87350IROC

iTrader: (2)

Guys, most modern cars use 3/16" rear brake lines. Most drum cars us a 1/4" or 6mm which is what our cars use even with rear discs. I can't stand the brake feel on these cars even with my big Wilwoods. I am beginning to think it may be the rear brake line is to large taking to much pedal travel for fluid to move the pistons. With that said, has anyone tried a 3/16" rear line and what are your thoughts? There are examples online of people converting old muscle cars to 3/16" rear lines and that making the brakes much more touchy, which I like.

Thanks,

John

//<86TA>\\

iTrader: (14)

I don't know anybody, you could be the guinea pig!

My only question would be can you get the proper fitting for the 3/16 line for the output og the prop valve?? Its 14mm 1.5 I think and I have never seen that fitting for the smaller line. With an adjustable valve it would not be a problem because you can use more common double flare ot AN flare fittings.

87350IROC

iTrader: (2)

Yeah that is the issue. However lucky me, I have a Wilwood adjustable prop valve already installed and it has 3/16" female inverted flare inlets and outlet. In fact I had to adapt it in order fit the stock larger lines. I was trying to figure out how to adapt the sizes until I read your post, thanks for the reminder.

Also, I contacted fine lines and they are willing to build the thirdgen front to rear line in 3/16". Very cool, this is going to be easier than I thought.

drperformance

iTrader: (5)

I was just wondering the same thing a few days ago. All modern cars run 3/16. I run four piston drag brakes up front and single piston ford explorer in the rear (9" rear) with a 1.032 master. The car stops at the track but leaves me wanting more.

3/16 to the rear?????

//<86TA>\\

iTrader: (14)

still interesting in the outcome of this.

however, in my head, the rear line could be 3/16" or 4" and would still only move as much fluid as needed to move the rear pistons, pedal effort would be the same since you cannot compress a liquid. Right?

as for newer cars, most newer cars will have abs, and most likely have a single 3/16 line to each wheel

loneroad

iTrader: (3)

im interested in this also. i just ran 1/4 to the rear, and the -4 to each caliper.

my thinking then was greater force across the piston. kinda like pushing the piston with a marker versus pushing it with a toothpick.

87350IROC

iTrader: (2)

No, the brake system is a closed hydraulic system. The only sizes that affect the brake pressure are the master cylinder size and caliper piston size.

We are talking about secondary effects that are minor (but maybe not insignificant) and affect the dynamic nature of the brake system, not the static pressure. The only two things that I can see where brake line size would affect dynamic feel would be the friction in the brake lines (smaller lines having more) and the compliance in the line itself (bigger lines having more).

drperformance

iTrader: (5)

Im looking at this more linear.

You hit the brakes and the piston in the master moves displacing fluid of a specific volume. Im thinking that the smaller line will in turn displace more linear movement at the caliper piston and result in more force.

Im sure this would also change the feel completely. Who knows I could be wrong.

Did factory rear disk cars also run 1/4 to the rear? I know drums need about 300psi of pressure while disks need upwards of 1000psi.

87350IROC

iTrader: (2)

Yes, my car is factory disc and has the 1/4" line.

Pressure is the same everywhere in a closed hydraulic system at steady state. Its simple fluid dynamics. The challenge comes from dynamics as the system is actuated. Brake line size does not affect the pressure at the caliper.

drperformance

iTrader: (5)

I agree pressure in the lines will be the same but the displacement should be more with the smaller line which would result in more caliper piston travel and more force applied on the pad/disk.

Correct???

sofakingdom

iTrader: (1)

A piston of x square inches moving y inches will displace x * y cubic inches of fluid, whether into a drinking straw or a fire hose.

Likewise, to move a caliper piston with x sq in of area y inches requires x * y cu in of fluid, regardless of line size.

Brake fluid is not compressible, so there's no effect from the sheer volume of fluid in there shrinking as pressure is applied. Well, at least, not that you could tell with your foot; but I suspect that the difference is SO small that the rise in temperature from the friction of the fluid rubbing against the line would be large enough to COMPELTELY SWAMP and therefore obscure any "compression" of the fluid.

Sounds like a bunch of hogwash to me. Now granted I'm only a mathematician and physicist by education and an engineer by trade (those other skills having limited market value), so I don't know much about stuff like that; but that's my opinion FWIW.

drperformance

iTrader: (5)

The more I think about this the more I get confused!

