It should be bleeding obvious

[ribbert]

I think about this every time it's posted.
Can someone explain to me why fastening the clutch lever to the bar overnight helps with bleeding?

Noel

Well, lots of interesting replies but no explanation. I guess it doesn't help at all.

Noel

[The General]

I think about this every time it's posted.
Can someone explain to me why fastening the clutch lever to the bar overnight helps with bleeding?

Noel

Well, lots of interesting replies but no explanation. I guess it doesn't help at all.

Noel

Surfaces exert forces. (Simple Really) If the surface area of say a bubble is reduced than there is less surface force resisting floatation. (bubbles less dense than surrounding fluid density should rise - unless another force is holding them back.) Note in particular example 3 here. -
4. Surface Tension in Fluid Mechanics
Did I win anything?   

[Pat Conlon]

That makes sense although I initially ruled it out, thinking that if the bubble is under pressure, so is the surrounding fluid...and with the fluid being under pressure, wouldn't that impede the rising of the air bubble?

I guess not enough to offset the reduced surface friction on the smaller compressed air bubble.

Very cool....Thanks Doug! Kudos.

[ribbert]

Did I win anything?   

Unfortunately not.
It's not really about surface tension.

It is a basic fact of fluid dynamics that with any bubble in any liquid, the smaller the bubble the more slowly it rises. It doesn't take long to think of many examples around the house or garage that demonstrates this.

Blow bubbles into the bottom of your beer glass with a straw and the large ones will immediately rise to the top. The tiny ones very slowly make their way to the top.

When you put fluid under pressure the fluid can't be compressed but the air (in the bubbles) can. This makes the bubbles smaller, and slower to rise. The flip side of this is bubbles expand as they rise, less pressure, and increase speed until they reach max velocity. Think about bubbles expelled from a scuba tank at depth and the size and speed when they reach the surface.

If you took a clear container of brake fluid and shook it vigorously then sat it on its base, you would hardly see the large bubbles because they rise to the top instantly as you right the bottle, the medium size ones rise over the next few seconds and smaller ones over the next hour and the tiny ones...... As a rule of thumb, shaken brake fluid should be left standing for 24 hours before use. If you drop a bottle of fluid you are about to use, stick it back on the shelf and get another one.

While smaller bubbles have less surface area and less friction, the fluid density remains constant and the smaller ones have less buoyancy.

In the case of the FJ clutch, the route from the slave to the M/C is all uphill and the air will naturally want to rise to the top anyway, but slowly. IMO all the tied lever overnight does is inadvertently allow time for this process to occur naturally.

Also, with the lever pulled in, as someone mentioned, the system is closed and the air will be trapped behind the seal so when you release it then immediately squeeze it to see how it feels there is a good chance you are pushing some of that air straight back into the system.

Hydraulic clutches are more difficult to bleed than brakes but there have been many methods posted here that make it easy.

The best solution is to get a stainless steel liner put in you slave and not have to worry about it as often.

As far as those that claim it works, but without offering an explanation, my view is we are ALL susceptible to the placebo effect.

Noel

[The General]

Did I win anything?   

Unfortunately not. Bugger
Blow bubbles into the bottom of your beer glass with a straw and the large ones will immediately rise to the top. The tiny ones very slowly make their way to the top. And if we`ve ground the inside bottom of the beer glass (now illegal) the bubbles will continue to form for some time
 
Think about bubbles expelled from a scuba tank at depth and the size and speed when they reach the surface.  Surprisingly, I think your beer glasses are deeper than ours!
While smaller bubbles have less surface area and less friction, the fluid density remains constant and the smaller ones have less buoyancy.  Correct. (But they do have buoyancy yes?)

In the case of the FJ clutch, the route from the slave to the M/C is all uphill and the air will naturally want to rise to the top anyway, but slowly. IMO all the tied lever overnight does is inadvertently allow time for this process to occur naturally.  Let`s leave two for a week (1 x placebo) and see
Also, with the lever pulled in, as someone mentioned, the system is closed and the air will be trapped behind the seal so when you release it then immediately squeeze it to see how it feels there is a good chance you are pushing some of that air straight back into the system.  Wouldn`t that be only a tini tiny bit maybe, but isn`t it no longer an enclosed system when the lever is let all the way out so some air at the top escapes to the top up chamber? (and if we keep pumping at the end of the lever travel?

The best solution is to get a stainless steel liner put in you slave and not have to worry about it as often.  Sounds like a good idea-esp if all the air has been bled

my view is we are ALL susceptible to the placebo effect. I agree

Noel

Note: The Surface tension (hence friction) surrounding the air bubble, is (subjectively) anchored  to the inside of the conduit in most non verticle places. (Do those bubbles attached to the side of the glass rise slower? esp in concave shaped glasses? me thinks so - now this is an experiment begging for my participation!  )

[ken65]

Holy Professor Sumner Miller Batman, he he

[ken65]

i use to work in a brewery , the best glasses are Headmaster imho,