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Thread: Direct Actuation Vs push or pull rod

  1. #11
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    Quote Originally Posted by Adam Farabaugh View Post
    Another advantage to pull/push actuation in my mind is that it makes integrating anti-roll systems much easier. I'm not quite sure how you would integrate ARBs onto a DASD without adding the bellcranks back in. Then again, who the hell really needs ARBs?

    I imagine that most teams would save 1 lb, maybe 2 lb at most by switching to direct-acting. Can anyone really tell me that those 2 lb makes the difference on the podium? I think not...
    Instead of settling in on the weight advantage, I would emphasize that reducing complexity/part count is MORE important because it potentially gets you off the workbench and onto the track sooner. This advantage is significant, don't underestimate it.
    Adam,

    You have hit on the 2 points that we found running DASD in 2013. We were trying to package the pushrod/rocker, and there was no real structure where we wanted to put things. But, we rotated the model around, and there was a straight line between the control arm and mount. Draw straight line, make a circle the size of DASD and done. As for the anti-roll devise, we had one designed, but not yet manufactured when it was time for first drive. So we re-calculated some wheel rates/anti-roll torque distribution, changed the front springs, and went testing. The car worked well enough that our drivers were always the limiting factor, so we left the extra bits off.

    -Matt
    Matt Davis
    University of Cincinnati
    Bearcat Motorsports: 2012-2013: Suspension guy

    Bilstein: 2013 - ??: Product Engineer

    This post is a collection of my own thoughts and opinions, and in no way, shape or form reflects the thoughts/opinions of my company, my university or anyone else but myself.

  2. #12
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    Quote Originally Posted by OspreysGoSWOOP View Post
    Why are you always so rude to Claude? The seminars he provides at the FSAE events are amazing and have provided learning opportunities to hundreds of young engineers.
    Osprey,

    Because of all those seminars he gives, Claude has a GREATER RESPONSIBILITY than anyone else here TO GET THESE THINGS RIGHT.

    This thread topic has come up many times on this Forum. It has been made clear many times that you cannot reduce a system's mass by ADDING mass to it. OBVIOUS!!! So obvious that even non-technical people get it. The fact that Claude and others keep repeating this nonsense makes me ... err..., frustrated. I was being VERY polite.

    "It's very clear that Claude places a lot of importance on being a life long learner."

    I can find the link if anyone wants, but I recently posted about this very matter. I gave at least a screenful of calcs showing how P/PR&Rs, especially with highish MRs (ie. around MR = ~1, = the "current fashion"), INCREASE the wheel-assembly's inertial resistance to acceleration. (This happens even with massless P/PR&Rs!) Frankly, the theory is so simple I should not have had to spell it out.

    (Edit: Here is the link to two consecutive posts on rocker-MRs. The second post gives the above calcs.)

    But has Claude learnt anything from that?

    Well, Claude???
    ~~~~~o0o~~~~~

    Adam,

    "I'm not quite sure how you would integrate ARBs onto a DASD without adding the bellcranks back in."

    Why not like on almost every production car ever made? Namely, a U-shaped-bar thrown under the car, with some drop-links going to the wishbones or uprights.

    But ... "... who the hell really needs ARBs?"

    Yes, indeed! (Edit: As Matt just confirmed, while I was typing this...)

    "I imagine that most teams would save 1 lb, maybe 2 lb at most by switching to direct-acting."

    I reckon quite a bit more, when you add in all the extra chassis tubes and nodes that students think MUST be there.

    "... I would emphasize that reducing complexity/part count is MORE important because it ... gets you ... onto the track sooner."

    Agreed, and this is the most important point.

    Unfortunately, it is most Teams' lack of any sort of big-picture thinking that has Chassis-Guy automatically designing in tubes and nodes for the rockers and SD mounts, even before Suspension-Guy has decided whether P/PR&Rs are even needed. This is all driven by the "pressure of fashion". A desire to "do what everyone else is doing".

    For example, a currently popular fashion that really irks me is the front rockers and SDs as high as possible on top of the nose. Often right in the driver's sightlines! It is hard to think of a single engineering justification for this. (I have heard "Err..., it makes the dampers easier to adjust!" <- Can you see why I get frustrated...).

