Pirate 4x4 banner

1 - 9 of 9 Posts

·
Registered
Joined
·
1,043 Posts
Discussion Starter #1
I read a ton of threads on disc brake conversions before I did mine. Both here and at other web sites. One common thing that stands out is guys saying that you move more fluid with disc brakes than drum. I'm not convinced that's true. Can someone explain it?
 

·
Registered
Joined
·
1,557 Posts
The piston on a disc caliper never returns to the bottom of the caliper's cylinder; it stays out, almost in contact with the rotor at all times. This feature is the reason discs are called "self adjusting". So as the pads wear, additional fluid must be available to fill the space behind the pistons.
With drum brakes, the pistons retract to the bottom of the cylinders after each application of the brakes. The volume never changes. The brake shoes require constant mechanical adjustment to stay in proximity of the drums. In the olden days, this required backing up and hitting the brakes, or crawling under the car with a large screw driver.
 

·
Registered
Joined
·
549 Posts
This could probably be moved to Newb...but here's my $.02.

It's a generalization that disc conversion require more volume and like most generalizations, it might be correct as often as it isn't. The obvious difference between discs and drums is the bore size difference between typical calipers and typical wheel cylinders. The stroke of the wheel cylinder, adjustment of the drum brakes, health of the residual valve, etc., all play a part in how much volume is actually needed. The same goes for calipers as not all calipers are created equally. Some have low-drag seals that retract the piston further to reduce brake drag and improve mileage. These require more initial volume than calipers without this feature in order to egage the pads and hence are best used with stepped-bore master cylinders (quick take-up).

I think it's fairly well accepted that disc brakes require more pressure than drums to work effectively. This is due in part to the smaller pad area relative to shoes and the fact that calipers are not self-energizing in the way that drum brakes are once engaged. Most systems are designed around 600 psi for drums and 1000 psi for discs. Higher performance disc systems with more rigid caliper designs approach 1500 psi.

So that's a gross oversimplification...and I didn't really answer your question...but this is Pirate :flipoff2:

If you're really interested in learning all about brake systems...keep researching. There's a lot of good info out there. :D
 

·
Registered
Joined
·
1,043 Posts
Discussion Starter #4
I guess if the mods think that this belongs in the noob section they will move it.:D
Didn't know about the low drag seals, interesting. However, I don't think that the conversions people are doing here involve a caliper that fancy. Do you?
I asked for an explanation because I don't claim to know every little aspect of a braking system.
A lot of the threads I read have people going up in size with their master cylinder thinking they have to move more fluid now that they have disc brakes. I don't think that's true. I think as a general rule you move less fluid(once the system is filled and bled) with disc than drum. Since that's the case you are now looking for more pressure not more flow. Seems whacky but to get more pressure you need to decrease the master cylinder size. Very few posts I read suggested that. And the ones that did were ignored.
The reason you get more pressure is because of the formula, Pressure=Force/Area.
Just trying to get a decent discussion going and maybe save someone an extra trip to the parts store.
 

·
Registered
Joined
·
293 Posts
That is why i change the factory 1/4" rear drum brake line to a 3/16 line for the disc brake conversion. On the same master, the 3/16" line will have more pressure and the 1/4" line will have more fluid volume. I never thought about it much as i just bolted everything on, and replaced all brake lines and parts and never had any problems after the first bleed.
 

·
Registered
Joined
·
3,206 Posts
Oi.

Drum cylinders and disc calipers work on the same exact hydraulic principle. To move the larger piston outward (any distance) is going to require more fluid than moving the smaller piston.

Can I say duh now? :)
 

·
Registered
Joined
·
1,043 Posts
Discussion Starter #7
Good point vintagespeed, so, you are schooled in hydraulics, that will help. I have some hydro schooling too.
The difference, I believe, is in in the travel of the pistons. The caliper piston moves very little, unless you have some serious rotor warpage, in comparison to the drum pistons. I think that is why the typical disc/drum setup has smaller lines to the discs than back to the drums. I only know of two reasons for this. There is not as much fluid movement to the discs, or, there is higher pressure going to them than there is going to the drums. The master cylinder could be doing that but I'm not sure. The ones I have rebuilt had the same size pistons so that wouldnt be the case.
 

·
Registered
Joined
·
549 Posts
onetonbb74 said:
That is why i change the factory 1/4" rear drum brake line to a 3/16 line for the disc brake conversion. On the same master, the 3/16" line will have more pressure and the 1/4" line will have more fluid volume. I never thought about it much as i just bolted everything on, and replaced all brake lines and parts and never had any problems after the first bleed.
FYI - Line size has nothing to do with pressure. You can run 2" DOM for your brake lines if you want to...fluid is incompressible and therefore the volume in always equals the volume out. The only thing line size effects is fluid velocity as based on the volumetric flow rate of the system. Therefore, brake fluid in a larger brake line will flow proportionally slower than fluid in smaller brake line as related by the cross-sectional area of the tubing (A=pi*r^2) but everything else is the same for a given system.

As within any system, there are losses due to friction and other factors. Larger lines can reduce those and also due to the slower velocities, can minimize cavitation through fittings which can create miniscule air bubbles in the lines (very similar to propellers, impellers, pumps, etc). While I'm sure that there were reasons to run 1/4" line in some OE applications, I've never had a problem running 3/16" pretty much everywhere.

6869704x4 - I didn't mean to suggest that I thought it was a worthless topic when I suggested that you move it to the Noob section...just that it is fairly basic general brake info and that's where I would search for it if I were looking. And you're right...there's a ton of misinformation out there on brakes, but there are also a few members and vendors on this board that really know their stuff.

As for the low-drag calipers, they've been around for decades but never really became prominent on the American iron until the 80's when the manufacturers were being pushed to decrease their mileage averages (CAFE). If you search Pirate under QT, quick take-up, stepped bore, dual bore...there are actually several threads discussing the potential benefits of such a setup, whether you're running standard calipers or ones with low drag seals. Common sense tells you that a large primary bore will help to keep a high pedal and that a smaller secondary bore will aid in developing higher pressures. It's a simple concept that minimizes the drawbacks associated with straight bore master cylinders.
 

·
Registered
Joined
·
3,206 Posts
6869704x4 said:
....The difference, I believe, is in in the travel of the pistons. The caliper piston moves very little, unless you have some serious rotor warpage, in comparison to the drum pistons.........
What makes you think that? A properly installed drum brake shoe should be turned to match the drum radius perfectly and when properly adjusted will have much the same tolerance in it's fit as a rotor/pad.

edit: Editing myself? Jesus.

To elaborate further, to move a piston that has twice the face area of another requires twice the fluid volume to move it the same distance.

So if we say that the caliper piston is 2" dia and the wheel cylinder pistons are .5" dia each (x2) to move them the same distance will require that the 2" piston have twice the volume of fluid as the 1" (.5 x 2) wheel cylinder pistons.

I'm sure this is no news to you. And it's a very simplified scenario. As for the M/C in the equation, a smaller bore M/C will work as long as it has enough travel in it's stroke to provide the necessary volume of fluid movement needed. When you upgrade to rear discs using a much larger piston bore you need to upgrade the M/C bore (or travel) as well.
 
1 - 9 of 9 Posts
Top