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#1235279 - 05/25/12 12:28 AM Re: Disc brakes: All the math that you never wanted to know [Re: dusted72]
feets Offline
Senior Management

Registered: 01/20/03
Posts: 26830
Loc: Irving, TX
Quote:

Ok so after reading through this whole thread I am still a little confused as to what the actual equations are used for. I know how to get the answers but why? Am I using those to get the figures for the stock setup compared to proposed setup? I am just unsure. In particular the purpose of the area of a circle and effective radius of a rotor? I did the math for both stock and proposed. What does it mean if the stock are of a circle is 6.78inches and the propsed is 7.06inches? Same with the Effective radius? Stock is 5.07 and proposed is 6.23. I just don't know. Also are there any braking books out there that are a must read? I am in Kuwait right now and will be returning back to the states within the next 2 weeks so if there are any recommended readings out there please share.





It's not too difficult to overcome a tire's friction with the ground. A basic brake package will do that.
When you get by going to larger brakes is the heat dissipation for repetitive stops. You also gain better feel over the brake function. In another thread I described it as tightening a bolt with a long wrench or a short one. You can break the bolt with either wrench but it's easier to feel the torque with the longer one. It's easier to feel and modulate the brakes with larger rotors.

The reason for the math is to make sure your stuff will work together. Will the master cylinder you're looking at work well with the calipers you have in mind? Will the pedal effort increase or decrease? Can you stop the car with one foot on the brakes or will it take both feet and your butt in the air to haul the car down?

In my case I copied an existing brake system. All my stuff is from a 2005 Mercedes S55. I didn't have to do anything except find an appropriate master cylinder.

The Pirate 4x4 site is great. I've been looking at it off and on for several years. It's got a lot of stuff that will work in many vehicles not just 4x4s.

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#1235280 - 05/27/12 04:43 PM Re: Disc brakes: All the math that you never wanted to know [Re: feets]
hooziewhatsit Offline
master

Registered: 10/05/07
Posts: 2629
Loc: State of Jefferson
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

This means my ok-ish braking is due to an issue/mismatch somewhere, and not just because they're 'ok'

Troubleshooting will have to wait for another day though
_________________________
If you ever find yourself in a fair fight, your tactics suck.

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#1235281 - 05/27/12 07:39 PM Re: Disc brakes: All the math that you never wanted to know [Re: hooziewhatsit]
feets Offline
Senior Management

Registered: 01/20/03
Posts: 26830
Loc: Irving, TX
It sounds like you either have air in the system or the master cylinder is bad.

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#1235282 - 06/15/12 07:18 AM Re: Disc brakes: All the math that you never wanted to know [Re: feets]
1964Polara Offline
enthusiast

Registered: 03/28/05
Posts: 281
Loc: Germany
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

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#1235283 - 06/15/12 11:28 AM Re: Disc brakes: All the math that you never wanted to know [Re: 1964Polara]
Uhcoog1 Offline
super stock

Registered: 05/12/05
Posts: 718
Loc: Houston Tx
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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#1235284 - 09/19/12 09:09 AM Re: Disc brakes: All the math that you never wanted to know [Re: Uhcoog1]
1964Polara Offline
enthusiast

Registered: 03/28/05
Posts: 281
Loc: Germany
To follow up this Thread...BTW best ever Thread on this board?

I've ordered the 15/16 MC from Dr.Diff and had it installed yesterday on my SSBS Setup and it improved a lot! Everything appeared what I was expecting. Longer pedal travel, softer pedal and much more braking force! After some testing brakes the pads where already starting to smell...I will change them asap...maybe even slotted rotors later.

By theory I got now 870 MC Pressure and 6213 brake Torque.
And if I want to further upgrade I still can hang up the MC pushrod and increase pedal ratio to 7:1 which will give my by theory more than 1000PSI & 7200 braking torque..

Great Thread! Thanx!!!

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#1235285 - 09/19/12 10:30 AM Re: Disc brakes: All the math that you never wanted to know [Re: 1964Polara]
feets Offline
Senior Management

Registered: 01/20/03
Posts: 26830
Loc: Irving, TX
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.

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#1972325 - 12/18/15 06:40 PM Re: Disc brakes: All the math that you never wanted to know [Re: feets]
48Heap Offline
enthusiast

Registered: 02/14/05
Posts: 351
Loc: Spokane, WA
Originally Posted By feets
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.


Attachments
Hot Rod Jan 82 Article.jpg

Description: High-Caliper Braking, HR Jan 82


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15 Challenger R/T 6M STP
74 Duster 360 -> original 4 speed car

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#1990226 - 01/13/16 06:56 PM Re: Disc brakes: All the math that you never wanted to know [Re: 48Heap]
feets Offline
Senior Management

Registered: 01/20/03
Posts: 26830
Loc: Irving, TX
Originally Posted By 48Heap
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.

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#2217370 - 12/19/16 09:16 PM Re: Disc brakes: All the math that you never wanted to know [Re: feets]
goldduster318 Offline
pro stock

Registered: 01/19/03
Posts: 1401
Loc: Lake Orion, MI
I think this calculator might really be what we're looking for. Pretty interesting since you can figure out how much you need for a 1g stop, etc.

http://www.jakelatham.com/radical/info/brake_calculators.shtml
_________________________
'70 Duster 470hp 340/A-833/8 3/4 3.23 Sure-Grip

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#2222870 - 12/29/16 02:06 PM Re: Disc brakes: All the math that you never wanted to know [Re: goldduster318]
feets Offline
Senior Management

Registered: 01/20/03
Posts: 26830
Loc: Irving, TX
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.

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#2298674 - 05/04/17 12:16 AM Re: Disc brakes: All the math that you never wanted to know [Re: feets]
CBODY67 Offline
member

Registered: 03/20/17
Posts: 5
Loc: TX
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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#2318644 - 06/09/17 06:57 PM Re: Disc brakes: All the math that you never wanted to know [Re: feets]
feets Offline
Senior Management

Registered: 01/20/03
Posts: 26830
Loc: Irving, TX
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.

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#2319212 - 06/10/17 10:45 PM Re: Disc brakes: All the math that you never wanted to know [Re: feets]
Mattax Offline
super stock

Registered: 02/28/10
Posts: 974
Loc: Phila. Pa.
There's a thread discussing Brake Lining Edge Codes.

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