Suspension Design

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Originally posted by Z:
Markus,

You pay $600 for ONE damper!!!? Why???

Do you realize you can win FSAE with next to no suspension at all?

"Wiping a corner off the car isn't that uncommon..." (because you hit a little bump?) should NOT be a goal.

Z

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Because stealing dampers would be criminal. We pay money for dampers because we don't have enough team members to make them. I won't make any guesses how the company comes up with that price but I'd believe it has something to do with the law of demand and supply.

And we don't do all this just to win. There's more to this competition than winning, at least for the majority of the teams.

So should the goal be that the car withstands everything? While being light and not made to withstand everything? You're talking against yourself...

Did I mention anything about bumps? These cars are also run on venues that have a chance of crashing (tire walls etc.). Everything will break at some point, a good engineer will design when and where. It might be beneficial to have (CF) a-arms that shatter in a crash to save the frame and damper. Otherwise you might end up here with your "much more rugged" control arms:

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"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]!!!"

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So pick your direction, you can't have both.

PS. You didn't comment anything on saving the dampers in the driveshaft problem. Running out of arguments?

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Since people keep claiming push/pull rod suspensions reduce unsprung mass - what effect does this have on your point score at competition? Have you actually tested the sensitivity of these cars to unsprung mass? Unsprung mass isn't even really the correct term, unsprung inertia is probably more appropriate. What happens when you hypothetically add a heavy large diameter bellcrank to the system when you go over a bump?

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This summer we tested our '12 car with 6kg mass added per corner randomly mounted either inside the car or attached to the uprights. Made multiple runs with two experienced drivers. Our conclusion was there were no statistically significance between mean lap time and position of the mass. There was a significant difference between the two drivers though. The test protocol did had its flaws (Suspension parameters was not adjusted for both mass setups, etc.) but it still shows a bit of insight about the relative influence of sprung mass.

Just food for thought.

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Originally posted by Francis Gagné:

This summer we tested our '12 car with 6kg mass added per corner randomly mounted either inside the car or attached to the uprights. Made multiple runs with two experienced drivers. Our conclusion was there were no statistically significance between mean lap time and position of the mass. There was a significant difference between the two drivers though. The test protocol did had its flaws (Suspension parameters was not adjusted for both mass setups, etc.) but it still shows a bit of insight about the relative influence of sprung mass.

Just food for thought.

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please tell me more about this test, i would like to know more details if that is OK

I suppose it comes down to the fact that the cars are not that sensitive, and the rookie drivers not world champions.

Yeah, we ran back to back tests, and the titanium wheel nuts consistently improved lap times by 34.86 microseconds.

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Originally posted by Warpspeed:
I suppose it comes down to the fact that the cars are not that sensitive, and the rookie drivers not world champions.

Yeah, we ran back to back tests, and the titanium wheel nuts consistently improved lap times by 34.86 microseconds.

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Ever since I've started looking into tire data I've started to think that if they really wanted to make this an engineering competition and not a driving competition, they'd force a really finnicky spec tire (with TTC data available for it, of course). One that only works well in a very small range of slip angles, is very sensitive to camber, and friction coefficient drops off rapidly with increased load.

When I was designing kinematics for our 2011 car, I really had no idea what I was doing. I made many of the mistakes that have been discussed on these forums- focusing too much on minimizing roll center migration without any understanding of why/if that is even important, no thought whatsoever into tire selection (other than "goodyear gives them to us for free"). Instead of looking for the camber that our tires like to operate at from the TTC data and designing the kinematics to achieve it, I just made sure camber gain was in the same ballpark as our previous cars. We had no idea how the ideal slip angle for our tires varied with load & what Ackermann % to use...the list goes on and on.

Looking back at how little I knew about suspension, how our wings didn't really make that much downforce considering their size, the car was ~480 lbs etc, I've always wondered how the hell that car won autocross in Michigan. After working with tire data for a while, all I can come up with is that FSAE tires just aren't very sensitive to weight and poorly designed kinematics, and we have good drivers. I honestly think any team can get top 5-10 in dynamic events by building a dead simple & reliable car (as long as it has no extreme design flaws and a decent running engine), finding some kid at your school with kart racing experience, and giving him 3 months to practice driving it.

