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Old 02-01-2006, 07:00 PM
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C4 Corvette Brake Questions

Hi All,

I need to know the following specs from an '87 Corvette:
  • master cylinder bore:
  • master cylinder stroke:
  • pedal ratio:
Also, is there a way to calculate the effect of the power brake booster as far as amount of line pressure increase?

I'm making a spreadsheet calculator that figures various aspects of braking components , pressures, volumes, etc. that will work for anyone, these specific questions are for my personal situation...

Thanks!

Russ

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Old 02-01-2006, 07:55 PM
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Look on www.napaonline.com
They list the bore (both of them in this case) and the size of the fittings.

FWIW I think it will be 1-1/8" bore. Don't worry about the stroke...that is what the bigger bore on the step bore master cylinder is for...it helps keep the peddle high and firm without using up much of the stroke. If you want to know the ratio I think you are just going to have to measure it.

How far are you going to go into this spreadsheet? Email me if you want some more help on it (triaged at gmail dot com).
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Old 02-02-2006, 08:30 PM
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Here's the brake calculator, let me know if you any other information added to it. It suits my purpose as-is.

According to this, the stock setup (2 1/8" F & 1 5/8" R) rear brakes have 58% of the line pressure that the front do, assuming the M/C has the same bore front to rear. By going to a dual M/C setup and using a 7/8" bore front with a 1 1/8" bore rear, my "4 front caliper" system will achieve a ratio of 60% instead of 58%. This can easily be fine tuned with the balance bar on the pedal.

Russ
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Old 02-02-2006, 08:49 PM
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Quote:
Originally Posted by S10xGN
Here's the brake calculator, let me know if you any other information added to it. It suits my purpose as-is.

According to this, the stock setup (2 1/8" F & 1 5/8" R) rear brakes have 58% of the line pressure that the front do, assuming the M/C has the same bore front to rear. By going to a dual M/C setup and using a 7/8" bore front with a 1 1/8" bore rear, my "4 front caliper" system will achieve a ratio of 60% instead of 58%. This can easily be fine tuned with the balance bar on the pedal.

Russ
Good start...

...Not a big fan of things locked with a password. I always lock stuff without a password just to keep someone from messing it up on accident but allowing them to see (and modify) all the equations if they want.

If you send me a unlocked one I could help you with all the mistakes
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Old 02-02-2006, 09:30 PM
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Password = RAS

All the mistakes??? Okay, fire away! BTW, all this info was straight from NASA's educational program... One request though, add in a vacuum booster (with formulas) so we can figure what it adds to the mix.

Russ
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Old 02-02-2006, 10:30 PM
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I didn't have much luck opening that file either. I'll just show what I find and you can see if it jives.

The C4 was made from 1984-1996 and used different calipers and masters throughout the years. Since you mention 2 1/8, I have to guess you have the 84-87? front calipers. Sometime after that they switched to 2 piston calipers in the front. I dont think any were step-bore, but the early ones might be. I think they messed with the pedal ratios too. I have down somewhere they they began using like 4:1 and then 5:1 with power brakes when they went to the larger masters. I don't know how accurate this is. I think they must have had some problems with the smaller masters due to piston retracion at the calipers, but even the 7/8 is large (caliper/master ratio) compared to their earlier standards on the C3.

I have the 84 master listed as 13/16, and 85-? later ones at 7/8 bore for several years, though I believe they eventually went up to 15/16, and then a 1" bore. I haven't looked them all up recently.

Anyway, line pressure will be near the same front and rear with any tandem dual port master, so I think you meant caliper piston force when you spoke of the percentage. Or I read it wrong. That'll do.

So, with a 2 1/8" front vs 40.5cc rear piston I get the rears as ~55-56% of the fronts, maybe as much as 58% depending on how accurate I am.
So yeah, with 4 fronts you'll want your dual masters, one to be about 55-60% of the other, and tune with the balance bar.
Remember, balance bars are for racing, when all braking is expected to be at threshhold all the time. For the street you might also want a rear proportioning valve, so you dont have to tune the rears so far down with the bar for just moderate braking.

