Suspension Design

Ben,

Spot on comments about giving the customer what he wants!

More generally,

While I am less convinced about the efficiency of semi-trailing arms and even less about live axles (actually I call them dead axles, I always have been convinced that you can make a good car with a McPherson.... but if you if you miss the window of good mix between of tire exploitation and kinematics design it will be more difficult to make a change. Providing some suspension pick up points adjustability, with a 5 links or a double wishbone you can get and/or adapt the heave and roll camber variations, roll centers, pitch centers position and movement to what the tires (and good drivers) "need" much better than with a McPherson. And if for whatever reason you are forced to switch to another brand of tire, your design could not be adaptable...

As a student I will most probably go for a double wishbone or a 5 links. As a design judge I regret there is a lot of monkey see monkey do and often students cannot explain WHY they choose the suspension type they use.

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Now I really enjoy suspension design. For this reason I have been suggesting, since 2005, that FSAE tracks should be laid out with a lot of real bumps, dips, corrugations, etc. This is in the slim hope that eventually some FSAEers would end up in the auto industry and actually start designing cars with good suspensions. But the bumpy tracks haven't happened, and are unlikely to for many reasons that have nothing to do with "educating young engineers".

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Z, as always you show your agnorance. A good mixture of ignorance and arrogance.

You obviously haven't been to many different FSAE/FS competitions, otherwise you would not make that statement. Something to remember, when reading Z's posts -> He is not a former FSAE team member.

Intended or not, many current FSAE/FS tracks have a lot of bumps.
The bumpiest of the bigger competitions probably being FSG, where the track layout is done in a way that quite some hard direction changes are directly "on/in" bumps by intention.

Fantomas

Double wishbones are an obvious natural choice at the front, but at the back, I am not so sure.

There can be a lot of problems with the potential for bump and compliance steer at the back with a double wishbone design, which seems to be a regularly occurring design problem area on many FSAE cars.

Good solid toe control at the back would have to be be pretty high on my wish list.

A single stiff rear trailing arm, with transverse upper and lower camber control links would give excellent toe and bump steer control while being simple.

Warspeed,

Speaking about compliance you obviously have not work with solid rear axle race cars (such as Nascar... but is Nascar racecar?) or Australian V8 Supercars

Yes I have seen a few solid rear axle race cars with less compliance than badly designed double wishbone rear suspension but I have designed, track tested and K&C tested enough race cars and seen many FSAE cars to know that you simply can design good front or rear independent suspensions and minimize compliance if you just make it a goal.

Don't tell a design judge that you choose solid rear axle over a double wishbone to minimize compliance; your argument will be less solid than your rear axle.

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quote:
Now I really enjoy suspension design. For this reason I have been suggesting, since 2005, that FSAE tracks should be laid out with a lot of real bumps, dips, corrugations, etc. This is in the slim hope that eventually some FSAEers would end up in the auto industry and actually start designing cars with good suspensions. But the bumpy tracks haven't happened, and are unlikely to for many reasons that have nothing to do with "educating young engineers".

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Z, Good to Baja, enjoy your bumps and keep your negativity for other chat rooms.

I still vote to get an understanding of fundamental handling before diving into the dickathon of kinematics.

Or perhaps more generally, suspension design comes as a result of a handling spec. If you don't know what you want from your handling, you cannot know what kind of suspension topology accomplishes it. If you don't know what kind of conceptual kinematic characteristics you want, you cannot design control arms and such.

For as much attention as it gets, kinematics are in my personal and professional opinion and experience largely secondary. They take a back seat to the big hitters of tire properties and dynamic load distribution (fore/aft, side to side, and diagonal). This is particularly so on a stiffly sprung "autocross car" with minimal suspension travel.

Don't focus on the secondary things and minutiae before you understanding the primary factors.

Ex FSAE,

I wish you could be a bit more specific in your comments.

There is no "dickathon" of kinematics. Nor any "dickathon" of aero, stiffness or damping.

