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Thread: D52 pad coeff.

  1. #41
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    Reference Post #4

    I didn’t want to leave this in-depth brake conversation without covering Total Brake Force. This only matters when we are in search of optimum track performance & therefore optimum braking performance.

    We already covered, when you run at the track, optimum lap times will only be achieved with optimum braking … and optimum brake bias (front vs rear). This was discussed in Reference Post #3 ... but just to make sure everyone is clear … optimum brake bias is when the driver can perform HARD, threshold braking ... without one end of the car locking up the tires.

    If the front OR rear tires lock up under threshold braking ... the front to rear braking bias needs to be adjusted. Once the bias is correct, Total Brake Force will be the final frontier affecting the car & drivers ability to achieve optimum lap times.

    How do you know if you have too little total braking force … or too much ?

    If the driver applies the maximum pressure their legs are capable of … to the brake pedal … assuming the F/R bias is worked out … and all 4 brakes don’t lock up … you don’t have enough total braking force … for that driver, car & set-up.

    If the driver applies moderate to high pressure … but significantly less than the maximum pressure their legs are capable of … to the brake pedal … assuming the F/R bias is worked out … and all 4 brakes DO lock up … you have TOO MUCH total braking force … for that driver, car & set-up.

    Before we move forward on the technical side, let’s get clarity on the human side. Assuming our goal is optimum track day performance, we have to take into account many factors about the car & track ... but also the driver. We don’t want to build a brake system that causes the driver to wear themselves trying to threshold brake.

    For optimum track performance, we need a brake system that allows the primary driver (you or whomever that is) to achieve threshold braking … meaning braking as hard as the car, tires & brakes will allow without locking them up … while only using 60-75% of the driver’s leg strength. If the brakes require the driver to use 90-100% of their leg strength … every corner … for xx laps … even well conditioned drivers are going to get tired legs. And tired legs lead to inconsistent braking & slower laps times.

    On the other hand, we don’t want the brakes to be so “touchy” … that the driver is locking up all 4 tires with moderate to high leg pressure on the brake pedal. In a panic stop (wreck in front of them) they may lock up all 4 tires & lose car control at that point … crashing … versus slowing & steering around it. This is key ... the brakes need to have good feel to the race driver, so they can modulate the brakes by feel. We do not want them to act as "on/off" switches.

    So, the goal for our brake system is to provide good, hard, balanced, threshold braking without the driver wearing themselves out. This is going to vary from driver to driver and is a very personal thing. I suspect for most of you … it is your car … you’re driving … so build it for you. I have ran cars the last 8 years for other drivers, and we were ALL happier when the braking was tailored to them. If someone else is the primary driver … I suggest you build the brake system tailored for them.

    Now, let’s get technical …
    My experience is mainly race cars, but all kinds … from Formula Fords (900# & 110 hp) … to Cup style stock cars (3500# & 900hp). I have seen brake systems with total brake force of 1600# on light race cars to over 6000#. It is possible to have too little braking force ... and possible to have to much braking force. I have dealt with both and neither situation is desirable in a competition vehicle.

    What’s right on one car is wrong on another. Heck, what is right for one driver … can be wrong for another. There is no optimum # … or correct number … or formula to achieve optimum brakes for each specific car & driver combination. But there are guidelines that can help you dial in yours.

    Here are some guidelines, tips & experience …
    1. Lighter cars need less total braking force & heavier cars need more.

    2. More traction/grip … allows you to use more total braking force.
    3. Anything … and everything … that affects the GRIP … affects the total brake force needed to achieve optimum braking.

    4. Install new softer and/or wider tires … with more grip … and now the car can use more braking force.
    5. Change the suspension (physically with parts or by tuning) … giving the car more grip … and now the car can use more braking force.
    6. Add some aero device that works at creating downforce … and now the car can use more braking force.

    7. Make the car heavier (turbos, cage, frame stuff, stereo, etc) … and now the car NEEDS more braking force.

    8. Smaller (ok, let’s say it) or weaker drivers … need more braking force built in. Bigger, stronger drivers require less braking force. It all has to do with leg strength & conditioning.
    9. All of my brake force formulas are based on 100# of pressure applied to the brake pedal. If the driver can only push 75# comfortably … we just lost 25% of our braking. On the other hand, if the driver is a big ole strong boy and is comfortable pushing 150# all day long … we just gained 50%. That is a big swing, and why the driver ALWAYS needs to be factored in the equation.

    Ok, once you know if you have too much, or too little, Total Braking Force … how can you fix it? You have a lot of options. I’ll outline them below with any major pros, cons or concerns . and you chose what works best for your individual situation.

    To ADD total braking force:
    A. Increase brake pedal ratio.
    Con: requires buying or making a new pedal. Could have fitment issues.
    Pro: Relatively low cost & no added operational costs.
    Concern: Do the math to insure ratio is optimum. Pedal travel will be longer.

