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good article on custom brakes, disc brake conversions Jump to page : 1 Now viewing page 1 [50 messages per page] | View previous thread :: View next thread |

Forward Look Technical Discussions -> Brakes, Wheels and Tires | Message format |

mikes2nd |
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Expert Posts: 4395 | They talkabout doing the math, will help you figure out the master cylinder you need.
https://www.hotrod.com/articles/avoiding-common-mistakes-how-to-make-your-street-rod-stop/
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jboymechanic |
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Expert Posts: 2007 Location: Muskego, WI | Definitely worth reading. | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | I disagree. Not worth reading. According to their reasoning, you get more pressure with smaller bores, so we should all go to .05" bores so we can get the maximum line pressure possible. They did briefly mention the volume limitation, but didn't bother to show you how to calculate the other constraints that prevents you from going too small. Also, who says 100 lbs is a good number for typical leg force? Where is the measured data? Men vs. women? How does this number change with a power brake booster in action? What is the justification for a friction coefficient of .5 and how much does it vary depending on pad materials etc? How did they calculate the effective radius that is essential for this final result? I do know how, but why go through the super easy part of the line pressure calculation in detail without giving the harder part of the calculations? Notice that their pad width is blank. That tells me that they just use a known or assumed value for the effective radius without actually calculating it out. No mention anywhere of how drum brakes changes these calculations. At the beginning they state that you need 900-1200 psi line pressure for disc brakes to be effective, and yet, they show that with a relatively small bore 15/16" master, you can only get around 700-800 psi. And their final calculation uses a pressure of only 600psi. One or more of their assumptions isn't true. This is why I stopped reading these magazines. I get tired of being treated like a kindergartner. You have to look elsewhere for real answers if you want to do this seriously. We are unable to change the pedal ratio very easily on our cars. Has anyone measured it? It would be pretty easy to measure if you have a pedal assembly available that is removed from the car. | ||

lozrox58 |
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Veteran Posts: 298 Location: Newcastle Australia | I think you need to read the article again. The 100lbs figure is a simple round number to make the calculations easier to understand. At no point does it mention that a human being (male or female) can only exert 100lbs force to a pedal. A typical person can exert their ideal body weight plus 20%. So a 200lb person can exert a pedal pressure of 240lbs. And yes your statement that a .05" bore will give maximum pressure is correct but then you master cylinder would need to be 28 feet long, so not very logical. | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | I fully realize what you are saying. My point is that reading the article again won't answer any of this if you don't know it already by some other means. The article is nearly worthless. | ||

jboymechanic |
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Expert Posts: 2007 Location: Muskego, WI | Nathan, The article is definitely very basic and aimed toward shade-tree mechanics, not engineers. I think the point is to at least bring some attention to an often overlooked subject and provide some brake system fundamentals. Same applies to hydraulic clutch systems. | ||

mikes2nd |
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Expert Posts: 4395 | well the best thing they mention is simply measure the pressure Anyone know what the stock pressure is for the drums? I think the problem is the amount of fluid you need to move and the amount of pressure you need to apply. They don't really mention about "how much" fluid you need to displace. | ||

