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jeffandre
12-27-2004, 09:51 PM
We have all heard that the front brakes do the most work, and of course weight distribution and how the vehicle weight responds to acceleration has an effect, but in the 'classroom' so to speak I have heard 2 basic formulas and need to know which is right.

In the past I read from somewhere in the old posts that the rear caliper piston area should be no more than 60% of the front caliper piston area. Then I read recently that the number is 40%. Which one is preferred for a 3200#+ 2nd Gen with fat tires and stiff suspension? My calcs are shown below for the various piston configurations I may be looking into when the time comes to buy front calipers to go with 14" rotors (I have 13" Eradispeed II rotors in the rear using C5 front calipers).

The C5 front calipers on the rear of the Camaro are 2-piston, 1.75 diameter each. C5 piston d = 0.875"; (0.875)(0.875)(pi) = 2.41"2; 2.41(2 pistons)(2) = 4.82"2. Since a forum post from a knowledgeable person stated that the total rear piston area should be no more than 40% (4.82/12 = 40%), (4.82/8.04 = 60%, per 12/23 posting?!?!) of the total front piston area I have determined the following:

2-piston - 2.76" d each; (1.38)(1.38)(pi) = 5.98; (5.98)(2) = 11.97"2 total piston area. 4.82/11.97 = 40.3%.
- 2.3" d each; (1.15)(1.15)(pi) = 4.15; (4.15)(2) = 8.30"2 total piston area. 4.82/8.3 = 58.1%.

3-piston - 2.25" d each; (1.125)(1.125)(pi) = 3.98; (3.98)(3) = 11.93"2 total piston area. 4.82/11.93 = 40.4%.
- 1.85" d each; (0.925)(0.925)(pi) = 2.69; (2.69)(3) = 8.06"2 total piston area. 4.82/8.06 = 59.8%.

4-piston - 1.95" d each; (0.975)(0.975)(pi) = 2.99; (2.99)(4) = 11.95"2 total piston area. 4.82/11.95 = 40.3%.
- 1.6" d each; (0.8)(0.8)(pi) = 2.01; (2.01)(4) = 8.04"2 total piston area. 4.82/8.3 = 60%.

5-piston - 1.75" d each; (0.875)(0.875)(pi) = 2.41; (2.41)(5) = 12.03"2 total piston area. 4.82/12.03 = 40.1%.
- 1.44" d each; (0.72)(0.72)(pi) = 1.63; (1.63)(5) = 8.14"2 total piston area. 4.82/8.3 = 59.2%.

6-piston - 1.6" d each; (0.8)(0.8)(pi) = 2.01; (2.01)(6) = 12.06"2 total piston area. 4.82/12.06 = 40%.
- 1.32" d each; (0.66)(0.66)(pi) = 1.37; (1.37)(6) = 8.21"2 total piston area. 4.82/8.3 = 59.2%.

8-piston - 1.38" d each; (0.69)(0.69)(pi) = 1.50; (1.50)(8) = 11.97"2 total piston area. 4.82/11.97 = 40.3%.
- 1.14" d each; (0.57)(0.57)(pi) = 1.02; (1.02)(8) = 8.17"2 total piston area. 4.82/8.3 = 59%.

In the end I may have to find a happy medium between 40-60% based on my front to rear weight bias and driving style. Once I guesstimate this, I will then have to get the right master cylinder (possibly a tandem with a balancing bar for fine-tuning), and then finish the package off with the best front pads that get the job done!

zbugger
12-27-2004, 10:05 PM
Jeff, thanks. I finished off a new bottle of advil reading your post. That still didn't help any. If anyone can help you after reading that, they are better than me. And the chances of that are excellent. Sorry I couldn't be of more assistance. Here's to another bottle of advil!!!!

jeffandre
12-27-2004, 11:40 PM
Z,
Wazzup?!?! I know what you mean, the tweaking my mind goes through to get close to the edge of insanity without completing falling into it. I figure this post will whip Vince into a frenzy and he'll help me get it all straightened out before he is out of the Oakland shop! :fingersx:

spanky the wondermuffin
12-28-2004, 04:18 PM
just want to be clear. are you asking which front caliper to choose to go with the c5 front caliper used in the rear?my answer to that would be don't try to use a front caliper for the rear.if that is not the ?,let me tell you my piston sizes as an example.front-wilwood superlite2 caliper,4 pistons of 1.75.rear-wilwood forged dynalite,4 pistons of 1.38.dual m/c with balance bar.front m/c-3/4.rear m/c-1.00.i've tried 5 or so different front calipers,a half dozen m/c's and 3 rear setups to get to where i am,which is happy with my brakes.hp books 1281,brake systems is a decent referance.