It is all about predicting (understanding, quantifying and simulating) and ideally measuring how much each N (or lb) of vertical load, each degree of slip angle, each % of slip ratio and each degree of camber influence each tire forced and moments and from there influence your car longitudinal and lateral grip, balance, control and stability. If the student can demonstrate what does what when and by how much and from there that kinematics is negligible so be it and I will be the first to admit it.

However, my experience with both race cars and passenger cars is that kinematics (and compliance)
1. Has a non negligible influence and sometimes an even bigger than expected influence than spring, ARB and damper. There is along list of screwed up passenger cars (starting with the Mercedes Class A and the first Smart) to prove it.
2. You cannot "patch" a wrong kinematics with "right" springs, dampers and ARBs.
3. Many automotive engineers underestimate the influence that kinematics has on TRANSIENT car handling.

Try to open a door with a force parallel to the door plane or a force perpendicular to the door plane but a few millimeters from the door hinge.
Try to get good traction and braking with 15 degrees of camber.
What is the point to have 1000 HP (or more) on a dragster if you have just a few 1/10 of a degree of toe out steer compliance (or toe out bump steer) on the rear wheels?

Archimedes, said: "Give me a lever long enough and a place to stand, and I can move the world."

A force is defined by a application point, a direction and an intensity. Worry about the first two before the third one.

Those are all certainly valid points.

Perhaps a more accurate statement is that the first step is to understand the relative influence of all your tools and design options, and establish a spec and design intent. Once that's established, all the piece parts fall into place and "design themselves" as it were. For this specific series I do maintain my opinion that getting the right CG location, load transfer distribution, tires, and diff are the big ones to worry about... and kinematics are just not as big a factor. I agree that in general that relative proportioning of what contributes to your handling can vary depending on platform. Even then, when I think about starting a design from scratch I begin by conceptualizing what I want from a bicycle model.. then expanded into a 4-corner rigid body model.. then adding inertia, and kinematics.

I do maintain as well that there is a dickathon of kinematics in this series. It's a trap I fell into when I was doing it. It's easy to get caught up in questions of "What VSAL do I want?" or "How much lateral roll center migration is acceptable?" or whatever. You can get too focused on the piece parts without understanding WHAT it is you're trying to accomplish at the system level. In a way, you're not seeing the forest because of all the trees in the way.

It wasn't until after I graduated that the light switch flipped and I came to realize that engineering is a top-down approach. You come up with something at a high, conceptual level.. and then the kinematics and kinetics (force elements) to achieve it are more or less explicitly defined. It makes the design process much easier.

Ex Fsae,

1. "Even then, when I think about starting a design from scratch I begin by conceptualizing what I want from a bicycle model.." I can't agree more with you: that is why we created the (free of charge) OptimumLap simple and simplified mass point software

2. "For this specific series I do maintain my opinion that getting the right CG location, load transfer distribution, tires, and diff are the big ones to worry about... and kinematics are just not as big a factor" Yep except there is a logical and a chronological order: you cam always decide different springs and ARB and damping later while you test the car while it will be difficult to change the suspension type and majorly change the kinematics as it will influence your car chassis (and other parts) design. Where I agree with you is that weight distribution Vs front / rear tire cornering stiffness ratio should be decided very early on, even before your kinematics.

3. " It's easy to get caught up in questions of "What VSAL do I want?" or "How much lateral roll center migration is acceptable?" " How boy there I am 100 % with you. If I come with this topic in design I will sure be more demanding and specific in the how / why / how much parts of thee questions. Maybe I should organize a seminar for design judges also. I am sure I will also learn from other judges too but there will at least be some common approach in our questions.

4."It wasn't until after I graduated that the light switch flipped" Ahhh You too? http://fsae.com/groupee_common/emoticons/icon_smile.gif Welcome to the club. I graduate and flip a new switch practically every month.

