Desired brake pressure

[SlimLim]

So, i'm installing aset of cobra/focus rs brembo 4pot calipers for the front, and mk4 2pots for the rear and i'm trying to calculate if i need to upgrade the master cylinder to retain the stock feel. I don't want to do this by trial and error.

How do you calculate if you need to upgrade the MC? I know the surface area needs to match those of the pistons surface area. However, is that the total piston area? Of the 3 (one side), or all 6 (taking into account both sides?) When working out the bias and force of teh brakes, this calculator suggests one side only http://www.tceperformanceproducts.com/bias-calculator/

So i'm also using this calculator. http://www.markwilliams.com/calculators.aspx
The TSRM states the pedal reserve distance is 50kg/110lbs/490N. So i'm inputting this value into the desired foot pedal box, but it appears i need to know the desired system pressure. So what is the stock desired brake pressure i need to achieve?

My train of thought is that once i work that out, it will give me the foot pedal ratio, then i multiply that with the MC surface area. And see if it matches the new piston surface area.

Am i even on the right track?

 

[SlimLim]

Anyone?

Would it be easier if i just list down the piston sizes, rotor sizes, then let someone else calculate it for me?

 

[hvyman]

Whats wrong with the stock master? Works for everyone else.

 

[SlimLim]

From my understanding, it doesn't work for everyone with bigger caliper pistons. Most people will lose stock brake feel if their increase the piston surface area beyond what the MC is capable of producing. I want to calculate if i need to upgrade, rather than work on a trial and error basis.

Some big brake kits will use more pistons and maintain the same surface area as the stock caliper pistons.

You also can't just upgrade to a bigger and badder MC as the larger you go, there is a decrease in line pressure (all things being equal). So i know some of the fundamentals, i just don't know the maths.

 

[gats]

Just a thought - can't you work out the effective ratio between the master cylinder S/A with respect to stock caliper pistons S/A?

Someone should know the stock diameters for master cylinder & caliper pistons, and I assume you know the diameters for your new calipers?
Then you can compare the change in ratios and whether you may need to upgrade the MC or not?!?

 

[SlimLim]

Just a thought - can't you work out the effective ratio between the master cylinder S/A with respect to stock caliper pistons S/A?

I can do this, but i don't thinkits the right way to doit. What is the ratio point that you need to upgrade to a new MC? For example, if the new surface area is 5% greater than stock, is this enough to need a new MC?

Calc the surface area of the pistons is easy, but do you use all 6 pistons in total SA? or do you compare using the SA one side of the caliper, therefore 3 pistons in the calculations to compare it against the stock single pots?

 

[supraguru05]

Are you trying to achieve the same pedal force vs brake torque as the factory brakes with your new calipers?

 

[SlimLim]

Hi Supraguru,

I'm trying to calculate the same pedal force required to lock up the brakes as stock. I was hoping to use some maths to determine if i needed a larger MC to achieve this.

I think the brake torque will be stronger given i'm using larger rotors and pads, and they are positioned further away from the centre of the rotor than stock.

Can you help me?

 

[hvyman]

Brake force is all in the caliper. It's all hydraulics. Rotor is just more mass.
The 4 piston use smaller pistons than the stick single. Not many people are using aftermarket mc with the front and rear bbk.

 

[supraguru05]

I neednyour new caliper piston and rotor diameters and number of pistons

 

[SlimLim]

Ok i had to disassemble the calipers. Here are the specs:

Fronts

- 4 piston brembo's 36mm (35.95mm) and 40mm (39.96mm) piston diameter. Rotors are 330.0mm x 28.0mm

Rears

- 2 piston JZA80 rears 40.4mm piston diameter. Rotors are 324.0mm x 16.0mm

Hope that helps...

 

[spiller]

i wouldn't bother upgrading to a new master cylinder unless you want proper bias control nick. if you are worried about a lack of feel through the pedal, disable your booster. it's really a matter of how serious you want to get and what you want to use the car for but i'll watch this thread with interest. the pedal in my car lacks a little feel but i think this could be due to the pads in there at the moment, i'm also going to disable my booster before i go to the track next. master cylinder upgrade (twins) will be the last thing i do.

 

[SlimLim]

Well I am fitting a tilton proportioning valve, so yes i am concerned about bias.

Why would you disable your booster?

 

[spiller]

a proportion valve doesn't actually control bias, it just alters pressure. the only way to truly control bias (other than altering "environmental" conditions, eg weight over rear axle, grippier tires, higher friction pads) is by using a twin master cylinder arrangement (as opposed to the factory tandem arrangement). have a read of the following links:

http://www.stoptech.com/tech_info/wp_proportioning_valves.shtml

http://www.centricparts.com/files/Centric White Paper C1-Pedal_Setup_Dual_Master_Guide.pdf

in regards to disabling the booster, when you are out on the race track under heavy braking, the booster can "mask" the lock up point making it difficult to determine the exact point where the tyres begin to lock up. By disabling the booster, you gain a much better feel through your braking foot as to when the tyres are about to lock up, making it easier to avoid lock-ups through pedal modulation. this is why race cars don't have boosters. Its definitely applicable when you are using a high friction pad with high initial bite.

 

[Poodles]

Race cars run different size masters so you have more hydraulic advantage in the system, not recommended on a stock master...

 

[spiller]

yeah usually when you go to an aftermarket arrangement the booster is eliminated for reasons above anyway...

 

[SlimLim]

Understand a proportioning valve is a bandaid. Interesting pointon the brake booster. What happens if you want to retain ABS?

 

[spiller]

i'm not sure about the ABS as mine has been deleted. I would think it wouldnt be affected at all though, the booster is disabled by disconnecting and plugging the vacuum line so nothing to do with the electronics or hydraulics of the ABS system...I think.