The science of mutant brakes...Im lost

Ok, I dont even know where to start in this equation. If it matters at all the car was originally 4 wheel drum with a dual circut master cyl. GVW approx 2900 lbs

Front end is being converted to a mutant disk brake setup but I guess the important part of the equation is it uses S10 calipers. Manufacturer of the conversion designed the kit to work with the stock MC as long as the check valve is removed and the rear drums are in place.

Now I need to swap rears and for the same money I can get a disk brake rear (from a ford explorer if it means anything)

Is there any logical way to figure out what the heck I need for a master cylinder? Or just try what I got and see how it works. Maybe add a proportioning valve as needed?

I'd start here: http://www.amazon.com/High-Performance-Brake-Systems-Selection-Installation/dp/1932494324/ref=sr_1_1?ie=UTF8&s=books&qid=1215654582&sr=8-1

David S. Wallens wrote: I'd start here: http://www.amazon.com/High-Performance-Brake-Systems-Selection-Installation/dp/1932494324/ref=sr_1_1?ie=UTF8&s=books&qid=1215654582&sr=8-1

A book? The only contact with print I have is GRM....lol....seriously though, thank you, I'll check that out.

Dave Coleman did a great writeup on choosing a Mc size years back. You might find it somwhere on the Sport Compact Car website.

JThw8 wrote:

David S. Wallens wrote: I'd start here: http://www.amazon.com/High-Performance-Brake-Systems-Selection-Installation/dp/1932494324/ref=sr_1_1?ie=UTF8&s=books&qid=1215654582&sr=8-1

A book? The only contact with print I have is GRM....lol....seriously though, thank you, I'll check that out.

Well, it's written by James Walker, a good friends of ours. Honestly, it sounds like you need to do some math and research to get your system in order. This book will have the formulas to get you heading in the right direction regarding prop valve, master cylinder sizing, and the rest.

I've read Walker's book, and it's a good one. You'll find a lot of his material in the White Papers section of the Stoptech website. Some of it reads much like the odd GRM article for some reason :)

Although I think it is going to boil down to sticking something on there, then adapting from that point. Assume you'll have the same size master front and rear, because dual masters are a pain to retrofit and stepped masters are rare. Do you have a choice of piston or rotor sizes with either your front or rear setup? If not, then you're going to have to hope that you have more braking torque at the rear than the front so you can install an adjustable prop valve and dial it in that way.

"I'd buy the book too!"

Seriously, if you are converting from drums to discs, there are a few general things to look out for:

1. Disc brakes usually have much larger fluid volumes than drum brakes (the pistons are bigger, after all). Best case this usually means the pedal will travel farther, but in an extreme case the pedal will bottom out before building adequate pressure. For this reason, a larger bore master cylinder may be required. This holds especially true if you are swapping out the front brakes and not the rear brakes. If you are swapping out both ends, it just gets uglier.

2. Stay away from the stepped-bore master cylinder. They are usually more complicated than they are worth.

3. If you are installing front disc brakes, chances are the overall system will be more front biased. In other words, you will be able to use more rear brake. Note this isn't always the case, especially if you are installing different rear discs at the same time, so be prepared to install an adjustable prop valve and fiddle with it in a nice, big, empty parking lot until you get your brake balance figured out.

There are about 15 other things to consider, but most of them are in the book, so read up! The first priority, though, is to make sure you have enough master cylinder volume that you can lock up the brakes with a reasonable amount of leg effort. The next priority is to dial in the brake balance. For a project like this, be prepared to test the system someplace remote before heading into turn 1 at warp speed.

James Walker, Jr.

David S. Wallens wrote: Well, it's written by James Walker, a good friends of ours. Honestly, it sounds like you need to do some math and research to get your system in order. This book will have the formulas to get you heading in the right direction regarding prop valve, master cylinder sizing, and the rest.

Was merely a joke, I ordered the book about 5 minutes after posting. Looks like it will be well worth the read.

On the front disk conversion it has been designed to work with the existing dual circuit master from the drums. This kit is on quite a few cars so I trust that its right. Its the addition of the rear disks (or even different drums since Im doing an axle swap) that concerns me. It seems most of the guys that have done swaps on these cars just kind of "play it by ear"....I like a little more reassurance especially with something like brakes.

Mr Walker, thank you for all the great advice here as well. To be sure I will test it remotely and safely. NJ inspection even has some really cool testing gizmos to figure out the braking force done by each wheel and the balance and so forth. This car doesnt require inspection but I may see if I can get them to run it across the test bed a few times so I can see where Im at.

yes get the book but also read this article!

http://www.hotrodheaven.com/tech/brakes/brakes.htm

best free brake article i've ever found

44

Um...I have a policy that I don't talk down anybody else's work, but I have some concerns with the technical accuracy of the material in the link above. Although there are some salient pieces of information provided, some statements are just wrong. For example:

"The two different size pistons allow the caliper to work at different levels without having as all the pistons operating at the same time. When you only need a little braking the 4 smaller pistons do the work, when you need to really stop the bigger pistons are activated. Great solution for street cars."

This is completely incorrect. All of the pistons work all of the time regardless of the number of pistons, floating, fixed, or otherwise.

