Inconsistent brakes

Started by motomadness, May 06, 2003, 08:37:10 AM

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tzracer

#24
QuoteBrian,

The inner diameter will affect lever travel.  The smaller the line, the fast the fluid moves through the line.  Expansion would hopefully be less with a SS braided line, but I think the smaller the id, the more "feel" you will get because the ratio of the volume of the master cylinder to the brake line is closer to 1.


Think volume flow rate, not fluid speed. In a smaller ID line, the fluid moves faster than in a larger ID line, but the volume flow rate is the same. How quickly you pull the lever determines the volume flow rate. The pressure change in the system travels down the hose at a rate much greater than and independant of, the fluid speed.

If the fluid is incompressible and the line does not expand, if the master cylinder moves, say, 10cc/sec, then the piston(s) in the calipers will move at a rate of 10cc/sec. The diameter of the line, nor the actual speed of the fluid will have any affect. The volume that goes in one end of the line comes out the other end, there is no place to store fluid in the line, regardless of its size.

To me, feel comes from removing expansion in the brake lines and flex in the brake calipers. I don't see where the volume of the MC and the brake line being the same would matter. The pressure in the system is the "signal", the brake fluid is the carrier of the signal. The signal can move much faster than the carrier. I think it was Pascal who first noticed this for a closed fluid system (that a change in pressure is felt throughout the entire system almost immediately).

Example: DC electric circuit, turn on a flash light and the light goes on very quickly even though electrons in a DC circuit generally travel at less than a millimeter per second.

Another example: suppose I set up a signal system from my office to my a$$istants office at the other end of the hall. I set up a tube big enough for a bowling ball,and fill the tube with bowling balls. When I put in a ball, one will fall out the other end. The signal (the falling ball) travels much faster than the ball I put in. Also the ball I stick in does not have to reach the other end for the signal to reach the other end.
Brian McLaughlin
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Nate R

GREAT explanation, Brian! The bowling ball thing really helpedme understand it.
Nate Reik
MotoSliders, LLC
www.motosliders.com
Missing my SV :-(

motomadness

Here are my thoughts:

I only mentioned speed because you mentioned volume flow in an earlier example.  True the fluid is incompressible, therefore the movement of fluid in the MC should match the movement of fluid in the caliper.  The key is to determine the efficiency ("feel") of the operation.  With rubber line, a lot of the force goes into expanding the line.  For the street, where most braking is slow, ratio of the rate of expansion force to rate longitudinal force is less than in racing conditions where the braking operation is faster and considerably harder => greater losses to expansion.  The effectiveness of the rubber line quickly falls outside of its peak performance design limit (not a rupture threshold), so one chooses the ss braided line.

Now, the thinner the line (the smaller the id), the lower the radial pressure exerted in the brake line (expansion force), and the higher the longitudinal pressure in the line.  Therefore the smaller brake line should require less force and travel at the lever for the same amount of stopping force at the caliper.  The more you can dial the expansion out, the better your assumed brake "feel" should be over the entire thickness of your brake pads, not just the first few 10ths of a millimeter.  I would assume other limiting factors would be brake fluid boiling.  The losses in the expansion, probably aid in keeping the fluid cooler due to less longitudinal motion (pressure waves), as well as the ability to generate enough pressure in the line during heavy braking at all temperatures.

tzracer

QuoteHere are my thoughts:

Now, the thinner the line (the smaller the id), the lower the radial pressure exerted in the brake line (expansion force), and the higher the longitudinal pressure in the line.  Therefore the smaller brake line should require less force and travel at the lever for the same amount of stopping force at the caliper.

Pressure is the same in all directions. By going to a smaller ID will not reduce the pressure on the walls. The pressure in the radial direction and the longitudinal direction will be the same regardless of ID. If the pressure were less in the radial direction, fluid would move in the radial direction, since fluids move from higher pressure to lower pressure until the pressures are the same.
Brian McLaughlin
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motomadness

QuotePressure is the same in all directions. By going to a smaller ID will not reduce the pressure on the walls. The pressure in the radial direction and the longitudinal direction will be the same regardless of ID. If the pressure were less in the radial direction, fluid would move in the radial direction, since fluids move from higher pressure to lower pressure until the pressures are the same.  


I should have used the word force, not pressure.

I am thinking more that the radial distribution of force of the fluid pressing on the brake line wall.  In theory, a larger line requires more lever motion to expand the line because there is more area for the expansion force to operate on.  Since the thinner line has less area, less lever travel would be required to generate the same expansion force of the larger line.  However, since more pressure is not generated in the thinner (as you said), more work effectively goes into moving the caliper pistons than into expanding the brake line.

tzracer

Here is what I think is happening. Both IDs have the same OD. The smaller ID has more wall thickness which results in less expansion (how much ?) and (possibly) better feel. This is comparing the same type of hose with only the dimensions differing. Another thing to take into account is that when changing brake lines, the fluid is also changed.

Some of it may be placebo effect. Would really have to perform a double blind test to find if the better feel is real or not. The difference may be so small as to not be noticeable.
Brian McLaughlin
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winders

Guys,

With the smaller brake line I.D., more finger pressure is likely to be required to move the pistons.

A bigger pipe will flow the same amount of fluid as a smaller pipe with less line pressure. That's simple hydraulics. So pushing X amount of fluid through a -2 line in 1 second takes more finger pressure than pushing X amount of fluid through a -3 line in 1 second.

I like the Galfer -2.8 lines because they have the same line expansion rate as the Goodrich -2 lines and require slightly less finger pressure. This improves feel and reduces fatigue.

And tzracer is right about the line size not affecting lever travel, assuming line expansion is the same.

Scott

Eric Kelcher

You guys have danced all around the answer. the expansion is a function of the elasitcy per square inch and with a smaller ID you have less surface area so less fluid is lost to filling the added volume of expansion and is directed to the expansion of the caliper stroke thus creating more piston movement.

Flip side with less volume of fluid in the lines the heat generated by braking has less fluid to heat and can cause brake fade/boil earlier. IE 100mph stop results in 10000 calories and brake fluid raises 100 degrees with 100cc of fluid decrease fluid to 75cc and brake fluid is going to get hotter 133  (not actual numbers for brake fluid or energy required to stop bik just an example)e
Eric Kelcher
ASRA/CCS Director of Competition

Eric Kelcher

FYI the lever effort is constant to get the resultant pressure at caliper; direct ratio of lever pressure regardless of flex of brake lines. the movement will be more with more expansion.
Eric Kelcher
ASRA/CCS Director of Competition

winders

Eric_Kelcher,

I didn't dance around anything!

You shouldn't make the assumption that a smaller I.D. line will expand less than a larger I.D. line. Galfer uses a line that expands less than the lines that Goodrich uses.

You wrote:

FYI the lever effort is constant to get the resultant pressure at caliper; direct ratio of lever pressure regardless of flex of brake lines.

That is true once the pistons have stopped moving. But, as long as fluid has to pass through the brake lines, more pressure will be required on the master cylinder side when using the smaller I.D. line.

Scott

Super Dave

That's it...

I'm converting to four leading shoe drum brakes.  Cables are much more consistent... ;D
Super Dave

EX#996

QuoteThat's it...

I'm converting to four leading shoe drum brakes.  Cables are much more consistent... ;D

LOL!!!!


Dawn   ;D
Paul and Dawn Buxton