57chizler |
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Expert Posts: 3482 Location: NorCal | If the MC diameter and pedal ratio are known you can compute the line pressure but the hard thing to determine is how much leg pressure is being applied to the pedal. | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | Since I made a mockery of their article, I feel some responsibility to illustrate the answers to the things that I brought up as much as possible. (Volume Calculation.jpg) Attachments ---------------- Volume Calculation.jpg (73KB - 100 downloads) | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | 3. Appropriate Friction coefficient There are more types of brake pad materials now than I knew existed. Here is what I have found in order from lame to high-tech: Asbestos Organic: ?? (no longer produced) Non-Asbestos Organic: ?? (fades, wears fast) Semi-Metallic: .28-.38 (higher rotor/drum wear) Low Metallic: .38-.5 (noisy) Carbon Metallic: ?? (better thermal performance) Ceramic: .33-.4 (clean) Carbon Fiber: ~ .6 (harsh transition, racing only) Most of these numbers come from this link: http://www.suscon.org/pdfs/bpp/pdfs/OEBrakePads.pdf The asbestos & non-asbestos pad friction coefficients likely start near a value of 0.2. I would say that a much more realistic number to use would be .3 for standard pads, and maybe .35 for semi-metallic & ceramic or .4 for low metallics. I can't see how you can justify using a value of .5 here since it is at the very top edge of what is possible. Edited by Powerflite 2019-07-24 9:53 PM | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | 4. Effective radius calculation. For torque calculations, you need to multiply the applied force by the radial distance from the center of the wheel. The force is pressure times area. But you can't just multiply the total pressure by the total area, and some random radius because the radius changes as you go out to different parts of the brake pad. So you have to look at very small sections of area on the brake pad. If the area is very small, the radius in that small area is nearly the same and you can multiply that area by it's radius & pressure to get it's contribution. Do this over the entire pad and add them up. This is the work of calculus and more specifically; using calculus to do an integration over the pad area. If the pad area looks like a simple annular sector (see picture), it is relatively easy to integrate over its area to find the resulting effective radius to use to calculate the torque. The result is: Torque = u * Pad Area * Pad Pressure * [Ra + w^2/(12*Ra)] Ra here is the average radius (R1+R2)/2, and w is the width R2-R1. u is the friction coefficient. So you must add this other term to the average radius to get your proper effective radius. If the pad isn't a nice annular sector, you can still do the integration, but the math can get ugly. We don't actually need to calculate the pad area and pad pressure because this is just the total force on the pad. That is given by the line pressure multiplied by the caliper piston area. So that's what we will use there instead. Keep in mind that the torque here is for one pad. You need to multiply it by 2 to get the total torque on one wheel, assuming that both pads are the same size & shape. Wheel Torque = 2 * u * Line Pressure * Caliper Piston Area * [Ra + w^2/(12Ra)] Edited by Powerflite 2019-07-25 1:16 AM (Annular Sector.png) Attachments ---------------- Annular Sector.png (32KB - 106 downloads) | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | 5. Drum brake torque The torque calculation for disc brakes is quite easy once you figure out the proper effective radius to use, but drum brakes are more complicated because the shoes amplify the torque action by its movement. This action has been studied in detail many years ago and the calculation can be found in Mechanical Engineer's Data Handbook and other similar references. Refer to the picture below. They use Tr for Torque from the right shoe. Assuming that the car is going forward, rename that one as the front shoe. Likewise the TL for left shoe torque should be renamed the rear shoe torque. K is the amplification factor, and the total torque on the wheel is given by T-front + T-rear. Of course the Force F here is the line pressure multiplied by the area of the wheel cylinder piston. You can measure that angle theta easily by measuring the arc length of the pad and dividing it by the drum radius. This will be the double angle in radians. Divide this by 2 to get theta to use to calculate the amplification factor K. Notice that the torque doesn't depend on the width of the shoe. A little counter-intuitive, but makes sense if you think about it. Edited by Powerflite 2019-07-24 11:59 PM (drum_brake_torque_equation.png) Attachments ---------------- drum_brake_torque_equation.png (164KB - 106 downloads) | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | 6. Available line pressure According to Jegs, the minimum line pressure you should see in your brakes is Disc brakes minimum pressure: 800 psi. Drum brakes minimum pressure: 400 psi. and according to Mark Williams Enterprises, you should design your system with at least 1200 psi available during an emergency situation. So for a disc brake system, it looks like there is good agreement on the 900-1200 psi requirement. A disc/drum car should use a true proportioning valve to increase the front pressure while decreasing the rear pressure. The typical adjustable ones just reduce the rear pressure without affecting the front pressure. The only factor that isn't taken into account yet is how much force can you or your wife really apply during an emergency situation. | ||

58coupe |
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Expert Posts: 1502 Location: Alaska | A couple of comments, I have the pedal assembly out of my 58 Plymouth removed and setting on a shelf and the pedal ratio for manual brakes on this car is about 7.65 to 1. Way back in junior high we had an apparatus to measure leg strength. You would put on a heavy leather belt and stand on it, bend your knees and connect the belt to the machine with chains. When you straightened your legs with all your might it would read out the numbers. I was surprised that many of us boys could pull about 600 lb with our legs. Long story just to show that 100 lb is not much push with our legs. | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | Wow, I didn't realize the manual pedal had that large of a ratio on it. I just measured my Dodge/Chrysler manual & power pedals and found: Pedal mounting bolt to pivot: 11" (same on both) Bottom of Pedal to Pivot: 12" (same on both) Manual Pivot to Push Rod: 1.375" Power Pivot to Push Rod: 2.625" I think measuring from the pedal bolt mount is a better number as that would give more of an average distance rather than from the bottom of the pedal. So that 11" value is what I am using. From this, I get: Manual ratio: 8:1 (A little more than what you got, but close) Power ratio: 4.2:1 (Much smaller than I expected!) The maximum force that can be had from the booster can be calculated based on it's dimensions. measuring inner dimensions would be more accurate, but I don't want to take it apart so this will have to do. See picture for the dimensions that I measured. The 1.5" center is very approximate and should be re-measured once you take it apart. The area is one full 6" circle added to a 2.75" x 6" rectangle, with a 1.5" circle taken out. Assuming that the motor can make 16.5 in Hg of vacuum (which is 8.1 psi) I get: Booster Area: 43 in^2 (This is the equivalent of a 7.5 diameter round booster - pretty small for a large car) Max Booster Force: 348 lbs This should help for doing line pressure calculations. (Booster Dims From Outside.jpg) Attachments ---------------- Booster Dims From Outside.jpg (30KB - 106 downloads) | ||

Shep |
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Expert Posts: 3204 Location: Chestertown, NY ( near Lake George) | I've done a couple of ft disc brake conversions in my shop. The owners insisted they didn't need power brakes, I said we couldn't get better than about 5.5:1 ratio, they couldn't be convinced til they complained about the required pedal pressure to stop. Told em so! Edited by Shep 2019-07-27 4:21 PM | ||

