Suspension Design

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Originally posted by vside:
I'm from the UK and am doing the suspension and steering on our first FSAE car build.

... I'm a little stuck on where to even start.

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This thread is a perfect example of

HOW NOT TO DESIGN A FSAE CAR!!!

(I was going to be much more blunt http://fsae.com/groupee_common/emoticons/icon_mad.gif , but decided to tone it down...)
~~~o0o~~~

Vside,

You indicate that you have no idea what you are doing, but "the chassis design has been done..." (!!!), so now you just need some help with the details...

Have you read Big Bird's thread on "Reasoning your way through the FSAE design process." (top of page)?

Have you done any high-level, big-picture, thinking of what is required to do well in this competition?

If so, are you really going to follow the advice above and build a car with the most complicated, and frankly unnecessary, type of suspension possible?

Have you already decided whether you will use pushrods-and-rockers, or pullrods-and-rockers? If so, why!!!?????

Do you realise that you only have to build a car that can travel ~30kms under its own power (and quite slowly at that) to beat almost ALL of the UK teams at the next comp?

Grrrrroooooaaaaannnnnnn..........

Z

I designed a chassis that was to specification so that we can start building something to get more funding from our university. We have to have something to show for by the end of the year else the whole project will take a back seat. So we have a basic chassis that fills the requirements. I am doing this project as an interest to maybe one day people can ask me for advice as I learn about what processes need to be done. These are the sort of things I am asking for help on. What is the best to use? What are others using?
Forums are to seek help from people who know what they are talking about and have experience in the design process and making of these cars and that is why I came here to ask for guidence to get me going and hopefully enable us to compete with a very basic car this year.
There is no need to caritassize for someone asking for advice. Because that is what the community is about is it not? And people have been helpfull so far and hopefully can help me more. Once the project is done and we have a car and a set of calculations that can be worked with then I will be able to help the following years at the university to develop something. It's about learning curves and I'm starting from where you must have done one day.

Thanks everyone for your help so far means a lot. Just on my way in to Uni I'm going to start weighing some of the parts we have at the moment and will start up with an excell sheet listing everything.

vside-

I understand the hoops you have to jump through with the university as a first year team as we are dealing with the same types of issues over here at CSU, Chico (gotta love starting a competing team for the very first time at the school!). The university will not invest in a project if they do not believe it will benefit the students/engineering program. If they want to see a chassis designed before they will support you, then you have to give them what they want.
What Z is trying to help you understand is that you cannot skip the fundamentals when designing the car just to get some money from the school. Don't take his post as a personal attack on you, but use it to learn what the "standard" is when designing a successful car. We spent a (very) long time reading and feeling lost just like you, in fact we still feel pretty lost, trying to figure out where to start. Z mentions a thread on this forum called Reasoning your way through the FSAE design process and everyone on the team needs to read this. Basically, the whole project is supposed to be designed from the top down with the concepts being put on paper first and the parts being the last things you design. This process is what separates building your car from being being a hobby and actual engineering. The university wants to see that you are actually doing engineering. They will show greater support for the team if they see your design process/intent clearly.
It's easy to talk about constructing a basic car, but there is so much that goes into even a simple car that you won't think about until you actually have to. I know it is exciting to hop on solidworks right away so you can get to the construction faster, but this is a huge project that needs hours upon hours of planning.
Anyway, I hope this post guides you in the right direction so that your team can be successful in your first competition.

- Josh

Oh, and I wouldn't even worry about the weight of your parts this year. Treat your car as a point mass with your target weight and work from there. You have to understand the basics before you get into the details.

@Mijo: That's one of the best first posts in a long time.

Starting from absolutely nothing is always pretty daunting for all of us.

The first thing is probably to establish some basic dimensions and an outline for the main frame in plan view, to enclose driver and drivetrain.
That should give you a fair estimate of the required wheelbase.

Full scale mock ups made from pvc electrical conduit can be very useful, and very quickly and easily modified.
What looks good in CAD may feel all wrong when you are actually sitting in it !

The next major decision will probably be tires, and the required final track width.

Once the major vehicle dimensions have been decided, you then have some firm numbers to start applying to the suspension design and geometry.

For a first effort, keep it all very simple and very conventional.
Read the rules VERY carefully.

