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Thread: When do Rear Brakes engage?

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    Question When do Rear Brakes engage?

    Hello All,

    I recently changed out front brake pads/rotors last week. After a long highway drive today out of curiosity I decided to poke my finger through the rims and feel how hot the rotors/drums were. Both front rotors were unbearably hot to touch. Both rear drums were cold to the touch and only mildly warm near the center where the Hub bearing is. My fuel economy hasn't changed, I'm still averaging 4.9-5.1 L/100km on the MID display when driving on the highway. So I do not think the brakes are dragging. Ive never tried to touch the rotors/drums after driving before. Is it normal for the drums to be cold when rotors are unbearably hot? Is it possible my rear brakes are simply not engaging? The parking brake feels/functions normally if that means anything.

    SOLVED thanks to Foama, see post #9 for solution if interested.


    Last edited by mohammad; 07-22-2021 at 07:07 AM. Reason: solved

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    Quote Originally Posted by mohammad View Post
    Hello All,

    I recently changed out front brake pads/rotors last week. After a long highway drive today out of curiosity I decided to poke my finger through the rims and feel how hot the rotors/drums were. Both front rotors were unbearably hot to touch. Both rear drums were cold to the touch and only mildly warm near the center where the Hub bearing is. My fuel economy hasn't changed, I'm still averaging 4.9-5.1 L/100km on the MID display when driving on the highway. So I do not think the brakes are dragging. Ive never tried to touch the rotors/drums after driving before. Is it normal for the drums to be cold when rotors are unbearably hot? Is it possible my rear brakes are simply not engaging? The parking brake feels/functions normally if that means anything.
    Had you been using them a lot before touching them, the rear brakes kick in at the same time as the front but at a lower intensity (brake bias)

    It's probably just rubbing somewhat because everything is new and in a slightly different axis to the old, did you give the brakes a good workout after you'd changed them or did you just keep driving as per usual?

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    Quote Originally Posted by skyblue View Post
    Had you been using them a lot before touching them, the rear brakes kick in at the same time as the front but at a lower intensity (brake bias)

    It's probably just rubbing somewhat because everything is new and in a slightly different axis to the old, did you give the brakes a good workout after you'd changed them or did you just keep driving as per usual?
    just drove as normal, i drove a bit over 50km I was expecting the brakes to all be warm, is there any easy way to see if the rear brakes are engaging correctly?

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    The rear brakes engage at about 20% force as the front. That said, the difference between front and back is dynamic and somewhat more complex, and the ABS controls the wheels from stopping rotation to prevent skidding.

    If you want to save fuel and $$, drive so the discs/rotors don't heat up much! Drive as if you were barefoot and thumbtacks were stuck on the brake pedal.
    I get around 4L / 100km on average (around 60mpg / US) and the way I drive the rotors are cold when I park the car. If I would heat the brakes any more than absolutely necessary, I would be wasting fuel!
    You may want to look at:
    https://www.ecomodder.com/forum/EM-h...ecodriving.php

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    mohammad (07-22-2021)

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    With my worn and squealing rear brakes, i notice i can "control" the squeal by how "hard" i press on the brake pedal. If i press the brake pedal at halfway i feel the fronts engage but i don't hear the rears' squeal. If i press the brake pedal hard and more than halfway then I begin to hear the squeal.

    I can't prove both front and rear are engaging at the same time but i can feel the fronts slowing me down and only when i stomp on the brakes do i feel like my brakes are really working hard to slow the entire car down.

    I assume both front and rear brakes start working as soon as you press the brake pedal. However i know and feel the fronts are doing most of the braking. If the rears' we're doing more work, i think the rearend of the car could fishtail and lose control in hard braking situations.

    Hope that makes sense.

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    mohammad (07-22-2021)

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    Quote Originally Posted by foama View Post
    The rear brakes engage at about 20% force as the front. That said, the difference between front and back is dynamic and somewhat more complex, and the ABS controls the wheels from stopping rotation to prevent skidding.

    If you want to save fuel and $$, drive so the discs/rotors don't heat up much! Drive as if you were barefoot and thumbtacks were stuck on the brake pedal.
    I get around 4L / 100km on average (around 60mpg / US) and the way I drive the rotors are cold when I park the car. If I would heat the brakes any more than absolutely necessary, I would be wasting fuel!
    You may want to look at:
    https://www.ecomodder.com/forum/EM-h...ecodriving.php
    do you have any reference for the claim that the rear brakes engage at 20% force? i thought all wheel slave cylinders received the same amount of fluid pressure unless a wheel locked up and ABS activated.(indicated by dash light).

    Do our cars have some sort of front/rear brake bias system in place?