Volume of a cylinder (brake line) = pie*radius squared*length

The smaller line will certainly hold less volume. If the volume the master displaces is fixed the fluid needs to go somewhere. I am thinking it results in more linear movement at the caliper with the smaller line.

Use you straw versus fire hose example. put a small amount of fluid in a straw and you will see it rise. put the same amount in a fire hose and you will be lucky to notice any change.

???????????????????

87350IROC

iTrader: (2)

Look, sofakindom and I are absolutely correct. Assuming an infinately stiff brake line, the size makes absolutely no difference on the pressure in the system. This is scientific fact. Pick up a basic fluids book if you are interested to learn more.

This thread is about pedal feel and dynamic nature of the pressure building in the system. It has nothing to do with static pressure. Please keep on topic.

BTW, I got my custom 3/16" line ordered today from Finelines.

John

sofakingdom

iTrader: (1)

The stop thinking about it.

Go back to the straw and fire hose example. Each one is already completely full, as a brake line would always be, and you try to put in one more molecule of fluid at one end.

OK: What has to happen in order for that one more molecule to go into the hose?

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Right: one molecule has to come out the other end. Doesn't matter whether it's a straw or a fire hose.

Now imagine that the thing putting "the next molecule" in, is the master cyl, and the other end goes to a slave cyl. What difference does it make what size the line is? A molecule is a molecule no matter what size line it's pushed out of.

Way too simple. Don't outsmart yourself over "pie" (why you would put pies into brake systems I can't begin to guess, but w/e) and "displacement" and other stuff like that. Keep it simple and it will become instantly obvious, even though you weren't paying attention in middle school science class.

drperformance

iTrader: (5)

HAHA

I contacted Strange & Mark Williams today. sofakingdom as it turns out you are wrong with one aspect (not all, 1 in 1 out is correct) and a DICK!

Long story short I'm switching to 3/16, but then again I have Strange on all four corners and on the firewall.

87350IROC

iTrader: (2)

Would you care to share what was wrong?

Everything looks good to me. But hey I'm just a mechanical engineer, what they heck do I know?

drperformance

iTrader: (5)

Small diameter... builds pressure quicker

Larger diameter... builds pressure slower

Drum brakes require more fluid volume and less pressure to operate as they need to span a greater distance and are mechanically locking. Disc brakes only have to span a fraction of that and require more pressure due to the volume of the calipers themselves and no mechanical advantage.

we can assume a given area of metal has a material constant for net expansion (or flex). We also know that a given length of tube has a surface that expands according to the square of the radius (A = Length * pi r squared, yep Pi again!). That means that the expansion of a tube varies according to the square of the radius of the tube times its material constant.

There's just no getting around it: the bigger tube will expand more under pressure if we make the following assumptions: (which are true in this case)
1. the wall thickness is the same,
2. the material is the same.
This may be small but they said it is felt!

With rear disks brakes the smaller 3/16 line is what was recommended by both companies. At least that is what I was told!

Apeiron

No, he's completely right. It's how hydraulic systems work fundamentally.

The effects of differences in the sizes of the hard lines are negligible, especially on relatively low-pressure systems like these. If you were talking about changing the sizes of the cylinders, that's something different.

drperformance

iTrader: (5)

By the way Im not trying to start anything just get to the bottom of this. I am switching to the recommended 3/16 line and will report back.

//<86TA>\\

iTrader: (14)

yeah, caliper piston or master cylinder size will create differences in pressure build, but not the line size.

Apeiron

Might want to check your math there. You've calculated the volume of a tube, not the area.

//<86TA>\\

iTrader: (14)

i like pie

iggy1991

Line size wont affect pedal feel. The line regardless of how big it is will move the same amount of fluid to the caliphers. I would look at a master cylinder from a different vehicle like a 4th gen and see if that makes a difference.

sofakingdom

iTrader: (1)

Well you're not "getting to the bottom" of anything; instead, you're bringing a pre-conceived notion in here that's PHYSICALLY WRONG and INCOMPATIBLE WITH REALITY, and refusing to understand the simple explanation of facts about how it really works, no matter how simple it's broken down for you.

