Bimmerfest BMW banner

1 - 20 of 22 Posts

·
Registered
Joined
·
55 Posts
Discussion Starter #1
Hello there,

Apologies if this has been answered before - I am terrible at searching the forum.

I know that brake fluid is hydrophilic and absorbs water from the air. But where does it get in contact with air?

The hydraulic effect in braking requires a sealed system to be able to pressurize the fluid, so it should be sealed down there in the brake pistons and ABS stuff. It remains the reservoir which has an airtight cap (or I think?). My geeky soul is at pain not knowing this.

Also if you feel like it please add some of the geeky facts you know about cars or other things.
Here are two that comes to my head rn:
The rear right tire wears fastest because the engine torque (bimmer engines are aligned along the car rather than sideways) increases weight on the left side.
Most of close door buttons in elevators do not work and are there to just give you the illusion of control. In most places doors need to stay open to accommodate handicapped needs.
 

·
Nuclear engineer
02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
Joined
·
17,466 Posts
It remains the reservoir which has an airtight cap (or I think?). My geeky soul is at pain not knowing this.
It does not. The cap is vented, otherwise the reservoir would collapse as the brake fluid is pumped to the brake cylinders (else why even have a reservoir).
 

·
Nuclear engineer
02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
Joined
·
17,466 Posts
The rear right tire wears fastest because the engine torque (bimmer engines are aligned along the car rather than sideways) increases weight on the left side.
It does not, certainly mine does not. You need to clarify your understanding of weight and of normal force and of Newton***8217;s Laws and the Third Law, ***8220;When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.***8221;
 

·
Registered
Joined
·
55 Posts
Discussion Starter #4
It does not. The cap is vented, otherwise the reservoir would collapse as the brake fluid is pumped to the brake cylinders (else why even have a reservoir).
I guess it would be impractical to make it as a closed system?

I know many french cars have a closed coolant system without a reservoir that becomes pressurized so that they can have a boiling point above 100C (plus what you get for additives).
 

·
Registered
Joined
·
55 Posts
Discussion Starter #5
It does not, certainly mine does not. You need to clarify your understanding of weight and of normal force and of Newton's Laws and the Third Law, "When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body."
It does, a BMW tech told me that without knowing the reason and gave me some BS reasoning. The torque from engine causes a counter torque from the body which put more weight on the right side. It is similar to the effect that makes the shift of weight from front to rear when you accelerate. You see the same in Helicopters, the tail rotor is to counteract the torque from the main rotor.
 

·
Nuclear engineer
02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
Joined
·
17,466 Posts
I guess it would be impractical to make it as a closed system?

I know many french cars have a closed coolant system without a reservoir that becomes pressurized so that they can have a boiling point above 100C (plus what you get for additives).
So French cars suffer no leakage to be made up from the reservoir expansion tank?

All mechanical liquid pumps’ seal packing is cooled and lubricated by weepage leakage of the pumped liquid, even AC freon pumps.

My X5 cooling system has a pressure relief cap on the expansion tank reservoir.
 

·
Nuclear engineer
02/2012 X5 35d M57Y CPO 99K miles NOKIAN WR G3 12K miles
Joined
·
17,466 Posts
It does, a BMW tech told me that without knowing the reason and gave me some BS reasoning. The torque from engine causes a counter torque from the body which put more weight on the right side. It is similar to the effect that makes the shift of weight from front to rear when you accelerate. You see the same in Helicopters, the tail rotor is to counteract the torque from the main rotor.
The tail rotor applies the Third Law. The engine accelerating, increasing angular momentum, does exert a First Law force = mass times acceleration.

A BMW mechanic is not a qualified teacher of basic physics, of high school physics.
 

·
Registered
Joined
·
55 Posts
Discussion Starter #8
For the reasons you mentioned, you still need to add some coolant every once in a while, but you do that directly into the radiator. Also the radiator lid has a pressure value that opens under a threahold pressure (there is a spring behind the seal). If engine overheats and coolant pressure goes too high coolant expels out on the pavement. ****ty design in my opinion!

Wrt, tire wear: they all are similar. Whe a body exerts torque to another, it takes the reaction which causes a chopper without a tail rotor be unstable and rotate around itself or the weight shifts back when accelerating.
 

·
Registered
Joined
·
306 Posts
For the reasons you mentioned, you still need to add some coolant every once in a while, but you do that directly into the radiator. Also the radiator lid has a pressure value that opens under a threahold pressure (there is a spring behind the seal). If engine overheats and coolant pressure goes too high coolant expels out on the pavement. ****ty design in my opinion!
What would be a better design?
 