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Originally posted by JT A.: I honestly think any team can get top 5-10 in dynamic events by building a dead simple & reliable car (as long as it has no extreme design flaws and a decent running engine), finding some kid at your school with kart racing experience, and giving him 3 months to practice driving it.

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ABSOLUTELY.

A strong and simple car that lasts the distance and actually finishes all in one piece is a triumph and a success.

When you have achieved that, then you can start thinking about what changes to make to make it better, faster, or even more simple.

As Stirling Moss once famously said "To finish first, first you have to finish".

Re: TMichaels - I do beleive there are a group of guys at Uni of Tas that want to build a car (they've been hanging around the past few FSAE-A comps). Why not get in touch with them and see where they're at - they'll probably be very receptive to a simple car design.

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Originally posted by JT A.:
I honestly think any team can get top 5-10 in dynamic events by building a dead simple & reliable car (as long as it has no extreme design flaws and a decent running engine), finding some kid at your school with kart racing experience, and giving him 3 months to practice driving it.

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I'll just take that and show it to our schools new team if you don't mind, sums it up perfectly.

Once i attended Formula Student (first and last year unfortunately) i really did find that out, a simple car with good, trained drivers will easily beat a much better and more sophisticated car that was just finished in time. the track layouts just don't allow car difference to be measured.

The competition is not really that tough, top 20 is possible to nearly everyone the only thing stopping each team from doing that is the team shooting themselves in the feet trying to "optimize" something or giving any item more importance than it deserves. but that does get you points in the design event if you can show for it.

Tobias,

"I have driven the [FSG] track several times myself and it is quite bumpy. Look at this picture, the rear wheels are completely up in the air. I don't know, if this is a setup issue, but no team managed to get over this bump without lifting both rear wheels off the ground."

I touched on the above issue on the Beam-Axle thread. Specifically, here is the section regarding the "Bump Map" (I might try to post such a map one day).

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It is instructive to draw a "map" of the type of bumps that ground vehicles drive over. On the the horizontal axis plot "Frequency" (Hz) as a log scale. On the vertical axis plot "Amplitude" (say, in metres) again as a log scale. Now, assuming sinusoidal bumps, the maximum vertical velocities and accelerations of the wheelprints are shown as two series of diagonal lines on the map (different slopes for V and A). FSAE is at the bottom-left of this map (along with forklifts, etc.), while desert racers are at the top-right.

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The key lesson learnt from such a map is that suspensions only work (and thus are only required) in a narrow range of the map. At the left of the map (very low vertical accelerations) the whole vehicle moves as one, with no suspension movement (other than perhaps twist mode). At the bottom right (high frequency, but low amplitude bumps) the body might travel quite smoothly, but even the tyreprints cannot accelerate fast enough to maintain contact with the road, so no grip. At top right (big bumps at high speed) the whole car necessarily leaves the road.

Note that the maximum downward acceleration of tyreprint, wheel-assembly, or whole car (so as to maintain contact with the road), is simply = (downward force)/(mass of object). Clearly, tyreprints can accelerate fastest, the whole car slowest, at 1G, and the wheel-assembly somewhere inbetween at 5-10G (= sprung/unsprung-mass x Gs).

The FSG cars you mention (medium amplitude bump at medium frequency) might maintain road contact with a better suspension (eg. longitudinally connected springing, but still with very simple control-arms). However, a better all round solution is more aero downforce, since it increases the car's downward acceleration to better follow the road.
~~~o0o~~~

"Let me suggest something:
You are talking a lot about how every team that would dare to follow your advice...
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why not offer a team nearby, I guess in Australia, to coach them ..."

Hmmm... So I have to find a very brave team, that is nearby, and is also prepared to mindlessly follow my advice...

Then...

"... even if "Monkey see, Monkey do" happens and all other teams start copying the winning concept...
... they will carry your thoughts into industry..."

Without them having the foggiest idea of what they are doing!!!

Tobias, how would the above improve the "education of young engineers"? (Please, no excuses... http://fsae.com/groupee_common/emoticons/icon_smile.gif).

Rather than giving them a fish, I would prefer they learnt how to fish...
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Luke,

Thanks for some rational analysis of direct acting spring-dampers. BTW, best way to prevent damper overheating is to have a soft spring-rate in the middle of the suspension's working range, so only low damping forces required, and use bump rubbers for the occasional overshoots.
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Markus,

"So should the goal be that the car withstands everything? While being light and not made to withstand everything?
... you can't have both."

Well, actually you can. The Model T Ford (~700kg), VW Beetle (~720kg), and Citroen 2 CV (~530KG) were all designed to be as simple as possible, while still being "mid-size four-passenger cars". These cars are all very light (weights dependent on spec), but they also have the rugged off-road capabilities that very few 2+ ton SUVs have today.

KISS really works.
~~~o0o~~~