"The effective radius of the 11” with a 2” pad would be 9” and the 13” rotor with the same pad width would be 11”."

This is absolutely not the method to calculate the effective radius of a rotor. It has nothing to do with pad height, never mind the fact that the radius of an object 13" in diameter cannot be any more than 6.5" in the first place.

"Before disc brakes all master cylinders had single reservoir. This was because you wanted to apply equal pressure to all 4 drum brakes. "

Not at all true. The single/tandem design is purely a function of government requirements for back-up operation in case of a single circuit failure. The effective displacement (and pressure output) of the dual master cylinder is identical to the single master cylinder assuming equal piston sizes.

"The design allows the application of two master cylinders being applied at the same, thereby doubling the volume output. Because of this high pressure output you will not need a vacuum booster."

In no way does switching to a dual master cylinder arrangement increase the pressure in the hydraulic circuit(s), nor does it reduce the need for a vacuum booster (which is a pressure issue anyway, not a volume issue as implied).

There are several more items to discuss here, but for fear of looking like a jerk I'll stop. Like any "free" information (this post included), you typically get what you pay for. Reader beware.

James Walker, Jr.

teamscr wrote: "Before disc brakes all master cylinders had single reservoir. This was because you wanted to apply equal pressure to all 4 drum brakes. "

Whew, yeah Im not up on all of this yet and even I know that one is wrong from personal experience.

My current project specimin is 4 wheel drum from the factory and carries a dual circuit. Oddly enough when equipped with the optional factory disk brakes it came with a single circuit master cylinder.

Mr Walker was kind enough to alert me to this thread. I haven't been online since 6PM yesterday so didn't know this thread existed.

I question Mr Walker's statement: "If you are installing front disc brakes, chances are the overall system will be more front biased. In other words, you will be able to use more rear brake."

For a given wheel diameter, a duo-servo drum has way more specific torque than a disc brake. So keeping all other parameters constant (wheel diameter, master cylinder piston diameters, existing proportioning valve if so equipped), installing front discs in place of front drums makes the vehicle more rear-biased, because for a given system pressure the front discs produce less torque than the front drums which they replace.

Because his statement is qualified by "Note this isn't always the case", I called him to discuss. His statement was made from the perspective of a typical consumer, where a "big brake" front disc kit would be installed in place of OE parts, inside bigger rims, possibly on a lowered vehicle that doesn't experience as much forward weight transfer under deceleration, and indeed in that case it is possible for a front disc swap to increase the front bias of a vehicle originally equipped with front drums.

So if you don't want to read his book, read my article in the June 2008 GRM. My article is kind of a Cliff Notes version of his book.

Mr. Walker is my dad. Did you call him too?

As far as everything else above in Mr. Caherty's post, I agree. The moral of the story is install and test your parts - it (the balance) can go either way depending on the extent of your modifications.

Thanks for the clarification, Pat!

JWJr

JThw8 wrote: On the front disk conversion it has been designed to work with the existing dual circuit master from the drums. This kit is on quite a few cars so I trust that its right. Its the addition of the rear disks (or even different drums since Im doing an axle swap) that concerns me. It seems most of the guys that have done swaps on these cars just kind of "play it by ear"....I like a little more reassurance especially with something like brakes.

Simply because it's been done on quite a few cars doesn't mean it's been done right! I learned this first-hand when I had to rescue a Jeep that had a brake problem. The front circuit had failed, leaving only the rears to stop the car. But this Jeep had a rear axle conversion that included new discs in place of the original drums, and there wasn't enough master cylinder to make them work. That little oversight put two people in the hospital.

A very interesting discussion with the Jeep wieners involved led to some eye-opening discoveries about what was considered "fine" in their world when it comes to brake system design.

Keith wrote: A very interesting discussion with the Jeep wieners involved led to some eye-opening discoveries about what was considered "fine" in their world when it comes to brake system design.

Agreed, I should have stated more clearly that the front conversion kit is a well engineered time tested piece which went through proper testing. If I was just doing fronts I would feel fine, but since Im adding in the rear disks things go all wonky and I need to check.

Similar discussions as your Jeep one have been held with the aficianados of my particular victim and I get similar results, they are looking for something other than what I am and sometimes the best thought and care does not go into it.

I like being able to stop, huge fan of good brakes (as anyone from the BABE crew will attest since I seem to have a reputation as the "brake nazi" on BABE)

teamscr wrote: Mr. Walker is my dad. Did you call him too? As far as everything else above in Mr. Caherty's post, I agree. The moral of the story is install and test your parts - it (the balance) can go either way depending on the extent of your modifications. Thanks for the clarification, Pat! JWJr

Do you two need a room?

Welcome to the fray JWjr. We have loved your work for many years, regardless of the stubborn decisions we make about part selection.

Need a room? Nah...his goatee freaks me out sometimes.

At any rate, we're just trying to emphasize that it's possible to have a bunch of right answers, and you need to take everything into account before making your own stubborn decisions. There is no single right answer, but there are a whole LOT of wrong ones out there!

JWJr

It's the freakish dome that wierds me out.