I was wondering which teams use flex joints in place of rodends where the control arms mount to the chassis. By flex, i mean flat plates of steel or composite material. Im not so concerned that they will create a virtual pivot point or that they will add a slight bit of stiffness, but i am slightly worried about them buckling or fatiguing, especially on the LCA. I wanted to know if anyone had any pictures or horror stories to steer me in the right direction. Also, what materials have people used?

This message has been edited. Last edited by: Vector006,

Did you think about how you are going to repair one after you hit something with your wheel?
I know they're cool, but if you can't repair it what's the point?

Igor

I don't see why they would be any harder to repair than a conventional wishbone?

If you made the flexure out of steel like Lehigh tested a couple of years ago (and like Jordan did in F1 in 1997) the wishbone is going to be a lot cheaper than if you spec an expensive aerospace grade rod end/speherical.

Something I would have liked to try given another year.

Ben

I agree with Ben, I would like to see a good flexure FSAE car. Nearly every non-steering joint in an F1 suspension is a flexure these days.
Flexures have cost and weight advantage over sphericals if done right, but they are less tolerant of design changes as they have a much smaller 'operating window' than a spherical. So don't change your mind about your suspension geometry!
As to his repair comment, I assume Igor is comparing a threaded rod-end with an integrated flexure, rather than an integrated spherical with an integrated flexure...
Ian

Not every wishbone mounting point on F1 cars is a flexure. In the rear of the Ferrari for instance there are still rodends because they allow more movement. For a lightweight fsae car there are issues with making the tab thin enough to allow movement but yet be tough enough to avoid buckling. 1/4" thick steel flexures will not allow for much movement, but 0.035" thick steel might have buckling or strain hardening problems. Titanium is a good alternative I guess.

F1 can use them on wishbones because their A-arms travel through small angles. SAE travels through larger angles, so designing flexures that don't fatigue may be more difficult.

I was indeed thinking of something else. We once had a discussion on integrating kevlar flex joints into the monocoque, which is kinda hard to repair.
Well, you can always bolt a door hinge there :-)

Igor

So does anyone remember which teams have done this? I can scarcely remember what the joints look like, and I want to try to dig up some photos.


Nart

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Originally posted by MikeWaggoner at UW:
F1 can use them on wishbones because their A-arms travel through small angles. SAE travels through larger angles, so designing flexures that don't fatigue may be more difficult.

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My other concern, with a much smaller scale car, and larger angles of travel, designing flex joints large enough not to fatigue would have a noticeable effect on the spring rate at the wheel of the car. You can design around this to a point, but someone would need to do a little development work to prove its feasible.

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Originally posted by Jon:

My other concern, with a much smaller scale car, and larger angles of travel, designing flex joints large enough not to fatigue would have a noticeable effect on the spring rate at the wheel of the car.

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Also, the addition of bending loads into the wishbones can't be very good either.

So does anybody remember which teams have done this?

I could be wrong, I'm fairly new to FSAE, but everything that I've heard and read indicates that there has never been an FSAE car that used flexures. Of course, that doesn't necessarily mean that research has not been done. I would be willing to bet that someone has looked into it.

Ben Steele

At the SAE Motorsports Conference in 2002 (Indianapolis) there was a FSAE team with a booth that had flexures on display. Not on the car, but they had built some samples and from what I understood were intending to run them in 2003.

I believe it was the Colorado School of mines. But I am not 100% on that.

Hey Jeremy

This might be a bit off the wall but I've seen polycarbonate (lexan) used in hinge applications. It has excellent tear strength and is fatigue resistant.

Mounted between two plates in double shear it would be pretty cheap and easy to repair as well.

Cheers, Ted

the problem with carbon is gluing it to metal. There are some pretty cool epoxies out there and I know that there is a thread somewhere in here on them, but why would you want to go through the hassle of making sure you have no stiction in the rodends when most teams have basic compliance issues in their a-arms and tie rods?

I have heard stories of a drexel car from a ways back that had some flex joints...From what I gather the car had composite flex joints (glass?)...maybe one of their memebers will chime in here eventually...

IMO if you ar going to do it, save some weight while you are at it. Make composite a-arms with composite flex joints. End the joint in a plate that you bolt to your chassis, and you're done. Should be less weight, fatigue shouldn't be as much of an issue, and you shouldn't have to worry quite so much about peel as you would if you tried to intigrate a composite flexure with a steel arm.

Edit: Just one other thing, on the bending loads, I would be willing to be that the bending loads are already in most of our setups since the sphericals do tend to stick, at least a little, even when new.


Travis Garrison
UW FSAE

Again, I'm fairly new so if I'm wrong you guys should feel free to tell me to shove it, but I have to disagree with you Travis. The bending loads you have to consider when using rodends are not even close to those you have to design for when considering flexures, even though there is some stiction, unless you somehow exceed the range of motion that your particular mounting system allows. I very much doubt that anybody would overlook something like that.
As far as carbon A-arms are concerned, one of our guys did a little bit of experimenting with making carbon tubes (for a clutch handle) and it turned out to be a pain in the butt. He never could figure out how to separate the part from the mold. That's not to say that someone who could devote a little more time couldn't figure it out and it would be awesome if they did.
I'm really in favor researching the flexure idea, though I doubt Auburn will be trying it this year. It'll be cool to see what you guys come up with.

Ben Steele

Lehigh, my former team was to my best knowledge the first team to have flexures on thier FSAE cars. I did the study as a senior project and the flexures were fitted to our 1999 test mule and successfully tested over an endurance simulation. Our 2003 car went to design competition with them however they werent run in the events; we just saw it as being too risky. Last competition (2004) we had full composite a arms with integral flex joints. They were half the weight of steel a-arms (5/8 x .035). These flexures worked however the FOS was very low they werent run. The high angle of our upper A arm was a contributing factor as well as our design being a pullrod system; the forces on the lower arms are simply too high to get the neccessary displacement and buckling resistance. My senior study was thorough using mathematical optimization to attain the geometry;
The design can be done, especially on the top; however the car should be designed with them in mind: 13 inch wheels helps, pushrod, low angle with horizontal. direct bolted & bonded connection to square tubing on chassis (not neccesary, double shear plates in bracket would be fine).
BTW: the main aussie judge (little fellow, really nice guy, not sure of the name) absolutely loved them: no rod ends in bending; no inboard rod ends, no "sticktion"...
I would love to share the report/pictures, just email me at tac5@lehigh.edu

I’m not certain but I think that Drexel beat you to the flex joints. The only picture that I can find of the car is in the FSAE history pdf.
http://www.sae.org/students/fsaehistory.pdf
Bottom right of page 5 (45) - the yellow car. Flex joints in the drive shafts as well if I am not mistaken

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Originally posted by Lehigh FSAE alum:
BTW: the main aussie judge (little fellow, really nice guy, not sure of the name) absolutely loved them: no rod ends in bending; no inboard rod ends, no "sticktion"...

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Could that be Pat Clarke?

I would love to see your report. I will send you an email soon.

Ted,
I would be concerned with the chemical resistance of polycarbonate in structural applications. It’s not resistant to many chemicals we use in/on/around our cars. It is not resistant (rapid attack or attack over short time period will occur) to acetone, brake fluid, lacquers & thinners, and gasoline to name a few. I have seen what acetone does to polycarbonate (thanks to Mike Waggoner’s demo) it makes it very brittle very fast.

A chart of chemical resistance for polycarbonate sheets

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Originally posted by Lehigh FSAE alum:
...Our 2003 car went to design competition with them however they werent run in the events; we just saw it as being too risky.

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I believe that's against competition rules, to show up in Design with something you're not running in dynamic events.