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Thread: Vertical mounted master cylinder

  1. #1

    Vertical mounted master cylinder

    Hello! I' m Lee from South Korea.

    Actually, I'm in charge of brake system for 2015 FSAE. I have several questions about vertical mounted master cylinder.

    In last competition, I saw that many teams installed vertical mounted master cylinder for Pedal system.

    My team will also use bearing mounted master cylinder which is 77 series one from Tilton engineering to reduce overall length of pedal box.

    But, I wonder how I can get pedal ratio.

    When using vertical mounted master cylinder, balance bar should be installed upward. From that, I got 3.29 of pedal ratio. I think it's not enough for braking force.

    Could you guys give me an advice of pedal ratio for vertical mounted master cylinder? I calculated 3.7 by looking over the following thesis.

    http://dspace.mit.edu/bitstream/hand.../191801934.pdf

    I wonder that those calculating method is right or not.

    Thanks.
    Last edited by shawnBaek89; 10-15-2014 at 11:43 AM.

  2. #2
    No one can answer that question without knowing your master and caliper bores and rotor diameter.
    What is your calculated pedal force per g of braking? If you can show the math that comes to that answer it will be much easier for others to help double check your work.

  3. #3

    vertical mounted master cylinder

    Thank you for your reply.

    I calculated pedal force per 1.5g of braking. I got 1.5g of data by installing G sensors in last-year car for 2014 FSAE.

    Here are several math things with picture to get pedal ratio.

    FBD.jpgFN1.jpgFN2.jpgFN3.jpg

    From those equations, I got 3.29 of pedal ratio.

    My team's car need 2871N of front braking force and 1248.6N of rear braking force for 1.5g braking.

    But, 3.29 of pedal ratio is not enough for us to generate braking force from Driver's foot.

    Based on 3.29 of pedal ratio with 60kgf pedal effort from driver's foot, it generates only 2111N of front braking force and 962N of rear braking force.

    It seems to increase pedal ratio. But, I want to know I did right things to get pedal ratio as picture that I attached.

    How do you guys derive pedal ratio? It's first time to design pedal system with using vertical mounted master cylinder for me.

    Could you give me an advice?

    Front and rear Master cylinder bore = 0.015875m
    Front rotor diameter = 0.187m
    Rear rotor diameter = 0.177m
    Tire diameter = 0.450m
    Caliper diameter = 0.0254m

    Thanks
    Last edited by shawnBaek89; 10-16-2014 at 02:36 AM.

  4. #4
    Senior Member
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    Lee (Shawn...),

    I started reading the MIT thesis you referenced, but after seeing ~half-a-dozen errors in as many pages I stopped.

    One of your diagrams has the length C varying from 162.88 mm to 135.048 mm. Really? Do you really think that you need, or can even measure, this length to an accuracy of microns!!!???

    Your reliance on the MIT thesis indicates that your teachers know nothing about car braking systems.

    Your ignorance of practical tolerances indicates that your teachers know nothing about real-world Engineering.

    I strongly suggest that you approach whoever is responsible at your school, and ASK FOR YOUR MONEY BACK!
    ~o0o~

    Meanwhile, if you put in the effort of posting ALL your calculations on this thread, then maybe someone here might be bothered checking them.

    Z

    (PS. BTW, I would aim for a Pedal-Ratio = ~2 (maximum of ~3).)

  5. #5
    Sure the 77 series has a stroke of 1.1 inches, but I doubt that you will use all of it, especially with one inch single piston calipers. I would estimate - purely internet speculation here - that the mc stroke is more on the order of half inch or so. Thirty+ degrees of brake pedal rotation seems a bit high to me. I imagine, to a certain extent brake pedal travel is a matter of personal preference, and personally, I like it to be very little - more like 10ish. I also got c final as 134.3376 - rounded off to the nearest 10 thou millimeter.
    Fitz Matush
    Auto Seat Tester

  6. #6
    Thank you for your reply.

    Yes, 162.88 mm to 135.048 mm means pull stroke of Tilton's 77 series master cylinder.

    This is just an assumption of pull stroke.
    Last edited by shawnBaek89; 10-15-2014 at 09:59 PM.

  7. #7
    Z,

    As a former brake system designer I have a few comments against your post. Our brake pedal travels .2" linear inches to achieve full lock of front and rear tires. Most of this travel comes from the fact that we have a very large pedal ratio to balance our system against the physically small components. This doesn't seem to have any adverse effects and one particular driver on a car swap day from another school had described the brake pedal feel of our car as "a pair of angel titties". Very easy to modulate input force, moves like a brick wall.

    A vague suggestion of pedal ratio (mechanical gain) of 2 - 3 (most road vehicles use a mechanical gain of ~3 - 5) isn't very helpful as this neglects the hydraulic gain from the system. 2 - 3 might be suitable with tiny master cylinders that would most certainly be at the limit of their fluid displacement capacity to move the caliper pistons. Sure it's nice for minimizing the stress in the pedal, but short sighted in the rest of the system design.

    Lee, pedal ratios on FSAE cars can vary wildly and tend towards the high side due to the minuscule components and high grip that must be overcome. Don't be surprised if you end up needing some ratio like 9:1 to accomplish your goals. On the flip side, aiming for Z's suggested pedal ratio of 2:1 requires fairly large brake system components that add more weight to the car than just reinforcing the brake pedal a little more. Also, the costs will be much higher to buy these larger components if that is a concern for you.

    I would also say that referencing the recommended pedal input force from the National Highway Transportation Safety Administration (NHTSA) was key for me to understanding how much is too much and too little for the drivers to handle. 60kgf sounds rather high to me. Maybe half of that would be more manageable? Try testing it out with your drivers and a scale to see what they can easily modulate.
    Kettering University Vehicle Dynamics
    Formula SAE 2010 - 2015
    Clean Snowmobile Powertrain 2012 - 2015

    Boogityland 2015 - Present

  8. #8
    MCoach,

    "one particular driver on a car swap day from another school had described the brake pedal feel of our car as "a pair of angel titties"."

    you mean the wife swap day

  9. #9

    Talking Wife swap, ya'll.

    Yes, certainly. Wife Swap Day.
    Kettering University Vehicle Dynamics
    Formula SAE 2010 - 2015
    Clean Snowmobile Powertrain 2012 - 2015

    Boogityland 2015 - Present

  10. #10
    Hello shawnBaek89,

    The methodology of designing braking system starts from your car vehicle dynamics, after you calculates your braking forces on each tire you will need to develop the hydraulic system that fits your calculation on a wide range. there are several parameters on the braking system starts from the pedal ratio - bias(front - rear) MC bore sizes calipers bore and rotor, to make your life easier try to fix some parameters and change the others then do some iterations and check your outputs . you may build a system that gives you the braking forces you need with a pedal ratio = 5 and driver force = 400 and so on . build a matlab code for your iterations and vary your inputs, creat a new methodology to chose the fit system to your car.

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