gmachinz
12-28-2004, 08:26 PM
I think the correct ratio for rear bias compared to front bias is no more than 40% for a starting point-then it gets into vehicle front/rear weight bias, tire choice, etc. I think there isn't enough thought in the typical equations to make up for quality tire compounds. For instance, a "sticky" tire is not near as likely to break loose on acceleration than would a fairly hard compound tire-so it would make sense that that same sticky tire would resist locking the rears up under hard braking too-but I think some of that depends on how much front end dive you experience which goes back to a properly balanced front/rear suspension which has to take into consideration the front/rear brake bias as well. Basically, a stiff front end would not let the rear come up as much. Then again not enough front to rear weight transfer inhibits good traction to a point-so it's a little bit of a trade-off anyway. I feel suspension design and tire compound choices can alter the equation of what "should" be correct almost to the point of having same size front/rear brakes-then it's a matter of correctly proportioning the fluid pressure......my experience is on the street though and not on a track so I could be a little off on that but that's my experience so far. -Jabin

David Pozzi
12-28-2004, 11:28 PM
I think a rear bias of 35% to 40% is good to shoot for, then if too high you can adjust it back using an adj proportioning valve. Excess rear piston size will increase the pedal travel.

jeffandre
12-29-2004, 11:29 AM
Thanks David,
That is what I am looking for, information to lead me in the right direction piston area-wise. The reason I went front C5 calipers in the rear is cost of parts of time (many pad choices for relatively cheap, easy to tweak on calipers with custom parts if needed, etc.). The C5 floating calipers were relatively easy to install and should be easy to dial in with the right mc and the current adjustable prop valve.

The front calipers will probably be Alcon's, and with so many piston choice setups available I figured I better get the math close before I start looking too hard.

The fairly well-balanced weight-wise 71 Camaro has minimal dive and with 345's on the rear and 295-315's on the front should ba able to utilize the giant brakes well on tracks such as Sear's Point and Thunder Hill. Of course 0-100-0 runs will be fun too! Thanks again,

David Pozzi
12-29-2004, 10:21 PM
spanky the wondermuffin,
Thanks for contributing your info.

Jeff,
I just noticed you have your front and rear percentages reversed!

"2-piston - 2.76" d each; (1.38)(1.38)(pi) = 5.98; (5.98)(2) = 11.97"2 total piston area. 4.82/11.97 = 40.3%.
- 2.3" d each; (1.15)(1.15)(pi) = 4.15; (4.15)(2) = 8.30"2 total piston area. 4.82/8.3 = 58.1%."

The 11.97 total piston area caliper has 58.1% of the total area.
The 1.320 caliper is 40.3%

Here is a little easier caliper displacement formula:

PISTON SQARE INCH FORMULA:
Pi /4 = .7853981635 times (bore squared) = area of bore in sq inches

OR:

Easier way to do it is: Piston bore squared X .7853981635 = area of bore in sq inches

To calculate the front percentage, I add front and rear capacity to get total piston area, then divide the front caliper area by the total area, = ft percentage.

For others reading this, you should only compare one half of a rear caliper to one half of a front caliper, a four piston caliper would only use two pistons (one side of the caliper) for calculation purposes.
You can compare two pistons of a rear four piston caliper to a single piston front caliper this way and it works.

I'm not good at math so I have to get a forumla out of a book and work some examples to make sure I'm doing it correctly, then insert my numbers.

Here is an example of a JL8 69 camaro which seems to have a pretty good F/R balance, the rear might not lock up with no proportioning at all according to some racers I've talked to, so I think a little more rear bias up to 40% and a prop valve would enable max rear braking.

As I said before, going larger than needed on the rear will induce extra pedal travel but going a little over perfectly balanced on the rear, allows a prop valve to be used and gives some adjusting "room".

JL8 discs BRAKE CALCULATIONS F/R 69 CAMARO

one ft cal piston 1 7/8" (1.875) 2.7611654186 sq in
two ft pistons: 1 7/8" each 5.5223308371 sq in 65%

One rear piston 1 3/8" (1.375) 1.4848934029 sq in
two rear pistons 1 3/8" (1.375) 2.969786806 sq in 35%

Front 5.5223308371 / 8.492117643 = 65%

For better accuracy, use all the decimal places in your calculations or the F/R percents will not add up to 100% The first couple of times I did it, I rounded off the numbers to three decimal places and it didn't work very well.

jeffandre
12-30-2004, 10:49 AM
David,
I am still trying to get this right. Using 2-piston 2.76" each for the front calipers will end up allowing the current rear calipers to account for roughly 40.3% of the area of the front calipers. If I go smaller in the front using 2-piston 2.3" each for the front calipers then the rear calipers will end up being roughly 58.1% of the area of the front calipers. I was thinking last night that this is not the way to determine the correct balance and your last response has shed some light on the mathematical equation that I really needed. I was wondering how I should actually determine the percentages, and your JL8 explanation did it perfectly. I will redo my figures and hopefully get them right this weekend. Thanks again,

Jeff

jeffandre
12-30-2004, 10:59 AM
In a nutshell I was trying to get the rear caliper area to be 40% of the front caliper area, instead of 40% of the total braking system. This is what I should be after right, having the rear 35-40% of the total piston area (when adding front and rear)?

