Master cylinder sizing

[Timmyevo6]

next season, i'll be running a pedal bias box in my evo6 race car.

how do i go about getting the correct size brake master cylinders for front/rear. and also for clutch and hydro handbrake?

cheers. tim.

[graham bahr]

typically cars use 0.6 for clutch and the same or maybe 0.7 for the front and 0.75 for the rear, dunno about handbrake but i'd guess 0.6, although if its a race your do you really need a handbrake?

[Belatti]

There are two ways:

a) you copy the master cylinder size of a similar car, with similar weight, center of gravity height, tire size, brake disc and caliper piston sizes...

or

b) You calculate it. For this you´ll have to guess the wheel torque needed, having Long G as an input is a good start.

[graham bahr]

Originally Posted by Belatti

There are two ways:

a) you copy the master cylinder size of a similar car, with similar weight, center of gravity height, tire size, brake disc and caliper piston sizes...

or

b) You calculate it. For this you´ll have to guess the wheel torque needed, having Long G as an input is a good start.

seeing as master cylinders for bias pedal boxes normally only come in 3 sizes thats rather complicating things!

small for the front big for the rear is usual rule of thumb and works irrespective of vehicle weight,

it has to be noted a you might still require an adjuatable portioning valve to reduce fluid to the rears

[Belatti]

seeing as master cylinders for bias pedal boxes normally only come in 3 sizes thats rather complicating things!

small for the front big for the rear is usual rule of thumb and works irrespective of vehicle weight,

it has to be noted a you might still require an adjuatable portioning valve to reduce fluid to the rears

You can install any cylinder you want to. Tilton for example has from 5/8" to 1" 5 more sizes in between, that would be 7 different sizes.

The pedal box is sold separated from the cylinders.

[phoenix]

Originally Posted by Timmyevo6

next season, i'll be running a pedal bias box in my evo6 race car.

how do i go about getting the correct size brake master cylinders for front/rear. and also for clutch and hydro handbrake?

cheers. tim.

Do you know your caliper piston sizes, front and rear, and pad areas, again front and rear?

Armed with that information - and the brake pedal ratio - a more informed 'guess' could be made.

[Timmyevo6]

Originally Posted by phoenix

Do you know your caliper piston sizes, front and rear, and pad areas, again front and rear?

Armed with that information - and the brake pedal ratio - a more informed 'guess' could be made.

i shall find this out!

[JohnD]

Calculation:
Ideal ratio of Caliper to M/cylinder is 23:1
For example, caliper pistons are 60mm wide.


Piston area= (60/2)^2 x Pi

Four calipers, so total area= (60/2)^2 x Pi x 4 (NOT 8!)

So area of M/Cyl should be; Diameter = (sqrt(Area/Pi)) x 2

So whole expression is (sqrt(((60/2)^2 x Pi x 4)/23/Pi)) x 2

put that into Google and you get: 25mm!

Or, one inch.

Simples!

John

John

[Tim Falce]

Originally Posted by JohnD

John wrote- Simple Calculation:

John

I put that into Google and ended up with this result.
These calculations don't take into account things like pedal ratio, the feel you want or whether the car is running a servo etc. I'd go with Graham's answer in post two and I bet he's closer to reality than you John.

[phoenix]

Originally Posted by JohnD

Calculation:
Ideal ratio of Caliper to M/cylinder is 23:1
For example, caliper pistons are 60mm wide.


Piston area= (60/2)^2 x Pi

Four calipers, so total area= (60/2)^2 x Pi x 4 (NOT 8!)

So area of M/Cyl should be; Diameter = (sqrt(Area/Pi)) x 2

So whole expression is (sqrt(((60/2)^2 x Pi x 4)/23/Pi)) x 2

put that into Google and you get: 25mm!

Or, one inch.

Simples!

John

John

Nice solution, John - but 1) where does this 'magic' ratio come from and 2) how does it take into account the brake pedal ratio?

I'm not saying that the 23:1 ratio is wrong, btw....

[JohnD]

Try Googling for "Master cylinder 23:1" - 6318 hits, plus two for this one. The ratio seems to be best known in the biking world, where the 'pedal' or hand lever ratio is closer to one.

It's not THE ratio, it's a ratio, and the pedal ratio is a seperate issue that needs to be integrated. Pedal ratio influences cylinder/caliper area ratio and vice versa - they are in effect two levers working in series.
And Graham's talking bias boxes - which was the original Q.
But you gotta start somewhere.

