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Thread: Brake system theory question for the gurus

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    Default Brake system theory question for the gurus

    I opted not to put this in the brake system forum since it's not a specific z car technical question. It's certainly applicable to a z car as much as any car, though. Here goes.

    I do not understand the popular opinion that installing oversize brake calipers and/or replacing drums with calipers benefits from a larger master cylinder bore. A larger master cylinder bore decreases hydraulic line pressure throughout the braking system for a given force at the pedal, this I fully understand.

    One argument is the moving of a "larger volume of fluid" with the larger bore. What does this have to do with anything? Basic analysis of any hydraulic brake system tells me that, if any one component is for some reason moving farther (such as a piston retracting farther from a rotor) simply becuz it's larger then something is wrong. The larger fluid volume in the system due to the larger calipers and/or pistons and/or added calipers in the rear seems completely independent of actually needing to move that volume any farther than before to operate the components. Is my thinking correct? The one thing that may make sense would be taking advantage of the decreased piston pressure to prevent the wheels from locking due to a now excessive amount of braking force when things are over sized.

    If I'm missing some key detail, please correct me. I just don't see a relationship between a larger master cylinder bore and better braking as it relates to increasing size and/or number of caliper pistons. Thanks, everyone.
    Last edited by BTF/PTM; 07-02-2010 at 05:49 PM.

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    bloody dam well dam it!!! I had a complete explanation typed out but I did a refresh on the page............

    OK, once more. This one will not be as good as the first.....

    "One argument is the moving of a "larger volume of fluid" with the larger bore. What does this have to do with anything?"

    Consider this, if you have a master cylinder bore of one inch that moves the caliper piston (diameter of 1") 1/4 inch, then what happens when the caliper piston is 10 inches. you understand this.....
    You could have the situation where the MC bore is to big and the brakes will be stiff, too small and you will run out of pedal stroke before the pads meet the rotor.
    When, under normal circumstances, you add larger calipers you are increasing the area swept by the pad (larger pads) and therefore you don't need as much pressure ( at the caliper piston) to achieve the same drag coefficient ( at the rotor).

    "One argument is the moving of a "larger volume of fluid" with the larger bore. What does this have to do with anything? Basic analysis of any hydraulic brake system tells me that, if any one component is for some reason moving farther (such as a piston retracting farther from a rotor) simply becuz it's larger then something is wrong."

    Yes you are right, the assumption that the distance traveled by the piston, at the caliper, to engage the rotor should be the same. But ultimately it is a matter of displacement so you have to move more fluid, see above.

    Most importantly, never underestimate the advantage of having good brake balance and pedal feel (that is why you make the changes to the MC when you make changes to things at the wheel). This is the ultimate answer to your question. It doesn't matter if you have your head wrapped around it or not. Many people before me have done the experiments. They tell the tale.

    I am sure I haven't explained this to my satisfaction.

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    Oh yeah I'm not a guru. I'm a simple monkey.

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    Registered User Curtis240Z's Avatar
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    Because brake pedal travel on stock pedal assemblies are limited, matching a larger MC displacement to larger calipers or drum to caliper retrofits keeps things close to normal.

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    Registered User doradox's Avatar
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    Quote Originally Posted by Free280Z View Post
    bloody dam well dam it!!! I had a complete explanation typed out but I did a refresh on the page............


    When, under normal circumstances, you add larger calipers you are increasing the area swept by the pad (larger pads) and therefore you don't need as much pressure ( at the caliper piston) to achieve the same drag coefficient ( at the rotor).
    The drag coefficient has nothing to do with pad area. It is a function of the pad and rotor material.

    Idealized maximum friction force is roughly (coefficient of friction)* (the force applied). Area doesn't matter. Push on the back of the pad with 10 lbs and the pad pushes on the rotor with 10 lbs regardless of area. How hard the hydraulics push the pad is a function of area though.

    Piston surface area is the key.

    If the ratio between the master cylinder piston area and caliper piston area is the same before and after the swap the pads will receive the same force for a given force applied to the brake pedal and the pedal will travel the same distance.

    If the ratio is changed and the caliper piston area increases then more force is applied to the pads. But you will push the pedal down further.

    Properly working brakes on a 240Z can lock the wheels at any speed. Therefore one could assume there is more than enough braking power available. However, if you use the brakes a lot and they begin to heat up that will no longer be true. Larger calipers and pads delay that loss from brake fade (they don't heat up as fast because they are bigger) and can continue to provide adequate stopping power for a longer time or under more extreme conditions.

    If you want more braking power you have several choices.

    Larger diameter rotors. Basically a longer lever.

    More force applied to the pads. (Install a bigger brake booster or fiddle with piston areas and live with a longer pedal stroke)

    Use brake pads with a higher coefficient of friction. This one can be dependent upon the intended use. For a track car a pad that has a high coefficient when the brakes are hot is a plus, but maybe not so desirable on the street where the brakes may be much cooler. So picking a pad designed for how you drive can be important.

    Push harder.

    Steve
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    Registered User Walter Moore's Avatar
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    A larger brake pad of any given material will produce more friction at any given pressure (PSI), however you are correct that the coefficient of friction depends upon the pad. The biggest benefit of larger rotors is more surface area to dissipate the heat generated when braking. With almost any brake pad material if it gets hot enough it generates less friction.

    But back to the original question, BTF/PTM did you understand the answer? The volume of fluid required to move larger, or multiple pistons is greater than the stock system required, and the "hydraulic pump" in a traditional automotive braking system is the master cylinder. If you increase the volume of fluid demanded you have to increase the capacity of the pump. (That or you have to pump the pedal multiple times at every stop.)
    '71 240Z, Because any fool can drive fast in a straight line.

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    Quote Originally Posted by Walter Moore View Post
    A larger brake pad of any given material will produce more friction at any given pressure (PSI), however you are correct that the coefficient of friction depends upon the pad. The biggest benefit of larger rotors is more surface area to dissipate the heat generated when braking. With almost any brake pad material if it gets hot enough it generates less friction.

    But back to the original question, BTF/PTM did you understand the answer? The volume of fluid required to move larger, or multiple pistons is greater than the stock system required, and the "hydraulic pump" in a traditional automotive braking system is the master cylinder. If you increase the volume of fluid demanded you have to increase the capacity of the pump. (That or you have to pump the pedal multiple times at every stop.)
    Yes, for a given pressure more area = more force.
    However, for a given force pushing on the pad if you increase the surface area of pad that means LESS pressure on the pad surface. The force on the pad is generated by your foot, brake pedal leverage, hydraulic piston area ratios.


    Force = area*pressure


    A larger diameter rotor is in effect a longer lever. For the same force applied to the pads the larger diameter rotor will generate a higher braking force at the tire. The increased heat dissipation benefit, as you said, is also true because the same amount of heat is generated regardless of any of the other factors.

    Steve
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    Thanks, everyone, once again the group's input really helped a lot. If I understand correctly, the reason a larger master cylinder is sometimes needed is as follows:

    Despite the cumulative piston travel remaining very small, there is still a larger volume of fluid spread over the increased (compared to stock) total area of wheel pistons, and thus the same larger volume of fluid must be displaced by the master cylinder to achieve the same pressure and piston motion, and thus the small piston has to travel farther to make up the difference. A larger piston displaces the same volume with less travel as it has a larger area.

    V = PI*r^2*h, with h being the key variable in this case.

    So basically, it's master cylinder travel, and thus brake pedal travel, that's key to the concept rather than braking pressure.

    As for the ratio, is there a link to some sort of equation or estimator or anything else that shows how this relationship works? Is it just a trial and error, personal preference thing?

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