Jeff

David Pozzi
12-30-2004, 02:47 PM
Yes, that's something that drove me nuts for a while when I first tried to do it. I tried to figure the percentage the same way you did, but you need the percent of total capacity. You add front and rear together, divide the front by the total to yield front percentage, divide rear by total to get rear percent.

spanky the wondermuffin
12-30-2004, 03:12 PM
at least consider reconsidering.those calipers might be cheap and easy,but will they make everything else expensive and difficult?a couple dynalites for the rear are like $80 each,superlites are $150.the rears can use cheap off the shelf brackets.the 1.75 front 1.38 rear combo is proven.the S-10 m/c will work- with the stock pedal ratio.a dual master/balance bar can be used and you'll know where to start with bore size.the dynalite would also be a choice for the fronts,in a 1.75 bore.wilwood says brake force is based mostly on piston size,not caliper size,and if you aren't wearing out pads before the end of a race you should use a smaller caliper/pad.

David Pozzi
12-30-2004, 07:50 PM
How do you like the Wilwood Superlite II calipers on the front? Have you experienced any tapered pad wear?

jeffandre
12-31-2004, 07:05 AM
Spanky,
Good point, got into the brake stuff while in the middle of the minitub and figured since I already had the calipers and rotors I might as well see if they would work. Since everything is together and finished on the rear I will probably go ahead and get the front lined out and then if I have problems I will reconsider the rear calipers. So much to figure out when it comes to overall braking system balance and I like the challenge so it should be killer, especially with my setup and usage plans.

Jeff

spanky the wondermuffin
12-31-2004, 07:56 AM
no taper visible to my eye,some idiot left my digital dail caliper on last time i used it so i can't get a true mesurement(battery dead).i think they are fine.i'm not even using the bridge bolts.test drive my car-you tell me.a couple regulars on the nasa ai circut have driven it and loved the brakes.

BRIAN
12-31-2004, 10:30 AM
What size rotors are you using with the Superlites? Are there good selection of pads?

jeffandre
12-31-2004, 02:59 PM
Yes, that's something that drove me nuts for a while when I first tried to do it. I tried to figure the percentage the same way you did, but you need the percent of total capacity. You add front and rear together, divide the front by the total to yield front percentage, divide rear by total to get rear percent.

Okay, math is done, although Alcon's typically have varying piston sizes so my mathematical calcs will be more for practice than anything else. At least I know what I am doing now and I know the piston area range that I need on the front calipers in order to keep the rears in the 35-40% range (7.2165 to 8.9347 square inches total front caliper piston area). Thanks for straightening me out David! I also like your idea of undershooting and using the balance bar and/or proportioning valve to increase the rear bias some, so I will work towards 35%. The last thing I need is for the rears to lock before the fronts, I've had the rearend out in front of me a few times, no fun...

For the rears to be 35% of the total braking balance the total front caliper piston area needs to be 8.9347, 40% is 7.2165.

David Pozzi
12-31-2004, 06:52 PM
Jeff, I would overshoot the rear since you can't increase the rear line pressure unless you have dual master cyls. If you have a single master cyl I'd go for 40% rear bias, then you can turn it down with a prop valve in the rear line.

The multi sized piston Alcons can be compared just calc each piston sq inches and add them up for one caliper half, then compare.
David

spanky the wondermuffin
12-31-2004, 07:11 PM
brian-see thread titled 'is there such a thig as too much brakes"

jeffandre
01-01-2005, 11:43 AM
Jeff, I would overshoot the rear since you can't increase the rear line pressure unless you have dual master cyls. If you have a single master cyl I'd go for 40% rear bias, then you can turn it down with a prop valve in the rear line.

The multi sized piston Alcons can be compared just calc each piston sq inches and add them up for one caliper half, then compare.
David

Okay, intended to go dual master for maximum adjustabilty depending upon conditions of brake pads, track, my abilities, etc. (rear prop valve already in car, may or may not be necessary with a dual master?). Got it on the multi-sized piston caliper math too.

Spanky has the right idea as far as going with a combo that is proven, I just got going and want to see it through. I may never do this again, so gotta get it right, so thanks for the help of everyone who has contributed to the brake forum over the years!

Ripper
01-03-2005, 02:13 AM
Wouldn't the easiest way to find out what bias to go with, be to find out the weight distribution during full braking (let's say it can be 70/30 or something) and then size the calipers after that (70% front and 30 rear)?

Or am I wrong?

You need to know weight distribution and center gravity height and the friction coefficient of the tires thought... This isn't impossible to find out though..