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Thread: New Pedal Assembly

  1. #1

    New Pedal Assembly

    Hi all,

    My name is Josh and I am fairly new to FSAE. Boston University is starting our FSAE team and is current doing lots of design work. Below is our current pedal box (which I designed today).

    Tilton 75 (.75, .625), custom 7075 pedals, 7075 brackets, all custom. Let me know what you think. Alternate potentiometer mounting options welcome..




  2. #2
    Tilton 75s because of lower cost?
    Why do you need potentiometers?
    You could go lighter on the accelerator pedal.
    Where is your over travel switch?
    Pedal faces that have sides are a big help: example https://scontent-iad.xx.fbcdn.net/hp...41&oe=55D8BD1A

  3. #3
    A few things.

    When you press the pedal what happens to the angle between the master cylinder pushrod and the master cylinder body?

    Is your pedal floor pivot sufficient to resist the loading without deflection in either the plate or the brackets? or failure of either?

    What is your pedal pivot exactly? is it a bearing or is it just a bolt?

    Is your master cylinder mounting plate sufficiently attached to your base plate to prevent bending/deflection under loading?

    Why does your accelerator pedal need to be the same as your brake pedal? why does it need to be as strong? does it need to be as strong?

    If you locate the pedals on the lowest mounting option is that web strong enough to prevent collapse of the pedal?

    Does the pedal box need to be a separate component? can it not be part of your monocoque/frame?

    That base plate looks heavy.

    Where is your throttle pull? do you need one or are you using electronic throttle control? if not, why not?

    Why all 7075? why not a lower grade? why not steel?

    Where is the over travel switch?

    What is your pedal ratio?

    What loads are you expecting as driver inputs?

    Can you make the pedal box shorter? if so how? why do you think I suggest this?

    Are you making your own pedal faces? Why not use commercially available ones? I suggest Tilton pedal faces, I've used them extensively for a simulator application and they're quite good, plus, business logic etc?

    Do the pedals require the same face geometry? for example, when I hit a brake pedal I don't load it the same way I do a throttle pedal.

    Why do you want the potentiometer there? I assume you're wanting to measure pedal travel but what about line pressures?

    Don't take any of this as me knocking your parts, anyone brave enough to post their work on here deserves respect in my eyes
    Aston University Formula Student - VD/Suspension guy.

  4. #4
    Hi guys, thank you for your responses. All advice is always appreciated.

    I guess i'll answer these in order, although most are meant to be thought-provoking, not answered exactly.

    The horizontal mounting of our master cylinders seems to be the best line up between desired force and minimizing angle between pushrod and body (MC). This seems to be where pivots (77) come in handy.

    We plan to use ball bearings or bushings (cheap, possibly sufficient) in our pedal's pivot assembly.

    There is a rib that holds our MC plate from bending about it's line (plane) of contact, and FEA confirms that this is plenty strong. Our forces distributed over the number of mounting bolts is also well within a comfortable safety zone (SF2).

    We are currently modeling the accelerator pedal as a bake pedal for ease of modeling, and it will eventually be replaced with a thinner, lighter pedal. No, it does not need to be nearly as strong as the brake pedal.

    Over-travel switch has yet to be added.

    We used an MIT study which stated that the average driver is capable of applying around 200 pounds of force to a brake pedal. Using this, we calculated the necessary moment arm for our balance bar to allow our system to reach 1100 psi, 100 psi lower than our pressure limit for calipers etc.

    We have some 7075 plate that isn't being used an see this as a good fit.

    Making the pedal box shorter would allow for a shorter frame, which would be lighter and cheaper. The materials for the box itself would also be lighter and cheaper.
    I see mounting them horizontally as the best option because they don't pivot..I feel like I'm missing something here. Will think it over some more.

    Potentiometers. I was under the impression that we are required to use two linear or rotary potentiometers to measure brake travel, and compare values. Two sensors being 10% off from one-another will trip the tractive system to shut off. Maybe this is what I get for trusting my electrical team and not double checking..

    Thanks for the help,

    Boston University Racing

  5. #5
    Senior Member
    Join Date
    Nov 2010
    NSW, Australia
    Hey Josh,

    Christian has asked the majority of questions that need asking, but just to build a bit on that:

    Nothing wrong with decent grade Al for your pedals, but for the box itself it can be very simple (and cheap) to use some structural steel sections (angle iron, etc.). We had an Al box once, and it needed several repairs. Not saying it's impossible (plenty of teams make great Al pedal boxes) but yours looks to be on the fatigue failure side of things (FOS 2:1 is not enough for repeated high loading on Al, especially if you've used the yield strength in your calcs).

    I definitely second the pedal sides suggestion. You bounce around a fair bit in these cars and the template rules mean that you are no longer snug in the cockpit... You either need a brilliant heel tray (not that easy) or some simple flat bits on the sides of your pedals (very easy).

    There are a few options for shortening the pedal box (it is quite long currently). Have a look at some prior art.

