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# Brake caliper piston area vs master cylinder piston area, any rule of thumb?

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Hi!

Just planning one of my brake setups in my car (will need a separate caliper/rotor setup for small winter tires), and wondered if there are any guidelines to sizing master cylinders vs caliper piston area?

My pedal box has a ratio of approx 5,2:1.

Calipers will be 6pot Brembo's "18z" with 30/34/38 pistons, equals 5494mm2 of piston area in each caliper. (360mm rotors)

Will be using the same calipers front and rear, so want to set a base bias selecting master cylinder size.

Thank you, will look at that!

I used it to work out why running the same calipers and discs front and rear was a really bad idea, even on a mid engined car!

The answer is - it depends... Those reading, please comment - it's been a while and I will probably forget, confuse something...

What you're doing it figuring out the best way to convert force at the pedal, and it's travel (moving the pads into contact with the discs, pad compression, hose and pedal flex, etc), into the maximum braking force at the track/road surface.

This may take the best part of an afternoon to go through all the way, but some highlights.

How much force can you apply to the pedal and comfortably modulate? For most people 50kg/490N (using 9.8N/kg for conversion) is the comfortably upper value, but some are capable of quite a bit more. I'd suggest starting with 500N force.

That pedal ratio will give a multiplication factor of 5.2, giving a total force applied to the cylinders of 2600N. As there is the implication of no vacuum/hydraulic assistance, that pedal ratio is a little lower than is usually found, but being able to select master cylinder bore size will largely compensate for it.

Now consider the brake forces you require at the tyres. if you have a 1200kg vehicle, and there is a potential friction co-efficent of 1.2 from the tyres, that means 1200 x 9.8 x 1.2 = 14112N of shear force at the track surface, which I'll round off to 14kN - I'll be rounding off a lot, it isn't critical as most data is going to be best estimate, anyway.

What is the loading on the front and rear tyres under braking - the load transfer? This is going to depend on the static loads on the tyres, the wheel base, the height of the centre of mass, and the braking 'G' - it's a little complicated to go into here, but you should find plenty under "braking load transfer", or similar, on the inter-web. If we assume, for the exercise, a static 60-40% front rear balance, a 3000mm wheelbase, a CoM of 450mm then we can say the CoM is 1200mm behind the front axle and under 1.2G braking the load balance moves to 660mm behind the front axle. This means the braking balance is going to be biased 3.55 times as much to the front as the rear.

NOTE! This is ignoring aero' and drag affects, which will also bias things, but it is a general guide, so, to continue...

Oh, or not - got to go, normal service will resume later.

Good discussion here guys. What Gord (is part-way through) explaining is on the right track. I'm sure he'll come back to expand on this, so not much more for me to add on this for now other than to say we will be producing a brake system course this year to help people spec their entire system, including these calculations being discussed here.

Thanks for the thorough explanation Gord! I understand the overall theory, but it's way too much info to process for now :)

Sounds good Tim, looking forward to see the course :)

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