Disc Brake Engineering Thread

[outcast]

I would just like to know if I could run 6 wheel disc brakes with upgrade to split brake system?

 

[mudguppy]

... Vehicles with disc in front and drum in rear need a split ratio master cylinder to put more fluid to the discs and less to the drums.

...and when you trace the lines of those split volume master cylinders, what you'll find is the large volume section at the end (i.e. closer to the front of the vehicle) is the chamber that feeds the rear drums and the small chamber feeds the front discs.

The reason is what Steve said - even though there is a large surface area of the caliper piston, only traveling a few thousandths results in an overall small volume displacement.

 

[peashooter]

Well now I'm just confused. Maybe we are comparing apples to oranges or something. But I keep looking this stuff up after each post reply here and I'm still seeing every article telling me the opposite. I'm not saying your wrong, just saying EVERY article I have read is saying discs use more fluid and have larger reservoirs. Maybe someday I'll do a disc system on my truck and figure it out for sure, but at this point I'm just going off of what I read. Below are a couple more quoted things I just saw. Can you point me to an article explaining your standpoint to help me wrap my head around this?

"As disc brake pads wear the caliper pistons will move outward. The fluid level in the master will drop more rapidly than the drum portion so you need more reserve."

"Disc brake reservoirs are larger than those for drum brakes. You will often see two reasons given for this:

1. First, because the pistons in a disc brake calliper are MUCH larger than the tiny pistons in a drum brake wheel-cylinder, disc brakes require more fluid volume to be displaced than drum brakes – requiring a larger reserve of fluid for operation.
2. Secondly, as disc brake pads wear, disc brake callipers are self adjusting. That is, the callipers only retract the piston just enough to prevent pad-to-rotor contact. Now, imagine you start with disc brake pads with ½” thick linings and you have a 4” diameter calliper piston. Every time you apply the brakes and the pads wear a little bit, the calliper retracts just a tiny bit less. By the time the pads wear to 25%, or 1/8” thick, the piston at rest will be .75” further out in its stroke than it was when the pads were new. That .75” behind the piston must be taken up by additional fluid – and in the case of a 4” diameter piston, the additional volume required is given by (pi[d/2]^2 * 0.75) or about 8 cubic inches. Multiplied by two callipers (one for each wheel) and that’s 16 cubic inches of extra fluid reserve required to compensate for pad wear. That’s much higher than the amount required to compensate for drum brake shoe wear. Therefore, disc brake reservoirs are larger than for drums because OEM designers must design a reservoir for disc brakes large enough that the brakes will still function even if Joe Public doesn’t check the fluid or add a drop between new pads and completely worn out pads."

-http://www.pirate4x4.com/tech/billavista/Brakes/

 

[mudguppy]

The answer is in the article:

... Therefore, disc brake reservoirs are larger than for drums because OEM designers must design a reservoir for disc brakes large enough that the brakes will still function even if Joe Public doesn’t check the fluid or add a drop between new pads and completely worn out pads."
-http://www.pirate4x4.com/tech/billavista/Brakes/

The fluid capacity for the entire range (i.e. pad life span) is more than the drum wheel cylinder - yes, the volume of fluid in a caliper piston can be greater than a wheel cylinder. However, upon activation of the braking circuit, the caliper uses much less fluid for a given pedal push than a drum wheel cylinder.

Do you need more fluid capacity to fill a caliper piston vs a drum wheel cylinder? Yes.

Does a caliper piston use a higher volume of fluid during a braking action than a comparable drum? No.

 

[rustystud]

View attachment master cylinder selection.pdfOk mudguppy and peashooter, your both right, and wrong . The calipers once they are filled only need a bare minimum of fluid to apply, but they do need to be replenished as the pads wear. Thus the larger reservoir .They also require a lager piston in the master cylinder versus a drum only system, to push more fluid ! The split ratio your talking about peashooter is because drum brakes only need about 800psi to apply, (and they need a residual check valve to keep about 3 to 4 psi in the system,) while disc brakes need about 1200psi. As far as the wear on a disc pad causing the piston to travel further is wrong. Anyone who has worked on disc brakes knows you have to force the piston back into the bore to install new pads. There is no spring pulling or forcing the piston back into it's bore. As the pressure is released after braking, the piston will relax and no longer put pressure on the disc only a very slight drag.
I will attach a PDF file explaining all this .

 

[ALFA2]

Hi steve6x6x6, do you know who makes those caliper brackets the one on the left picture ? Would appreciate any help in that direction. Thanks, ALFA2.

 

[steve6x6x6]

Hi steve6x6x6, do you know who makes those caliper brackets the one on the left picture ? Would appreciate any help in that direction. Thanks, ALFA2.

Not sure witch pic you are looking at, I make everything in all the pic's i posted in this thread.