"PS. You didn't comment anything on saving the dampers in the driveshaft problem..."

I have no idea what happened to your dampers/driveshafts. Honest ... I was not there ... I didn't do it ... http://fsae.com/groupee_common/emoticons/icon_smile.gif

But I am sure that spending excessive money on dampers is a poor use of resources. And spending even more time and money building deliberately fragile wishbones/pushrods&rockers, just to "save the dampers", is also a waste of resources.

Focus on the "big picture"...
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Francis,

"This summer we tested our '12 car with 6kg mass added per corner randomly mounted either inside the car or attached to the uprights...
... there were no statistically significance between mean lap time and position of the mass...
Just food for thought."

This is confirmation that FSAE tracks are at the bottom left of the "Bump Map". The vertical accelerations of the wheels over the "bumps" (???) is small enough that large variations in unsprung-mass don't really matter.
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Hmmm ... half-way down page 6 already...

[Note to self] Better start (get back to?) discussing KISS suspensions soon... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

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Originally posted by Z:
Markus,

"So should the goal be that the car withstands everything? While being light and not made to withstand everything?
... you can't have both."

Well, actually you can. The Model T Ford (~700kg), VW Beetle (~720kg), and Citroen 2 CV (~530KG) were all designed to be as simple as possible, while still being "mid-size four-passenger cars". These cars are all very light (weights dependent on spec), but they also have the rugged off-road capabilities that very few 2+ ton SUVs have today.

KISS really works.

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None of the mentioned cars are "very light" especially compared to their spec. I'd say medium weight, or you could give them light if you where looking through heavily shaded KISS-glasses while being almost pope-drunk.

I've personally driven off road with a VW beetle and the front suspension got damaged badly. The speed was low, there was no big bumbs or impacts to front wheels yet the suspension was bent enough to be undriveable. I don't remember if the frame stayed intact or needed repairing too. This was actually closer to just driving off-road than crashing.

So at least the beetle is not very light nor very withstanding to harsh use...

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"PS. You didn't comment anything on saving the dampers in the driveshaft problem..."

I have no idea what happened to your dampers/driveshafts. Honest ... I was not there ... I didn't do it ... Smile

But I am sure that spending excessive money on dampers is a poor use of resources. And spending even more time and money building deliberately fragile wishbones/pushrods&rockers, just to "save the dampers", is also a waste of resources.

Focus on the "big picture"...

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With only a very limited mechanical understanding you should be able to visualize a couple of possible cases of what could have happened. There is not many ways how a driveshaft can jeopardize a DA-damper or push-/pullrod.

But if you weren't there and didn't see it, does it mean that it didn't happen? And in no way could there be a benefit in that situation from using rockers compared to DA? Of course there can't, after losing a damper we just fly to Australia and pick a new one from the damper-growing tree...

Wishbones are fast to make and cost very little money. Even "cheap" dampers will need more time and money, and waste more resources.

Teams can of course decide where they want the weak link or convince themselves there isn't one - false sense of reliability is a really good character for an engineer, especially in the future career. We had some good examples of that in history with trades like bridge and ship building.

We chose a-arms, not only because they save the damper, but because they save the frame, the wheel height sensor and the corner assembly too. And the shattered CF arms can't injure the driver and even though Finland is known to raise a lot of good drivers we still can't afford to lose any of them...

By the way, I love how they use the soft materials in 2CV to absorb impact energy. Namely the face of the driver.
http://crash.citroenz.biz/2CV/2cvcrash.jpg