a-arm aerodynamics

[Christopher Crowe]

I'm sure there's a reason this isn't done, but I don't know what that reason is...

Cladding the entire a-arm on a car with essentially the same inboard pickup point altitude (no antis). That is, fill in the triangle between the front and rear pu points and the ball joints with a light strong material, making the a-arm a great big filled in area -- and a kind of symetrical wing...

On chassis dive, you'd get a downward slope of the triangle and some crude, turbulent downforce. On squat, the opposite... which, in a lower power car could be neat...

I know there are sanctions against building in an a-arm that acts as an aero device (by causing it to angle up and down)... but this wouldn't be that. It's just be filling in the natrual-born A in the a-arm and just using the chassis dynamic pitch to do the rest.

Started thinking about this when I found myself wondering if the filled-in triangle wouldn't produce less drag than the usual two tube arangement (with its two wakes; and the associated aero mayhem that comes with it, etc).

Anyway, I know people have doubtlessly run this through their brains... but I wondered if anyone in the history of the world anyone has actually tried it, measured it and etc?

Chris

[starkejt]

The Campbell/Kephart "Yellow Banana" FF (Vestal) had them mostly enclosed in one iteration of the suspension, IIRC.

[aims20]

I don't have too much wisdom to add, but for the 2011 Virginia Tech Formula SAE car, I had a few concepts sketched out in my logbook dated December '09. Some simple hand-calcs with an airfoil shape one of my colleagues conceived/we were experimenting with was showing that at 100 mph you could potentially make nearly 100 pounds of downforce with all of the A-Arms. That was neglecting 3-dimensional airflow effects and assuming that not all of the A-Arm Area was usable (steering tires /packaging).

We never got around to putting it in CFD, (or FEA for that matter) and the 20 lbs of down-force at 35 mph that we were designing for (auto-cross style car), as well as the lack of development on that particular wing profile meant that it quickly became an unattractive challenge in our chaotic college environment. It was however, on a car whose front/rear track width was 49"/47" with a wheelbase of 65"

The other issue is, depending on the design and the amount of the downforce that is generated, you add another varying component of stress to the system. More Stress is bad for weight. More varying stress is even worse for fatigue... and weight... and weight in the A-Arms is all sorts of bad news: more translational inertia, more yaw rotational inertia, and more unsprung/half-sprung mass to control. Not to mention the added component of jacking forces you will have on the car.

As a result, either the weight, or the cost (Carbon/other composites, etc.) of the part (or the rest of the suspension system if it has to be designed to compensate it) goes up rather dramatically (Unless its your backyard, no-liability project).

Bending in A-Arms doesn't do you many favors since buckling (at least to the way we were designing the a-arms) was the primary failure mode. If our FSAE team could ever get the money, the time, and the focus, we would LOVE to strain gauge our linkages. Our current model for sizing is so simple, and putting it into FEA has been done, but validating all of it (FEA constraints as well as loads) is something we are trying to get done this year. It would also give us some real insight to design the suspension. If I had the time, I'd follow through with it to figure out how many points we'd gain in in the FSAE competition from it.

It could certainly be done. I'd be surprised if those crazy sculpted arms on F1 cars didn't take that into account. A clever structural designer, with the right tools would know how to develop the load paths to minimize the influence of the added loads/mass since there are parts of the A-Arms that aren't quite "carrying their share of the weight" (excuse the pun, haha).

Andrew Spencer
Virginia Tech Mechanical Engineering
2012 VT FSAE Lead Test Engineer

[Wren]

I don't think the A-arms pitch when the car is on the brakes. They travel down in the front and up in the back, but only the chassis pitches.

Unless it was all welded steel construction, I would consider it bodywork.

I don't have a lot of faith that the air getting to the rear a-arms is clean at all. Especially not clean enough to try to make downforce from. I question what direction the air is even flowing back there.

There is a lot of other low hanging fruit to be had before trying to extract a few pounds of downforce in a certain condition from the A-arms. Get the frontal area and Cd down and go with it.

[Wright D]

[FONT=Verdana]Front A-arms typically live in an up wash environment generated by the front wing. Some food for thought... [/FONT]

[Christopher Crowe]

I was mostly wondering about reducing drag by filling in a-arms -- not really the effect of chassis pitch giving a tiny bit of downforce.

Dustin, the "up wash" environment is doubtlessly true in the front. And maybe filling in the a in the a-arms would actually cause lift! I dunno...

Anyone else have any thoughts in this area? I find it a neat academic exercise at least... But then what if a lot of drag were removed from those damned tubes that are hanging out there in the wind.

Wider (y-axis) wing struts seemed to be employed more often toward the end of the biplane era. Is there anything in any of this?

C

[Mike Holland]

Doesn't this cover it?

9.1.1 [FONT=Univers][SIZE=1][FONT=Univers][SIZE=1]G. It is not permitted to construct any suspension member in the

form of an asymmetrical airfoil or to incorporate a spoiler in the
construction of any suspension member. Symmetrical streamlining

of suspension members is permitted.
[/SIZE][/FONT][/SIZE][/FONT]

[starkejt]

Doesn't this cover it?

