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Thread: Vertical mounted master cylinder

  1. #11
    And a word to say i did read mit paper that shawnBaek89 mentioned and there are some errors.

  2. #12
    Thank you for all of you guys kind reply.

    To Ahmad rezq,

    What is the error of MIT paper? Is there any wrong about calculation of pedal ratio from MIT?
    Last edited by shawnBaek89; 10-17-2014 at 02:38 AM.

  3. #13
    Senior Member
    Join Date
    Mar 2005

    Quote Originally Posted by MCoach View Post
    ... one particular driver ... described the brake pedal feel of our car as "a pair of angel titties".
    This has me baffled.

    I am guessing that he meant the feel of BOTH pedals, accelerator and brake? In which case "angel titties" is probably a good optimisation goal, though hard to put numbers to.

    But ... if you described your girlfriend thus, then might she not get very upset that you are suggesting that she has one that is very firm, pert, and upwards pointing, while the other is soft, squishy, and sagging down to the floor?

    Back to brakes.

    "Very easy to modulate input force, moves like a brick wall."
    I AGREE with this as a major design goal. Namely, a rock-hard brake pedal with minimal movement.

    This contrasts with many newbie Brake-Guys, such as the OP, who seem to think that "the ideal" is to use almost all of the Master-Cylinder's stroke. So they aim for a pedal travel of several inches. <- WRONG!!! (Are you paying attention, Lee?)


    "A vague suggestion of pedal ratio (mechanical gain) of 2 - 3...
    ... isn't very helpful as this neglects the hydraulic gain from the system ...
    ... might be suitable with tiny master cylinders ...
    ... short sighted in the rest of the system design ...
    ... requires fairly large brake system components ...
    ... more weight ...
    ... costs will be much higher ..."
    All above comments are WRONG!

    In fact, the reason I only mentioned the pedal ratio was to see who would repeat the above false arguments. It is a common mistake.

    I reckon the main reason that most FSAEers automatically assume that the pedal ratio must be around 5 is that the pedal is one of the very few brake-system parts that the students actually make (disc is the other). And ... "since everyone else uses a pedal ratio = ~5, we should too..."!

    Very disappointing that so few students think for themselves...

    So, here are a few brief suggestions on how to get rock-hard brake pedals (much easier than "rock-hard abs" ).

    Obviously, the aim is to minimise unnecessary freeplay, and flex of stressed components.

    Freeplay at the pads is mainly controlled by the piston seals, so not much you can do there. But reducing "pad knock-back" from warped discs or sloppy wheel-bearings helps. Freeplay at the pedal includes the small but necessary stroke needed to uncover the MC's "refill hole". This stroke is multiplied directly by the pedal ratio. So lesser pedal ratio = lesser MC freeplay = better.

    Flex is reduced by lowering structural stresses. For given size components, this is easiest done by lowering the forces acting on them. Now consider these components and the forces acting on them, starting at the wheel-end first.

    1. THE CALIPER - A greater normal force Fn between pads and disc means more squashing of the pads, more spreading of the caliper, and hence more flex. Less normal force Fn = less flex of pads and caliper = better. For a given required deceleration force at the wheelprint, Fn is mainly determined by Radius.wheel/Radius.disc, and by the "Mu" of the pads.

    So reduce R.w/R.d as much as possible, perhaps easiest by starting with small radius wheels and tyres. Then choose the highest Mu pads that are available, economical, durable enough, and have the right "feel". There is NOT a great deal to be gained in this area if you are using off-the-shelf parts.

    2. THE HYDRAULICS - Greater hydraulic pressure means more squashing of the brake-fluid, more expanding of the brake-lines and other wetted components, and hence more flex. Less hydraulic pressure = less flex = better.

    So aim to get the required Fn pad-disc force with a lesser (but still practical!) hydraulic pressure. This simply means CHOOSING CALIPERS WITH LARGE PISTON AREA! More details below. (Interestingly, another recent thread has the OP worrying that his pressures are too low, because they are lower than everyone else's!!!)

