BBK questions - Any Automotive Engineers here??

[KenO]

So, I'm working on a BBK for my car, and need a little help with the math. Trying to get the same fluid displacement as the stock single piston calipers. I think I have the fronts setup, but would appreciate somebody checking my math, but need a little help on the rears. Stock calipers are single piston floating calipers. New calipers are Outlaw Racing, preferably 6-piston front but can stick to 4-piston, 4-piston rears (unless somebody has a better idea?? I'd prefer to stick with Outlaw calipers over Wilwoods or anything else, they're local and the owner is working with me on machining a set custom for me) .


Front Caliper Piston: 57mm
Rear Caliper Piston: 40mm


57mm x 1 is same fluid displacement as 1 1/8" x 4 pistons.

4-piston: (2) 7/8" plus (2) 1 3/8" equals slightly more displacement than above.

6 piston: (2) 1.38" + (4) 1.25" = same displacement as 57mm piston


What about the rear caliper? The piston is significantly smaller, and I'd prefer to use larger/smaller piston sizing in order to reduce pad taper. This is for a mostly daily driver, occasional autox/track car.

 

[SteveSRT8]

Aaaargh, math is way out of my league.

if it were me I'd call Dave Zeckhausen at Zeckhausen racing. He's a great resource and will share his immense knowledge over the phone with you.

Most ABS systems can tolerate a bit of displacement volume increases, but the consequences are large if you get too big. I guess you are not using a matched master cylinder...

 

[scurvy]

Equivalent area calculations are pretty easy... and brakes are extremely simple hydraulic systems.

If I understand your numbers correctly, the stock front pistons are 57mm. The four piston calipers have two pistons at 7/8" and two 1-3/8"; six piston calipers have two at 1.38" (did you mean 1-3/8"?) and four at 1-1/4". We'll do all of this in inches since it appears to be how things are sized.

The area of a circle is pi*radius^2; or since 2*radius = diameter, the area can also be expressed as (pi*diameter^2)/4.

The stock pistons are 57mm - divide by 25.4 to get diameter in inches (2.2441"), their area is (pi*2.2441^2)/4 = 3.9552 square inches.

Four piston replacements have a total piston area of (2*(pi*0.875")/4) + 2*(pi*1.375^2)/4 = 4.1724 square inches, which is 5.5% larger.

Assuming you meant 1-3/8" for the six piston calipers, they have a total piston area of (2*(pi*1.375^2)/4) + 4*(pi*1.25^2)/4 = 7.8785 square inches, nearly twice the piston area as stock.

Needless to say you would double these figures if you had two calipers in the front braking system!

The stock rear piston is a single at 40mm which has an area
Click to reveal..
left as an exercise to the reader.

 

[KenO]

Originally Posted By: scurvy
Equivalent area calculations are pretty easy... and brakes are extremely simple hydraulic systems.

If I understand your numbers correctly, the stock front pistons are 57mm. The four piston calipers have two pistons at 7/8" and two 1-3/8"; six piston calipers have two at 1.38" (did you mean 1-3/8"?) and four at 1-1/4". We'll do all of this in inches since it appears to be how things are sized.

The area of a circle is pi*radius^2; or since 2*radius = diameter, the area can also be expressed as (pi*diameter^2)/4.

The stock pistons are 57mm - divide by 25.4 to get diameter in inches (2.2441"), their area is (pi*2.2441^2)/4 = 3.9552 square inches.

Four piston replacements have a total piston area of (2*(pi*0.875")/4) + 2*(pi*1.375^2)/4 = 4.1724 square inches, which is 5.5% larger.

Assuming you meant 1-3/8" for the six piston calipers, they have a total piston area of (2*(pi*1.375^2)/4) + 4*(pi*1.25^2)/4 = 7.8785 square inches, nearly twice the piston area as stock.

Needless to say you would double these figures if you had two calipers in the front braking system!

The stock rear piston is a single at 40mm which has an area
Click to reveal..
left as an exercise to the reader.

Yes, I meant 1.38", lol. I don't mind going SLIGHTLY bigger wrt total fluid displacement, but I need to keep it the same bigger or smaller proportion front & rear. This is keeping the stock master cylinder. I just can't figure out for the life of me what my piston options are going to be to replace that tiny rear caliper.

 

[scurvy]

So just to verify you meant 1.380" inches diameter for the six-pot front caliper pistons, not 1-3/8"? I realize in decimal inches they're only 5 thou off but they are not the same!

Re-reading your first post, am I correct in interpreting it as meaning you're going to use the 6-piston calipers on the front but want to use the 4-piston calipers at the rear?

If that is the case, let's look at the ratios.