    Not sure if there's new rules since I've left, or if you're doing an electric thing and it's specific for that, but brake operation is only really measured via brake pressure (plus the over-travel switch, which is typically just a toggle switch).

    What's the sloped bar behind the brake pedal? Looks like you won't actually be able to actuate the brakes....

    Only other thing is to make sure you have a sturdy mounting to the frame.

    UoW FSAE '07-'09

  6. #6
    Don't have much time so apologies for the short reply but i'm just going to shortcut a little bit for you here. I'll update for the other questions later.

    In short, if your MC pushrod is parallel with it's body unloaded, what happens when your pedal travels on its arc?

    The location on your pedal which your pushrod is located drops vertically, not much but enough. This then means that at full pedal application your pushrod is no longer parallel to the cylinders axis, thus inducing some side loading / friction into the master cylinder. Whilst +/- 5 deg is shown as 'acceptable' for most MC's, i attribute this to being to instil confidence in 'homebrew' kit cars. My thoughts are that when the pedal is fully loaded the pushrod axis should be colinear with the MC body axis so that all the force applied goes into moving the fluid rather than some being lost as friction.

    So basically, move your MC pushrod to a 'fully loaded' state and then design around that. What this means is that with the pedal relaxed the pushrod is no longer coaxial with the body but that's ok because you're not applying much load initially.
    Aston University Formula Student - VD/Suspension guy.

  7. #7
    Jay, Christian,

    Thank you for all the useful feedback. I've made the walls of my brake pedal significantly thicker, now producing a safety factor of roughly 6, using the yield strength for 7075. Not sure if the best thing to do from here would be running fatigue analysis, or taking an experienced FS member's advice; leaning toward advice, since there are unknown scenarios that fatigue analysis certainly won't account for. Our gas pedal is also being made significantly more narrow, as it carries very little force.

    Some changes: considering 6061 as a cheaper alternative, which would mean potentially adjusting thicknesses etc to meet strength standards, which I plan on improving anyway. Considering your steel bracing idea, Jay.

    With fixed support MC, I am limited in my angling options for shortening the assembly. As a first year team, our goal is to make the thing move, and we might stick with our horozontal plan. The next week will be spent exploring angled MC options, after which we'll make a decision and stick with it.

    What FOS should I be designing to? There are many variables, like an unexpected hit from the side of a pedal etc that I cannot calculate forces for. Could you share what FOS you usually use?


  8. #8
    Forgot to ask: how do I know the displacement of my MC's at full force? It certainly won't be the full travel of the MC, maybe half? Has a lot to do with air in brake lines, rotor thickness, and other factors. Doesn't seem like something I can predict. Certainly wont be at either extreme, though, which is something I should take into account. Any thoughts?

  9. #9
    Senior Member
    Join Date
    Dec 2009
    PERTH, Western Australia
    In order of biggest effect to smallest effect.

    Air in the line, compliance/float in all joints, pad travel to contact the disc, line stiffness followed by caliper flex and finally master flex.

    eliminate the first two by correct design and assembly, pad travel depends a lot on the condition of the seals in the caliper but is generally pretty good, if the rotors are warped, then they can push the pads a little further into the caliper.

    Line stiffness is a big one, we run stainless aircraft hydraulic tube to the rear of the car ($10 worth if you find a nice supplier), then use quality braided line.

    Caliper flex can only really be prevented by buying quality calipers, we use brembo P32s and 1/3rd of the travel is due to the spreading of the caliper, the rest is line flex, pad travel etc (our brakes have very little travel anyway).

    Master flex, the same as caliper.

    in a perfect world, full force displacement is next to nothing, in reality, it is easier to build it properly, measure each of the sources and make modifications to suit.
    ex-UWA Motorsport

    General team member 2013-15, Vehicle Dynamics Team Lead 2012
    Project Manager 2011, Powertrain minion 2009/10

  10. #10
    Senior Member
    Join Date
    Nov 2010
    NSW, Australia
    Hi Josh,

    I've been out of uni for a while now but your materials lecturer should be able to fill you in on fatigue life for various materials (J.E. Shigley's Mechanical Engineering Design has some fatigue design in it, for example). But briefly: the problem with just FEA'ing your model and finding a FOS based on the material yield strength is that a) you probably can't machine it as perfectly as you've modelled it, introducing stress raisers etc. b) you might not be loading it exactly as per your FEA, as you've alluded to, and c) the yield strength is not appropriate for fatigue measurements. For example, I did a Google for 6061 vs. 7075 and found this: "ar-15lowerreceivers.com/6061-vs-7075-aluminum-whats-difference/" which puts 7075 yield as 73kpsi but fatigue strength at 23kpsi. Obviously you'd have to check the numbers but that might give you some ideas. I can't remember what FOS we designed for when I was on the team.

    As for shortening the pedal box, what's wrong with running your M/C's underneath? Running them towards the driver via a pull system? There are nice options such as these that can help centralise mass but at the end of the day the most important thing is that they can lock all 4 tyres.

    UoW FSAE '07-'09

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