brake lever zip tie physics question

[Val]

Hi All,

any ideas how brake lever zip tie physics works?

My educated guess is by tying the brake lever we increase the pressure in the system. However the entry to the master brake reservoir is closed.

Increased pressure makes somehow all the small tiny air bubles that make the brake soft, to join together and flow at the top of the system to the master brake reservoir as one big buble.

When you cut the zip tie and release the brake lever the big buble goes into the master brake reservoir. Job done the brake is nice and firm.

The question is what is the physics explanation?

Note the above explanation is not valid for dissolved air. For air that is dissolved works Henry’s law: the weight of gas dissolved in a liquid is proportional to the pressure of the gas.

Means that we can release the dissolved air only by decreasing the pressure NOT by increasing it. That is how carbonated drinks work. We have high pressure carbon gas in the bottle on the top of the drink. When we open the bottle the pressure has been decreased and based on Henry’s law the dissolved carbon gas forms bubles which get releases in the air - the hissing we see.

When hydraulic oil temperature increases or static pressure decreases, air solubility is reduced and bubbles can form within the fluid. This release of dissolved air is known as gaseous cavitation.

Obviously zip tie can't work for dissolved air. So the zip tie method must affects entrained air, air that is suspended in the brake fluid and normally exists in the form of small bubbles.

For entrained air bubles Boyle’s law must work right? However Boyle’s law say if you increase the pressure the volume decreases?

What exactly physics makes the zip tie method work (small tiny bubles to join) ??
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[talkToTheHat]

I think we have a transition from air bubbles to dissolved air involved. Increased pressure causes small air bubbles to dissolve in the fluid. Sudden decrease in pressure may cause air to drop out of solution if it has become saturated. Furthermore a change in temperature overnight will have similar effect so this will be most noticable if compressed whilst the fluid is warm and released whilst the fluid is small. The suddenly precipitated air forms rapidly and is likely more mobile in the solution, rising to the top.

It may also be the case that the problematic air remains in solution.

It may also be the case that the small bubbles remain within the brake fluid.

If you just can't get a good lever feel, it could be that you are buying brake fluid with a high dissolved air content. I flushed my brakes with cheap fluid from a motorcycle discount store and researching air solubility has given me a hint as to why. Could also be that i need to replace my brake lines. I think next though is a flush through with some motul brand fluid. I have had good results on the past with that.

It might be more use to do the zip tie thing then flush the system with plenty of fresh fluid.
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Bandit. does. everything.

[Beehive Bedlam]

Get a life.

[talkToTheHat]

I'm sorry, are you feeling so overwhelmed by the scientists in the thread you can't come up with a better comment than that pathetic attempt at an ad hominem attack. Come back when you are equipped to talk fluid physics or motorcycle maintenance.
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Bandit. does. everything.

[Pete.]

You people talking about dissolved air in the oil being a problem should try clamping the brake hose just above the caliper and then seeing how far you can pull in the lever.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[talkToTheHat]

Dissolved air is not so much of a problem, it has a minor effect on the compressibility of the fluid. It's when the dissolved air precipitates out into little bubbles that causes big problems.

Ever left a glass of still water to stand overnight and seen bubbles form on the side of the glass?
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Bandit. does. everything.

[Chuffin Nora]

[Rogerborg]

Conceivably it's woo.

Leave system overnight, bubbles rise, brake firms up just because.

Cable-tie lever, bubbles rise, brake firms up because cable tie.

Place bacon on seat, leave system overnight, bubbles rise, brake firms up because bacon on seat.
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GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[MCN]

I think you'll find the 'trick' only applies to 'Break Levers/Handles' which are 'tied back'.

There is no law of fizix to explain or justify the myff.

Just bleed the brake properly and forget time over distance equations with levers and Galileo Gallipoli (Who, sadly, is another on the long list of those who'll not be competing in this year's IOM TT.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[bladeblaster]

I have no scientific explanation for this particularly, however it was always my assumption that leaving the level zip tied over night was so that a constant pressure for a long period would gradually push any small remaining trapped bubbles through the system and to the bleed nipples on the caliper.

So whenever I have used that method I have always opened the bleed nipples again before releasing the lever. Occasionally this has resulted in a few bubbles coming out.

[Rogerborg]

There's no movement in the fluid though, and the air bubbles should rise.

Ideally they'll rise up to the master cylinder and escape there when you release the lever. When I do this ritual, I release very slowly and then give some little jiggles to try and tease the air out.

Bacon on the seat may be more effective. Actually, reverse bleeding is likely more effective, it's my method of choice now.
____________________
Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[weasley]

We did this a few months ago.

I think the main conclusion is that we don't call them zip ties over here.
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Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[mauzo]

[techathy]

My theory with nothing to back it up - With the system open for expansion the bubbles can expand as they experience lower pressure and due to the viscosity of the liquid causes enough friction to hold the bubbles in place. By not allowing the bubbles to expand they will float & combine so near the top of the brake hose to form a small number of much larger bubbles. When you realise the brake these large bubbles can escape.

[almostthere]

I tried bacon on the seat it was gone the next morning fucking seagulls brakes were just the same though
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[Wafer_Thin_Ham]

All this perfectly good bacon going to waste.
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[growler]

I was under the impression that cavitation worked the other way round, releasing the gas from solution when pressure is decreased not increased
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DILLIGAF

[Rogerborg]

[Pete.]

Tiny bubbles rise more slowly than larger ones, so quite how compressing them makes them more likely to rise to the top I don't know.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Wafer_Thin_Ham]

[jjdugen]

It could just be that leaving the system pressurized overnight allows the pistons/pads to bed themselves firmly against the disk. If one of the pistons hangs up slightly, the lever requires more movement to displace it, giving a soft feel.

I claim my Nobel prize and the award of millions of dollars, Thank you.
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[Kickstart]

Hi

My theory is that it just allows the air to all get right to the top of the system, and when the lever is released the pistons push back slightly and push some of the air in to the reservoir. Also when you next pull the lever the air sat in the m/c is right by the hole in to the reservoir, and for the first few mm of lever travel it is easier for the air to bubble in to the reservoir than to push the pistons (after pulling / releasing the lever the will be a bit of fluid between the piston and the air).

All the best

Katy
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[Nemo]

[techathy]

[MCN]