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
This is just speculation and I've yet to do it myself but I suspect that as long as the clutch is not leaking you put a constant pressure on the fluid so that the air bubbles trapped in nooks and crannys is slowly forced upwards to the bleeding point and thus escapes throught the bleeding valve. Also the fluid stays still while the pressure is applied as opposed to when pumping so all the teensy bubbles that might permeate the fluid congregate to larger pockets of air.
Just my two kroners worth.
Quote from: fj316 on June 20, 2013, 06:46:59 AM
Just my two kroners worth.
Amen to kroners.... Never have enugh of them... :good2:
Brake fluid absorbs quite a bit of air (or one of the gasses that make up air), and fastening the clutch lever to the bar (raising the pressure on the system) can cause some of the small air bubbles to dissolve into the fluid. I have removed this absorbed air by placing new brake fluid in a vacuum chamber and evacuating it several times. Using the degassed brake fluid provides a firmer feel at the lever and makes bleeding easier too. I do not know for sure, but I think brake fluid likes to absorbs Oxygen which makes up about 21% of air at sea level.
Quote from: ribbert on June 20, 2013, 06:06:08 AM
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
It doesn't help at all. It's fantasy dreamed up by someone who can't bleed up in the right way.
All the posts aboput bleeding clutches and brakes, with a gizmo called this or that is nonsense.
I just use the finger and thumb method everytime and it takes about 5 to 10 minutes to bleed the whole set up, and that's from dry.
I've definitely noticed greatly improved brake lever feel after using an old mini-fridge motor to pull a vacuum on the brake bleeder while bleeding. Between that and jiggling the lever all the air is worked out quickly.
"Finger and thumb method"...do tell! Always interested in a simple approach. :morning2:
Thanks,
Jeff
Seems like the method is recommended whenever someone is having trouble bleeding the system. In that case, pulling the lever to the bar would produce negligible pressure in the system.
Plus, when the lever is pulled, the path back to the master cylinder is blocked so any air that rises would still be trapped until the lever was released. Seems like the same bleeding would be accomplished by just walking away from it overnight.
Hooligan
OK, i have posted this before, but here go's again,
so you've got a totally re-built system, no fluid, fill the reservoir, crack open the banjo or the bleed nipple and remove completely, then put you finger over the hole or your finger and thumb if you've taken a banjo union off a hose, hold it there and pump the lever a few times, release your digit and you will hear a sucking or popping sound, this is the fluid starting to travel out of the reservoir down the hose, after a short while of repeating this procedure a load of fluid will come out of wherever has been held by your digit, now put the bleed nipple or banjo bolt back in and start to bleed up with the tube off the nipple in the jar, don't forget to keep topping up the reservoir.
Works everytime, don't be fooled by magic gizmos and hold the lever bollocks, honestly.
Thanks ...makes sense.
I keep the tube attached the bleeder elevated above it so it fills with fluid and eventually pumps into the jar. It usually a matter of pumping the air out. I flushed two FJ clutch masters in the last two weeks this way with no issues.
Jeff
Quote from: ribbert on June 20, 2013, 06:06:08 AM
Can someone explain to me why fastening the clutch lever to the bar overnight helps with bleeding?
It's the placebo effect.
Dan
As I said a couple days ago I was skeptical about this zip tie method but seeing is believing.
A friend of mine put a zip tie on his front brake lever, not to bleed the brakes but to keep the bike from rolling in the trailer.
After he cut the zip tie and unloaded the bike he said his front brake lever never felt so good.
So just a couple weeks ago I changed my front calipers, I bled them till no air was coming out of the bleeders and had a good pedal (lever)
So just for the F@#K of it i put a zip tie on my lever for the night, and sure as shit the next day the lever was much tighter than it was the day before.
so those of you that think it's bullshit have obviously never tried it.
I know I didn't answer the question, and didn't have any great words of wisdom, or scientific formulas to back up my theory but I do know it works.
Try it, you'll like it
Maybe the air bubbles do float up some :scratch_one-s_head: , can't see throuh the line to tell, but once the cable tied lever is released, perhaps it pulls the air And fluid that's rissen? Up to the top,out into the master :scratch_one-s_head:
Simple time, letting the bubbles settle to the top.