·
Registered
Joined
·
905 Posts
Gee, 30+ years in auto, atv, MC repair trade plus a few as a votech instructor ***8230;. Guess I need to go back to school to learn these new geeky facts.
 

·
Registered
Joined
·
905 Posts
To understand why the right rear tire unloads under acceleration read this article. True, drag cars use a 'banjo' 3rd member, but physics is physics.

Another comment about 'French' cars having no means of expansion for the cooling system can not be correct. Fluids expand when heated. If there is no place for this expansion to occur, the system will fail. The easiest way to allow expansion is to let the expanding fluid compress a gas (air) in a half filled closed container. Otherwise some means of expansion must be provided, such as a bellows tube or flexible 'bladder' that physically changes it's dimensions to allow expansion. Talk about failure waiting to happen …. Also putting the system under pressure will raise the boiling point by appox. 3 degrees F per pound of pressure. The referenced chart show that plain water under 16 psi will boil at appox. 250 degrees F. (121C)

Now for the brake question: Most modern reservoirs have a diaphragm that separates the brake fluid from the atmosphere. The diaphragm allows for the necessary changes in fluid levels while reducing the exposure to moisture laden air. So how does moisture enter the system? Through the seals around the caliper's pistons. When used, the calipers as well as the fluid heats up and expands. Drive through a puddle, the caliper rapidly cools, the fluid contracts and pulls the water in past the boots and seals. Not a lot, but it adds up. Normal heat cycles will also allow some moisture to enter. Another little know fact is the square piston seal actually flexes (rocks) when in use. It's this action that pulls the piston away from the pads when releasing the brakes.

At least that's how I learned it back in the day.
 

·
Lost but making good time
'11 335xi; '03 330Ci
Joined
·
5,617 Posts
Most of close door buttons in elevators do not work and are there to just give you the illusion of control. In most places doors need to stay open to accommodate handicapped needs.
The door open/close buttons in an elevator are not an illusory feature nor provided for our convenience. They are used for manual control of the doors by firefighters when the lift system has been switched into fire-service operation. In that mode all automatic functions are disabled; the cars operate only under direct control of firefighters.

Doors have photocells or other obstacle detectors to accommodate slow-boarding passengers. Some systems do accept open/close requests from the buttons but that is not universal.
 

·
Registered
Joined
·
55 Posts
Discussion Starter #14
About the tire wear: The article is too long and I am a bit tired, but I am pretty sure about the engine torque effect (may be not the only reason but it is a major one for sure). It is something that happens in many settings. For example, some jet skies tilt a bit when you accelerate.

Correct, there is some air in the radiator that leaves room for expansion (which increases the internal pressure and hence higher boiling point). In fact, a sure way to screw up the system is to fill the radiator above the max point. Pressure goes too high and the radiator lid value opens and you get boiling coolant pouring out.

With brake fluid: It is interesting that water still goes through the seals and it adds up to a significant amount.

To understand why the right rear tire unloads under acceleration read this article. True, drag cars use a 'banjo' 3rd member, but physics is physics.

Another comment about 'French' cars having no means of expansion for the cooling system can not be correct. Fluids expand when heated. If there is no place for this expansion to occur, the system will fail. The easiest way to allow expansion is to let the expanding fluid compress a gas (air) in a half filled closed container. Otherwise some means of expansion must be provided, such as a bellows tube or flexible 'bladder' that physically changes it's dimensions to allow expansion. Talk about failure waiting to happen …. Also putting the system under pressure will raise the boiling point by appox. 3 degrees F per pound of pressure. The referenced chart show that plain water under 16 psi will boil at appox. 250 degrees F. (121C)

Now for the brake question: Most modern reservoirs have a diaphragm that separates the brake fluid from the atmosphere. The diaphragm allows for the necessary changes in fluid levels while reducing the exposure to moisture laden air. So how does moisture enter the system? Through the seals around the caliper's pistons. When used, the calipers as well as the fluid heats up and expands. Drive through a puddle, the caliper rapidly cools, the fluid contracts and pulls the water in past the boots and seals. Not a lot, but it adds up. Normal heat cycles will also allow some moisture to enter. Another little know fact is the square piston seal actually flexes (rocks) when in use. It's this action that pulls the piston away from the pads when releasing the brakes.

At least that's how I learned it back in the day.
 

