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Re: Disc brakes: All the math that you never wanted to know
[Re: Supercuda]
#1235262
05/19/12 11:49 PM
05/19/12 11:49 PM
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Joined: Feb 2010
Posts: 18,493
Granite Bay CA
Kern Dog
Striving for excellence
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Striving for excellence
Joined: Feb 2010
Posts: 18,493
Granite Bay CA
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Quote:
When you decrease the pedal ratio, you increase the stroke in the M/C. Power brakes systems use less pedal ratio, therefor more stroke at the MC for a given pedal travel.
Same bore M/C going from manual to power sends out more volume for a given pedal travel.
That's what is different.
I thought that the travel of the piston in the master cylinders was about the same. I might be operating on a "fuzzy memory" here, but when I bench bled the master cylinder, I bottomed out the piston. When I put the master cylinder in the car, it appeared that the with the brake pushrod in the M/C, it bottomed out only a fraction deeper than the brake pedals travel. I'm certain that you don't want the pedal to stop 2 inches off of the floor because the M/C ran out of stroke.
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Re: Disc brakes: All the math that you never wanted to know
[Re: Kern Dog]
#1235263
05/19/12 11:57 PM
05/19/12 11:57 PM
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Joined: Jan 2003
Posts: 28,118
Irving, TX
feets
OP
Senior Management
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OP
Senior Management
Joined: Jan 2003
Posts: 28,118
Irving, TX
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Some disc/drum masters don't have enough stroke for rear disc brakes. If the pedal it's a rock solid stopping point deep in the stroke you may want to check that out.
I know it's theoretically possible but I haven't experienced it myself.
The hot rod has a D150 manual disc/drum 1-1/8" unit pushing fluid to the AMG disc brakes.
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]
#1235264
05/20/12 12:37 AM
05/20/12 12:37 AM
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Joined: Jan 2003
Posts: 1,692
Seattle WA
RichV
top fuel
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top fuel
Joined: Jan 2003
Posts: 1,692
Seattle WA
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Quote:
EFFECTIVE RADIUS OF A ROTOR
To work with the actual braking ability you really need to find what is called the effective radius of your brake rotor. To do that you need to know the caliper piston diameter (only the biggest one for calipers with more than one piston) and the diameter of your rotor.
Effective radius is the total diameter of the brake rotor plus that same diameter AFTER you subtract the diameter of the piston. Then, divide by 4.
Use a "big" Mopar 11.75" rotor and the 2.75" caliper we used above.
11.75" minus the 2.75" piston is 9 inches.
So, 11.75" plus 9" is 20.75".
Divide by 4 and you get a 5.18" effective radius.
Effective radius of the rotor is unrelated to the piston diameter. It is the center of contact for for the brake pad to the rotor. Typical undergrad engineering calculation put it outboard of the center, but the taper wear of pads typically moves back inboard. So the center of contact is good compromise.
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Re: Disc brakes: All the math that you never wanted to know
[Re: RichV]
#1235265
05/20/12 07:27 AM
05/20/12 07:27 AM
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Joined: Jan 2003
Posts: 28,118
Irving, TX
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OP
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Irving, TX
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Quote:
Effective radius of the rotor is unrelated to the piston diameter. It is the center of contact for for the brake pad to the rotor. Typical undergrad engineering calculation put it outboard of the center, but the taper wear of pads typically moves back inboard. So the center of contact is good compromise.
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.
Now, I'm NOT going to measure every stinkin pad in the world and spoon feed you the information.
If you want to use the calculation, go for it.
If you want to gather all the info from each individual setup and present it to the person requesting it in a 24 hour 7 day format then be my guest.
Again, put down the engineering texts and reread the part where I said these were good enough to get the average guy in the ball park without requiring a high level of math or engineering degree.
Now, are you frothing at the mouth or trying to accomplish something else?
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]
#1235266
05/20/12 09:17 AM
05/20/12 09:17 AM
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Joined: Sep 2007
Posts: 14,889
up yours
Supercuda
About to go away
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About to go away
Joined: Sep 2007
Posts: 14,889
up yours
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You can tell the A body guys. The A body power setup uses an offset crank assembly to lessen the pedal ratio You can see it in this link http://www.ebay.com/itm/1967-MOPAR-MOPAR...R-/290681535604
They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
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Re: Disc brakes: All the math that you never wanted to know
[Re: feets]
#1235268
05/20/12 09:39 AM
05/20/12 09:39 AM
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Joined: Sep 2007
Posts: 14,889
up yours
Supercuda
About to go away
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About to go away
Joined: Sep 2007
Posts: 14,889
up yours
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Sorry, I do not own any power brake A bodies, I think my last one was in 1984. Since then they have all been manual brake setups. I am not a fan of power brakes in A bodies.
They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
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Re: Disc brakes: All the math that you never wanted to know
[Re: dusted72]
#1235272
05/22/12 10:55 PM
05/22/12 10:55 PM
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Joined: Jan 2003
Posts: 28,118
Irving, TX
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OP
Senior Management
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Posts: 28,118
Irving, TX
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Thanks, but remember this stuff will only get out in the ballpark. There are lots of other things that go into real brake system equations. Some of it has been discussed in this thread.
Feel free to use this as a basic guide. If you're saving your pennies to build something when you rotate home there's enough time to run lots of numbers.
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]
#1235273
05/24/12 12:15 PM
05/24/12 12:15 PM
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Joined: Oct 2007
Posts: 3,953
Oregon
hooziewhatsit
master
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master
Joined: Oct 2007
Posts: 3,953
Oregon
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Here's a file that compares manual to power brakes. It looks like it takes the surface area of the booster and current vacuum to calculate the additional pressure provided. Then it has two lines for each manual and power so you can compare different sized MCs. I think the table on the right is different foot pressures. For power it then adds the pressure provided by the booster (I think it's looking at the 9"). Although I think it needs to take into account pedal ratio somewhere in there  As always, use this at your own risk  edit: cleaned up file available a few posts down 
Last edited by hooziewhatsit; 05/25/12 12:10 AM.
If you ever find yourself in a fair fight, your tactics suck.
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Re: Disc brakes: All the math that you never wanted to know
[Re: dusted72]
#1235277
05/24/12 08:55 PM
05/24/12 08:55 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:
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.
Go here, read it all, and use the excel:
http://www.pirate4x4.com/tech/billavista/Brakes/
The purpose of the math is to determine how much braking force you need and what balance you need. The usable brake torque is determined how much grip your tires have with the road surface (coefficient of friction between tires and surface). You can calculate the force needed by the caliper, and the master cylinder needed to produce that force. The diameter of the rotor determines the leverage.
The brake bias needed changes through the 'curve'. Assuming a 54/46 (f/r) weight bias when static in my duster, a .1 G braking even will require very close to a 55/45 ideal bias. It will change, ending up at a 75/25 ideal bias at a 1.3 G braking event. You need to calculate weight transfer at different G braking events.
Does that help?
-'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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