Message-ID: <36861554.B1F13B@regandesigns.com>
Date: Sun, 27 Dec 1998 11:09:08 +0000
From: Brent Regan <brent@regandesigns.com>
To: lancair.list@olsusa.com
Subject: Brake Line Sizing
Mime-Version: 1.0

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Scott writes:

<< 1/8" tubing has an ID of .065", so particulate could be a problem
(though I
 really don't know....). my vote would be 3/16" which has an ID of .132"
and
 would also be more damage tolerant.

 a thought, since you will need to go flex at the point where you
connect to
 the brake reservoirs and brakes anyway, why not just run flex all the
way?
 (you will need 3 lines and 4 connections for your design per brake) I
ran
 666-3 all the way. very easy and reasonable. you can see a picture of
my
 brakes lines at: >>

The reason to use the smallest diameter brake line and to use hard line
is to reduce the system springiness (increase spring rate). You want the
highest spring rate (stiffest) brakes so that you have fine control of
the brakes and a firm pedal. Springiness comes mostly from four sources;
Flexure of the linkage, caliper and seals,  compressibility of air
trapped in the system,  compressibility of the fluid used and expansion
of the tubing used due to system pressure.

While there isn't much that can be done about the first item, we can
work on the last three. Getting all the air out of the brake system is
the most effective way of optimizing brake performance. Fresh brake oil
from a sealed container should be degassed at low pressure for several
hours before use and then slowly introduced to the system to reduce
foaming.

Compressibility of the fluid is reduced by reducing the fluid volume
between the master cylinder piston and the caliper and the best way to
do this is by making the line size as small as possible. Anything with a
bore greater than 0.030" is overkill. If you have foreign objects larger
than 30 microns in your brake system, you need to overhaul it.

As it works out a small bore line is stiffer, for a given wall
thickness, than a larger line. This is because the hoop stress in the
line equals the pressure times the radius of the bore divided by the
wall thickness or P*R/T. This stress divided by the lines material
modulus and multiplied by the bore gives the strain. Steel has the
highest modulus at about 30 million PSI, aluminum has about 10 million
PSI and plastics have a modulus of a few hundred thousand PSI.

To summarize, to build the stiffest brake system you should degas the
fluid, bleed all the air out of the system and use small diameter steel
tubing. Also, never reuse brake oil. Once it has been in the system,
throw it away (properly).

Wishing you prosperity in the New Year.

Brent Regan
www.regandesigns.com