), or fit a different master cylinder. But there aren't really any direct replacements as far as I know.
Trondelond
Active Member
Ooo, thanks! Will do!