But that's OK; you go right on ahead and believe whatever you like, it won't hurt me one bit. Except of course that we're talking about BRAKES here, and if you don't understand how they work but mess with them ANYWAY, I can only hope no one is anywhere close to you - least of all, ME - when your "experiment" goes horribly wrong somehow.

drperformance

iTrader: (5)

Ok guys as recommended by strange engineering I switched my rear brake line to 3/16 from 1/4 this weekend and I wanted to report my findings.

My set up:
-strange 1.0032 bore master
-strange 4 piston front brakes
-ford explorer rear brakes (strange economy)
-fronts ran straight off master rears through a willwood proportioning valve

With 1/4 rear line to the brake hose on the rear end I had the proportioning valve wide open and the rear wheels never came close locking up. With 3/16 rear line I did have to dial back the valve a little but it feels like the bias was the only thing changed and the initial hit to the rear brakes is faster?

87350IROC

iTrader: (2)

Awesome, that agrees with what others on the net have said, and is why I started this thread. Its not that they are getting a whole lot more pressure but you get better initial bit due to lower compliance in the system. So you would say the brake pedal feel is more grabby or instant?

direngrey

You guys are thinking too hard imagine garden hose turn tap on full water comes out now put you finger over it what happens it sprays everywhere at a higher press.

Now brakes
Smaller line more pressure less flow
Larger line less pressure more flow
Increase flow to increase pressure

Ideally u wanna never lock up so u can steer but keep maximum force alot ppl can't do that so they made abs I guess
Force on caliper
smaller line more force but it won't move as fast
Larger line less force but moves faster

I'm pretty sure they design it cuz they need different back pressures to work correctly.

87350IROC

iTrader: (2)

Sorry, you are under thinking it.

The brake system is a closed hydraulic system whereas a hose is open. There is basically no "flow" in a brake system. The pressure is driven by the input piston (master cylinder) and the output piston (caliper). The size of the pipe doesn't matter from a strictly static point of view.

If you plugged the hose, it would be the same pressure to matter the hose size.

Think about a fuel pressure gauge. The orifice for the measuring membrane is much smaller than the fuel rail, yet still reads the correct pressure.

John

direngrey

It's an open system single acting the membrane just keeps air from touching the fluid doesn't take much to let air in and out of between the lid and membrane. Your foot is the pressure relief .

87350IROC

iTrader: (2)

No, the reservoir is not pressurized. It is merely a holding tank for extra fluid. It is not part of the high pressure system.

The brake system is really no mystery. I suggest a quick Google search for more info.

direngrey

I never said it was pressurized. the membrane acts as flexible barrier between air coming in under cap and the fluid. It's still an open system. it's not closed. Closed system have no air coming in or out of their reservoirs. stop giving people wrong information.

87350IROC

iTrader: (2)

double post

87350IROC

iTrader: (2)

I have been very polite with you. I will continue to be polite as long as you stop calling me a liar.

It is obvious you do know know what you are talking about. The pressure side of the brake system is a closed system. If it wasn't you would blow up your reservoir the first time you pressed the pedal.

Here is how it works. There is an orifice that allows fluid to travel from the reservoir to the master cylinder piston. The sole purpose of the reservoir is to hold enough fluid to allow the system to remain full as your pads wear down of the course of 50k miles. In an engineering sense, there is now flow in the system.

As soon as you press the pedal the piston moves and the orifice is now blocked from the reservoir. The system is now closed and all pressure remains in the high pressure side (master, lines, calipers). If the system wasn't closed the plastic reservoir would pressurize (>1000psi) and explode.

Here is a picture.


I suggest taking a look at an entry level fluids text book.

This is a good one to start.


By the way, the reservoir is air tight. If it wasn't your hydraulic fluid performance would degrade very quickly. The membrane you are talking about is not actually a membrane. Its purpose is to allow pressure equalization inside the reservoir WITHOUT introducing air into the system. As the fluid level decreases with pad wear, you would get a low pressure condition inside the fluid volume. The flexible fluid barrier compensates for this lock of volume to keep the fluid reservoir equalized with atmospheric pressure. So the hard plastic cap has a vent to allow the pressure on either side of the barrier to equalize. But no air is ever intended to make it from one side of the barrier to the other.

John

SlickTrackGod

Guys, brake fuild does compress slightly. the larger line will reduce the intial pressure, but ultimately the pressure will equalize to the same force when the pressure remains constant. its the intial pressure compression that is higher in the smaller diameter line.