I think you may be limited on cylinder choice with the dual master setup. As far as I know, you can only get 3/4, 7/8, and 1" with most set ups. You can easily go smaller than stock tandem with a dual master and balance bar.

3/4~.44sq"
7/8~.6sq"
1~.7854sq"
1 1/8~.994sq"

.44 is about 56% of .7854, so I would get those.
The smaller master will also give you better line pressure, esp. with manual brakes. Don't worry about the stroke. A single master for the dual system will have enough stroke, and since each master only operates 2 brakes instead of 4, without the piggyback of a tandem master, you get half the pedal travel anyway (that is, if you were to use the same bore as the original tandem).

If I was to make a rule of thumb, I would say not to go below 60% bore size on the duals vs. the original tandem. The 1" is easily above that 7/8 or even 15/16 OE. The 3/4 is also. I have not tried going smaller than that, it is just a guess.

With using 4 fronts you are also changing the design of the rotors, and the front vs rear fade that the manufacturer had tested and presumably designed into the system... plus you are using it all on a different car, so you might not get it tuned perfect the first time, but I think you're on the right track.

But I'm just some yahoo on the internet spewin'. Let us know how it all works out.

P.S. I think stoptech.com has a new pdf file about setting up dual masters/balance bar.
/edit

Last edited by yesgo; 02-02-2006 at 10:38 PM.
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Old 02-03-2006, 10:12 PM
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OK, I did find one mistake and it's now corrected, along with several other changes (BTW, this is in Excel 2002 format). It now has provisions for the balance bar. Again, if you need to unlock it, the password is "RAS" (no quotes). Yesgo, I agree on Stoptech, they have some excellent reading!

Russ
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Old 02-04-2006, 12:06 PM
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I just had to download something so I could read it on OS X, but it looks OK to me.

I'm not sure I understand "force on m/c (PSI)", But it's not all that important to the balance. I measure force on m/c as lb/force which would be pedal force multiplied by pedal ratio... making force on m/cs 312.5lbs @ 50lb pedal load. That just basically changes all the numbers themselves, but not their relationship with one another.
156 appears to be half that, but I dont understand the PSI in that measurement.

You also will usually lose about 10% of your actual psi from calculated psi because the system is not 100% efficient. That percentage seems to go up with longer pedals, and down with larger masters, so I cant really predict it accurately.

Like I said, what you have looks fine to me for your purposes.
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Old 02-04-2006, 01:04 PM
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I'm having a hard time here, by my calc's, if I use a 7/8" bore M/C for the front circuit, I come up with a caliper piston travel of 0.085" per side. That's NOT MUCH!

(M/C bore / 2) ^ 2 * 3.14 * stroke = volume (front line circuit)
(0.875 / 2) ^ 2 * 3.14 * 1.00 = V
(0.438 ^ 2 * 3.14 * 1.00 = V
0.191 * 3.14 * 1.00 = V
0.601 = V (volume moved by front circuit M/C)

Caliper volume = V / 2
CV = 0.301

Caliper stroke = CV / ((Caliper bore / 2) ^ 2 * 3.14)
CS = CV / ((2.125 / 2) ^ 2) * 3.14)
CS = 0.301 / (1.063 ^ 2) * 3.14)
CS = 0.301 / (1.129 *3.14)
CS = 0.301 / 3.545
CS = 0.085"

I just do not see how this can be enough movement to safely apply the brakes and stop a car.

On top of that, the instructions that came with my Wilwood pedal show a recommended M/C bore of 3/4" (0.750) for their "GT" series calipers which are 4 piston jobs (2 @ 1.88" & 2 @ 1.75"). This amounts to a combined area of 10.36 sq in per caliper (mine are 3.54 sq in) which results in a stroke of only 0.029"! That's "4 sheets of paper" worth of travel from a full 1" stroke of the M/C, or a pedal movement of 6.25" using my pedal assembly's ratio of 6.25:1!