John

[Timmyevo6]

the pedal box i'm using has a 5.1 ratio.

my front calipers are ap 6 pots, rear calipers are OEM bremo 2 pots. not certain of piston size etc.

no servo unit. not going to bother with hand brake. i've lots of help in the pits, someone can put their toe under the tyre!

i want good brakes, driven a car with no servo before and i understand you need to STAND on the brakes to get them working well.

[JohnD]

[QUOTE=Timmyevo6;2593076] not going to bother with hand brake. i've lots of help in the pits, someone can put their toe under the tyre!
QUOTE]

Timmy,
See Blue Book, Specific Regulations for Car Racing (G), "Brakes
108. Brakes should comply with Statutory
Requirements as to the construction of Motor Vehicles.
If there is no mechanical system available for applying
braking effort to at least two wheels, there must be two
hydraulic systems so that braking is maintained on at
least two wheels in the event of failure of the first
system."
http://www.msauk.org/uploadedfiles/m...Car_Racing.pdf

I think you'll find that the scrutes expect a hand brake, if only to stop the car rolling out of the inspection area - eg Mallory.

John

[Timmyevo6]

Originally Posted by JohnD

I think you'll find that the scrutes expect a hand brake, if only to stop the car rolling out of the inspection area - eg Mallory.

John

good point well presented, thanks for that.

handbrake it is then!

[JohnD]

A tandem master cylinder, with another master in series on the handbrake lever, to the rears (or fronts!) is perfectly acceptable for racing. ("Two hydraulic systems") For road use you MUST have a mechanical system, but as my MoT man says, there's also a cable from the lever - how can he tell if the hand brake works by cable or hydraulics, as long as it works?

John

[GORDON STREETER]

Brakes should comply with Statutory
Requirements as to the construction of Motor Vehicles.
If there is no mechanical system available for applying
braking effort to at least two wheels, there must be two
hydraulic systems so that braking is maintained on at
least two wheels in the event of failure of the first
system."
A bias pedal box actually meets these regulations.
However just run a master cylinder off the handbrake lever in the rear brake line, job done.

[Timmyevo6]

for a hydralic system, does it go,

pipe from rear master cylinder, into handbrake master cylinder, out of hand brake master cylinder, into T piece and the from the T piece into each rear caliper?

Don't panic on my 'lack of knowledge', i do the driving and bill paying, my brother is the engineer.

[GORDON STREETER]

Yes its really that simple, just run the handbrake M/C in the rear brake line.

[Belatti]

sorry I dont get it... with one master cyl for the front and one for the rear... you have got 2 separated circuits and you dont need handbrake, is that right?

BTW, the 5 to 1 pedal ratio is very reasonable and typical

for an integral calculation you should do what my brake supplier has in a program he made, that is based on a previous excel he had
basically the program has black fields where you put pad compound (mu database), wheelbase, CoGh, rotor(s) size(s), caliper piston sizes and amount, master cyl size and pedal ratio, tyre diameter, force made by your foot and a long G estimation...

when you have some X amount of blank fields completed the program estiumates the others... its easy to do with an excel spreadtsheet


However thats more than what you Timmy need, just telling in case someone likes the idea

[Tim Falce]

With a bias bar and two master cylinders, which you are saying equates to a tandem system, you wont have any brakes if one system fails as the pedal will go to the floor and all pressure of the bar will go to the side of least resistance.

[Timmyevo6]

Originally Posted by Tim Falce

With a bias bar and two master cylinders, which you are saying equates to a tandem system, you wont have any brakes if one system fails as the pedal will go to the floor and all pressure of the bar will go to the side of least resistance.

indeed, however, if you have a hand operated master cylinder, it'll still function. i think.

[Tim Falce]

Originally Posted by Timmyevo6

indeed, however, if you have a hand operated master cylinder, it'll still function. i think.

Of course it will providing it's not operating on the system that has failed.

[Belatti]

I would have thought that as pump stroke is rather short, you would reach the bottom and be able to brake with the onther circuit...

[GORDON STREETER]

Tim, I'm not saying it will work, its just that it complies with the regs as far as the wording goes.
Having had a "complete" brake failure at Brands some while back I know that a bias bar tandem system has its "shortcomings"

[Tim Falce]

Gordon, I fully understood the meaning of your first post on it but I was just pointing out that having two system doesn't mean it's a fail safe. What happens when one side of a tandem cylinder system goes down, do you still have any stopping power with that? After all when you bleed one brake on this type of system the pedal will go straight to the floor won't it.