    But, with expert Design Judges telling the students that it is a good idea because it "...reduces unsprung mass...". Groooaaannnn!!!
    ~~~~~~~o0o~~~~~~~

    Maybe off-topic, but consider the current spread of Ebola.

    The death count in Africa is now about 5,000, but will surely climb (ie. over next few months, or ~year). This is despite Ebola being NOT very contagious, and only killing ~20% of infected people who get reasonable health care. But Africa is the Dark Continent, and it is in a permanent Dark Ages.

    So, what about the spread of such diseases in the Western world?

    Well, last time that sort of thing was common was ... yes, you guessed it ... during the last Dark Ages! And very common it was... (ie. Black Death, etc...).

    So far the Ebola virus has only managed to get a tiny foothold in the West. In one instance it managed to hitchhike over to America. When the virus's host eventually succumbed to its effects, the host was placed in the best possible medical care in one of the most advanced First-World hospitals on the planet. And then ... the tiny little virus managed to infect two more of the nurses in said hospital!

    Short story, the piss-poor EDUCATION given to the nurses in that First-World hospital allowed the little virus to win that battle. That was an example of the sloppy, lackadaisical attitude that is now also rife in Engineering education, as shown here. It is the sort of attitude that allows anyone, but especially the experts, to blurt out any old opinion, on any matter at all, and the stupid and uncritical youngsters swallow it whole.

    Medical expert, "Oh no..., you don't need a full body suit, just this little face mask will do..."

    FSAE expert, "Oh no..., you don't need DASDs, they just increase your unsprung mass..."

    You young people will be here a lot longer than me.

    You reap what you sow...

    Z
    Last edited by Z; 10-31-2014 at 10:20 PM.

  3. #13
    I imagine that most teams would save 1 lb, maybe 2 lb at most by switching to direct-acting. Can anyone really tell me that those 2 lb makes the difference on the podium? I think not...
    Considering the weight and simplicity Direct Acting will also save the whole car 4 brackets = 8 welding points if you are welding your brackets(space frame chassis), 8 ball joints (with their own tolerances) , 12 fasteners.
    Last edited by Ahmad Rezq; 04-22-2015 at 01:29 PM.

  4. #14
    I suppose there is no correct answer to what is better. The question is, is it worth the effort? Reducing unsprung mass vs. extra time to spend on other parts of the car. Which grants you more points? What makes the car go faster? Do linear motion ratios matter? What is the maximum unsprung mass you can work with? Those are the questions that really matter. Push or pullrod or direct acting or whatever system is just a reflection of the answers to those questions. And I guess that's true for all parts of the car.
    Daniel Muusers
    Formula Student Team Delft
    2010-2015

  5. #15
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    The thing that has bothered me the most about the use of push/pull rods (not just in FSAE but in general) is that everyone seem to want to these high motion ratios for some reason. Then when you ask them, what is your damping ratio at the wheel? They don't have an anwser. This already shows that most people have no-idea why they are putting push rods into their suspension. Don't forget that motion ratios that give faster damper speeds are going to amplify the damper friction too. So moving from a motion ratio of 0.5 to a motion ratio of 1 will double the friction hysteresis coming from the damper.

    I do agree however, that setting up a direct acting layout with such a small damper is not easy. You either need to a: lengthen the damper,or b: extend the chassis so the damper can connect to a sufficiently outboard part of the suspension. Both of these solutions are going to add weight. However, you still have the advantage of having 8 less bearings on each axle, each contributing it's own friction, compliance and free play.

    Regarding the unsprung mass argument - Erik is right. Push rods will increase the total unsprung "inertia". People often forget that the mass/inertia of the moving part of the damper is reflected back to the wheel proportional to the square of its motion ratio. So again, if you go from a motion ratio of 0.5 to 1.0, you quadruple the damper's contribution to the overall unsprung mass. Then you have the extra inertia of the push rod and the rockers. Its frustrating to continually see this oversight - since its first year mechanics.

  6. #16
    My initial thought (couple years ago) about direct actuation is that you either put an a-arm in bending, or you have to have a large damper.
    Recently, pretty sure it wouldn't be too hard to extend the damper by making a sort of cradle to align the damper body with a link (see attached).