And before anyone debates whether brake fluid compresses or not, these systems were designed in the 80's with standard everyday DOT3 fluid that holds moisture. The systems ovr time are subjected to heat from the calipers, so yes your fuild does compress.

The older the fuild is, the more effective the smaller line will be with initial bite.

87350IROC

iTrader: (2)

No argument from me. That is what I have been saying this whole time. Not to mention with the smaller line, the less initial pressure drop to the metal lines expanding. Of course this is a benefit regardless of the condition of the fluid. If you made the wall thickness bigger to compensate for the larger line, this argument would go away.

Nothing is perfectly incompressible. It is essentially incompressible and assumed to be for any engineering analysis. Dean, I think what you are referring to, but didn't specifically mention is the compressibility of H2O is when it boils and changes to a gas? H20 in the liquid form is "incompressible".

In my opinion, I have done no analysis on this, the largest contributors to system compliance from the hydraulics is:

1. fluid compressibility due to water vapor (assuming high H20 saturation and hot condition)
2. flexible brake lines
3. caliper flex (depending on caliper)
4. caliper and master seals
5. hard line flex
6. fluid compressibility due to water vapor (assuming low H20 saturation)

John

SlickTrackGod

Yes, that is the whole problem with moisture buildup inside hydraulic brake fuild systems. Moisture turns to steam vapor obviously at 212*. once it's vapor- its basically trapped air and it will increase line compression and reduction of initial fluid travel.

articwhiteZ

iTrader: (56)

would like to see the book/listing that states fluid will compress.
even slightly...to a point that you would feel it....just saying.
and im not talking boiling old brake fluid.with water in it..from age and the steam turning to gas and then compressing..just saying..

ps expl.. water has air in it..and will not compress.. but will boil.and turn to gas...just the thing dot 3 and 4 brake fluid will not do. untill over heated...just a expl. it's called Hydrolics.. when 02 Molecules.hang out together..you can compress them..when there not there..ya cant..not even a smidg..just me maybe..and any hard hydrolic line that.that moves.will fail in time..just sayn..it's how i keep jets in the air. and landing..just me.

SlickTrackGod

ANother one Read the post above you. I didn;t realize it was amatuer hour and I had to be so specific

articwhiteZ

iTrader: (56)

with a post like this.. we would feel it was Amature Hour..Come on... dont hate

SlickTrackGod

Typical- you leave out the rest of my quote and take it out of context. I was leading into how brake fuild develops moisture- even when sitting in a can on a shelf.

So lets get techinical. Do you realise that brake fuild has a shelf life? Do you know how soon it begins to develop moisture? Do you know how hot your brake fuild system runs in the caliper area and thus the lines leading to that area? do you know how much back and forth the fuild travels entering and exiting the caliper in the rear line assembies whent he brakes are applied each time?

Brake fuild does compress because it starts to develop moisture in it almost immediately- and as I am sure you are aware, your brakes get much hotter than 212* when you drive on them an average trip so with that said you have a slight amount of air that develops in the brake fluid system immediately and gets worse over time. Hows that for amatuer hour?

articwhiteZ

iTrader: (56)

ill stick with my post..fluid will not compress.. even if it's pi$$.
look into it..

what floats...rocks...rock..Verry small rocks.. "Monty python"

dont hate the playa..hate the Game!

SlickTrackGod

Brake fuild with moisture in it contained inside a line and heated will you idiot- How many times does someone have to explain this to you?

But you just another one of those asses going around on TGO making trouble

articwhiteZ

iTrader: (56)

hey sparky. you should check into some new meds. and get the blood pressure under control.

im sure it would help with the name calling. after all.. nobodys taking your lunch money any more.....Are They?

you should also look into what dot3 is made of. and unless your standing out side in the rain/down poor.. with your hood off.. filling your master cylinder.
then it might absorbe enough water. to make a dif when you take your car to a road race track and run it all day!..
and the brake system you say Designed back in the 80s..started running back in the late 50s.
and the key word is ..will ABSORB! over time... not made to hold water..so if your going to get all huffy...well

Facts are Facts...just get them straight is all.

you may retort.. with a witty comeback...come on now.. you can do it.