Some quick replies...
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Dash, Thanks for "a real life testimonial"...
~~~o0o~~~

Ben,

"FSAE is an engineering competition with the stated goal of creating a car for the weekend autocross racer. If the weekend autocross racer expects to see pushrod and rocker suspension because F1 cars have it, then giving the market what it wants is a sound engineering judgement for an engineer who wants to remain employed." (My emphasis.)

That might be a good business decision, but it is not engineering. Essentially, any fool can make that decision... But it takes a good engineer to build a truly fast car, and I reckon "weekend autocrosser" would rather pay for that...

I note that another stated goal of FSAE is to encourage "creativity and imagination". Copying what every other fool is doing is in breach of the C&I goal. But no team loses points for that, because, despite my repeated requests, no official or judge has yet publically supported C&I!

Anyway, from the point of view of winning the competition, there are very few points available for "bling" and many more points for building a fast car.
~~~o0o~~~

Owen,

Firstly, I do agree with you that most FSAEers are from the right end of the "Effort" spectrum. The reality of all the hours that MUST be put in soon weeds out the slackers.

Unfortunately, "the system" is sending the students in the wrong direction. By this I mean senior design judges who clearly have a poor understanding of Classical Mechanics, but nevertheless pressure students to follow their faulty advise (eg. the much repeated "Pushrods and rockers are good because they can lower your unsprung mass"!!!, and other such codswallop...).

I cover this in a bit more detail below, but briefly, most FSAE suspensions are so poorly designed that they reduce the car's potential performance. Unfortunately, the students never learn of this, because no one tells them. In the short term future I would like to see at least some good engineers out there designing cars with good suspensions. (That's for my children's sake, I'm happy with cars that are older than me...).
~~~o0o~~~

exFSAE,

"I still vote to get an understanding of fundamental handling before diving into the dickathon of kinematics.
...
For as much attention as it gets, kinematics are in my personal and professional opinion and experience largely secondary. They take a back seat to the big hitters of tire properties and dynamic load distribution (fore/aft, side to side, and diagonal). This is particularly so on a stiffly sprung "autocross car" with minimal suspension travel."

Agreed!

In a bit more detail, on smooth track racing (=FSAE) the biggest influences on vehicle DYNAMICS are overall mass distribution, tyre types/sizes/pressures/cambers/toes (none of which require a "suspension"), then engine power, compliances, etc., with suspension spring rates, dampers, and kinematics some way back (depending on details). And should significant aero be allowed, as it is in FSAE, then aero can trump all!

Stressing one point again, the tyre camber and toe values have an initial "static" setting that DOES NOT REQUIRE A SUSPENSION. All that "kinematics" does is to vary these settings according to spring rates and applied loads. For smooth track, stiffly sprung cars, the variation possible is very small. In fact, variation from compliance effects can be much greater, and more adverse.

"Don't focus on the secondary things and minutiae before you understanding the primary factors."

Or more bluntly, "Don't try to make babies by pulling your dick." (Oops, apologies to exFSAE... http://fsae.com/groupee_common/emoticons/icon_smile.gif)
~~~o0o~~~

Claude,

"There is no "dickathon" of kinematics.
...
However, my experience with both race cars and passenger cars is that kinematics (and compliance)..."

"Kinematics" and "compliance" are two entirely different subjects. (FWIW, Kinematics is a field of Applied Mathematics where the idealised geometric bodies are perfectly rigid, frictionless, massless, etc...) By conflating the two subjects you are misleading the students.

And then there is the whole issue of the "2 x 2-D" version of kinematics that is almost universally taught to FSAE students, even though it is wrong!

(And BTW, in Mechanics it is very misleading to say "A force is defined by a application point...", because ANY point along the force's Line-of-Action is as good as any other point...)
~~~o0o~~~

Bottom line is that FSAE students spend an inordinate amount of time on suspension design, but come out of the whole process with a very poor understanding of how suspensions work.

Something is clearly wrong with this system of "education".

(Ooops, a bit longer than intended...)

Z

(Edit: Posts are coming thick and fast... Last two posts I missed, but agree with...)