I just don't see it, guess I'll be calling Wilwood Monday...

Russ
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Old 02-04-2006, 07:06 PM
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On fixed opposed piston calipers you only calculate the pistons of one side. If the opposing pistons are different sizes, you would take the average.

Basically an opposed piston caliper is fixed, so each side has half the travel as a non opposed sliding caliper piston, multiple piston or not.
It's been a while since I did that stroke math. I'm a bit tired. I'll see if I come up with the same, maybe tomorrow....
But the OE on those cylinders was 7/8" diameter, and probably about 1.5" stroke, of which I hope less than half is used when operating optimally. On the tandem master you have to calculate both circuits and add together for total stroke, while on the dual master system you keep them separate.
You are definitely trying my knowledge. You may be correct that the 3/4 might be too small, but thus far, I think it should be fine.
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Old 02-04-2006, 09:09 PM
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Found one more mistake and added a few things, including comments for each block. Uploaded revision 3. Password "RAS" will still unlock the sheet. Isn't it so much nicer when you can just tab to the cells you need to enter data into? Don't know where Triaged went...ya still checkin' things out? Need more input...

Russ
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Old 02-05-2006, 12:45 AM
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Sorry for the late reply...I went climbing for the weekend

I see you fixed the force on each MC. That was one of the first things I noticed. Note that this is not the same with a tandem MC.

One other thing of note is the listing of force with the units of PSI. That is incorrect. The units should be lb. or lbf. (which stands for pounds force)

Also don't forget that from the factory the rotors were not the same front to rear. There will be a different effective radius working on each end of the car. On vette's there might also be a difference in tire radius front to rear to take into account....

...while you are taking all that into account it might also be nice to include the coefficient of friction of the pads so you can figure out the braking torque.

...while you are doing that you might as well provide the front/rear weight distribution and the height of the vehicle center of gravity to figure out how much traction will be at each end of the car.

...after doing that you could determine the max deceleration rate and the peddle force required for it.

...It might also be nice to add in the ability to plug in calipers with many (and even different diameter) bores.

Some things to help you in your quest...
-Look up SAE J-866a. That is where the coefficient of friction and pad edge codes are.
-Look up tire coefficient of frictions...about 0.8 works for rubber (i.e. normal car tires not drag slicks) on asphalt.


Thanks for providing the password...

This attachment shows how someone I know on another forum tackled the problem. Note that he "solves" for the MC bores...he kind'a came at the problem in through the back door.
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Old 02-05-2006, 07:02 AM
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D'oh. Thanks Triaged. Now I know how why it's 156 instead of 312.
2 masters. D'oh again
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Old 02-05-2006, 04:35 PM
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Triaged: Interesting calc's, there's a lot of work in that sheet! I ASSumed a few numbers and my ratio was just about what I had come up with earlier (0.875" front & 1.125" rear). Pedal force was quite a bit higher than expected, though.

I'm still puzzled by the calculated "stroke" of the calipers as figured by the volume of fluid moved through the master cylinders. Doesn't that movement seem inadequate?

Russ
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Old 02-05-2006, 04:47 PM
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Quote:
Originally Posted by S10xGN
I'm still puzzled by the calculated "stroke" of the calipers as figured by the volume of fluid moved through the master cylinders. Doesn't that movement seem inadequate?

Russ
You will find that the balance bar will pivot as to match the proper displacement to each end. That does of course have its limits. Ideally you should have no peddle travel...obviously this is not the case in real life. My S10 (with B-body brakes, MC and blazer rear disc) has only about 1" of travel when the brakes lock up.

With most aftermarket MC's you will need to shim the MC so the balance port (not sure if that is the proper name for it) is just about closed. If you do not shim the MC's you will end up with way too much travel because the first bit of travel will just pump the fluid into the res. Some MC's come shimmed (I know AP's do) but most don't.
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