Direct Actuation Vs push or pull rod

[Ahmed Aly]

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

[Drew Price]

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.

[dynatune]

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

[Claude Rouelle]

And another reason: you reduce the non suspended mass.

[Claude Rouelle]

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?

[Claude Rouelle]

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

[Z]

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...)

[OspreysGoSWOOP]

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.

[Kevin Hayward]

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

[Adam Farabaugh]

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.

[mdavis]

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

[Z]

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 (http://www.fsae.com/forums/showthread.php?11396-how-to-design-a-bell-crank-for-push-rod-suspension&p=118722&viewfull=1#post118722) 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

[Ahmad Rezq]

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.

[DMuusers]

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.

[Tim.Wright]

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.

[Goost]

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

[mdavis]

Austin,

Your design looks a lot like what we ran in 2013: https://scontent-a-dfw.xx.fbcdn.net/hphotos-xaf1/v/t1.0-9/164946_549367231780755_881470777_n.jpg?oh=ef039bc4 d5df516e3b76f3f52a5e4785&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

[Tim.Wright]

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??

[Alumni]

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?
407

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

[Z]

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...)

[Claude Rouelle]

One additional information about the pushrod Vs pullrod debate.

Pushrods mainly work in compression and buckling while pullrods work in tension. I have worked on a LPM1 with pullrods where we saw pullrod length difference before and after the race by as much as 1.25 mm. Of course they did not elongate the same length and that really messed around the corner weights and the car handling. Pushrod length variation under fatigue is something you want to consider in your calculations.

On another note.... A debate with Z on this forum would not be productive; I already mentioned it. His unnecessary aggressive and uncompromising language made any confrontation unworthy.

That being said, I can be wrong, I have been wrong a few times in the past and I could be wrong again. Nobody has to agree with me. In fact that is written on page 3 of my seminar.

But in this seminar (next one is in Melbourne 4 days and 898 pages) there is much, much engineering truth and sharing of 37 years of experience, perspectives and knowledge than there are mistakes. My company has dozens of satisfied customers who, thanks to our input, win races and championships on 4 continents. In the last few years 3 major OEM have asked us to perform consulting jobs for them, they are very satisfied and the amount of work we do for them keep increasing so I guess there must be a few things that I and the guys working in my company are doing right....

I made a choice to spend a serious part of my life sharing my experience, knowledge and passion for vehicle dynamics and race car engineering with students at FSAE / FS competitions and in seminars that are offered at a ridiculously low price compared to the ones I gave in the industry. I am open to criticism when they are made with a minimum of courtesy and ideally by people who have proven records of on-track race car engineering success.

Why Z negatively focuses on me in this forum, I don't know and frankly I don't care. Z debate is closed.

[Z]

Nobody has to agree with me...

But ... [I have]
... 37 years of experience, perspectives and knowledge ...
... dozens of satisfied customers who, thanks to our input, win races and championships on 4 continents ...
... 3 major OEM ... very satisfied ...
... sharing my experience, knowledge and passion ... at a ridiculously low price ...

Claude,

This issue of whether or not P/PR&Rs "reduce the non suspended mass" is a simple mechanical matter. It can be easily explained, as I did near top page 2.

But instead of giving the students any reasonable explanation of your viewpoint, you have (again!) given them nothing but this.

http://www.trulyfallacious.com/wp-content/uploads/2013/08/appeal-to-authority-doctors-and-camels.png

https://classconnection.s3.amazonaws.com/651/flashcards/1167651/jpg/appeal_to_authority1329063498961.jpg

(Students may wish to google "Appeal to Authority", "Argumentation from Authority", or "Argumentum ad Verecundiam", for more background on this fallacious way of arguing a viewpoint.)
~~~o0o~~~


Why Z negatively focuses on me in this forum, I don't know and frankly I don't care. Z debate is closed.

Claude, I DO NOT focus on you. I focus on the many Vehicle Dynamics mistakes you make on this Forum, yet refuse to acknowledge.

Your refusal to clear up issues such as above is as bad as telling the students blatant lies. Even if you charged the students nothing, the very fact that you promote yourself as an "authoritative educator" means that you have a RESPONSIBILITY TO NOT MISLEAD the students.

It is one thing to share your "experience, knowledge and passion". I have no problems with that. But it is utterly IRRESPONSIBLE of you to fill these engineering students' heads with nonsensical pit-lane mumbo-jumbo.

If you personally have difficulty understanding the VD, which IMO is the crucial issue here, then perhaps you should hire someone who does understand it.

But leaving these issues unresolved, and then saying that the "debate is closed", helps no one.

Z

[Tim.Wright]

I shall try to sum up the problem in a formula:


EffUnsprungMass =
WheelTyreMass +
WheelCarrierMass +
PushrodMass x (i_PushRodDisp/WhlZDisp)^2 +
PushrodMOI x (i_PushRodRot/WhlZDisp)^2 +
RockerMOI x (i_RockerRot/WhlZDisp)^2 +
DamperPistonMass x (i_DamperTravel/WhlZDisp)^2

Look at the last term. If you increase your motion ratio, you increase your unsprung mass.

[MCoach]

Everyone here seems to be concerned about reducing the unsprung mass to a minimum (quite sensibly, of course), but has anyone given thought as to what unsprung inertia helps damp your tire oscillations at the contact patch? Tires with super floppy sidewalls have some interesting properties...