braking force, quick-take-up,


VW Alignment Tech
08-22-2006, 10:04 PM
I'm into self study so I got a book without the answer booklet. I'm trying to pass A5 brakes this fall. I'm having trouble figuring this (and future questions, I'm sure) out.

1)while discussing what affects the amount of pressure exerted by the brakes, Technician A says the shorter the line the more pressure there will be. Technician B says braking force will increase if the size of the pistons in a master cylinder are increased.

I choose both A&B. If a brake line is short, that means (to me) there will be less distance to travel (meaning less power lost) and hence, more/greater braking force. Also, I would think that if the piston size in a master cylinder is increased, that would mean a greater volume of fluid to displace, therefore resulting in greater force when the brakes are applied.

2)While discussing quick-take-up master cylinders, Tech. A says this design allows for increased braking power. Tech. B says this design is only used on drum brake systems.

I don't know how to answer this question as I've no real idea what quick-take-up means. Just as well, I would think that a QTU MC could be used on drum and disc systems.

3)While bleeding a brake system, Tech A. loosens the brake line fitting at the MC if a bleeder screw is seized and cannot be loosened. Tech B. uses shop air to push the fluid and air from the wheel units to the MC.

I would say Tech A. has the better bleeding system (although I'm sure it's quite messy). Tech B's method (as I see it) stands a very good chance of damaging the system.

KiwiBacon
08-23-2006, 01:23 AM
The length of a brake line has no effect on braking power. A shorter line is stiffer and may give better pedal feel, but it's never going to be a deal breaker. When your brakes are applied the fluid is stationary.

Volume of fluid displaced is pretty much irrelevant (provided you're not running out of pedal travel). Think in terms of pressure and area.

Force = pressure * area.
So a bigger piston will have more braking force for the same line pressure.

curtis73
08-23-2006, 01:31 PM
Agree and disagree :)

Length of the line (or for that matter size of the line) has nothing to do with pressures. The fluid is not compressible, so 1500 psi at the beginning of the line is 1500 psi at the end of it. If the opposite were true, the front left brake would get much more pressure than the right rear and the brakes would have to be designed with that in mind... but they aren't. In truth a little pressure drop can be experienced, but its on the order of 5 psi which doesn't amount to a hill of beans in a system that can reach nearly 2000 psi.

Be careful with the logic behind sizes of master and slave cylinders (I refer to slave here as any wheel cylinder or caliper... the driven cylinder that is). Increasing the size (that is to say, area or diameter) of the master cylinder will reduce line pressure for a given input force. If you push with 100 lbs on the pedal, a 1" master cylinder will give LESS pressure to the line than a 3/4" master. The difference is that the 1" master displaces more fluid, so while it might take 140 lbs on the brake pedal to get the same line pressure, it will take less travel to achieve that pressure.

534BC
08-23-2006, 04:22 PM
1 The length of the lines practically will not change the pressure at the outlet, it will be equal at inlet and outlet regardless of length. It will decrease the response time perhaps, but not to notice in any of our cars with a liquid brake fluid. It is hydraulic brake fluid and does not compress. It is hydrostatic (almost like a mechanical linkage) Tec A is onlt correct for the time the fliud is moving. Tec B is wrong , decreasing the piston size of the master cylinder only will increase the force exrted at the brakes or decrease pedal effort.

2 I have no idea what is meant by quick take up systems.

3 Tec A will bleed only what is upstream (the master cyl) and the rest of the systems that is downstream will not get bled. Tec B will lbleed what ever air and/or fluid that the valving may allow out , but replace it with air. Bad idea.
Tec C will heat the bleeder and free it up before bleeding system in the normal way or use the lines at the wheels.

Moppie
08-23-2006, 05:57 PM
Tec C will heat the bleeder and free it up before bleeding system in the normal way or use the lines at the wheels.


Fire plus brake fluid = :uhoh: :uhoh:
Tech C better becareful how he heats that bleeder.