9.1.1 [FONT=Univers][FONT=Univers]G. It is not permitted to construct any suspension member in the [/FONT][FONT=Univers]form of an asymmetrical airfoil or to incorporate a spoiler in the
[/FONT]

[FONT=Univers]construction of any suspension member. Symmetrical streamlining [/FONT][FONT=Univers]of suspension members is permitted.[/FONT][/FONT]

That would not stop you from fairing in the wishbones with sheet metal welded to the links in effort to reduce drag.

[Mike Holland]

There was discussion above as to getting downforce out also. Then I would think it becomes illegal.

[starkejt]

There was discussion above as to getting downforce out also. Then I would think it becomes illegal.

I agree.

[Brands]

On the various F1/Champ Car/IRL wind tunnel models I've been involved with we always optimized wishbone leg angles within the relevant regulations. We would often end up with, for example the fwd leg nose down and the rear nose up, but maybe different on the top wishbone and again some other combination at the rear. There wasn't much in it but in F1 land worth doing. I believe Tyrrell tried a fully enclosed top wishbone in the late 90's but I think it was banned pretty quickly. The regulations now specify a relationship between the section thickness and chord length. As Wren says there's plenty more performance elsewhere to be had, but for fun it might be worth getting the tin & tape out!

[Wren]

Doesn't this cover it?

9.1.1 [FONT=Univers][SIZE=1][FONT=Univers][SIZE=1]G. It is not permitted to construct any suspension member in the[/FONT][/SIZE]

[SIZE=1][FONT=Univers]

form of an asymmetrical airfoil or to incorporate a spoiler in the

[/FONT]

[/SIZE]


[SIZE=1][FONT=Univers]construction of any suspension member. Symmetrical streamlining[/FONT]

[/SIZE]
[SIZE=1][FONT=Univers]of suspension members is permitted.[/FONT][/SIZE]
[SIZE=1][FONT=Univers][/SIZE][/FONT][/SIZE][/FONT]

I don't think he is proposing an asymetric airfoil or a spoiler (gurney). Just a piece of metal between the legs of the a-arm. It would basically be a wider version of the DB-1 rocker arm. Nothing illegal about it, even if it does make downforce.

[Rick Ross]

Tyrrell ran a solid front A-arm in 1996. It was banned by the FIA as a movable aerodynamic device.

[TimW]

The Campbell/Kephart "Yellow Banana" FF (Vestal) had them mostly enclosed in one iteration of the suspension, IIRC.

[starkejt]

Wow, a photo of Dan wearing long pants. Very impressive.

[Christopher Crowe]

-- in that the device would be symmetric, no low-pressure side, no high pressure side. Any additional downforce would only come from the changing pitch of the chassis -- just like the car's entire body.

I didn't know the FIA outlawed it. The current SCCA regs surely don't seem to... and well, hmmm... I assume the FIA got rid of it to keep some last vestigae of the classic open wheel monoposto look...

The Vestal pic is a neat example of the idea; although I think the greater gain would be up front where the flow is generally less mangled.

I hope some of the aero folks will weigh in on this concept. I'm sure some study somewhere has been done.

Thanks for the thoughts.

C

[iamuwere]

You could defeat the anti-rule weinies and do something like Audi did with the R8 rear wing. Smart, those Audi geeks are.
http://www.mulsannescorner.com/audir8-01-6.html First section.

[R. Pare]

I don't see anything in the rules that would preclude welding in sheet in between the a-arm legs - as long as it is a permanent part of the arm, it would be hard for someone to claim that it is a "fairing" and therefore try to claim it as "bodywork" - generally, the test as to what constitutes "suspension" is that if you take it off and the corner collapses, it is "suspension" (not written in the rules, but the components described have that verifiable function).

While it looks/sounds cool, and would most likely have some drag benefit, more than likely you would get lift on the upper arm off of the front wing wake, and god only knows off of the lower.

Would be interesting to set up a yarn tuft grid in between the legs and see what is there.

[Reddog]

It is EZ to do ... go for it.

[rperry]

It's a good idea, but how much pitch will you really see? You can look up "thin airfoil theory" to roughly approximate how much 2-D CL you can expect per degree of change. Then cut that number down by quite a lot since your a-arms are not thin airfoils of infinite span.

There is much more to gain from properly angling each suspension arm, which will either require a properly good CFD model (not floworks or the like), or a lot of time in the wind tunnel. Though you could get away without a rolling ground tunnel, I still imagine doing 1-2 CFD runs would be cheaper.

[R. Pare]

There is much more to gain from properly angling each suspension arm

Not allowed, since the rules state that the arms have to be symmetrical about a horizontal axis - tho' everyone cheats that to some small degree.

[Rick Kean]

Not allowed, since the rules state that the arms have to be symmetrical about a horizontal axis - tho' everyone cheats that to some small degree.

Ahhh, the 'real' benefit to raking the chassis...

[JHerscher]

Next time you're on a test & tune day, turn your in car camera on the suspension and see what the tufts of yarn you cleverly placed there are doing. Next session, fill in the gap with racers' tape, and check to see any difference.

Probably will raise more questions than allow you to draw any conclusions, but...

As someone else stated, probably a lot more to be gained elsewhere, but still an interesting intellectual exercise.

John