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Thread: Direct Actuation Vs push or pull rod

  1. #1

    Direct Actuation Vs push or pull rod

    hello my friends

    our team think using direct actuation spring damper system because it's simple in manufacturing and some other reasons, so I did a search to know its disadvantages and i found that:

    1-it cause more drag
    2-prevent modifying the motion ratio (difficult to adjust the spring rate)
    3-more weight in lower control arm

    I want to know what are the other reasons that make the (push/pull) system better than direct actuation system
    Ahmed Aly
    Cairo university

  2. #2
    Those are the three big reasons.

    Another reason might be that using a push/pull rod with a rocker allows you to use smaller spring/damper assemblies. With a direct acting setup the dampers need to be quite long to reach from the unsprung assembly to the chassis, which usually gives you a weight increase.

    Keep in mind that for most of the competition, the speeds of the cars are low enough that aerodynamic drag from the spring/damper assemblies on a direct-acting suspension may well be negligible for your car.
    _______________________________________

    Northwestern Formula Racing Alum
    Head Engineer, Frame/Suspension 2006-2009

    My '73 Saab 99 Road Race Build

  3. #3
    There is one more reason why somebody would look into a "rocker suspension". The rocker geometry would allow the possibility of creating a non linear motion ratio (at least more non-linear that a directly actuated spring) simulating the effects of non-linear bump stops but without the "cost" of hysteresis that these parts usually have and allowing a very precise definition of wheel rate in that operating point. The concept however is usually applied on wing cars with significant down force allowing the mechanical balance over ride height to change significantly to compensate/work better together with aerodynamic balance.

    Cheers,
    dynatune, www.dynatune-xl.com

  4. #4
    And another reason: you reduce the non suspended mass.
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  5. #5
    Drew,

    "Keep in mind that for most of the competition, the speeds of the cars are low enough that aerodynamic drag from the spring/damper assemblies on a direct-acting suspension may well be negligible for your car"

    "MAY well be negligible?" ... Did you quantify that?

    Drag... and downforce... and cooling efficiency?
    Last edited by Claude Rouelle; 10-30-2014 at 08:07 PM.
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  6. #6
    Ahmed,

    And another reason: calculate the effect on roll, pitch and yaw inertia that damper + spring units have if they are inboard or direct actuation: I can tell you it will not be 1 %

    And the same token look the influence of the CG height of at spring + damper units located above (pushrod) and under the chassis (pullrod) and the influence on pitch and yaw inertia of damper + spring units pointing (after the rocker) towards the center of the car or towards the front or rear of the car
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  7. #7
    Senior Member
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    Does anyone ever think these things through!? Groooaannnn
    ~o0o~

    "... direct actuation spring damper system ... disadvantages ...
    1-it cause more drag."


    Rubbish! Most FS/FSAE push/pullrod&rocker systems have their SD stuck out in the wind. So same drag there. PLUS the additional drag from the push/pullrod, AND more again from the rocker.
    ~o0o~

    "2-prevent modifying the motion ratio (difficult to adjust the spring rate)."

    DASD MR can be modified by changing the angle of the SD.

    BUT why would you want to!!! Are not the little clicky things on the dampers there so YOU DO NOT HAVE TO MODIFY MR!? And see other thread from yesterday discussing very easy way to quickly make fine adjustments to spring-rates. Or just swap the springs!

    (BTW, how many FS/FSAE Teams have EVER changed their rockers during a competition, because they suddenly needed a different MR? Would not a spring swap be just as easy?)
    ~o0o~

    "3-more weight in lower control arm."

    Utter BULLDUST! Only a complete FOOL would swallow this lame BS.

    As should be blindingly obvious, a DASD system is lighter by the weight of the push/pullrod & rocker.
    ~o0o~

    "Another reason might be that using a push/pull rod with a rocker allows you to use smaller spring/damper assemblies. With a direct acting setup the dampers need to be quite long to reach from the unsprung assembly to the chassis, which usually gives you a weight increase."

    Same utter BULLDUST as above.

    Spring strain energy, and damper capability to dissipate energy, are the same for both systems, so same mass requirements. And the length of a DASD is no different to that of the pushrod. But the DASD has NO rocker, so all-up it is SMALLER, and has LESS TOTAL MASS.
    ~o0o~

    "... one more reason ... rocker geometry would allow the possibility of creating a non linear motion ratio ... simulating the effects of non-linear bump stops but without the "cost" of hysteresis that these parts usually have and allowing a very precise definition of wheel rate in that operating point. The concept however is usually applied on wing cars with significant down force allowing the mechanical balance over ride height to change significantly to compensate/work better together with aerodynamic balance."

    As noted in quote, this "rising-rate" can be done at least as well with bumps stops. In fact, bump stops give much greater scope for non-linear variation (think about it!). And they can be made without hysteresis, although, oddly enough, the dampers are there to ADD hysteresis! (See off-road racers' "hydraulic bump-stops".)

    Aggresive rising-rate works best with conventional (ie. narrow-minded-"suspension-engineer"-thinking) "third-springs", where it helps control aero-platform Heave and Pitch motions. It is not at all good for the corner springs because it causes the body to lift during cornering. So, for good cornering, falling-rate corner-springs are better (also just covered on other thread).

    So, given that both rising and falling-rate can be good and bad (at the same time!), what do most racecars end up running with their corner-spring rockers? Yep, linear, or as close to it as they can get.
    ~o0o~

    "And another reason: you reduce the non suspended mass." !!!!!!!!!!!!!!!!!!!!

    So, Claude, you are telling us that you have invented the fabled "Anti-Inertial-Drive-System"!!!!!

    I guess you can forget about living out of a suitcase giving seminars to students, because soon you will be a SQUILLIONAIRE!!!

    (On the other hand, you might want to check your sums... Or post them here and I will tell you where you went wrong.)
    ~o0o~

    Why is everyone so keen to follow "fashion", without ever thinking about it???

    Why are there so few rational thinkers anymore???

    (Oh, yes..., failed education system... grumble... )

    Z

    (PS. Claude just posted a few more... Same-same...)
    Last edited by Z; 10-30-2014 at 09:07 PM.

  8. #8
    Quote Originally Posted by Z View Post
    So, Claude, you are telling us that you have invented the fabled "Anti-Inertial-Drive-System"!!!!!

    I guess you can forget about living out of a suitcase giving seminars to students, because soon you will be a SQUILLIONAIRE!!!

    (On the other hand, you might want to check your sums... Or post them here and I will tell you where you went wrong.)
    ~o0o~
    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.

    It's very clear that Claude places a lot of importance on being a life long learner. I imagine he's a very busy person but he consistently participates in almost every post here about vehicle dynamics (regardless of post quality).

    This is a forum for civil discussion and respctful exchange of ideas. We should try to foster those attitudes.
    www.OspreyRacing.org
    University of North Florida

  9. #9
    Senior Member
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    While Z may have been rude his comments were correct. I'm sure it was an oversight by Claude.

    Please note that while smaller the mtb style shocks are no lighter than the larger 1/4 midget style shocks, that provide a great base for direct acting shocks. Put in the rods, rockers and extra mounts and the push/pull rod is definitely heavier. Also please note that the wishbones do not need to be heavier to take a direct acting shock.

    Kev

  10. #10
    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.
    Penn Electric Racing

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