Master cylinder size for a given/known caliper ?

**hawk1903** · 05-08-2017, 18:21

Dear friends,

Could you please help me with a practical formula/maths to choose the correct MC for a known caliper ?
I have looked up in the net for the answer but it seems that all the explanations were somehow too complicated.
Basically, I am trying a determine the correct MC size for the OE Ford Connect caliper within the pedalbox set up. ( it is front calipers in my case )
The scenario is a bit like running the Sierra OE style single piston rear calipers on the rear of the Escort on the Atlas or Englis axle.

Cheers

**Graham** · 05-08-2017, 19:26

problem is, there is no fixed formula, some prefer a really firm pedal but little travel, others the opposite way round, pedal ratio has an effect, and so on, its a black art!

**hawk1903** · 05-08-2017, 19:49

Originally Posted by Graham

problem is, there is no fixed formula, some prefer a really firm pedal but little travel, others the opposite way round, pedal ratio has an effect, and so on, its a black art!

Hello Graham,
Exactly. I know what you mean. For example, on most of rally build Escorts, we have a typical set up with Cp4567 calipers and 0,625 MC for the front and either 0.700 or 0,750 MC for the rear CP2577 calipers. We like their pedal feeling and the nice adjustability thru the pedal box. With the correct pads for the event and temps, they are sweet.
However, this time, I am trying to improve the pedal feeling and power on a kit car which has mentioned Ford OE single piston calipers and a typical pedal box set up which I have a chance to choose the MCs.
The pedal ratio is 7.5
Yes, there are lots of formulas as I mentioned which still involves that black art as you mentioned but I just thought that there may be a practical approach to this matter

Cheers
Kerem

**MK1_Oz** · 11-08-2017, 13:57

I respectively beg to differ. There is one master cylinder that provides the best/maximum braking effort for a given set of components. This however is a little different from pedal feel. What you may find a good pedal feel may not give you 100% braking effort however.

After researching brakes and reading text books for weeks I put together this spreadsheet - feel free to use it (unless you compete in Australian tarmac rallies

). From the specs of my car and using a few assumptions (coefficient of friction of the tyres being one) it calculates the theoretical maximum torque that can be applied to the tyres before they lock up => 100% braking. It then calculates what master cylinder and brake bias position is required to match this theoretical maximum.

After I used this model to change my master cylinders the car was chalk and cheese. I had listened to the usual rubbish that the 0.xx" MC must be on the back etc etc and just had aweful brakes. My stage times were a huge improvement after I made the changes.

Happy for anybody to point out any flaws in my model (nicely, no tossers) as discussion leads to learning. Pretty confident in it though....

**hawk1903** · 13-08-2017, 13:00

Originally Posted by MK1_Oz

I respectively beg to differ. There is one master cylinder that provides the best/maximum braking effort for a given set of components. This however is a little different from pedal feel. What you may find a good pedal feel may not give you 100% braking effort however.

After researching brakes and reading text books for weeks I put together this spreadsheet - feel free to use it (unless you compete in Australian tarmac rallies

). From the specs of my car and using a few assumptions (coefficient of friction of the tyres being one) it calculates the theoretical maximum torque that can be applied to the tyres before they lock up => 100% braking. It then calculates what master cylinder and brake bias position is required to match this theoretical maximum.

After I used this model to change my master cylinders the car was chalk and cheese. I had listened to the usual rubbish that the 0.xx" MC must be on the back etc etc and just had aweful brakes. My stage times were a huge improvement after I made the changes.

Happy for anybody to point out any flaws in my model (nicely, no tossers) as discussion leads to learning. Pretty confident in it though....

Hello,
Many thanks for the above model. I will study and digest this information.
At this stage, may I ask couple of questions please ?
I believe your car have the typical OE style pedals and modified to a pedal box system.
Have you taken the pedal ratio somehow into account ?
On the table, driver force on the pedal is 45 kgf, is this an average force or you have actually measured it ? It seemed a quite big force to me.
How about the pad's material and their biting character taken into account ? and the same question goes for the tyres in a way.
Overall, when it comes to real life racing, harder pads and harder tyres should have an effect, should not it ?
Regards

**MK1_Oz** · 14-08-2017, 01:51

I have a GRP4 Fabrications pedal box that I believe is a full custom jobbie.

Pedal ratio is factored in (4.18) between spreadsheet rows 6 & 7 - the formula is referencing a data table on another sheet (Calculator-Data).

I did alot of research to find the 45kgf that I used. I did some rough measurements with some scales but it was difficult to correctly measure. Remember this is a rally car so this is representing a typical full pressure situation.

Brake pad coefficient of friction is factored in between rows 22 & 23 and again the formula is referencing the data table on the other sheet. I have used 0.48 which has been taken from the manufacturer.

Tyre coefficient has been taken into account between rows 36 & 37 and again the formula is referencing the data table on teh other sheet. I have used 1.1 which is a best guess based on basic internet research. The manufacturer would NOT give me this info!

IT is not the hardness of the pad as such that effects braking but the coefficient of friction although to a degree they seem to be related. A harder tyre will more likely give less braking ability as teh softer the rubber the more it moulds to the road surface and in doing so the more it increases the surafce area.