Beam Axles - Front, Rear or both.

Jay and MCoach,

Given that FSAE students are known throughout the universe as the "Masters of Floppy Linkages", I must assume that you are kidding with your above comments. This assumption supported by Jay's humorous question "Do you intend to hammer the locating pin in so it's a nice fit with zero compliance?", and MCoach's suggestion that my brake system has "two 30" ... [pull-]rods" when quite clearly they are closer to 10" in length. Also MC's acknowledgement of his prior mastery of floppiness.
~o0o~

Bill,

You suggest that students, such as above, should get busy FEMing when they want to determine which linkages are floppier (or stiffer?) than others. Sadly, I suspect that it is the FEM itself that is the root cause of this problem.

Two generations ago all students were given "calculators", and now hardly any can do simple sums.

One generation ago all students were given word-processors with "spell-checkers", and now hardly any can spell simple words.

Today all engineering students have access to structural-analysis programs (eg. FEM), and now hardly any can analyse simple structures.

The problem seems to be that the young-engineer thinks that their job is simply to give an "executive level" description of the structure (ie. its rough topology), and the computer-minions will then go away and optimise it. Unfortunately, not even the cleverest computer-minion can turn a sow's ear into a silk purse, or make the major topological changes needed to turn a bad structure into a good one.

(Edit: And since the young-engineer has lost the ability to analyse simple structural topologies, they cannot distinguish the good ones from the bad ones. So we all end up with highly optimised sow's ears...)
~o0o~

Goost,

Thanks for the reference. I will look it up. Given that it is 1904, it should be good! :)

(Edit: The example shows that efficient cantilever beams are NOT triangular structures, as sometimes assumed, but rather are somewhat fatter in the middle...)
~o0o~

Finally, any students pondering the above issue of "stiffness of brake-linkages" might compare the FBDs I mentioned in previous post with those of an "almost vertical M/Cs" linkage. Keep in mind that my last sketch has the "pedal motion-ratio" at about 2:1 (foot : pullrod motions), whereas most FSAE brakes are typically at higher ratios (roughly 3:1 -> 5+:1).

Then explain how a linkage that converts the driver's NECESSARILY ~horizontal foot-force, of, say, 2 kN maximum, into TWO almost vertical forces of 6 -> 10 kN, can ever be more structurally efficient than a linkage that keeps all forces close to in-line, and at low magnitude (ie. with 2 kN foot-force my linkage has each pullrod at 2 kN tension, and the pedal-tray longitudinal member at 2 kN compression, with negligible chassis stresses/strains forward of FRH, etc...).

PLEASE SHOW ALL FBDS!!!

Or, to put it another way, I will happily wager your first year of real wages that my pedal-tray is stiffer, stronger, and lighter than yours! :)

Z

Z,

Not sure if you're intentionally ignoring my question or you actually think I was being 'humorous' (I wasn't). I'm truly interested to know your intended idea for securing your brake system. I completely agree with the overall concept (and as I mentioned earlier I understand that it is much more structurally sensible (assuming mounting rigidity) than the near vertical m/c's), but I plainly don't understand the secure location of such a device. What sort of tolerances do you envisage on the square hole to the locating arm? On the locating arm to the pin (and the top/bottom of the square hole)? Is there other hardware that you would add in a more detailed design?

Jay,

A key point here is that the driver only ever PUSHES on the pedals. Mostly they push very hard on the brake-pedal, and somewhat more softly on the accelerator-pedal. They never PULL on the pedals (well, not unless throttle stuck!).

In the main post covering this I mentioned that the front of the pedal-tray is constrained left-right and up-down by some "slide-rails". These would be extremely simple in my version, and I am aware of many FSAE teams doing similar slides, albeit in a more complicated way. Anyway, these slides would have a simple, springy, "friction pad" incorporated that suppresses any "rattling" of the tray. The rear locating-pin slide, at base of FRH, may also have something similar.

So, while the driver is not pushing on the brake pedal, the whole pedal tray just stays where it is. It follows that the locating-pin can have very loose tolerances. Any "slop" is taken up the first time the driver pushes the pedal, after which NO MORE MOVEMENT.

Furthermore, a practical locating-pin hole tolerance is about 0.5 mm. This translates to maximum fore-aft movement of the brake-pedal of ... 0.5 mm! So even if the pedal tray rattles around and moves fully backward after each brake application, there is still only 0.5 mm of slop at the pedal. I doubt it is possible to adjust brake-M/Cs so that the necessary pedal movement to close the "refill-hole" is less than 1 mm. So the lesser locating-pin slop is really a non-issue.

Can anyone who is really proud of their rock-hard brake-pedal post how much pedal movement they have, measured at the foot-pad and wrt the seat-back, between zero foot-force and 2 kN?

This is a number that all Teams SHOULD have!