When you pull the lever, you're actuating a little piston. Pull the lever in just a tiny bit while watching, and you'll get a face full of brake fluid. This is because the master cylinder resevoir is open to the rest of the system when the lever is fully away from the grip. When you pull the lever in, it tries to raise the pressure in the system, but the bleed to the space above is still open, so it squirts out the top and into your face. Keep pulling on the lever, and that bleed is covered up as the seals pass by. At that point, it's a closed system, from the master cylinder's piston seals down to the slave.
So, pulling the lever in to the stop and walking away leaves you with a closed system. No big deal. Walking away for a few hours lets the tiny bubbles migrate up through the slightly thick fluid (thick compared to say, water), until they're resting against the piston seals. When you then undo your lever and let it release, they cheerfully shoot up into the resevoir, and the next pull of the handle shoves fluid in, rather than air.
This is why when you bleed the system, you really only need to move the end of the lever an inch or so if that, but most people will persist in bringing it all the way to the bar, making themselves tired and cranky for no good reason. It's really not a particularly difficult system to bleed, but it can be a bit twitchy to prime the master so that it moves fluid rather than air. It's a little like trying to pour water into the top of a glass rather than starting at the bottom, a tiny fraction at a time. I think people have difficulty with it because they make it a lot more work than it really needs to be and give up early because they are wasting most of their energy and not really getting anywhere with it. I don't even open the bleed at the slave until I've gotten some headway made on getting the system mostly full of fluid, because the air is going to naturally want to go out the top, rather than shoving it out the bottom. Once you've got the system working but still mushy, that's when it's time to open the bleed at the bottom and start shoving fluid through gently. Brake fluid is slightly thick stuff and doesn't want to be forced through like water!
Quote from: andyb on June 20, 2013, 04:04:47 PM
Simple time, letting the bubbles settle to the top.
When you pull the lever, you're actuating a little piston. Pull the lever in just a tiny bit while watching, and you'll get a face full of brake fluid. This is because the master cylinder resevoir is open to the rest of the system when the lever is fully away from the grip. When you pull the lever in, it tries to raise the pressure in the system, but the bleed to the space above is still open, so it squirts out the top and into your face. Keep pulling on the lever, and that bleed is covered up as the seals pass by. At that point, it's a closed system, from the master cylinder's piston seals down to the slave.
So, pulling the lever in to the stop and walking away leaves you with a closed system. No big deal. Walking away for a few hours lets the tiny bubbles migrate up through the slightly thick fluid (thick compared to say, water), until they're resting against the piston seals. When you then undo your lever and let it release, they cheerfully shoot up into the resevoir, and the next pull of the handle shoves fluid in, rather than air.
This is why when you bleed the system, you really only need to move the end of the lever an inch or so if that, but most people will persist in bringing it all the way to the bar, making themselves tired and cranky for no good reason. It's really not a particularly difficult system to bleed, but it can be a bit twitchy to prime the master so that it moves fluid rather than air. It's a little like trying to pour water into the top of a glass rather than starting at the bottom, a tiny fraction at a time. I think people have difficulty with it because they make it a lot more work than it really needs to be and give up early because they are wasting most of their energy and not really getting anywhere with it. I don't even open the bleed at the slave until I've gotten some headway made on getting the system mostly full of fluid, because the air is going to naturally want to go out the top, rather than shoving it out the bottom. Once you've got the system working but still mushy, that's when it's time to open the bleed at the bottom and start shoving fluid through gently. Brake fluid is slightly thick stuff and doesn't want to be forced through like water!
Well explained andyb. I have used this method on two occasions. Once for the clutch and once for the front brakes. Both times, much better lever feel was achieved by the next day. It must be said, however, that before tying the lever back, I had bled all the air out of the system by the usual process. I think that the tying method only evacuates a very small amount of air that normal bleeding may not shift. I only recommended this in a previous post because I have had first hand experience with it actually working. Pete.
Quote from: ribbert on June 20, 2013, 06:06:08 AM
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
Quote from: ribbert on June 21, 2013, 07:22:14 AM
Quote from: ribbert on June 20, 2013, 06:06:08 AM
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 (http://www.youtube.com/watch?v=MUlmkSnrAzM#)
Did I win anything? (popcorn)
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.
Quote from: The General on June 21, 2013, 03:25:18 PM
Did I win anything? (popcorn)
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
Quote from: ribbert on June 21, 2013, 08:46:15 PM
Quote from: The General on June 21, 2013, 03:25:18 PM
Did I win anything? (popcorn)
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! :drinks:)
Holy Professor Sumner Miller Batman, he he
i use to work in a brewery , the best glasses are Headmaster imho, :good2: