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jclarkdawe
09-20-2012, 04:55 AM
Cutting brake lines just seems so cliched.

So bad guy pops the hood (easily done), and loosens one of the brake lines under the master cylinder, draining out some of the brake fluid. (Yes, it leaves a puddle, but how many people would notice a puddle under their car?) Bad guy then tightens the brake line and adds some water into the reservoir, which means the brake light shouldn't come on.

As the protagonist drives down the road, doing normal braking, the brake fluid heats up. But because there's a good amount of water, the water starts boiling, creating air in the brake lines. Brakes suddenly fail.

This sound right?

Would a mechanic notice any unusual smell?

How would a mechanic diagnose the problem? Or would he just bleed down the whole system and start with fresh fluid?

Thanks for any help.

Best of luck,

Jim Clark-Dawe

ironmikezero
09-20-2012, 06:32 AM
Jim, it typically wouldn't work that way; the steam expands and causes the pads to engage the rotor (or drum). It's far more likely the brake will seize. See the link...

http://www.afcoracing.com/tech_pages/fluid.shtml

jclarkdawe
09-20-2012, 07:28 AM
I love competing authorities with different results. I'm finding sources that say my result. And I don't know enough to know which is right, although I'm seeing the possibility of both being right.

Brake fluid is designed to not compress. Water can be compressed, but not by much. Water converting into steam expands, which would push against the rotors or drums, as you describe. But steam can also be compressed, if I remember right. Air can very easily be compressed.

I'm wondering if what happens here is initially there's some level of increased pressure against the rotors as the water turns to steam, although maybe not enough to be noticed by the average driver. Then as the driver stops using the brakes, the steam cools down, creating air pockets???

And for a further question, is the brake fluid used in racing going to change the equation here?

Best of luck,

Jim Clark-Dawe

anguswalker
09-20-2012, 01:23 PM
But steam can also be compressed, if I remember right.
Yes, but not to a volume less than that of the liquid water from which it originates.

What could happen (I imagine- I'm guessing now because I'm not 100% sure of the mechanism of the hydraulics) is that

1) Protagonist applies brakes.
2) Water boils, causing brakes to seize.
3) Protagonist , mystified, stops the car.
4) Before cooling down, steam escapes from the brake fluid/water mixture.
5) When the mixture cools down the volume is therefore less and more fluid is drawn from the reservoir. When the system is empty protagonist applies the brakes and there is no pressure.

This would probably have a better chance of working if Bad Guy siphoned out all the fluid from the reservoir and replaced it all with water. This is dependent on which is denser. If water is denser than hydraulic fluid then fine- it would get drawn in first. If not, the water BG added could sit in the reservoir for months before it got drawn in. Time for an experiment I think...

The problem is that the brake pressure warning light would be showing by the time the protagonist totally lost pressure.

Another problem-
Yes, it leaves a puddle, but how many people would notice a puddle under their car?
Probably a fair proportion in the case of brake fluid. It has an odd, almost fluorescent appearance, not like water or oil. Better if BG drains the fluid into a container.

What I guess would then happen is that the brakes would feel very spongy, because of the fluid removed from the system. causing the protagonist to pump the brakes. This would draw water in from the reservoir (but see above- only if the water was denser or there was only water in the reservoir). Then the brakes would seize as the water boiled, and if
a) the protagonist was already in trouble because he/she was approaching a corner too fast because of the spongy brakes and
b) Initially only one front brake seized
then they might end up in severe difficulties.

By the way, I am starting to get worried that you are actually not an author at all but a psychopathic horse-torturing murderer seeking to validate your sick fantasies by gaining the implicit approval and support of your online acolytes. Please reassure me that that is not the case.

Either that or do the decent thing and hand yourself in to the nearest secure correctional facility.

WriteKnight
09-20-2012, 04:59 PM
I'm always a bit thrown off by the bad guy 'popping the hood'. Modern vehicles are latched up tight - and require access to the interior to pop the hood. I'm guessing he has a key or the door is unlocked?

Loosening the lines at the master cylinder will bleed out the master- but the lines will still be full of fluid. It will take a while for the water to displace the fluid.

When the water heats up, the calipers will seize. As if someone were putting on the brakes. Not an abrupt transistion - but fairly fast. The vehicle will stop. You'll smell the calipers burning.

jclarkdawe
09-20-2012, 05:18 PM
I'm always a bit thrown off by the bad guy 'popping the hood'. Modern vehicles are latched up tight - and require access to the interior to pop the hood. I'm guessing he has a key or the door is unlocked?

On a fair number of pickup trucks, you can reach through the grill and grab the cable that leads from the cab to the latch. Pull on the cable and it does the initial pop. Second latch is then easily popped.

I'm lazy. Half the time when I open the engine compartment on my truck I pop it in the way I just described. It's too much effort to walk back to the cab.

Best of luck,

Jim Clark-Dawe

Lil
09-20-2012, 05:27 PM
I'm just curious. How long would it take before the driver ran into problems?

The reason I'm asking is that I was looking for something that would have the car break down a couple of days (400-500 miles) down the road.

WeaselFire
09-20-2012, 06:18 PM
The reason I'm asking is that I was looking for something that would have the car break down a couple of days (400-500 miles) down the road.
That wouldn't be brake fluid. And water in the fluid is noticeable almost immediately, water and brake fluid have different compression rates.

If they can get under the hood, there are a million ways to sabotage the vehicle. Sanding or shaving a belt, loosening a spark plug, remove the air filter, etc.

Are the details of the sabotage important? If not, you don't need to be too detailed.

Jeff

anguswalker
09-20-2012, 07:06 PM
OK, here's another suggestion, which I think might work better than the water:

Bad Guy crimps or kinks the metal pipes running to both front brakes (so that no fluid can pass through). Brakes would feel pretty normal at low speed and there would be no fluid lost, hence no puddle on the road or brake pressure light. However when protagonist tries to stop suddenly at speed (they're a reckless driver, right?), with no braking force on the front wheels they would immediately go into a rear wheel skid and slow down barely at all.

jclarkdawe
09-21-2012, 06:13 AM
Yes, but not to a volume less than that of the liquid water from which it originates. And if I remember right, when you compress it that much, you get some massive pressure.

What could happen (I imagine- I'm guessing now because I'm not 100% sure of the mechanism of the hydraulics) is that

1) Protagonist applies brakes. Actually a series of braking actions, such as normal driving does. but causing a gradual increase in the fluid temperature.
2) Water boils, causing brakes to seize. I'm thinking that the protagonist applies the brakes, holding the pedal down to gradually slow down. A split second later, the brake fluid hits the boiling point, causing the brakes to seize and car skids to a stop.
3) Protagonist , mystified, stops the car. Protagonist is a vet whose schedule has been blown to hell. Gets out, glares at car, lifts hood and stares at engine. Tries calling mechanic but in a dead zone for cell. After a few minutes, she gets back in truck and finds brakes are no longer lock. She starts driving again.
4) Before cooling down, steam escapes from the brake fluid/water mixture. Which creates air pockets in the lines.
5) When the mixture cools down the volume is therefore less and more fluid is drawn from the reservoir. When the system is empty protagonist applies the brakes and there is no pressure. You get into trouble with air in a brake line before then. Air pockets in a brake line is why you have to be so careful about bleeding the brakes. It's easy to get a little bit of air in the line at the high points. Problem with the air is it compresses, rather then pushing against the rotors to apply them. So the brake fluid doesn't need to be empty before the brakes fail.

This would probably have a better chance of working if Bad Guy siphoned out all the fluid from the reservoir and replaced it all with water. This is dependent on which is denser. If water is denser than hydraulic fluid then fine- it would get drawn in first. If not, the water BG added could sit in the reservoir for months before it got drawn in. Time for an experiment I think... Water and brake fluid mix easily. It's one of the reasons why brake systems have to be air tight.

The problem is that the brake pressure warning light would be showing by the time the protagonist totally lost pressure. True, but brakes can fail from air pockets without the brake pressure warning light coming on until the point of failure. (Been there, done that, don't need to do that again.)

Another problem-
Probably a fair proportion in the case of brake fluid. It has an odd, almost fluorescent appearance, not like water or oil. Better if BG drains the fluid into a container. It's going to be on a dirt driveway with a protagonist who is thinking of other things.

What I guess would then happen is that the brakes would feel very spongy, because of the fluid removed from the system. causing the protagonist to pump the brakes. This would draw water in from the reservoir (but see above- only if the water was denser or there was only water in the reservoir). Then the brakes would seize as the water boiled, and if
a) the protagonist was already in trouble because he/she was approaching a corner too fast because of the spongy brakes and
b) Initially only one front brake seized
then they might end up in severe difficulties. Protagonist debates going the long way, or the way going down the big hill. She figures the brakes might lock up, but not fail, so why not go the quick way.

By the way, I am starting to get worried that you are actually not an author at all but a psychopathic horse-torturing murderer seeking to validate your sick fantasies by gaining the implicit approval and support of your online acolytes. Please reassure me that that is not the case. I'm gaining approval? This is how you win approval around here? Strange.

I've started a new novel which is going to be a mystery/thriller. I want to be creative.

Either that or do the decent thing and hand yourself in to the nearest secure correctional facility.


Loosening the lines at the master cylinder will bleed out the master- but the lines will still be full of fluid. It will take a while for the water to displace the fluid. This is one of the things I'm wondering. Some sources seem to imply it happens fairly quickly, while others do not.

When the water heats up, the calipers will seize. As if someone were putting on the brakes. Not an abrupt transistion - but fairly fast. The vehicle will stop. That works for initial warning. You'll smell the calipers burning. Will make sure to add in smelling the brakes.


OK, here's another suggestion, which I think might work better than the water:

Bad Guy crimps or kinks the metal pipes running to both front brakes (so that no fluid can pass through). Brakes would feel pretty normal at low speed and there would be no fluid lost, hence no puddle on the road or brake pressure light. However when protagonist tries to stop suddenly at speed (they're a reckless driver, right?), with no braking force on the front wheels they would immediately go into a rear wheel skid and slow down barely at all. I thought about kinking, and it's quick and easy to do. And it can happen for no reason. One problem is the vehicle is a one-ton pickup, but rather loaded with vet supplies. An empty pickup can lock up the rear end real easy, but add a load and the rear end doesn't tend to slide. And it's been done.

Thanks for all of the help.

Jim Clark-Dawe

benbradley
09-21-2012, 06:34 AM
...
As the protagonist drives down the road, doing normal braking, the brake fluid heats up. But because there's a good amount of water, the water starts boiling, creating air in the brake lines. Brakes suddenly fail.

This sound right?
Why does the brake fluid heat up? Is it because it's compressed when you press the brake pedal? The compression will be released, and the fluid will cool down when the pedal is released.

Is it because the brake cylinder is next to the brake pad? Either way, I find it hard to believe the fluid would get hot enough to boil water under normal braking. Is the driver/protagonist delivering mail?

jclarkdawe
09-21-2012, 07:07 AM
Why does the brake fluid heat up? Is it because it's compressed when you press the brake pedal? The compression will be released, and the fluid will cool down when the pedal is released.

Is it because the brake cylinder is next to the brake pad? Either way, I find it hard to believe the fluid would get hot enough to boil water under normal braking. Is the driver/protagonist delivering mail?

Brake pads create a lot of heat in stopping, so the system is designed to deal with heat. DOT 3 brake fluid has a dry boiling point of 205 C (401 F) and a wet boiling point of 140 C (284 F). The wet boiling point is when the brake fluid has absorbed 3.7% water by volume.

Terrain is hilly and the driver tends to book more service calls then she should in a day, meaning she drives faster then she should. Not enough to normally cause brake problems, but definitely enough to heat the system.

Thanks for the thoughts,

Jim Clark-Dawe