Front/rear stock caliper piston area ratio is 2.031:1

Outlaw 6 piston/4 piston area ratio is 1.886:1 so you would have more rear brake bias... but not by much more, only 7%.

 

[KenO]

Originally Posted By: scurvy
So just to verify you meant 1.380" inches diameter for the six-pot front caliper pistons, not 1-3/8"? I realize in decimal inches they're only 5 thou off but they are not the same!

Re-reading your first post, am I correct in interpreting it as meaning you're going to use the 6-piston calipers on the front but want to use the 4-piston calipers at the rear?

If that is the case, let's look at the ratios.

Front/rear stock caliper piston area ratio is 2.031:1

Outlaw 6 piston/4 piston area ratio is 1.886:1 so you would have more rear brake bias... but not by much more, only 7%.



Did I do my math wrong? I could have sworn the 6-pot with the piston sizes mentioned was nearly the same as the single 54mm stock piston? And yes, I meant 1.38, sorry. Remember, we're only counting piston area of 1 side of the fixed caliper when calculating & comparing to a single piston floating caliper.

Also, fwiw here are the specs of the Outlaw 4000 caliper:

http://www.outlawdiscbrakes.com/3000_4000.html

 

[scurvy]

Originally Posted By: KenO
Did I do my math wrong? I could have sworn the 6-pot with the piston sizes mentioned was nearly the same as the single 54mm stock piston? And yes, I meant 1.38, sorry. Remember, we're only counting piston area of 1 side of the fixed caliper when calculating & comparing to a single piston floating caliper.


Brain [censored]!

I've been going on the equivalent area of all four piston bores and reckoned the stock calipers had pistons on both sides. Take my numbers for the Outlaw calipers and divide them by two.

Stock 57mm fronts = 3.9552 in^2
Stock 40mm rears = 1.9478 in^2
F/R ratio is 2.031:1

Outlaw 6-piston = 3.9393 in^2
Outlaw 4-piston = 2.0862 in^2
F/R ratio is 1.888:1

So yes, the 6-pot calipers have the same piston area within half a percent.

 

[KenO]

Originally Posted By: scurvy
Originally Posted By: KenO
Did I do my math wrong? I could have sworn the 6-pot with the piston sizes mentioned was nearly the same as the single 54mm stock piston? And yes, I meant 1.38, sorry. Remember, we're only counting piston area of 1 side of the fixed caliper when calculating & comparing to a single piston floating caliper.


Brain [censored]!

I've been going on the equivalent area of all four piston bores and reckoned the stock calipers had pistons on both sides. Take my numbers for the Outlaw calipers and divide them by two.

Stock 57mm fronts = 3.9552 in^2
Stock 40mm rears = 1.9478 in^2
F/R ratio is 2.031:1

Outlaw 6-piston = 3.9393 in^2
Outlaw 4-piston = 2.0862 in^2
F/R ratio is 1.888:1

So yes, the 6-pot calipers have the same piston area within half a percent.



Great, I was correct, thank you! Now [censored] to do about a rear caliper..... I need to find out if that is too much of an increase in rear bias.

 

[scurvy]

Originally Posted By: KenO
[Great, I was correct, thank you! Now [censored] to do about a rear caliper..... I need to find out if that is too much of an increase in rear bias.


It's less than 10% additional bias toward the rear... could that be made up for in pad selection?

 

[KenO]

Originally Posted By: scurvy
Originally Posted By: KenO
[Great, I was correct, thank you! Now [censored] to do about a rear caliper..... I need to find out if that is too much of an increase in rear bias.


It's less than 10% additional bias toward the rear... could that be made up for in pad selection?



For me - absolutely. But, I'm building 2 sets initially, and I'm not sure I want to put that burden on somebody else, for a streetcar. Your 4-pot calcs - what piston sizes did you use again?? Using (4) of the 1.12" pistons on the 4-piston caliper looks to be about equivalent to the stock 40mm rear caliper?? Just how important is differential piston sizes??

 

[scurvy]

4-piston area calculation based on your sizes:

"4-piston: (2) 7/8" plus (2) 1 3/8" equals slightly more displacement than above."

(2*(pi*0.875^2)/4) + (2*(pi*1.375^2)/4)/2 = 2.0862 in^2

Using four 1.12" pistons gives an area of one side of

(4*pi*1.12^2)/8 = 1.9705 in^2 just over 1% difference from the stock rear pistons. Couple those with the 6-pot fronts and you've got a F/R ratio of 1.999:1, within 2% of the stock ratio. Sounds like that one's a winner.

 

[KenO]

Well [censored]. Was just informed that the 6-piston caliper is ONLY available in a 1.62/1.12/1.12 configuration. Not a horrible difference, but..... Wonder if thats going to effect pedal height with the stock master cylinder or not?