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Re: Disc brakes: All the math that you never wanted to know
[Re: hooziewhatsit]
#1235281
05/27/12 08:39 PM
05/27/12 08:39 PM
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Joined: Jan 2003
Posts: 28,116
Irving, TX
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Irving, TX
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It sounds like you either have air in the system or the master cylinder is bad.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: Disc brakes: All the math that you never wanted to know
[Re: 1964Polara]
#1235283
06/15/12 12:28 PM
06/15/12 12:28 PM
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Joined: May 2005
Posts: 723
Houston Tx
Uhcoog1
super stock
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super stock
Joined: May 2005
Posts: 723
Houston Tx
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Quote:
GREAT THREAD!!!!
I have 64 B-Body with SSBC-A156 Disc Brakes and poor braking performance with a hard pedal feel.
Specs:
Rotor Diameter: 11,25
4 Pistons each 43 (mm I guess) = 1,69' x 2 = 2,39 for the calculator
Master Cylinder bore: 1 1/32 = 1,03'
Pedal Ratio not measured, taken 7' from the file
With the specs I have calculator shows torque 3100 and pad movement 0,0047. When I change to a 15/16 MC torque raises to 3824 but pad movement just 0,0039.
If I change the brake pedal move from 5 to 6 inches due to other stroke pad movement goes up to 0,0047.
So I might improve braking torque about 20% with a 15/16 Master Cylinder. I was thinking of buying a 1975/1976 A-Body master but they are all power units. Any other 15/16 Master recommendations?
Many thanx in advance
Dr Diff (Cass) is the only one I am aware of that sells a 15/16 manual master with the groove to keep the pushrod in place. He machines the groove in place himself. I just bought one, and it's sitting waiting to be installed.
-'02 Dodge Viper Ex-World Challenge racecar
-'73 Duster, 6.1 based 392 hilborn hemi, tko600, full floater rear 9", Hellwig custom bars, viper brakes, built for road course
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Re: Disc brakes: All the math that you never wanted to know
[Re: 1964Polara]
#1235285
09/19/12 11:30 AM
09/19/12 11:30 AM
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Joined: Jan 2003
Posts: 28,116
Irving, TX
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I'm glad you found it useful and were able to improve your brakes.
Remember the braking torque is not a true value. I added it to show what kind of difference in braking power you would have when changing parts.
Brake pads will not have a perfect friction value of 1 like the math shows. In the real world you'll be between .35 and .45 and the brake torque will drop to that percentage.
Still, it allows you to compare the changes in hydraulic forces assuming no change in brake pad compounds.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: Disc brakes: All the math that you never wanted to know
[Re: feets]
#1972325
12/18/15 08:40 PM
12/18/15 08:40 PM
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Joined: Feb 2005
Posts: 351
Spokane, WA
48Heap
enthusiast
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enthusiast
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Posts: 351
Spokane, WA
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What this calculation does is average the diameter of the rotor and the diameter of the rotor minus the piston diameter. Doing that finds you the center of the piston. That is going to be pretty close to the center of the pad. I know this is an old thread, but I kind of tripped over it just now and just have to ask a question. The calculation of the effective radius of the rotor completely confused me. So I read through the thread and found this post explaining what the calculation is supposed to do. My question is, if the calculation is supposed to average the diameter of the rotor and the diameter of the rotor minus the piston diameter, shouldn't the piston diameter be doubled? A larger diameter minus a smaller diameter doesn't return the inner diameter, it returns the diameter of the circle that would pass through the smaller circle. On the other hand, a larger diameter minus a smaller diameter divided by 2 would return the number I believe you are looking for. Another way to calculate it is half the diameter of the rotor, minus half the diameter of the piston. For example, a 11.75" rotor has a radius of 5.875", and if you laid a 2.75" piston on the edge of the rotor and came back in to the center of the piston 1.375", the radius from the center of the rotor to the center of the piston would be 4.5". The calculation as it stands returns 5.1875" which is only .6875" less than the radius of the rotor. Am I missing something? Is that not what you are shooting for, the distance from the center of the caliper piston to the center of the rotor? Not trying to start a fight, or nit pick anything, just trying to understand the math.  High-Caliper Braking, HR Jan 82
15 Chrysler 200S 3.6
15 Challenger R/T 6M STP
74 Duster 360 -> original 4 speed car
a.k.a. DionR
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Re: Disc brakes: All the math that you never wanted to know
[Re: 48Heap]
#1990226
01/13/16 08:56 PM
01/13/16 08:56 PM
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Joined: Jan 2003
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Irving, TX
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shouldn't the piston diameter be doubled? Yeah, you're right. I guess I blew It on that one. It's easiest to think radius of the rotor minus radius of the piston. Looking back I guess I was hung up on the diameter since that had been used in other calculations. Kinda funny that it only took a few years for someone to catch that.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: Disc brakes: All the math that you never wanted to know
[Re: goldduster318]
#2222870
12/29/16 04:06 PM
12/29/16 04:06 PM
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Joined: Jan 2003
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Irving, TX
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That calculator is kinda neat but it's making the same assumptions mine does and then some.