    3. THE PEDAL - Greater pedal ratio means greater stresses in the pedal structure, and in the MC-push/pullrod(s), and in the many connecting joints including the bias-bar, and in all the surrounding structure out to the MC(s) mounts, and hence more flex. Less pedal ratio = less stresses everywhere for a given amount of structure = less flex = better.

    So, summing up, if the OP chose calipers with, say, 1.5" diameter pistons, and ALL ELSE AS BEFORE, then he would have less than half the hydraulic pressure, so roughly half the flex there (it is not quite linear). Furthermore, for the same foot-force on the brake pedal and the same MC(s), he would only need half the pedal ratio, which gives roughly ONE-QUARTER the flex there (the relationship is roughly quadratic).

    Note that the calipers would NOT need to have any more mass because the spreading force on them is unchanged, so structurally they can remain the same. They just have bigger "holes" in them. However, the brake pedal and associated structure around it (ie. including the MC mounts, etc.) would be much simpler, lighter, stiffer, ... = better.


    "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 ..."
    "0.2 linear inches" is GOOD ENOUGH, IMO. It seems that the rest of the system must be well-detailed.

    However, if you want an even rock-harder brake pedal, then you can use smaller bore MC(s) fitted at a lower pedal ratio. Given that your foot pedal only travels ~5 mm, and the MC thus moves much less (ie. /PR), you should not have any problems with "... fluid displacement capacity to move the caliper pistons."

    Or you can go all the way and try bigger caliper pistons (or more of them), and an even smaller pedal ratio.

    Bottom line, just because everyone else does something stupid, does not mean that you should too.

    Last edited by Z; 03-31-2015 at 11:12 PM. Reason: ... details ...

  4. #14
    I think your stress analysis of the system adequate, and yes my style is loose and fast, but that' fine by me.

    I think I will take your advise on suspension topics on this topic, aiming for what is an acceptable amount of compliance and movement rather than "optimizing" the system.
    My goals (when working this system) were pushing the limits on minimizing mass without giving up the "angel titties" feeling. I got quite a chuckle of out of it, but he certainly was not describing the gas pedal. So in our case, a high pedal ratio, high stresses, high temperatures, and tiny calipers are the compromise for a weight and cost efficient system that provides a system that reacts with respect to the drivers (exceeded!!!) expectations.

    Main thing here is packaging. I don't think we'd be able to fit a caliper with 1.5" pistons in our system. Hell, I spec'd 0.070" clearance between the caliper and wheel shell and we were only working with 1" piston caliper. So it's not like I was not trying to get what I could out of the system. We're using the smallest master cylinders that we can so there is not an easy change in this design. To a driver, anything under .25" seems to be irrelevant, they just don't notice.

    Kettering University Vehicle Dynamics
    Formula SAE 2010 - 2015
    Clean Snowmobile Powertrain 2012 - 2015

    Boogityland 2015 - Present

  5. #15
    Thank you for all of you guys comments.

    It would be really great help for me to design the awesome brake system!

  6. #16
    Junior Member
    Join Date
    Jan 2009
    South Dakota
    From a Human Machine Interface perspective, there's a reason that newer combat aircraft use control columns (centre-sticks) that function only on force input.. they deflect only enough to allow strain gages to function. The human body can control things much more accurately when moving things in 3-d space is removed from the equation. Most driving instructors tell you to concentrate on flexing only the larger muscles in your thigh to modulate braking force.

    One would think this should also apply to throttle control, but I haven't seen anyone try that yet.

    Packaging sure would be a lot easier with a brake and/or throttle by wire system fed from a pressure sensitive pad bonded to the inner skin of the front bulkhead....

  7. #17
    Senior Member
    Join Date
    Mar 2005
    Quote Originally Posted by shawnBaek89 View Post
    It would be really great help for me to design the awesome brake system!
    Some more suggestions for "Brake-Guys" to think about.

    1. To repeat above point, a "rock-hard" brake pedal with good "feel" is a good goal to aim for. MCoach's ~5 mm of movement of the brake pedal qualifies as rock-hard. It is generally easier to achieve this rock-hard-edness with a low pedal ratio, and lowish hydraulic pressures.

