Suspension Design

[Price]

I believe Carrol Smith makes a comment in one of his books that if you're really clever, you could have a 'dry' set of rockers and a 'rain' set of rockers, so if the track gets damp or wet you can just swap the rockers to simultaneously raise the ride height and soften the wheel rate without having to corner-weight the car again like you would have to with a spring change.

Drew,

I was under the impression that this was the historic reason that rockers were developed and used in the first place. Also, I'm pretty sure you could have both "dry" and "setup" positions with the same rocker with two different mounting holes for either the push/pull rod or the damper.

I also have not seen too many examples of FSAE cars with outboard mounted direct actuated dampers that did not have significantly low motion ratios(spring travel/wheel travel), or a drastically(in my opinion) digressive nature. Both of those reasons would push my choice to rockers.

[Warpspeed]

Changing over an entire outboard coil shock is both fast and easy.
And what you replace it with can have been previously set up during testing for correct corner weight, wheel rate, ride height, and correct damping.
This is a vastly more flexible and much more easily understood system to have.

The late Carroll Smith had quite a bit to say about rising rate suspensions, and the evils of over complexity.

[js10coastr]

I also love the fact that the site says "thank you for contributing to this forum"

[woodsy96]

I agree with Brent and drew, designing with rockers and pushrods/pullrods defintely makes life easier and acheiving more/less linear motion rations without inducing massive bending loads in a lower wishbone, especially the shocks generally used in FSAE. Trying to design a chassis to reach out to a node for a Kaz 7800 to directly actuate against sounds troublesome.

Other advantages:

-your shock is less susceptibe to damage inside the chassis than hanging in free space if/when something breaks.
-Pushrod shocks are way easier to access to make changes

[wagemd]

Originally posted by Brent J. Butler:
The team voted against it. The primary reasons cited included "No one else does it, so it can't be good", "Double wishbones didn't break last year", and "Carroll Smith said double wishbones are the best and he knows more than you" (I am the only "suspension guy" on our team). Some of our teammates were actually quite angry at us for "wasting the summer working on a different design" (even though most of these team members did zero work themselves). Frustrating, but I try to look at it as another way that FSAE is preparing me for the real world.

Ah yes, the good ol' design by democracy... Sad to hear you had a cool suspension concept that got shot down by someone who was designing a dash and someone else that cobbled a muffler together. Some decisions are better left to the people who know what they are doing. - Learned that the hard way.

[Warpspeed]

Ah,
But this whole "thing" is a DESIGN COMPETITION.

I can imagine the look on a design judges face when you answer by telling him you did it that way because everyone else does it that way.

If you can come up with a strikingly practical, original and clever idea, and can justify your decisions, and DARE to be different, a jaded design judge may become truly fascinated.

[Pete Marsh]

Originally posted by Brent J. Butler:
The team voted against it. The primary reasons cited included "No one else does it, so it can't be good", "Double wishbones didn't break last year", and "Carroll Smith said double wishbones are the best and he knows more than you" (I am the only "suspension guy" on our team). Some of our teammates were actually quite angry at us for "wasting the summer working on a different design" (even though most of these team members did zero work themselves). Frustrating, but I try to look at it as another way that FSAE is preparing me for the real world.

This is a pretty sad story really.

Pete

[Z]

Brent,

I agree with Pete above. The "real world" part is true, and probably why some old codgers around here are so grumpy...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~o0o~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~

Before moving on (back?) to suspensions, better get this one out of the way first... (please read with lots of ).

The Many Advantages of Push/PullRods and Rockers (and other fairy tales).
================================================== ==========
The following compares P/PR&Rs with "direct acting" Spring-Dampers that go from chassis to outer-end-of-whatever-suspension. All the "reasons" below appear somewhere on this forum.
~~~o0o~~~

1. "Direct acting SDs are too short to reach the end of the wishbone, so require complicated structure going out to the wheel."
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Piffle. Unscrew the spring-base/BJ-eye on the end of the damper piston rod, and make up a new, longer one. So now your modified SD is as long as your old pushrod. (See next...)
~~~o0o~~~