Keep in mind that mine was designed to give a quick and dirty idea of the size master cylinder you need for specific calipers and rotors. The rest of it was done just for grins.
There are some serious calculators out there that go much deeper than mine or the one above but I seriously doubt many people here will have the information required to make them work correctly. I know I don't have that kind of info on any of my setups.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: Disc brakes: All the math that you never wanted to know
[Re: feets]
#2298674
05/04/17 01:16 AM
05/04/17 01:16 AM
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Joined: Mar 2017
Posts: 5
TX
CBODY67
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TX
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When I started reading "the math", I was certain the pad/rotor friction coefficient would be "a value" as "a constant" of sorts. Which is fine for the calculations used.
On the edge of brake pad material is that pad's "birth certificate". Listing a code for the manufacturer, batch/date manufactured and/or other ID, AND the pad's coefficients when "cold" and "hot". Letters from "C" to "F", I believe. The letter code is two letters, first being "cold" and the second being "hot".
Question might be just how much difference there might be in these coefficient levels?
When vehicles started to be downsized in the 1980s, brake components got smaller, too, for the lighter cars. Pad dimensions were also decreased as metallic pad compositions became more common . . . less pad, more stopping power (brake torque). On many current "supercars", the rotors have grown to larger diameters and brake pads seem to have (again) become smaller in surface area.
Another "given" will be a constant surface finish of the rotor itself. A shiny smooth used surface might have less "bite" than a freshly cut and patterned or new (with factory in-broken-in contact surface) rotor.
The key curiosity is the difference between the pad co-efficient letters?
Thanks for all of the great information!
CBODY67
66-CL42, 67-CE23, 70-DH43 Each under about 25K built. Numbers decrease with options and colors! How'd I manage that?
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Re: Disc brakes: All the math that you never wanted to know
[Re: feets]
#2318644
06/09/17 07:57 PM
06/09/17 07:57 PM
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Joined: Jan 2003
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Irving, TX
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I haven't got a clue about the pad/rotor friction. There are too many variables to even play that game. That's why I used the magic value of 1. It shows the difference made when you change the other components.
That's what the thread was about. I concentrated on hydraulics and dimensions. You can use whatever pad and rotor material you like and the performance change will vary based upon the effectiveness of the mechanical system.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: Disc brakes: All the math that you never wanted to know
[Re: Mattax]
#3114634
01/19/23 07:28 PM
01/19/23 07:28 PM
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Joined: Aug 2003
Posts: 43,970
Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,970
Bend,OR USA
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Last edited by Cab_Burge; 01/19/23 07:30 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Just hooked up my pressure gauge. Engine off I'm getting ~400psi at the front calipers. Engine on, I'm getting 700psi, and if I pump it half a dozen times, it'll go up to 900psi (with the pedal bottomed out (?)). This is actually very close to what the booster excel file predicted 