 

[SP5]

First of all, I must confess, that while a Chevy pick-up, with front discs/rear drums is not the same as a duece, there are still a few similarities. I have done a rear disc conversion, with great results.
The above posted article omits a very important factor. In the typical vehicle conversion, the rear drum brakes are the Bendix duel-leading shoe design, the duece is only a single leading shoe, there is only servo action on one shoe in each drum, thus that article is misleading, as to the nature of using it as a referance, pertaining to fluid pressure or piston travel.
What is important, is that the residual valve(s) must be removed when drums are changed to discs, and an adjustable proportioning valve be installed in the line leading to the rear brakes, in order to prevent the rears locking-up before the fronts, (much depends upon weight carried in the rear).
There is much less volume of fluid used to energize the discs, and in the conversions I did there was no need to change the master cylinder, a drum MC provides more than enough volume of fluid, but a power boost system is def needed to provide the needed pressure the calipers require.

 

[steve6x6x6]

Tests have been preformed on the dual circuit 2 1/2 ton brake system with a pressure gague in the brake line. Panic stop will generate 1,800 PSI, this the working pressure for disc brakes.

 

[SP5]

Sure, a little bit of spherical geometry will show that in a drum brake, in which the shoes are fixed at the bottom, and only spread-out at the top by the WC, results in a brake system in which only about one half of the total area of the shoes contributes any real braking effort, and in a single-leading shoe system, the "forward", (servo), shoe is picking-up about 6o% of the effort.
The area of the shoes down by the anchor bolts/pins contributes almost nothing, as there is no mechanisim to force that part of the shoe to contact the drum, except by the forced flexing of the shoe under high pressure.
In this type of design, (duece), a large percentage of the volume movement of fluid is simply used to expand the upper area of the shoes into contact with the drums, before any real contact pressure can be applied.
Hey, it's a design thats basically from the middle/late '30s, what can you expect.

 

[rustystud]

If you are going to an all disc system , you will need a larger reservoir as disc brakes use more fluid. Mudguppy was wrong to say that the larger reservoir in a master cylinder goes to the rear drums. Just go out and check for yourself. When the caliper is activated it moves out only a small amount, but it does not retract back in all the way. It stays out barely touching the disc. So fluid must come from the reservoir to fill the void left by the piston. The rears, using a brake cylinder, travel the whole distance out until they meet with the shoe resistance against the drum. When released they are pushed back all the way in their bore by spring pressure, also pushing the fluid back into the master cylinder reservoir. This shows that very little fluid is needed in a drum system, as it recycles itself. That's also why the master cylinder does NOT need a larger reservoir for the rears. I do believe a stock master cylinder will work for a "all wheel disc system" if you add a large external reservoir, and use 2 of the old style air-pacs (which have longer strokes) in a split system. Sure the system will not have the same pressure as a modern all wheel disc does, but if the pressures can be as high as "Steve6X6X6" says it can then, the greater braking ability afforded by using disc's will compensate for the lower pressures overall, at least in my opinion.

 

[rustystud]

First of all, I must confess, that while a Chevy pick-up, with front discs/rear drums is not the same as a duece, there are still a few similarities. I have done a rear disc conversion, with great results.
The above posted article omits a very important factor. In the typical vehicle conversion, the rear drum brakes are the Bendix duel-leading shoe design, the duece is only a single leading shoe, there is only servo action on one shoe in each drum, thus that article is misleading, as to the nature of using it as a referance, pertaining to fluid pressure or piston travel.
What is important, is that the residual valve(s) must be removed when drums are changed to discs, and an adjustable proportioning valve be installed in the line leading to the rear brakes, in order to prevent the rears locking-up before the fronts, (much depends upon weight carried in the rear).
There is much less volume of fluid used to energize the discs, and in the conversions I did there was no need to change the master cylinder, a drum MC provides more than enough volume of fluid, but a power boost system is def needed to provide the needed pressure the calipers require.

The rear drum brakes will uses the same amount of fluid regardless of what type of shoe activation used. The deuce rear brake cylinders are not a single servo system, as there are 2 pistons in each cylinder. The difference between the deuce and a modern system is the anchoring of the shoes. The modern style allows the shoes to float causing the front shoe to apply first, then using the force of the drum to drive it into the second shoe which is applying also, then causing it to wrap back up to the first shoe again. This really puts a bind in the drum so to speak ! Basically that's how it works, if you want the whole technical jargon there are books available. The deuce system just uses the cylinder to push out both shoes at once, and of course most of the braking is done at the top of the shoes especially after there is some wear on them. But they both uses the same amount of fluid in their 2 piston design. Yes and no on the volume of fluid needed for a disc. Yes they only need a small amount to apply, but they never give all this fluid back to the master cylinder as some is used to fill the void left from the piston not going all the way back in it's bore. If you have done disc brakes before, you must remember having to push in the disc piston back into it's bore before you could install new pads.

 

[SP5]

Don't get "reservoir capacity" mixed up with "fluid requirments" for effective braking.
From full "on", to full "off" the discs are only moving a few thousands of an an inch.
A drum system moves a lot of fluid "back and forth" between the MC, and the WC, much more than discs, due to shoe clearence and retracter springs.
Discs reqire more pressure, but very little fluid is actually moved between MC and caliper.
Reserve reservoir capacity with discs is only needed to make-up for the faster wear-down of the pads, it has nothing to do with the amount needed to energize the brakes.