Iv used compressed air to force bleed brakes. Infact you can buy kits that seal the master cylinder making it easier.
I simply stuffed a rag around the air duster, and used it to seal the top of MC.
As long as the MC is full of fluid, and you only use a light pressre on the duster, and stop before the MC empties, its a very, very effective way of force bleeding brakes.
Of course you should never need to do this to a modern car, if you have to it is a good sign something else is badly wrong with system some where.

534BC
08-23-2006, 06:05 PM
Yea, exactly. heat casting around bleeder with it shut. Brake fluid doesn't burn very well anyways. I'd be more concerned with rust popping in my eye.

Moppie
08-23-2006, 06:29 PM
Brake fluid doesn't burn very well anyways.


But when it does burn its not very pleasant to be around. Infact having inhaled some of it, I would say its quite toxic.

VW Alignment Tech
08-26-2006, 12:29 AM
So the only real way for there to be increased braking force would be to have increased braking pressure (meaning I'd have to step on the brakes harder)?

Also, how can it be that brake fluid doesn't move (is hydrostatic)? When you step on the brakes, you activate the master cylinder to push fluid through the brake lines to the calipers (movement). Just as well, when you bleed the brakes, you can see the brake fluid leaving through the bleeder screw (movement again).

:banghead: (as I try to understand fluid dynamics)

534BC
08-26-2006, 12:42 AM
It moves (as a column) but does not compress, it is not spongey like a vapor or like air brakes. It moves just as if you had a mechanical connection to the brakes hardware. The fluid is just easy to route around corners, lol.

The fluid from the mc will never begin to reach any of the brake cylinders or pistons in a normal brake application. There is not that much volume moving. In a small line size the fluid probably doesn't even move a foot in length.

As far as increasing braking force there are many ways to get more force. In addition to stepping on pedal harder there is dropping mc piston size , increasing leverage in mc linkage such as longer pedal, shorter throw or moving actuating rod closer to pedal pivot. At the opposite end (the brakes end) doing the opposite things will increase force. Increasing piston area by increasing diameter or number of pistons, larger wheel cylinders. I am not sure about increasing the surface area of the pad/shoes, but increasing the diameter of the rotor or shoes will gain braking power.

It sounds like you have a real application you are trying to use this on? Any and all efforts to gain pressure on a given system will increase the amount of travel your foot has to do making the same total amount of work. Adding a "booster" of some sort will give the increase of power without increasing the travel. That's like cheating though, lol Is this cheating on a test?

KiwiBacon
08-26-2006, 02:37 AM
So the only real way for there to be increased braking force would be to have increased braking pressure (meaning I'd have to step on the brakes harder)?

Also, how can it be that brake fluid doesn't move (is hydrostatic)? When you step on the brakes, you activate the master cylinder to push fluid through the brake lines to the calipers (movement). Just as well, when you bleed the brakes, you can see the brake fluid leaving through the bleeder screw (movement again).

:banghead: (as I try to understand fluid dynamics)

Brake fluid travels through the lines when you start to apply the brakes and when you remove your foot from the pedal.

But when you're braking (i.e. pads in contact with the rotors) the fluid is stationary, the pressure changes as pedal force changes.

Knap
08-26-2006, 06:59 PM
Worth a read

http://www.teamscr.com/grmbrakes.html

Brake fluid does contain dissolved air

KiwiBacon
08-26-2006, 07:22 PM
Worth a read

http://www.teamscr.com/grmbrakes.html

Brake fluid does contain dissolved air

There was one bit in there that didn't make sense to me. They said that bigger slave pistons made for more compliance.

I suggest that bigger slave pistons require less line pressure for the same stopping force, hence line compliance is less of an issue.

I also believe that many people are terrible at modulating brake pedal force, but can modulate pedal travel with relative ease.
These type of people can get more feel from a system with more compliance.
But a system with less compliance will be a lot more responsive.

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