Thread: 3/16" vs 1/4" brake line

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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.

Originally Posted by 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..

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

Bob