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Does anyone happen to know the stroke of a stock 1 1/8th inch bore AD master, or for that matter the wheel cylinder distance between pistons when compressed? Im guessing no more than 3/4 of an inch for the wheel cylinders, but the master i have no idea. I'm putting together a "brake optimizer" where i can adjust parameters and was hoping to benchmark it against a stock setup. Any help would be great!!

OKAY after some "engineering estimations" I've come up with the rears will lock up on dry asphalt at 120-ish pounds of pedal effort at about 2.75-3 inches of pedal travel (from top most position, your mileage will vary depending on shoe gap, brake lines, ect...). This is assuming a stock drum drum system. To the well calibrated right foot of you guys driving stock trucks does this seem "in the ball park"? These calcs were done assuming no proportioning valve.

Unloaded, or at max gross weight? Rear brakes on big rigs are designed to stop a loaded truck and the driver is supposed to be aware of the tendency for the rear brakes of an unloaded truck to lock up and drive accordingly. The proportioning valve needs to be installed between the driver's ears.
Jerry

I should have been more clear, assumed weight was 3400 lbs with a weight distribution of .65 front axle .35 rear. I also assumed no force transfer while braking since MOI information on AD trucks is a little fuzzy . Stopping distance will allude me unless i can model between the "edge codes" provided by SAE. I'm just writing a program for pedal travel and service brake application force that will put me in the ball park with correct pedal ratios and caliper volumes, accounting for compressibility of the brake fluid. And before someone says brake fluid is incompressible i agree the glycol-ester is but the up to 7% dissolved air (by volume) isn't. (SAE J1705, Appendix A, A.2.2.8)

What are you trying to do exactly, and why?

I'm trying to put together a disk brake system using OEM components, or aftermarket if i have to, that will work well and compliment the stock-ish nature of my truck. If i can work out the kinks in my assumptions I'll be able to use this script to size brake components and relationships on other projects as well. Assuming vehicle dynamics on the computer is a lot cheaper than throwing parts at a project i might not be happy with. At least if I'm not happy with it after i purchase parts i can confidently blame myself for the assumptions i made being wrong.

Even a rookie wrench jockey understands that a huge amount of stopping force is transferred to the front brakes. If you don't factor that into your algorithm, you've just got another absolutely worthless GIGO situation. Good luck!
Jerry

I completely agree! Fortunately at this point its only purpose is to provide amount of "pedal squish" and maintain acceptable levels of service brake force while messing with the master, and new front calipers. I still need the same amount of squeeze to make friction! I was asking for a gut check response from people on the board with the 120 lbs of service brake application to lock to the rears on asphalt. Its a "butt dyno" measurable item to baseline what was tolerable pedal effort for an AD truck in favorable conditions. I don't have a drivable bolt so i can't check it myself. Weight transfer to axles, and or left right transfer in cornering is outside of the scope of what I'm trying to do at this point. This is more of a comparative result than an absolute value if that makes sense.

So it's basically an exercise in futility that you can delude yourself into believing is worthwhile. Are you also applying for a government grant to finance your time and effort?
LOL!
Jerry

I wouldn't say it isn't worthwhile, it currently performs its original purpose of comparing the non linear pedal travel to a given range of service brake application pressures and the hydraulic advantage present in the OE system. I can easily see system pressure as a function of application distance and effort. Any improvements in that system leading to either a greater mechanical advantage or efficiency is an improvement without a "empirical" answer saying the braking distance improved X to Y. That is what i mean by a comparative improvement rather than it being driven by a predetermined goal or result.

Its taken me about 3-4 hours to write it and document the SAE specs and equipment dimensions. There is a lot of potential in this script to be used as a vehicle dynamics improvement tool, but key pieces of information most notably all 3 axis of CG are missing. They are obtainable, but not with the resources i currently have. Just because you may not understand the tool, or in this case script, being used doesn't make it worthless. Just trying to add a little math to this hobby of old trucks, rather than an ad hoc method of throwing parts at things until a desired result is found.

I've heard of people overthinking a lot of things but this sure tops them all. Glad you are exercising your brain cells, it will keep your brain young. Hang in there Jerry, make him earn it.
Meanwhile I have a headache!

Lets go about this a different way. Find documents on a car or truck of equal weight and back fill your program with that information. If you have hard numbers from the manufacturer, won't the pressure still apply to your truck? It will be easy to measure the pedal ratio and from there figure the travel amount, that should be the only difference. Rear lock up is going to be different on every truck due to tires, road, adjustment, shoe grade, drum surface, etc..... so forget about figuring that in. In worst case, you have to install adjustable valve to limit rear application.

I suspect the pressure you are after can be obtained several ways, either from master cylinder bore size, caliper bore size, or wheel cylinder bore size. I suggest looking a kit already made up and sourcing parts from it.

When looking for disk for my truck, I found a GM car of equal weight and length that used the same type of disk. The rears are standard drums ( close enough ) to original car I pulled parts from. The proportioning valve worked just as it should have and pedal effort is what I expected for manual disks. No surprises at all since I choose components from the correct car. I did install an adjustable proportioning valve to limit rear brakes due to lock up, but then that was also expected, trucks always lock the rear brakes.

Joe

Let me offer an alternative process. Find a mid-1990's Dodge Dakota pickup and look at the the brake system- - - - -those trucks are almost clones of the 1950's Chevy trucks in size and weight, but they use a well-designed disc/drum brake system that's 3 decades newer. it's already been engineered and road tested. It works. Duplicating the system, or just scavenging the parts and adapting them to the GM platform will save you a lot of time and effort, and the results will be useful instead of being some engineer's pipe dream.

My brother has several advanced engineering degrees including a doctorate in plasma physics. He does a lot of consulting work for the automotive industry, including "computer modeling" parts. The part is designed on a computer, then run to failure by the same computer without ever being manufactured. His projections are almost 100% accurate.

He charges his time out at the rate of $1,000.00 a day.

Jerry

Guys!! The script currently works for what its intended to do!! It compares pedal effort, line pressure, and pedal travel. Like i said the lock up stuff was a quick extension of its intended purpose that could be used as a quick gut check.

Jerry, it might be that the dodge calipers and master work out to be correct. I'd like to not add a brake booster so ratio and pedal travel potentially will change for my desired max pedal effort and this script will tell me that. Next step is pulling manufacture information for bores, and fluid capacities and seeing where I'm at. I'm not here to argue or beat my chest. I'm also not here to be told that this is a waste of time, this has been quick, easy, and painless so far. At minimum i can build a mouse trap as good as the engineers 70 years ago. I appreciate the dodge suggestion and will investigate the one first, my second thought is a 4Runner so i could steal its disks to boot.

A booster just makes your leg stronger. I got over being macho on a brake pedal 50-something years ago with a 47 Ford business coupe. Virtually everything I've owned since then has had some sort of power-assisted brakes, whenever possible.
Jerry

How do you account for the friction differences in different brake lining materials in your calculations? Just curious, not knocking your project.