Z

My 30" length was based on me sitting in our car and what I'd be able to reach as a driver belted in. If it were to be 10" I don't think even our lankiest, orangutan-armed driver would be able to reach it based on the Percy requirements of seating distance from pedal face. I think the 2:1 ratio is not achievable without significantly affecting the weight of the rest of the system, either much larger calipers, rotors, etc. and that, to me, would take greater precedence over putting the pedal is less bending based on our system engineering requirements. FSAE usually has a high pedal gain because the rest of the components are sized for the likes of a mountain bike to minimize overall mass. If your human can reliably interact with his environment with X deflection of Y part in the overall goal of putting the damn car across the line the fastest, then mass be damned, that car will be built to minimize mass and live with the consequences. Considering our pedal already weighs on the order of .1 to .2lbs, I think it's a moot point to wager and becomes a freshmen level lower member swinging contest of "my part is more optimized than yours." ...And there are much larger fish to fry.

I chose to study our '09 car because I knew it had problems and wanted to understand the extent of the problems. It left a lot of room to dig as it was well documented. I wanted to understand it to understand how to basically rewrite our design process for this system this year. It wasn't like I had any design input on that so I can't quite speak for the evaluation process used for the design. Oh my, has this year been fun.


As to the sliding pin joint, could a linear motion bearing assembly be used? As in something like this? I think a full metal version may be more equipped for the job.
http://m4.sourcingmap.com/photo_new/...120_ux_g03.jpg

we use 3/4 x .049 SHS sliding on 5/8 x .049 SHS rails (thats ~.6mm of play), both 4130. The pins were made from some 6mm bar and the holes were hand marked and drilled with a 6.5mm drill. We added some springs to the pins to allow quick adjustment. You never notice the (small) play in the assembly because, as Z says, you only ever load it in 1 direction.

Quote:

---

Originally Posted by Goost

What is often considered the seminal paper on structural optimization gives a fascinating introduction to how to think about these concepts.

A.G.M. Michell. The Limits of Economy of Material in Frame-structures. Philosophical Magazine Vol 8. 1904.

Attached an excerpt. I think this can be found on Google Books. Short story - make everything parallel/perpendicular between loads and constraints. Where curvature is needed to 'connect-the-dots', replace parallel/perpendicular with tangent/normal (in a circle or a logarithmic spiral).

Doesn't only apply to frames either - in many practical circumstances applying topological optimization to a 'smooth' shape (e.g. monocoque) will result in a frame-like 'optimal' anyway (e.g. space-frame-like strips of reinforcement within the monocoque wall).

So assuming you have access to water-jet, maybe the ideal beam axle looks something like this [attached] cantilever...

---

Goost,

Thank you very much!

I am putting this reference up again because it is a VERY GOOD paper! HIGHLY RECOMMENDED!

Students who get the gist of this paper will go on to be very good structural engineers. One specific example of this "gist" is that the triangles in spaceframes should be as "compact" as possible. Put the other way around, the very long and narrow triangles found in many FS spaceframes (eg. SISs with triangles ~0.8m wide by ~0.3m high) are BAD DESIGN! Also, Michell's last section on minimal structures to resist torsional loads should be very helpful to Frame-Guy.
~o0o~

When trying to download the above paper (which, as per modern practice, was mostly hidden behind numerous "pay-walls"), I came across this paper (also good, because also very short).

"What was wrong in Michell’s paper of 1904?", Mariano Vazquez Espi, Jaime Cervera Bravo, v2.0: January 19, 2012.

I mention this paper because it gives an interesting insight into the modern academic system (ie. YOUR educations, and the "spiral down the S-bend to Idiocracy" rants you may have noticed hereabouts :)). The answer to the title question is given here.

"4. CONCLUSION - The Writers conclude giving an answer to the title question: nothing is wrong in Michell’s paper - or at least, the errors pointed out by Rozvany do not exist. One of the Writers warned time ago (see Cervera Bravo, 1982: note 90) against the careless rewrite of old texts, ...
...
Perhaps the main moral of this story is that the peer-review method - that was so useful during XVIII and XIX centuries with Royal Society’s format - performs somehow bad nowadays..."

Z

Looks like the 2015 Aus comp is going to have a few beam axle cars attending.

- ECU (assuming they stay beam)
- UWA (assuming they attend)
- ADFA
- Uni SA
- UQ

Maybe more?

http://www.scienceagogo.com/news/201...runc_sys.shtml

We are well past the 10% point, at least in the Australian competition. Wonder when we will start too see this concept internationally.

Mitchell,

Besides UWA I'm not familiar with those teams.
Do you have links to photos?

-William

ECU - https://www.facebook.com/ECU.Motorsport
UQ - https://www.facebook.com/UQRacing
ADFA - https://www.facebook.com/academyracing
Uni SA - https://www.facebook.com/UniversityO...aliaFormulaSae


As far as pics of the beams go, there are none for this year yet. I can share this though. We are using a triangulated 4 link de dion.

Attachment 502

Will that be with mechanical interconnection again Mitchell?

Anyone other than UWA using a beam at the front as well?

Pete