2. "Push/pullrods make it easier to adjust ride height without changing spring pre-load, or damper stroke position."
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Yawn. Add an adjustment screw in your extended damper rod (1. above). This would look like the adjustment you currently build into one end of your push/pullrods, and serve the same purpose.
~~~o0o~~~

3. "P/PR&Rs allow a Motion Ratio close to the magical 1:1."
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So what? What's so special about 1:1? Well, apparently...

3a. "MR=1:1 gives lighter springs."
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Crap. A short travel, thick wire, stiff spring that, due to a different MR, gives the same wheel-rate as a long travel, thin wire, softer spring, will have very similar mass. The strain energy (= force x distance) that the two springs must absorb is the same. So if they are both made from the same quality spring steel, with same energy/mass capability, then they have the same mass.

3b. "MR=1:1 gives more effective damping."
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Groan. These are very small cars, racing on very smooth roads. They do NOT need a lot of damping. (Geez, they don't really need any suspension at all... ) Who runs their dampers at their stiffest settings, and feels they need even more stiffness? Worst case, just fit slightly larger diameter dampers with shorter stroke.
~~~o0o~~~

4. "The rockers allow a constant, or slightly rising wheel-rate."
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Well, to keep Claude happy, I'll just call this one "pure, unadulterated, intellectual masturbation"!

Firstly, it is easy to get similar, approximately constant (= good), wheel-rates with direct acting SDs.

Secondly, the rapidly rising rate suspensions that work great for absorbing bumps on dirt bikes are NOT at all suitable for circuit racing cars. Which is why no such racecars use them. Great off-road, but not on track. Also, if your FSAE car has to cope with bumps (perhaps at the old-parking-lot test track), then good bump rubbers are the best option.

Thirdly, this is a big subject, but read this thread about droop-limited suspensions to see that often severely "digressive" (= falling rate) springing can be desirable (I start at the bottom of page 1 ).
~~~o0o~~~

5. "They can lower the car's CG."
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Yes, agreed! Pullrods certainly do allow the SDs to be mounted lower, for a slightly lower CG. But for some reason the majority of cars (from pics I've seen) use pushrods with the SDs mounted as HIGH as is physically possible! Ie., on the very top of the nose, or well up above the final drive. See next...
~~~o0o~~~

6. "The dampers are easier to adjust."
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Ah, yes, that makes sense now. Mount them as high as is physically possible...
I can picture it...

"Sure, all our talk about low CGs is just blarney to keep the judges happy. But when our driver dived into the pits during Enduro, and I gave it that lightening quick two-clicks of extra hi-speed bump... (didn't even have to bend over!)... well, that won us the comp!!!"

Yes. Sure.
~~~o0o~~~

7. "Unsprung weight may be decreased..." (<- This one from a senior Design Judge!)
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So, clearly, the more push/pullrods-and-rockers you use, err ... perhaps in series and/or parallel ... , then ... err ... the lower the mass becomes...

"Err..., thinking ... thinking ... YES!!! THAT'S IT!!!!
Geez, forget about FSAE!! We've just invented the fabled ANTI-GRAVITY DRIVE!!!
Yippeeeee!!!!! Now off to the stars, and beyond!!!"
~~~o0o~~~

8. "They can save your damper in an accident."
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So, something like...

"Wow ... that was a bad one! Totally wiped out the corner. And that bone sticking out of the driver's leg doesn't look good.
But HAPPY DAYS! The damper doesn't have a scratch!"

Or, taking this thinking a bit further...

"Ha! Those other teams may scoff at our "tungsten-cobalt ballistic umbrella" (patent pending). But we'll have the last laugh! [insert maniacal laugh]
Oh, yeah... When the sky starts to fall and all the other teams are struck down in the middle of Enduro... [add much more maniacal laughter]!!!"

This sort of "design for the worst, most improbable, case..." is quite common in industry.
~~~o0o~~~

9. "They make it easier to do fully interconnected springing of all wheels..."
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I agree with this one (in fact, I think I'm the only one pushing it...).

But, even though the one and only (?) team using fully interconnected springing for the last decade has been very successful with it, no other team seems to want to take the plunge. I would suggest cable "pullrods", possibly running over pulleys rather than via rockers, as a good way to interconnect front and rear suspensions.
~~~o0o~~~

Probably missed some, but methinks that people using some of the above "reasoning" are mainly kidding themselves...

You will find a lot more of that sort of reasoning when you get real jobs...

Z

[Warpspeed]

On the interconnected spring thing...

Has anyone ever tried fitting longitudinal "antiroll bars" down each side, linked between front and rear suspension, to increase wheel rate in pitch ?

I have never seen this done, but it would be a rather different approach to the anti dive and anti squat problem.

[Gruntguru]

Tony
Quite the opposite of what Z proposes.