    2. Where to put the MCs?

    The general big-picture rule for packaging components of these cars is "Everything heavy to be low down and close to the CG". This reduces CG-height and Yaw-inertia for better all-round performance. (This rule also applies when packing the family wagon for holidays. )

    The common location of MCs in front of the pedals puts them far away from the CG. This is NOT TOO BAD, because the MCs are quite light.

    BUT (!!!) this also pushes the HEAVY front-bulkhead and IA about a foot (0.3 metres) forwards of where it could be. It also makes the chassis heavier by an extra 0.3 metres of unnecessary and heavy structure.

    So, why not put the MCs and bias-bar parts, say, hmmm...,
    ... under the driver's left knee?

    Plenty of room there. Just use a pull-rod (or two) coming back from the left side of the brake pedal, at a pedal ratio of about ~2.

    End result is that the chassis can be shorter, lighter, stiffer, and with much lower yaw inertia. And (!) bias-bar adjustment can be done by the driver, whilst driving, with his left hand on a small knurled knob on the end of the bias-bar. Too easy! (I have seen a Team spend almost a year trying to design an electric servo-motor controlled bias adjuster, before they eventually gave up because too hard...)

    3. Question: Why is almost (*) every bias-bar ever made mounted HORIZONTALLY?
    Answer: "Because that is how everyone else has done it before, so we have to do the same!". Grooooaaannnn...

    A conventional brake pedal and single MC is a planar linkage lying in a vertical (side-view) plane. So, as a planar linkage, all its joints can be simple revolutes.

    A conventional brake pedal with a horizontal bias-bar is a 3-D linkage. That is, for everything to work properly, more "flexibility" is required in the joints than can be had from a few simple revolutes (ie. more DoFs needed). So, usually some Ball-Joints are used. But the BJs in typical bias-bar linkages can have excessive friction, which adversely effects the biasing of "brake balance". So suppliers of "real racer's brake stuff" keep the same overall layout of horizontal bias-bar, but they then replace the BJs with more expensive and complicated "crossed needle-bearing trunions" (ie. joints that look like UJs) to reduce friction.

    BUT (!!!), making the simple conceptual change of mounting the bias-bar VERTICALLY, with the two MCs mounted one-above-the-other, allows the whole linkage to be planar again. Now only a few, simple, low-friction, needle-bearing revolutes (or even simpler "knife-edge revolutes"!) are needed to eliminate the friction problems.

    I can imagine some very simple such arrangements mounted under the driver's left knee, near the cockpit left wall. Simpler, stiffer, lighter, less friction, easier to make, so cheaper, etc., etc...


    (* PS. The earliest Citroen ID/DSs, introduced ~1955, had AUTOMATIC (load sensitive) F/R brake-balance adjustment via a bias-bar that was mounted in the same plane as the brake pedal. I know of NO other use of such "common sense" (= "the least common of all the senses..."). Later Citroens used an even simpler, but equally effective, system.)
    Last edited by Z; 10-22-2014 at 11:23 PM.

  8. #18
    Senior Member
    Join Date
    Jun 2007
    Perth Western Australia

    There is a rule requiring the brake components to above the chassis lower surface (T7.1.7), which combined with the template rules, makes that solution very difficult. Especially for those with low mount steering.

    I believe the common interpretation is that an additional "guard" does not count as part of the "chassis" if it is lower than the mandated leg bay lower tube.


  9. #19
    Still I believe there is room left where the rack mounts (or thereabout). I believe Tokai university (Japan) used a similar system to what Z describes back in 2007 (?). BTW that car was really "out-of-the-box" thinking...

  10. #20
    You can build such systems you want if you know how to compromise with the other systems if you wanna build back master cylinder have a cup of tea with the steering guy also the ergonomics guy chassis and set your design. don't make it individually and send the design to the chassis guy you will find pieces of rocks falling from the sky on your head ask you to edit your design. backing to the backward master cylinder mount.
    if you want to build this system keep in your mind these things
    1 - a back masters mount will save some chassis weight.
    2 - if your rack is at the bottom you must know the space you have to place your masters
    3 - think of the evacuation of the car as the driver will have a long travel passing the masters the rack and the steering column

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