Step 1 - Decide which, as a first year team, is more important to you. A driving car, or a great design in CAD / on paper that you firmly comprehend.

Once you decide on the former, just get something - anything - together, with the most simple suspension you can come up with. Equal length parallel arms? Fine. I'd wager that there will be SO much you guys run through that "optimal" suspension will be the least of your concern by the end.

On a more serious note if you want to do a proper job of it, my 2 cents would be to take an approach that many teams don't. First thing is to forget about all this crap of roll center migration, swing arm length, caster, kingpin, motion ratios, blah blah blah. All secondary stuff that's just a distraction from a good fundamental understanding of handling dynamics.

That's why this stuff seems so complicated when you first approach it - there are so many damn piece parts and widgets of the fully assembled car and it's easy to get caught up in all of that rather than peeling it all away to the fundamentals.

I'd say start with a bicycle model of the car, and a tire model that's cornering stiffness versus load. Or maybe you if you're brave make it a rigid 4 corner with a CG height and conceptual lateral load transfer distribution. If / when you have a good fundamental understanding of how all those come together to shape your handling.. then the actual mechanical aspect of the car (the springs and motion ratios and such) is easy. Understand and make the design choices at a high conceptual level and the pieces parts will design themselves.

Not to mention if you have understanding at that level you'll be way beyond the majority of the paddock. Or conversely, if one doesn't fully understand the handling of a rigid body model - why on earth would they feel like they're ready to go to a more complex version with bump steer, camber curves, etc?

This thread is about "Suspension Design", so it might be worthwhile to consider some of the big-picture issues related to FSAE suspensions.

For instance, how much "ride comfort" is required? Well, FSAE is (sort of) racing, so comfort is not a priority. But even if it was, FSAE tracks are billiard table smooth, so there is really no need for suspension to provide "comfort".

Ah, but what about cornering? That is, isn't suspension supposed to be important for "handling and grip"? Well, it is well known that go-carts corner at lateral Gs at least as high as FSAE, if not higher, and they have NO suspension. And there are countless other examples of effectively suspensionless cars winning race series. So, again, suspension is NOT necessary to go fast around corners.

Really, the only reason FSAE cars need suspensions is that the Rules mandate them. Nevertheless, I have seen an FSAE competition that was won, very convincingly, and against high standard opposition, by a car that had its springs so highly preloaded that on track it had NO suspension movement whatsoever. The appropriate quote here, usually attributed to Colin Chapman, is "Any suspension will work, if you don't let it..."

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".

So, instead of FSAE being a venue where young engineers start to learn about good suspension design, we have a situation where the teams spend a ridiculous amount of time designing and building the most complicated suspensions possible, but which are also (mostly) useless! There is not, and doesn't have to be, any real thinking involved in this process. Brainless mimicry suffices.

To put it bluntly, the vast majority of you FSAEers are simply pulling your dicks, suspension-wise. http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~o0o~~~

Errrrr, so what exactly is the grumpy old-fart getting at???

The Three-Step-Dance that will take us into the Next Dark Ages.
================================================== =

1. LAZINESS.
Today's students have grown up in an "Every kiddy gets a gold-star!" education system. Not all, but certainly some of said students have learnt that they can do just fine with next to no effort.

"Hey, doing background research, like homework, and reading, and trying to figure out stuff by myself is really hard. But that's what these forums are for, eh? I just have to ask something like,
'...it's really baffling me ... to get more funding from our university ... I just need a rough guide of where i need to start ... a set of calculations that can be worked with...'
and yahoo!, all the hard work gets done for me."

2. DON'T THINK, JUST FOLLOW.
The gist of above student's plan is simply to copy whatever the majority of the flock are doing. It is not necessary to understand anything. It is mostly just a matter of talking-the-talk. Fortunately, this requires no more intellectual capacity than a parrot.

"Squawk!... We've minimised our roll-centre migration... optimised the camber-compensation... kept the motion-ratio at a sensible value of 1:1, with slightly progressive rates... and next year we're fitting two pushrods-and-rockers in series, because if one can reduce unsprung mass, then two will obviously be even lighter! ..."

All of which, of course, is bulldust!

3. THIS IS THE REALLY SCARY BIT.
After spending a year or two mastering the above gibberish, and becoming firmly entrenched at the centre of the flock, our hero starts recruiting as many newbies as he can to boost the flock numbers. There is nothing more blissful for a sheep, than being at the centre of a humungous flock... (it's a fear-of-wolves thing...).

"... one day people can ask me for advice ... Once the project is done ... I will be able to help the following years ... "

And thus is established a self-perpetuating downward spiral of STUPIDITY...
~~~o0o~~~

For FSAE, the double-wishbone-with-push/pullrods-and-rockers suspension is a complete and utter wank. As noted above, no thought, just mimicry.