 

[ATPTac]

Don't get "reservoir capacity" mixed up with "fluid requirments" for effective braking.
From full "on", to full "off" the discs are only moving a few thousands of an an inch.
A drum system moves a lot of fluid "back and forth" between the MC, and the WC, much more than discs, due to shoe clearence and retracter springs.
Discs reqire more pressure, but very little fluid is actually moved between MC and caliper.
Reserve reservoir capacity with discs is only needed to make-up for the faster wear-down of the pads, it has nothing to do with the amount needed to energize the brakes.

This is probably the best layman's way to explain the difference between the two systems.

Once you install calipers and pads onto a rotor, pump the brakes up, and top off the master cylinder, the amount of fluid used for disc brake application is negligible. Its not so much about moving fluid at this point, its more about "compressing" it, for lack of better terms, in order to get the caliper to squeeze the pads onto the rotor. And I'm sure someone is going to chime in and say you can't compress brake fluid, I was using that to simplify the explanation and get across the point that there really isn't any fluid "flowing" from the reservoir.

As long as you're checking your brake fluid level on a regular basis (which you should be regardless on your pre-trip), the fluid level going down as pads wear out shouldn't create an issue if you stay on top of keeping it topped up.

 

[SP5]

As an addition, No, the duece brakes are not duo-servo, only the front shoe has any servo, (wedgeing) action. The rear shoe is called a "trailing" shoe, and can only have any brakeing action depending upon WC pressure only.
The front shoe is actually doing about 60% of the brakeing effort.

 

[rustystud]

Don't get "reservoir capacity" mixed up with "fluid requirments" for effective braking.
From full "on", to full "off" the discs are only moving a few thousands of an an inch.
A drum system moves a lot of fluid "back and forth" between the MC, and the WC, much more than discs, due to shoe clearence and retracter springs.
Discs reqire more pressure, but very little fluid is actually moved between MC and caliper.
Reserve reservoir capacity with discs is only needed to make-up for the faster wear-down of the pads, it has nothing to do with the amount needed to energize the brakes.

The drum system is moving fluid back and forth and under ideal conditions would never need any fluid at all. The disc system will use brake fluid to replace the fluid lost due to the new position of the piston. The piston is continually moving closer to the rotor after every application of the brakes, however so small. That is why there is a larger reservoir with the discs instead of the drums.

 

[rustystud]

As an addition, No, the duece brakes are not duo-servo, only the front shoe has any servo, (wedgeing) action. The rear shoe is called a "trailing" shoe, and can only have any brakeing action depending upon WC pressure only.
The front shoe is actually doing about 60% of the brakeing effort.

OK SP5, this thread was about being able to use the stock master cylinder in a 6 disc system. We are now entering the Twilight Zone here. Yes you are correct in the proper name of this style of brakes, but the fact I was trying to convey is that BOTH pistons move in the wheel cylinder. Now I can tell you have recently gone through "brakes 101". I remember that time well, I still had a body that worked then but we do not need to bring into this discussion all the minutia about this old style used in the 30's to this in the 50's. All it does is detract from the conversation ,which is "can a deuce have disc brakes" . Trust me I know all about fluid capacity versus fluids needed, like I told someone else in another thread I have 36 years as a "ASE" master mechanic, I do know about brakes, especially brakes 101 . So lets get back to some constructive talk here.
We need to determine the size of master cylinder needed, the pressure requirements and yes reservoir capacity. Also there is the air-pacs to consider . If you had read the report on master cylinders I already posted you would be up to speed with where I'm at. Steve6X6X6 brought up an excellent point, that is the system in stock form can reach pressures of 1800psi which is right in there for disc brakes, now can it handle the volume of fluid needed to apply pressure to 6 calipers. I feel it can if you use the Long stroke old style air-pacs, and a larger fluid reservoir. If you also go with the "all stainless system" that "Peashooter" is selling, that would make an awesome system.
I do have to say one more thing about your post here, of course the brakes are not as good as modern ones, that's why we're trying to change them !

 

[SP5]

You are right, the stock MC, with the airpak, is well suited to developing the pressures needed to operate a disc system. I know we are all in the same book, and prob in the same chapter, but we, (maybe me), just get the pages mixed up.

 

[steve6x6x6]

OK, just take a deep breath and step away from the keyboard.

 

[mudguppy]

... Mudguppy was wrong to say that the larger reservoir in a master cylinder goes to the rear drums. ...

I was wrong. I was remembering that the rearmost chamber actually went to the front circuit on most disc/drum setups - I was mis-remembering that the front chamber was the smaller, thus feeding the rear.

Thanks for taking the time to point out that I was wrong. Made your day I'm sure - happy to help.

The point I was making earlier is what SP5 did a better job explaining - discs 'use' much less fluid than drum cylinders during braking, so volume / flow is a negligible factor.