Manual vs. Power Brakes

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This topic contains 11 replies, has 6 voices, and was last updated by  gkesseru 7 years, 1 month ago.

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  • #234415

    edsnova
    Participant

    @edsnova

    This “manual brake” problem confuses me. I got into it with a commenter on a political thread recently as well. I think there is a technical fix. Bear with me, please.

    My experience: As I’ve said before, my first car was a 1967 Chevy Nova. That was the first year for the dual master cylinder. The car had four-wheel drums and manual brakes. I believe there was a booster option for the front disc option, which about 300 people bought from the factory. The car weighed 2900-odd pounds and could also be ordered from the factory with the code L-79, 327 V8 generating 350 horsepower.

    Mine was a 6 cylinder but, being owned by a teenager, by and by it grew a pair, and by the early 1990s it had a 350 and front discs from a later Nova (the early brakes were both rare and dodgy). Never did I replace that original master cylinder, nor did I install any kind of proportioning valve. I heard I might need to, but the car stopped just fine without any additional stuff.

    The car. Stopped. Just. Fine.

    Why would my 2900-pound, 300-odd horse ’60s American crate be able to stop without trouble using an unaltered 1960s drum brake MC and my spindly-ass right leg–while grown men today worry themselves sick over not having a power bosster in their 2000-lb car built with 1980s/1990s/maybe newer parts?

    This is not a rhetorical question. Several of you have much more automotive experience and knowledge than I, and I am eager to hear your thinking. Being me, I of course have a theory, which is crack-pot but which I offer for your amusement:

    What if mid-1960s brake master cylinders were better suited to manual braking? Because power brakes were then fairly new and rare options, and cars (in the US, anyway), were generally pretty big, stands to reason that the lever forces, reservoir size and hydraulic passages of the earliest dual MCs must have been sized better for human leg actuation. Maybe later MCs, made for power brakes and discs generally, have different mechanical properties that make boosters more necessary?

    Please discuss. I am eager to learn.

    #253303

    pmossberg
    Keymaster

    @pmossberg

    You might be on to something Ed.

     

    I recall my grandfather’s ’65 Plymouth with power steering and power brakes. Both incredibly over-boosted. Mom usedto say, when she drove her dad’s car, all she had to do was “…look at the brake pedal…” and the car would stop. Could that be because the vacuum booster was working with a master cylinder designed for manual brakes?

    PMOSSBERG2012-12-10 09:39:13

    Paul Mossberg
    Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
    2005 Intermeccanica Roadster

    If you own a TDr and are not in the Registry, please go to http://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)

    #253304

    dan-r
    Participant

    @dan-r

    $.02 Worth if you need power brakes on a 2000 pound car maybe you should not be driving or you need to be exercising more??? if all parts are working service brakes work just fine . Kinda like power steering ,,have we all got week or lazy ?? I have had many cars and trucks with power nothing ..cars in the old days not even an option ,,,,Dan

    #253305

    mrlmd
    Participant

    @mrlmd

    I had an old Dodge from the fifties (like a tank) and it had power steering by Armstrong.

    #253306

    pmossberg
    Keymaster

    @pmossberg

    Don’t think anyone is arguing whether or not a TD replica “needs” power brakes.

    Ed’s speculating and asking some questions about how the systems work.

     

    Paul Mossberg
    Former Owner of a 1981 Classic Roadsters Ltd. Duchess (VW)
    2005 Intermeccanica Roadster

    If you own a TDr and are not in the Registry, please go to http://tdreplica.com/forums/topic/mg-td-replica-registry/ and register (you need to copy and paste the link)

    #253307

    edsnova
    Participant

    @edsnova

    Right, Paul. It’s come up a couple times here.

    Found a string on the Ford Muscle Forum asking similar questions. There does seem to be some interest in this subject. And since several of the TDs on the forum have been tweaked, I was hoping we might all profit from some ginned-up knowledge.

    #253308

    gkesseru
    Participant

    @gkesseru

    Having read the discussion on the links, I have some basic engineering thoughts. The brake system works as a series of mechanical systems the purpose of which is to take longer and smaller pressure movements of your leg and translate these into shorter and larger pressure movements of a slave cylinder. The first force multiplier is the brake pedal lever. The pedal length from the pivot shaft to the pedal is longer then the length from the pivot to the MC rod attachment. If this ratio is 4:1, for example then a 4 inch movement of the pedal at 100 lbs. gets converted to a 1 inch push of the cylinder at 400 lbs. So the first place you can play with the system is to use pedals with different length ratios between the top arm and the bottom arm. The second force multiplier is the difference between the size of the Master Cylinder bore and the slave cylinder bore (in surface area). A 1 inch MC bore pushing fluid on a 1/2 inch slave bore further translates that original 1 inch at 400 pounds movement into 1/2 inch at 800 pounds movement. The bigger the MC bore, the more effective the brakes are. So the second place you can play with the braking power is the selection of MC bore size. Additionally during braking the car naturally pushes down on the front tires and actually lifts some of the rear, a proportioning valve is used to send more pressure to the front brakes than to the back to keep the back brakes from locking up. This is especially needed when the back brakes are drum and the front are disks (like in a Pinto based kit). If you have a disk/drum system make sure that you have a proportioning valve that is made for a disk/drum car. The final selection possibility is the size of the rotors. A larger diameter rotor moves the braking point further out the rim closer to the contact edge to the tire . The mechanical ratio here is the distance from the center of the axle to center of the caliper cylinder a compared to the distance from the center of the axle to the edge of the tire. The larger the rotor diameter, the greater the stopping power (because of the distance ratio, not because of the surface area of the rotor). The surface area of the rotor does come into play only because there is more metal to heat up, so brake fade happens later. So…using a MC from a Power Brake car in a manual brake kit car matters only if the size of the MC bore is different. Want more stopping power in a manual brake system? Your choices are: move the MC connection point on the brake pedal, use larger bore MC, and finally a larger diameter rotor.

    #253309

    gkesseru
    Participant

    @gkesseru

    Further thinking about it, I got it backwards in one part: a small MC is more powerful because it is the lbs/square inch of the MC that gets sent to the slave cylinder as the same lbs/square inch.  Pressure in a fluid is the same everywhere within the same container. So a small MC bore connected to a much bigger caliper Cylinder is where the mechanical advantage occurs. Larger movement of the MC ends up as a smaller movement with greater force at the slave cylinder.

    #253310

    edsnova
    Participant

    @edsnova

    I think you’re onto something, G. Those lever forces would seem to be easy to measure & think about (mostly). Which means any of us with the no-stoppie problem should be able to think of how to fix it, then look for the pars or do the mods needed.

    #253311

    donsmg
    Participant

    @donsmg

    I agree with most of gkesseru’s engineering thoughts except for the master cylinder versus slave cylinder diameter effect on brake force. The smaller the MC piston area relative to to the slave piston area the greater the force multiplication. That means changing to a smaller MC piston bore will require less pedal pressure but at the same time more pedal travel to accomplish the stop.

    Don Loveless
    Ford Pinto Fiberfab
    Rochester, MI

    #253312

    donsmg
    Participant

    @donsmg

    Opps, thats what happens when you take a break while writing a reply and don’t check for updates before hitting the send button.

    Don Loveless
    Ford Pinto Fiberfab
    Rochester, MI

    #253313

    gkesseru
    Participant

    @gkesseru

    This guy is the Phd of brake systems

    PMOSSBERG2012-12-19 10:05:34

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