57chizler |
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Expert Posts: 3482 Location: NorCal | The typical FL MC has a 1 1/8" bore so the math says, with an 8:1 pedal ratio and 100 lbs on the pedal, you should have around 800 psi line pressure. I'm still wondering how easy it is to apply 100 lbs to the pedal, especially for an old guy. All of my scales are too bulky to fit on a brake pedal. Edited by 57chizler 2019-07-27 4:42 PM | ||

58coupe |
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Expert Posts: 1502 Location: Alaska | Nathan, I was measuring to about the center of the pedal pad but if you measure to the bottom of the pad it would be closer to 8 to 1. 57 chizler, think about this, every time you are walking, you are pushing your total weight on each leg against gravity. | ||

lozrox58 |
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Veteran Posts: 298 Location: Newcastle Australia | I'm still wondering how easy it is to apply 100 lbs to the pedal, especially for an old guy. All of my scales are too bulky to fit on a brake pedal. Easy enough to find out with a common set of bathroom scales. Sit down in a hallway with the scales against the wall and you back against the opposite wall. Push on the scales to see what you can achieve. I achieved 71kg (156lbs) without too much effort. An emergency brake would be more. | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | One bit of info that I left off of the booster calculation is the booster actuation lever ratio. The booster force is applied 6.5" above the pivot (on a '57-'59 power pedal) which, divided by the push rod distance, gives a ratio of 2.476. So the alleged 348 lbs available from the booster can apply a force of 862 lbs at the master cylinder if needed. That's a nice addition to the total force requirement. Assuming an adult woman can apply at least 150 lbs of force to the pedal during an emergency situation, that gives a total of: 348*2.476 + 150*4.2 = 1492 lbs, and results in a line pressure of 1501 psi with a 1 1/8" master. That should be plenty sufficient with some room for error in the calculation on a power brake system. The equivalent manual brake setup would provide: 150*8 = 1200 lbs, and results in a line pressure of 1207 psi with a 1 1/8" master. This confirms the general feel that a manual system is harder to stop, and provides less total pressure available than a power system. However, interestingly, starting at a foot pedal force of 227 lbs, the manual system provides the same line pressure as the power system. If you are able to apply more force than that, the manual system will out-perform the power system due to the long 8:1 pedal ratio from your foot, compared to the 4.2:1 ratio on the power setup. But that's assuming that the brake pads & shoes react well to the resulting high line pressure that that force would provide. Edited by Powerflite 2020-12-06 9:26 PM | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | Here is my spreadsheet that I use to calculate these things. To use it, just fill out the "Input Parameters" and "More Input Parameters" sections at the top with your own numbers, and read the results in the yellow shaded boxes. There are separate pages for Drum/Drum, Disc/Drum, and Disc/Disc. I hope it is useful to someone. I have it filled out for my '57 Chrysler with 12" brakes. The section on deceleration is lacking. I would like to be able to calculate 60-0 times, but it needs more work & understanding to do that; in terms of how much braking force can realistically be used due to tire traction and how much the car's weight will lunge forward etc. Detailed info like center of mass location & tire performance would be needed for that. The numbers I currently have in that section are just guesses. Attachments ---------------- Brake System Calculation.xls (237KB - 9 downloads) | ||

mikes2nd |
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Expert Posts: 4395 | I should webify that | ||

56D500boy |
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Expert 5K+ Posts: 6894 Location: Lower Mainland BC | . Impressive Nathan. Not sure how a total pedal travel of 7.88 odd inches is even possible. Manual transmission? I like your master cylinder stroke of 1.55 inches. REFERENCE: http://www.forwardlook.net/forums/forums/thread-view.asp?tid=75155&... | ||

dels56 |
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Veteran Posts: 114 | I just read all the above and darn near fell asleep doing so. In your calcs you only forgot one thing. The compression of the brake fluid. And how that adds to peddle travel. DOT 3 won't compress as much as DOT 5. This darn Covid, I have not much else to do! Del S | ||

Powerflite |
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Expert 5K+ Posts: 7128 Location: So. California | Dave, the total pedal travel is at the pedal mounting bolt at the bottom of the arm. The total travel at the master cylinder is the line above that. Well, Del, here is your new insomnia aid! You're welcome! I don't use Dot 5, and there is no compression in Dot 4 that I use. If you use Dot 5, you will need to calculate the total volume in your system, including in the lines, to get the compression effect. Edit: I updated the file with the total pedal travel for the power brakes as well. I had only calculated it for the manual brakes before so I included the power brake calculation on this file as well. Edited by Powerflite 2020-12-31 1:18 AM | ||

dels56 |
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Veteran Posts: 114 | I just had to add a bit for absolutely no reason. You did a lot of work that will probably help someone along the way. Yes there is compressibility in every liquid and there is a difference between the two. The glycol base is stiffer than the synthetic but probably wouldn't change your calc by 0.0001% Happy New Year. Del | ||

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