3/16" vs 1/4" brake line

[jhal]

I am redoing a pickup frame that has OE 1/4" brake line to the rear drums. My life would be simpler converting to 3/16" to the rear - the front already has 3/16". Does anyone know if the reduced diameter tubing to the rear would cause a braking problem (disc in the front) ?

Thanks in advance, Hal

[Henry Rifle]

Denny,

I respectfully disagree. I can't see any reason why 3/16 tube would decrease the pressure. The only effect it could have is a little more friction. Fluid velocity would change (increase) in smaller tubing, but the pressure won't change.

Hal,

In my experience, using 3/16" line will not cause a problem or decrease your braking power in any way. I have used it on every street rod / muscle car / custom I've ever built. Even used it on a 170 mph super comp drag car.

The main factors in brake systems are the master cylinder size, the wheel/caliper cylinder size and the brake pedal ratio. Line size could hurt if you went very, very small, but as long as you stick with 3/16", you're fine.

[skids72]

Actually, the smaller diameter tubing will increase brake pressure for the same pedal movement. Will it be a problem? Maybe not...

The proportioning block may equalize any difference (?)

Two cents,

-Chris

[29arod]

check out or call one of the brake vendors for verification.

(http://www.ecihotrodbrakes.com/brake_facts.html)
(http://www.inlinetube.com/) Inline tubing

"The smaller the tube, the more pressure it creates and the better the brake system will work. Disc brakes require about 1000 lbs of pressure and drums about 300 lbs pounds. Disc brake cars use 3/16” tubing and drum brake cars use ¼” tubing. If you are building a street rod that has front discs, the entire system is typically plumbed in 3/16” tube." (Call Inline tube for technical assistance)

http://www.inlinetube.com/"

[jhal]

Thanks for the thoughts. I am glad to hear someone has done it with no problems. Yes Chris, I agree that the proportioning valve should adjust out any pressure difference.

[rspears]

I would say that another way. The smaller diameter tube decreases overall system volume slightly, but once filled you are simply moving fluid from the master cylinder to the slave, and both volumes and areas are unchanged. Your system pressure remains the same - you will see no difference between 3/16" and 1/4" lines. Jack has it right, IMO. Just my $0.02.

[Henry Rifle]

Actually, the smaller diameter tubing will increase brake pressure for the same pedal movement. Will it be a problem? Maybe not...

The proportioning block may equalize any difference (?)

Two cents,

-Chris

Chris,

Again, I disagree. Your statement is contrary to basic hydraulics. There's no reason for the smaller diameter tubing to increase brake pressure for the same pedal movement. The tubing is just a conduit between the two pistons. You get the same pressure whether you use 3/16" tube or 2" tube.

A-Rod,

Frankly, that statement on Inline Tube's website is pure baloney. He's right about the amount of pressure required for disk and drum brakes, but he's totally off base about the effect of tubing size on pressure. He doesn't even need to take fluid mechanics. Eighth-grade physics would suffice.

[skids72]

Henry,

The key to my statement lies in the "same pedal movement". Pressure is related to volume temperature and a constant related to the substance being compressed by PV = nRT. For the same stroke of the master cylinder piston, the pressure will be higher in a system with a smaller diameter tubing of the same length because the volume is smaller. Just like an open garden hose vs. one with a smaller opening, for the same volume of fluid, the pressure is higher.

No offense, but check your baloney meter

-Chris

[bobscogin]

"Pressure is related to volume temperature and a constant related to the substance being compressed by PV = nRT. "

Isn't that some of that Ideal Gas Law stuff? As a practical matter, does it really have any relevance to hydraulic brake systems other than possibly theoretical?

" Just like an open garden hose vs. one with a smaller opening, for the same volume of fluid, the pressure is higher."

As long as one end of the hose is open, I see your point, but once the brake pedal is depressed and the fluid is static, the pressure is everywhere the same regardless of line size. Cap off your garden hose to stop flow, and check pressure at the input and output. Would it not be the same at both points for a static fluid whether a garden hose or brake system?

Bob

[Henry Rifle]

Chris,

My baloney meter is perfectly calibrated, and, unfortunately, just about everything you say is incorrect in the case of a brake system.

I don't dispute what you say about pressure change in the garden hose. Every time the brake line size question comes up, someone brings up the garden hose. However, the analogy is doesn't fit. The garden hose is an open system, and a hydraulic brakes are a closed system. Apples and oranges.

PV = nRT? Surely, you're not going to quote the ideal gas law as applicable to noncompressible fluid flow, are you? Even if it worked with hydraulic fluid, it's not germaine to this discussion.

The governing equations for this type of system are based on Pascal's Principle, which states: "Pressure is transmitted undiminished in an enclosed static fluid." And, although brake fluid does move somewhat, it's basically in a static state. It moves a bit, but very slowly. The relevant equations are:

Fw = (Aw x Fm) / Am, where:

Fm = force applied by the master cylinder piston (lbs, not psi)
Am = area of the master cylinder bire
Fw = force applied by the wheel cylinder piston (psi)
Aw = area of the wheel cylinder bore

and,

Xw = (Xm x Am) / Aw, where:

Xw = distance travelled by the wheel cylinder piston
Xm = distance travelled by the master cylinder piston

Using these two equations, if the wheel cylinder bore is 4 times the area of the master cylinder bore it will apply 4 times the force, but the master cylinder piston must move 4 times as far.

Note that the volume of fluid in the system has nothing to do with either calculation. Assuming that the sysem is full, the master cylinder piston is going to move the same distance with the same pedal stroke, and it's going to exert the same pressue on the wheel cylinder piston, regardless of line size.

Denny,

I don't disagree with your reaction time explanation. Fluid in a smaller diameter line will have to move faster to displace the piston the same amount.

[bobscogin]

Chris,

My baloney meter is perfectly calibrated, and, unfortunately, just about everything you say is incorrect in the case of a brake system..

LOL! We must have our baloney meters calibrated by the same lab. I see it the same way, as evidenced my previous post.

Bob

[skids72]

Okay... where do I find my crow? I'm hungry...

[rspears]

check out or call one of the brake vendors for verification.

"The smaller the tube, the more pressure it creates and the better the brake system will work. Disc brakes require about 1000 lbs of pressure and drums about 300 lbs pounds. Disc brake cars use 3/16” tubing and drum brake cars use ¼” tubing. If you are building a street rod that has front discs, the entire system is typically plumbed in 3/16” tube."

Not to belabor the point, but the vendor quote above is misleading because of the first sentence. On a higher system pressure a smaller tube is used due to the higher burst strength of the tube - less internal surface area for the pressure to affect leads to the tube being capable of carrying more pressure within its yield strength. That, however, does not mean that the system pressure is higher with the smaller tube. The tube is able to carry more pressure due to its dimension. Jack's explanation is right, IMO.

[skids72]

No wonder my brakes don't work right... I've been using freon all these years!

[jhal]

Thanks to all for the discussion and information - I am off to build with 3/16".