EDIT to add.
when talking about somthing/pointing out somthing.
always make sure to be specific. dont run it all into a ball of junk.
this way the myths stay out of it...and stay away from the name calling
it's to child like...you can use the tex to be specific. if you know how to put it into a tex making it specific.
not ..just.. ya its sticky..yuck.

nosajwols

iTrader: (1)

The boiling point of water increases with pressure....

The difference in feel is likely due to one of many factors, none of them is due to the smaller line.

New line means new fresh fluid.
New fluid... maybe there was some air in the old line, new fluid fresh bleed.
Old line may have become weak (or was weaker) and had some "expansion" under pressure, new line does not.

This is a hydraulic system not a pneumatic system.

SlickTrackGod

The fuild is not under pressure when the brakes are releases and it is stitting static in a hot caliper. It is not always presurized like a radiator cap. It does not boil inside the line, it boils inside the caliper and is drawn into the line simply from moment each time you press the brake pedal. The smaller the line, the more the dynamic position of fuild exiting the caliper

nosajwols

iTrader: (1)

Well you are right in a way... but if you have such a large water problem that it is congregating at the calipers and boiling--forming gas (after all brake fluid is NOT hydrophobic) I think a brake system flush should be something to look into, not line replacement. As the pressure builds all that water you have in your caliper will revert back to liquid BTW... STG Usually I find your info on the ball, not in this case.

Back on topic... Modern cars also have TWO lines to the back (at least decent ones) since the car has either a cobination or all of the following:

-Cross linked brakes (each half of the master does one front and one rear brake, for example front left and rear right). The modern idea is that if there is a failure it is better to have one front and one back than say only back...
-ABS, TCS, ESC, etc...... with control module under the hood, requiring separate rear lines per side.

Two 3/16 lines = 0.0552 sq.in in area. One 1/4 line = 0.049 sq.in. The two 3.16 lines are actually ~10% bigger not smaller... Still goes back to replacing the rear line with one smaller line is doing to opposite... This is also why most old cars usually have one 1/4" to the rear and two 3/16 (one each way) from the split to the calipers.

If there was any flow at all the larger line will have a lower pressure drop in the line. Many people get this confused since they get pressure drop and pressure loss mixed up. There is a small amount of flow but in the grand scheme of things it is a small amount...

Back to why things feel different, it is either black magic (a violation of the laws of fluid dynamics) or just the act of replacing the line and the fluid fixed a problem that had nothing to do with line size (air, water, line flex, line age, etc) and is now being propagated as a line size solution.

SlickTrackGod

You are missing the case and that exact cure. It is the very reason why racers flush their brake systems frequently and replace fluid. Performance cars need this more often also- I have to replace the fluid in my Vetruck at least once a year becasue the brakes start feeling spongy and not as percise. What I am refering on a normal performance street car is something that happens over a few years time, not an everyday occurance- I am talking about a long term effect. If moisture is built up in the system over years, and then you heat the system, you are going to put a little more on the table with the larger rear lines- Am I saying change the lines? I would just change the fluid more often...like I do anyways regardless of line size.

hrspwr

iTrader: (3)

I am no expert, but thought I would throw this into the mix.
Is it possible that the 'feel' of the pedal when switching from a 1/4" line to a 3/16" line is because the fluid is being distributed equally among all the lines? Naturally it is easier to push fluid in a larger diameter tube than a smaller one. I understand that the prop valve controls this to an extent and that is why you had to adjust your valve after the switch?
The only pressure difference would be the one through the line, once it exits the line into the wheel cylinder or caliper, pressure needed for x amount of force is the same.

nosajwols

iTrader: (1)

Not that the balancing you mentioned is an issue but...

Cross section etc. Two 3/16 is ~ equal to one 1/4 (~10% difference) so "balanced" would be two 3/16 to the fronts (one each wheel) and one 1/4 to the rear split into two 3/16 (one each wheel).

OR

Two 3/16 to the rear and two to the front (all equal)....

Having one 3/16 to the rear that is then split into two 3/16 at the rear end is not balanced in any way shape or form.

87350IROC

iTrader: (2)

You guys crack me up.

Does anyone understand the concept that this is a CLOSED hydraulic system?

There is negligible flow. The line needs to be just big enough to allow the small amount of flow without adding to much boundary effects. The smaller the line, the less it will deflect for the same wall thickness. Simple as that. "Flow balancing" or whatever you want to call does not exist. There is "no" flow in a closed system.

John