    B. Install calipers with larger pistons.
    Con: Expense of buying new calipers.
    Pro: No added operational costs.
    Concern: Availability, may require pad change, fitment issues. Pedal travel will be longer.

    C. Install master cylinder(s) with smaller pistons.
    Con: Expense of buying new master cylinder(s), but less than buying calipers.
    Pro: Relatively low cost & no added operational costs.
    Concern: Pedal travel will be longer. Don’t go so small the M/C lacks sufficient fluid volume.

    D. Install rotors with larger diameter.
    Con: Expense of buying new rotors… probable added operational costs from rotors that cost more to replace … and increased rotating weight negatively affecting performance.
    Pro: Just the additional braking force.
    Concern: Availability & fitment issues.

    E. Install brake pads with higher friction numbers.
    Con: Expense of buying new pads now … and probably added operational costs from pads that wear out quicker and/or cost more per set.
    Pro: Smaller initial expense than buyer calipers. May, or may not, cost less than master cylinder(s).
    Concern: Getting high enough friction pads to do the job … and not wear out in everyday street driving.




    To REDUCE total braking force:
    F. Decrease brake pedal ratio.
    Con: requires buying or making a new pedal. Could have fitment issues.
    Pro: Relatively low cost & no added operational costs.
    Concern: Do the math to insure ratio is optimum. Pedal travel will be shorter.

    G. Install calipers with smaller pistons.
    Con: Expense of buying new calipers.
    Pro: No added operational costs.
    Concern: Availability, may require pad change, fitment issues. Pedal travel will be shorter.

    H. Install master cylinder(s) with larger pistons.
    Con: Expense of buying new master cylinder(s), but less than buying calipers.
    Pro: Relatively low cost & no added operational costs.
    Concern: Pedal travel will be shorter.

    I. Install rotors with smaller diameter.
    Con: Expense of buying new rotors.
    Pro: Probably less operational costs & reduced rotating weight positively affecting performance.
    Concern: Availability & fitment issues.

    J. Install brake pads with lower friction numbers.
    Con: Expense of buying new pads now.
    Pro: Smaller initial expense than buyer calipers. May, or may not, cost less than master cylinder(s). Probable reduced operational costs from pads that last longer and/or cost less per set.
    Concern: Some lower friction pads do not do well in high temperature environments. Study the compound graph & know your operating temperature range.

    That’s all for now.

    There’s still lots more to know about brake systems, from fluid characteristics, heat transfer, ducting, rotors types, bedding pads, rotors, proportioning valves, brake lines & much more. If someone has specific questions, feel free to ask.


  2. #42
    Join Date
    Jun 2006
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    Katy,TX
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    So a related update . The '81 TA I just test drove after a few years being down and redoing interior, and then dash and suspension. Just before I had thrown the old Bendix organics on the factory rear discs . Could lock them up easily and the original to the car fronts did not seem to be doing much. Figured old tired calipers and pads so threw a 1LE on and using D614 larger Hawk pads on the front. Even after a fair amount of stopping to bed in the fronts rears lock. So either the organics grab too easily or my factory prop valve is bad.

    Options swap on some other rear pads, replace factory prop valve with a T and adjustable rear.
    1978 Black Trans Am 455 Edelbrock heads 10.99@124.5 through mufflers on pump gas
    1981 Trans Am 400 stock type motor
    79 Camaro getting a 500" 695 hp IA2 Pontiac motor
    1965 GTO project car
    470ci/Chevy dual quad 409 604 HP 64 Impala SS project
    2004 Pulse Red GTO

  3. #43
    Join Date
    Mar 2005
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    Walton, NY
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    Skip, you sure the Hawk pads on the front aren't the problem? HPS? I never had a good feel with them.

    I ended up going 13.4 Z51 front with some Advance "Wearever Gold" GG friction rated pads.

  4. #44
    Join Date
    Jun 2006
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    Katy,TX
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    Hawks could be but the other calipers and previous pads on the car originally when I got it and driving OK before the rear end and rear calipers got freshened had the same issue. So far only a few miles on the Hawks.

    Granted these are old school organics from the 80s and have great cold stop. Back then the semi metallics were terrible cold. A buddy who was a mechanic and worked on those old ratchet rear calipers said sometime the rear semi metallics would expand with heat and since the piston can no go back any with the ratchet bind so organics were better.
    1978 Black Trans Am 455 Edelbrock heads 10.99@124.5 through mufflers on pump gas
    1981 Trans Am 400 stock type motor
    79 Camaro getting a 500" 695 hp IA2 Pontiac motor
    1965 GTO project car
    470ci/Chevy dual quad 409 604 HP 64 Impala SS project
    2004 Pulse Red GTO

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