Brake MS sizing vs pressure question

[Jasper]

compare the piston area to the master cylinder area.
remember that if you have changed from sliding calipers to a fixed caliper, you neeed to multiply the sliding caliper piston area by 2.

Nope, the applied force will be the same, the piston travel is shared between the two sides of the caliper.

Edit: Scratch that! Stu obviously meant if you used the piston area from both sides of the caliper in the equation...

As an aside, are you using a single diaphragm booster? If so moving to a tandem booster will increase the amount of assistance, while keeping a firm pedal.

[kingmick]

Welcome to the world of brakes the inexact science.
matching master cylinders to brakes can be a bit hit and miss at time. Main reason is drivers like diffrent things and when you start using racing calipers diffrent calipers with the same piston area will work diffently.
This is one, even very good race engineers stuff up from time to time because of the variables.

[kingmick]

Answer to your question! no unless the driver hates the feel or you make big changes to the piston area layout, there is no braking benefit from changine master cylinders, the calipers make the diffrence and the pedal setup you use can also make a diffrence..

[Celica RA45]

joel i use 4 spots on the front and rear with no booster the fronts are Ap and im using .625 on the front and .7 for the rears and there ap as well
also smaller master size ,it will push more fluid to brakes ,more travel on pedal and more pressure on brakes
bigger master ,harder pedal and less fluid to brakes ,less pressure to brakes
hpoe this helps

[Jasper]

joel i use 4 spots on the front and rear with no booster the fronts are Ap and im using .625 on the front and .7 for the rears and there ap as well
also smaller master size ,it will push more fluid to brakes ,more travel on pedal and more pressure on brakes
bigger master ,harder pedal and less fluid to brakes ,less pressure to brakes
hpoe this helps

Smaller MC = less fluid to brake per unit stroke = more pedal travel

Bigger MC = more fluid to brake per unit stroke = less pedal travel

Otherwise, spot on.

[Celica RA45]

smaller master ,more line pressure more travel pedal
larger master less line pressure and very hard pedal
i think thats what i said jasper .front rotors are 310mm with 4spots and .625
rears rotors 255mm with 4 spots and .7 master

[oldcorollas]

Welcome to the world of brakes the inexact science..

it is only inexact science if you don't know what you are doing

[kingmick]

it is only inexact science if you don't know what you are doing

Oh people know what there doing but using pen and paper calculations only work most of the time!

Shame even the best race engineers have to try it in practice to see if it works, when they could just calculater them all!

so stew, how do you calculate on paper a drivers pedal feel of what they like?
e.g Some drivers like a long throttle some like a short throttle, some like stiff and some like soft.
Even with knowing piston areas you can usually get it right or close, but never every time.If you make a pedal to hard with the right calculation you can make brakes to hard to use limit braking, to soft and you affect reaction takeup.
Most road cars its even harder because of pedal and MS flex.

[oldcorollas]

a bodgy engineer will just do it on paper.

a scientist will take the driver, test his leg strength, and ask how firm they want it (put them in other cars with a pressure testing device and measure the force they put in)

then, you can design the system pretty close to that.

just because someone is an "engineer" doesn't mean they know how to do it properly.

Shame even the best race engineers have to try it in practice to see if it works

wel.. duh.. no point never trying it in practice i know you don't like maths, but the idea is to give as close as possible a starting point.... if you have a driver that actually knows what they want, it makes a big difference (as opposed to ones that only think they know what they want, or can't give the right feedback)

[kingmick]

I dont mind maths, saves time.
your saying what im saying
99% of engineers are the same on paper, its there practical skills of interpreting what they are told,see and have a feeling for that makes some shine .
I like maths, i just dont like maths only

[oldcorollas]

99% of engineers are the same on paper, its there practical skills of interpreting what they are told,see and have a feeling for that makes some shine .

so you are saying that... 1% of engineers, which are good, are good because they are unscientifically lucky?
i suppose an engineer makes a lot of guesses, rules of thumb, and interpreting what they are told and see and have a feeling for, and then hopes it will be good after they calculate it..

meh, thats just unscientific scientist would meaure braking force for a given pedal travel, then work out the volume of fluid needed "post MC", then measure the driver, then match the two systems...