    Without checking the math, I think you have a bit of control over motion ratios with this too. Assume SLA, damper ~connected to the lower/upper ball joint and the end of the rod pinned to the chassis, then you could control:
    1) the linear motion ratio - moving the inboard mounting point up and down (chassis Z)
    (this is actually from changing the angle, but that's almost the same on typical FSAE suspension geometries)
    2) progressive (in rebound) characteristics by moving the inboard point forward or backward (chassis X)
    3) progressive (in bump) characteristics - moving the inboard point toward or away from the wheel-set (chassis Y)
    of course you can flip the terms as you like for bump/rebound, and flip the directions if you move the outboard point instead of inboard.

    ~~~

    Still agree on the point that ARBs are complicated to implement without (as we seem to be trying to avoid) placing a control arm in bending.
    Interestingly by putting a rocker on the chassis mount point of the direct spring/damper, it then connected through an ARB, the corner springs and roll spring are in series... that may open some interesting opportunities for the bold
    Attached Images
    Austin G.
    Tech. Director of APEX Pro LLC
    Auburn University FSAE
    War Eagle Motorsports
    Chief Chassis Engineer 2013
    Vehicle Dynamics 2010-2012

  7. #17
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    Austin,

    Your design looks a lot like what we ran in 2013: https://scontent-a-dfw.xx.fbcdn.net/...85&oe=54F5D1D6

    As for your roll device with rockers, that sounds a lot like what Monash ran in 2013 on their rear suspension. They are running TTX25's direct acting with no caps or anything like that to extend the damper. I think the installation of their dampers is very well done, and can be easily modified to provide a very lightweight frame, even with 2014 rules (a fellow alumni and myself are going to put one together at some point to prove to our former team that it can be easily done).

    -Matt
    Matt Davis
    University of Cincinnati
    Bearcat Motorsports: 2012-2013: Suspension guy

    Bilstein: 2013 - ??: Product Engineer

    This post is a collection of my own thoughts and opinions, and in no way, shape or form reflects the thoughts/opinions of my company, my university or anyone else but myself.

  8. #18
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    So maybe your lower arm has to be a little heavier to cope bending loads. But if this weight increase is less than the weight of a push rod, rocker with 2xbearings, 3xbolts nuts washers + the associated extra attachments on the chassis...

    What exactly is the problem of putting the arm in bending??

  9. #19
    This thread is a great example of how far down the rabbit hole most teams have gone under the belief - one reinforced by the design judges - that they are building a mini-F1 car, rather than learning how to engineer.

    That fact that some students can no longer visualize how a standard OEM anti-roll bar mounts in mind boggling.

    I'd further like to point out that nobody has yet pointed out how much time committed by teams to designing, manufacturing, and setting up fixture for bell cranks and additional mounts.

    And lastly, as more teams discover that making proper bell cranks/motion ratios/etc. is difficult, has anyone else noticed the trend over the past year or two of teams (determined to incorporate that bell crank into their design,) end up with something close or very similar to this?
    Untitled.jpg

    Perhaps this is the attempt at direct actuation with FSAE sized shocks?

  10. #20
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    Quote Originally Posted by Claude Rouelle View Post
    And another reason: you reduce the non suspended mass.
    Claude,

    There is now yet another thread where a student is asking how to design 'bell-cranks" (aka P/PR&Rs), so they can get the magical MR = 1.

    No doubt the student thinks they must do this, because, amongst other lame reasons, it will reduce the non suspended mass.

    No doubt the student believes the above, because "Claude Rouelle, THE pre-eminent authority on this subject, has said so!". (And you, Claude, have certainly said so here.)

    So, Claude, as an educator of the next generation, do you think you are being responsible in misleading the students this way?

    Would not some sort of retraction, or at least clarification, be appropriate? (Edit: Or, IF (?) you have supporting arguments for your view, then please post them.)

    Note that Bill Mitchell once wrote an SAE paper explaining his "mea culpa" in advising FSAE students to avoid "migrating RCs".

    Z

    (PS. Note that in Alumni's image above, the pushrod could be screwed directly onto the damper-shaft (after removal of the damper's bottom-eye), and the rocker and its frame node (+ 1 x revolute, + 2 x BJs, +++) would be eliminated. Sooo easy...

    And, if you must have ARBs, then look at any production car, or any old racecar...)
    Last edited by Z; 11-29-2014 at 10:16 PM.

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