    On a side note, does your gasoline come with 10% ethanol?
    Last edited by mohammad; 07-22-2021 at 06:27 AM.

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    Quote Originally Posted by mohammad View Post
    just drove as normal, i drove a bit over 50km I was expecting the brakes to all be warm, is there any easy way to see if the rear brakes are engaging correctly?
    Yeah, jack up one of the wheels at the back and then spin it, then have someone push on the brakes.

    You could try using the brakes a bit to wear in the new rotors and pads but it'll happen on it's own eventually.

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    Quote Originally Posted by mohammad View Post
    do you have any reference for the claim that the rear brakes engage at 20% force? i thought all wheel slave cylinders received the same amount of fluid pressure unless a wheel locked up and ABS activated.(indicated by dash light).

    Do our cars have some sort of front/rear brake bias system in place?

    On a side note, does your gasoline come with 10% ethanol?
    First point. Here fuel contains either 5% or 10% ethanol, depending on what you buy. The 10% is sold a bit cheaper, but you get maybe 2 to 3 % worse mpg.

    Braking: A few makers have the rear brakes start out at about 50% force of the front (some BMW fo instance), most others usually around 20%. With further progression of braking force, the rear don't progress as much as the front, and at strong braking ABS sets in. Some cars have a braking-pressure-aleviation-valve in the brake lines before the rear brake cylinders (some Fiat for example). The basic braking principle is in practically the same on all cars on the road today.
    The general (static) force-relationship is a function of several things. The friction a particular brake design generates in relation to the applied force, the hydraulic ratio also plays an important role. The master cylinder has a diameter of so-and-so, the cylinders in the axles have such-and-such diameter. The diameters of the slave cylinders generate a particular force versus the applied pressure. Different diameters mean different pressures meaning different forces. You could read it up with a bit of searching in the net.

    A "sportier" driver not only causes more total brake wear, but in particular causes the rear brakes to wear out much faster in relation to the front brakes than a modest driver.
    Hard braking = a sharp increase in rear brake wear.

    A real "sporty" driver may need a set of rear brake shoes after wearing out only two sets of front pads, for a hypermiler the rear linings can easily last the life of the car. My rear linings have about 80% of lining material left after about 60000miles (100000km). A so-called "sporty" driver might be screaching metal on metal on the rear brakes after maybe only 6 000 miles.
    Last edited by foama; 07-22-2021 at 07:14 AM.

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    Quote Originally Posted by foama View Post
    First point. Here fuel contains either 5% or 10% ethanol, depending on what you buy. The 10% is sold a bit cheaper, but you get maybe 2 to 3 % worse mpg.

    Braking: A few makers have the rear brakes start out at about 50% force of the front, most others usually around 20%. With further progression of braking force, the rear don't progress as much as the front, and at strong braking ABS sets in. Some cars have a braking-pressure aleviation part in the brake lines before the rear brake cylinders (some Fiat for example). This basic principle is in all cars built in the last 60 or so years.
    The general (static) force-relationship is a function of several things. The friction a particular brake design generates in relation to the applied force, the hydraulic ratio also plays an important role. The master cylinder has a diameter of so-and-so, the cylinders in the axles have such-and-such diameter. You could read it up with a bit of searching in the net.
    Thanks for telling me, I had no idea these vehicles come with a brake bias system in place, i just assumed all slave cylinders received the same amount of brake fluid pressure.

    ANSWER: Apparently the fancy name for this system is EBD(Electronic Brakeforce distribution) as the name suggests it varies the amount of brake fluid pressure each wheel receives.(generally less brake force is applied to the rear wheels as they are more likely to lockup). The purpose of the system is to reduce the probability of a wheel locking up and activating ABS.


    Also with regards to fuel economy, if I average 5L/100km then we take into account that you have a 7.5% more efficient 1L engine and we take into account you use lower ethanol fuel than me(lets say 2.5% more energy dense), that means that you have a 10% efficiency advantage over me. Adjusting for that, my comparable fuel economy goes from 5L/100km to 4.5L/100km which means im not driving super ultra significantly worse than you like i would have guessed i was.
    Last edited by mohammad; 07-22-2021 at 07:07 AM. Reason: solved problem

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    Quote:
    " i just assumed all slave cylinders received the same amount of brake fluid pressure."

    Yes, they usually do before ABS sets in, unless they have a pressure-aleviation-valve which our cars don't have.

    A slave brake cylinder with you-name-it diameter will generate four times more pressure with the same hydraulic brake fluid pressure applied, than a slave cylinder with only half of you-name-it diameter. That resulting force works onto a certain design, which in turn generates a certain braking force. Just simple physics...



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