there are only unknown variables if they have not been measured
an engineer that is worth the air they breath would not stuff around with "feeling". they would do it right the first time.

pedal feel and travel is a function of the MC size (ie, how much fluid it pushes) and pedal leverage. if you have the freedom to change both, it would be easy to set it up right without stuffing around

[kingmick]

No no no! they "99%"{1% for hopless ones} get the same answers with there caluclations on paper, But its a much much lower number of them that have the practical skills of interpreting what they are told,see and have a feeling for that make the cut.
But the trouble is, its to complex to "just do it the first time".

[oldcorollas]

Hi all, I recently 'upgraded' my master cylinder from a 14/16th to a 1" bore.

last post on this.

14/16th's
r = 2.54 x 7 / 16 (1.11125cm)
A = Pi x r^2 = 3.8795 cm^2

1"
r = 2.54/2 (1.27cm)
A = Pi x r^2 = 5.0671 cm^2

a change from 14/16 to 1" bore with no other changes would give an increase in pedal force required of 30%, and also reduce pedal travel by 30%


so... for the same pedal input, you have 30% less braking...

15/16th
r - 2.54 x 7.5 / 16 (1.190625cm)
A = Pi x r^2 = 4.4535cm^2

this is about a 15% increase over the 14/16ths MC...

ie, 15% harder pedal, with 15% less travel.


now, leverage..
the brake fluid pressure is a result of the force on the MC and the area of the MC

ie, pressure = force/area, P=F/A

with leverage, if you double the mechanical leverage, you get twice the force, but you also get twice the pedal travel.....
taking the above example tho:
1" bore is 30% area increase in bore size... this means for a given pedal force, the brake fluid pressure DECREASES by 30% (F/A) and also 30% decrease in pedal travel (since same travel moves 30% more fluid).

now, if you increase the leverage by 30%, you increase the F by 30% and get your brake fluid pressure back to the old 14/16" level, BUT, now you have increased pedal travel by 30%.....

so you are back to the same situatio as before

so... for a given brake fluid pressure (ie, equal braking force in the calipers) and a given brake pedal input (firce applied to pedal) if you change master cylinder size and change the pedal ratio, then there is no difference in pedal weight or travel!!!! (i could do it in algebra, but i cbf, and Mick hates maths )

so to sum it up. the pedal travel multiplied by the pedal force required for a given braking force are a FIXED function of the calipers, not the master cylinder!!!!!
you can change the pedal travel and the pedal force by either changing master cylinder size OR pedal ratio, however, the product of pedal travel and braking force is still fixed.

so, to decrease pedal travel, and increase pedal effort needed, increase master cylinder size or decrease pedal leverage.

to increase pedal travel, and decrease pedal effort needed, decrease master cylinder size or increase pedal leverage.

changing the caliper piston size has the opposite effect as changing master cylinder size.

to actually increase braking force for a given pedal force, increase disc size , OR, increase the booster size to one with more multiplication.

[clubagreenie]

Lock Thread and sticky

[oldcorollas]

ok, alegbra.. lets see if asci works

--------------F2----------------F1
-O-----------B-----------------C
-o------------o-----------------o
---------------||
---------------||
------------|------| MC piston
------------|------|
------------<-d->


O is pivot point,
B is point where pedal arm pushes master cylinder,
C is the brake pedal.

the leverage of the pedal arm is OC/OB

so, if your foot has a force F1,,
then the force at B, (F2) is
F2 = (F1xOC) / OB

now, if the master cylinder piston area (A) is gotten from the diameter, d, ie
A = Pi x (d/2)^2

brake line fluid pressure, P, (ie, pressure of fluid in master cylinder) is force / area

ie pressure = F2 / A
P = (F1 x OC) / (OB x Pi x (d/2)^2)

so, if you make pedal longer, or MC to pivot shorter, or decrease piston size, you increase line pressure.

rearranging


P x Pi x (d/2)^2 = (F1 x OC) / OB

so you can see here, with fixed line pressure and fixed pedal ratio.. if you increase master cylinder size (d), you increase pedal force (F1)