There are many alternative, much simpler, and much more suitable suspension types that can be used. Very briefly, and roughly chronologically, the major suspension types are,

1. Beam-axles (eg. on all vehicles ever, can be live or De-Dion at rear),
2. Sliding-pillar (eg. Morgan and Lancia, and most motorbikes and aeroplanes at front),
3. Lateral swing-axles (eg. Tatra (still on their trucks), can be high or low-pivot),
4. Leading and trailing-arms (eg. Citroen 2CV front and rear, and most motorbikes at rear),
5. Semi-leading/trailing-arms (eg. F100 at front, and many mid to late 1900s cars at rear),
6. Strut-wishbone (eg. McPherson at front, strut at rear),
7. Double-wishbone (eg. originally mostly at front to allow shorter wheelbase with front engine),
8. 5-link (eg. recent fashion, mainly for NVH reasons).

Of these, any of the first five are more than adequate for a winning FSAE car. They are also structurally much simpler than double wishbones, allowing quicker design and build of the whole car. So more time for the more important big-picture issues!

To repeat, FSAE CAN BE WON WITHOUT ANY SUSPENSION MOVEMENT AT ALL! However, about +/- 1cm of approximately vertical wheelprint movement can be advantageous. The requirements of FSAE suspension are simple. They can be provided by any of the simpler suspension types.

Rational criticism welcome. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Quote:

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Originally posted by Z:

To repeat, FSAE CAN BE WON WITHOUT ANY SUSPENSION MOVEMENT AT ALL!
Z

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I'd agree with this statement. After running low on funding we snatched the dampers off the 2010 car and fitted it with steel beams instead. Its a tad rough over the bumps, and the Torsen differential doesn't like taking hard corners ( tire deflection lets it lift). Still wicked fast though!

Z: 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.

I think you're on the money with a lot of what you say, but the example of pushrod/rockers doesn't illustrate your case at all well in my opinion.

Ben

Z,

While I agree with many of your sentiments about "gold stars" and "kids these days", I have to say that you may be harping at the wrong crowd. Generally (not absolutely), people who engage in FSAE are not the type of student you are describing. In most programs, it is entirely extra-curricular and very under supported. Keep in mind that most teams are all engineering students, and have little time which can be considered extra to their curriculum. There are many choice things I could say about the typical FSAE-er, but rarely would the words "lazy" or "stupid" be among them.

The learning curve is steep, and it is not always possible to understand the fine details behind WHY a certain roll center migration or other parameter would be good/bad in the ~2-4 months OR LESS in which the student has to design thier suspension system. Starting from a higher level and picking goals which may or may not have been "copied" is still a valid design decision. The engineering comes in when you validate your choices and continue to learn why they were a good or bad idea. Unfortunately, this cycle is often forced to repeat itself every few years as well, since students are very cynical of their former leaders' designs, or simply want to learn it themselves. Which I think is completely fair, since this is an educational competition.

On that note, I would also have to respectfully disagree about how the typical double A-arm, rocker style suspension is apparently the worst thing to happen since rear wings on hatchbacks. From a learning perspective, this type of suspension design almost forces the designer to have a basic understanding of vehicle dynamics. There are many choices one can make which allows the creation of your own, UNIQUE system that you think is appropriate. It also provides easy and effective adjustment of many parameters, allowing the would-be engineers to test and validate thier design. I am not a suspension guru, and will not pretend to know more than I do, but I see this as a massive benefit for this type of system over some of the others you suggested. I am not saying that they would not work, or that "status quo is best (F1 does it)", but that if the goal is to learn the most about engineering, motorsports and automotive trickery the typical FSAE suspension is not a bad place to start.

I would also like to make note that most of us recognize that less suspension travel can be advantageous, but there are compromises to be had. Rookie drivers are not used to lightning-quick, spine-wrenching vehicle characteristics. We also typically test in parking lots (or whatever can be found), so there is not always a smooth surface to run on. There is also the infamous bump at FSG, and last year at Lincoln one team hit the ~3/4" bump in the autocross and lost thier suspension. I suspect this was caused by a number of factors, but I think we can all agree that would be a devastating thing to have happen, especially if you were the suspension designer!

That was not supposed to be such a long post, but I think sometimes it is forgotten that us FSAE-ers have to live in a world of compromise, just like everyone else. The best choice for one reason is not always the best choice in the end, and sometimes the better decision lies with the easier path.