brake pressure calculator???

[fordracer]

Does anyone know how to figure hydraulic brake pressure and the effects of changing master cylinder/caliper piston sizes will have on pedal feel and stopping???

[mickmate]

Start with a pedal ratio. 6:1 is a good number. Put 100 lbs of leg force on it you get 600 lbs going in on your master linkage. Now calculate the area of piston on your master cylinder. .750" dia = Pi x R squared = .441" are you with me so far?

[Dwight]

keep going.
do we need to measure our pedals to check the ratio?
is there a way to measure the pressure on the front and rear wheels?
Dwight

[mickmate]

Pedal ratio is the length between pivot and cylinder versus pivot and pedal center so yes measure those lengths or assume 6:1 as it will be close.
You can measure the pressure with an inline gauge or we can calculate it.

[427sharpe]

A simple tool called a brake pad apply pressure tester. Waekon BEQ0197. There are some knock offs of good quality on Ebay most of the time, less than $100. Remove caliper, install sensor end between pads you want to check (2 wheels at a time) and have a cohort press the brake pedal. Instant gratification!

[mickmate]

Lets say we've got dual masters so half the force goes to the front master 300 lbs. P = F/A = 300/.441" = 680 lbs/sq in. Lets use this master on a 4 piston Wilwood with 1.75" pistons. 2(pistons per side) x Pi x Rad sq = 2 x 3.14 x 1.75/2 sq = 4.808 in sq. Force = Press x Area = 680 x 4.808 = 3269.44 lbs per side or 6538.8 for the front. If you calculate the rears out the same way putting your numbers in of course you'll have a total number on your stopping force. The difference between them is the brake bias. You want at least 60% of your braking on the front wheels. Check my maths and throw in the center of gravity, weight, coefficient of friction on the pad material and a few other things and you'll have some good theory to put into practice. There's a good chart here that calcs for you if you put your numbers in. TCE Performance Products - Producing Winning Results since 1993

[Tom Kirkham]

Ford Racer,
Are you starting from scratch, or modify an existing set up?

To answer your basic question as you increase leverage (hydraulically or mechanically), pedal travel will increase for a given pressure, this will give the sensation that the pedal is softer. The typically ways of increasing leverage are: increasing the pedal ratio, larger total surface area for brake calipers (on opposed piston calipers only count pistons on one side), smaller master cylinders, larger rotors (assuming the calipers are moved out as well), or smaller diameter tires.

mickmate: I am surprised that your link, did not mention tire diameter.

Be careful! 60% front weight bias is a very general number. I am sure it is too much for a Porsche 911 and way too little for a front wheel drive econobox. For most applications too much front bias is better than to little (this will give understeer). Autocross cars are however an exception.

[mickmate]

Good point Tom, this one takes wheel sizes into consideration. http://sth2.com/Z-car/Brakemath.xls
That is a ratio again between the force we've calculated on a rotor of whatever diameter working on a tire of whatever diameter. It's hydraulic principles in practice like a lever. Making the master smaller gets you more pressure but with a longer stroke to get it. That as Tom said gives a softer feel. With a track car you want the most even braking you can get. That is dependant on a huge number of variables hence a lot of cars using a balance to adjust between front and rear on track cars. What you don't want is too much rear stopping power as that makes the car swap ends so if you're going to err do it on the front bias as that will at least keep the car straight even if it's not giving the most braking effect. The book I'm looking at says if it's loose going in to a corner on braking there's too much rear brake, if it's pushing too much there's too much front brake. Testing will tell!

[rdorman]

Send me an email. I have an excel spreadsheet that will do it all for you.