·
Registered
Joined
·
55 Posts
Discussion Starter #15
Correct it also serves in the manual control for fire service. But under normal usage, it serves the purpose to give people the feeling of control.

The handicapped person still needs some time to get to the door (where the detector is), so in most places in US there is a minimum requirement for doors to stay open, running the button useless (I read in some places in Europe its not the case). Yet, everytime you get on an elevator you see people using it to ease their rush.

The door open/close buttons in an elevator are not an illusory feature nor provided for our convenience. They are used for manual control of the doors by firefighters when the lift system has been switched into fire-service operation. In that mode all automatic functions are disabled; the cars operate only under direct control of firefighters.

Doors have photocells or other obstacle detectors to accommodate slow-boarding passengers. Some systems do accept open/close requests from the buttons but that is not universal.
 

·
Registered
Joined
·
905 Posts
About the tire wear: The article is too long and I am a bit tired, but I am pretty sure about the engine torque effect (may be not the only reason but it is a major one for sure). It is something that happens in many settings. For example, some jet skies tilt a bit when you accelerate.
You only need to read the 1st couple of paragraphs to understand the torque reaction.

With our IRS, the torque is applied to the body through the differential mounting hardware. A little speculation here: Many people have complained about 'sag' on the right rear. On my old E92 it was 2". You couldn't notice it by sight but it was there. When I replaced the suspension and had both sides with equal ride height, it was much easier to spin the r/r tire. I wonder if this 'sag' is a design feature to overcome differential torque under acceleration.
 

·
Registered
Joined
·
55 Posts
Discussion Starter #17
Ok read it now. Could not quite understand how the suspension system can be adjusted to counteract the engine torque effect on weight shift. so you actually can have the rear right showing less wear? what is your experience?

I changed all 4 tires together 13K mile ago and now the rear passenger is noticeably more worn out. I don't do tracks or anything but I normally drive like an animal.


You only need to read the 1st couple of paragraphs to understand the torque reaction.

With our IRS, the torque is applied to the body through the differential mounting hardware. A little speculation here: Many people have complained about 'sag' on the right rear. On my old E92 it was 2". You couldn't notice it by sight but it was there. When I replaced the suspension and had both sides with equal ride height, it was much easier to spin the r/r tire. I wonder if this 'sag' is a design feature to overcome differential torque under acceleration.
 

·
Registered
Joined
·
905 Posts
Ok read it now. Could not quite understand how the suspension system can be adjusted to counteract the engine torque effect on weight shift. so you actually can have the rear right showing less wear? what is your experience?
The point you're missing is torque (force vector) developed at the differential is what unloads the r/r tire. Reread the paragraph titled "Twists and Turns" and forget engine torque for the moment. Pay attention to the part that starts "As viewed from the rear of the car, …".

Remember, our diffs. are mounted to a subframe bolted to the body so the diff. twisting action is transferred to the body, rather than the axle housing.
 

·
Registered
Joined
·
55 Posts
Discussion Starter #19
The point you're missing is torque (force vector) developed at the differential is what unloads the r/r tire. Reread the paragraph titled "Twists and Turns" and forget engine torque for the moment. Pay attention to the part that starts "As viewed from the rear of the car, …".

Remember, our diffs. are mounted to a subframe bolted to the body so the diff. twisting action is transferred to the body, rather than the axle housing.

Oh now I get it... the torque pushes body clockwise but the axle is torqued counterclockwise, so r/r wheel is transferring less weight to the ground but it appears the opposite because the two torques are compressing the spring on the r/r wheel (pushing wheel and body towards each other). Is it right?
 

·
Registered
Joined
·
905 Posts
Oh now I get it... the torque pushes body clockwise but the axle is torqued counterclockwise, so r/r wheel is transferring less weight to the ground but it appears the opposite because the two torques are compressing the spring on the r/r wheel (pushing wheel and body towards each other). Is it right?
Not quite. You are correct in thinking the engine torque twists the body clockwise (from the rear of the car). The differential creates 2 forces. One lifts the front of the diff. counter clockwise (as seen from the right side) about the center of the ring gear (axle centerline). On a solid rear axle, this contributes to spring windup and wheel hop. The other force is the ccw (viewed from the rear ) rotation of the diff., caused by the interaction of the pinion and ring gears. With a solid rear axle this twisting action unloads the r/r tire.

An understanding of physics will give you a totally different view of the world. You don't have to be able to do the math to enjoy the theory.
 
1 - 20 of 22 Posts
Top