cause the judges don't like (rod ends in bending)

[eilonoz]

the last disscution about that was at 2008 and it sais there that FSAE judges will never accept rod ends in bending as good engineering. is this true? and is it really bad engineering?
a spherical bearing mounted in a plate is a pretty heavy part

[Owen Thomas]

They will not accept it as good engineering because a threaded connection in bending is - mechanically - bad engineering.

The threads on a rod end, even when very well manufactured, produce massive stress raisers. This usually means to have a rod end that will not break, you will have to go to a larger size. Or they will break. Or, you will get "lucky" and they will just bend; good luck adjusting them now that the threads are distorted!

I can't definitively say that it is bad design, because I do not know your situation. If using rod ends instead of a spherical bearing & holder saves you a thousand man-hours and a good chunk of your budget (unlikely), and you can prove it, then the judges should accept your choice.

[MCoach]

^^This.

Also, look into Euler buckling and bending theory.

Think about the design that goes into those.

you have a tube with a large moment of inertia compared to a rod end with a very small moment of inertia. Put those parts into bending and buckling scenarios, especially considering teams like to use the very tiny #10 rod ends compared to the 1/2 - 3/4" tubing that they thread them into. I think we just found what will fail first...

Now, on the other hand, the sphericals mounted in some sort of bucket allows the tube to be the part more likely fail.
Sphericals and buckets don't have to be that much heavier than a rod end either.

[Claude Rouelle]

The fact tat you wonder if REiB could be that bad engineering let me wonder if you are serious and even what kind of engineering skills/experience your teacher(s) have...

But... OK let's say Rod End in Bending is not a bad thing (I can see Pat Clarke close to a heart attack here) but let's say...

With REiB, for a given load you will have a big stress so you will need a stiffer rod end. Inevitably that will mean a heavier rod end. And how many rod ends do you have in one car? There will probably also be more compliance in your chassis so you need a stiffer chassis so also probably heavier.

Yeah but F = ma. So for a given lateral G(a) more mass (m) means also more force (F), which means more compliance which means need of stiffer parts and so on ... and you just started the crescendo to disaster.

And there is even a flaw in the above reasoning: in simple, basic terms the lateral G (and longitudinal G too) depends of the tire coefficient of friction .... which decreases with the vertical load. So is simple terms heavier means less potential grip

Work smart; work on the causes of the bending by trying to limit them before you work on the consequence by making them heavier.

It is possible to make a car light, stiff, relatively cheap and easy to manufacture. that is the quintessence of a good engineering design and execution

[Dunk Mckay]

I also don't see to many situations where you would want a rod end in bending?

I mean if it's in bending it means you've got at least 3 attachment points, the locations of these are generally going to be defined by some kinematic simulations/predictions/calculations, etc. So why do you want to adjust them?

I know some people might think that they are good for amber adjustments, but while it changes you camber it also changes your kinematics.

If you want to use it to test out various kinematic setting then...maybe, but just adjusting the outboard locations is only gonna get you so far? You can adjust inboards as well, but then you need to have adjustable suspension clevises as well! And you're still constrained by the angles of your wishbones.

Just spend the time getting your kinematics right the first time (or get a good baseline to develop for next year) and use some other camber adjustment method and stop being lazy. We use shims, not the best perhaps as the can be overtightened and compress then loosen, but simple to design and make and will work fine if you take care of them properly.

[eilonoz]

i'll give the example that made me think about it.
i wanted join together the tie-rod and the lower a-arm. here is a link to a picture of it:
https://docs.google.com/docume...6_fQmHVfWceXjm0/edit (https://docs.google.com/document/d/1O48eIVkndwkbGia42lIVBrUvrpn16_fQmHVfWceXjm0/edit)

one option is to place a spherical joint in a plate which welded to the two rods (upper picture). now, the major force at this joint is applied from the rear a-arm rod at acceleration and braking, while the tie-rod is almost unloaded. when cornering, the tie-rod and the a-arm rear rod, loaded at less than one quarter of the highest (brake/accel) load.
so, in case of a rod end (bottom picture) i'll get about 8500 N of "good" force and 300 N of "bad" force, or 1000 N of "good" and 2000 N of "bad".

i cant see whats wrong with this design.

if there is somthing i believe we cant argue about is that per newton, rod end weighs less than sphere & plate

[eilonoz]

my bad. try this link
https://docs.google.com/docume...16_fQmHVfWceXjm0/pub (https://docs.google.com/document/d/1O48eIVkndwkbGia42lIVBrUvrpn16_fQmHVfWceXjm0/pub)

[Dunk Mckay]

Among many things, you gonna have issues when you tried to adjust your toe on the rear. Because the axis on which the outboard rod end moves in and out is fixed, it's distance from the spherical bearing on your urpight is going to change. So you're going to have to build some sort of adjustment into your uprights.

There are a ton of other issues with this design, and I'm sure someone will highlight them, but I'm tired after a long day of build. So I'll finish by saying that if someone asked to put something like that on my team's car I'd tell them to go away and think about what they'd done.

[Charles Kaneb]

eilonoz,

Build a little model version of both A-arms in this picture. Wood or cardboard is OK for this test - you'll see exactly what's happening if you use cheap materials.

Slip bolts through the center of the bearings. Install nuts. Clamp the inboard bolt and nut assemblies in a vise. That simulates bolting your assembly to your chassis. You don't have to have spherical bearings; assume the spherical joint or the spherical part of the rod end to be infinitely stiff.

Yank really hard on the tie-rod (called a toe-link at Texas A&M).

On the spherical bearing one, the load path to the bolt goes through solid material all the way to the spherical joint, and acts directly in tension on the spherical bearing, and goes right to an aligned double shear load through the bolt.

On the rod-end one, the load doesn't pull directly through the threads of the rod end. It instead pulls on the side of them. Any rod end has a smaller cross section area anyway than a plate with a spherical bearing...

Staking dies and a press are cheaper than going up a couple sizes on all the rod ends.

If you can't afford spherical joints and plates with cutouts, it might be worth trying to make thin-walled plastic bushings inside a tube with a through-bolt.

[Z]

Originally posted by eilonoz:
... FSAE judges will never accept rod ends in bending as good engineering. is this true? and is it really bad engineering?

Ford GT40 front suspension (note upper wishbone).
http://www.racingicons.com/gt/1076-007.jpg

Ford GT40 rear suspension (note outer end of lower wishbone at left of pic). These pics from a replica, but similar to original.
http://www.autohausclassics.com/images/gt40-suspension.jpg

So, 1960s, and a 7 litre big block Ford is pushing a ton-and-a-half+ of GT40 racecar down the Mulsanne straight at close to 400kph (reports "in excess of 240mph").
On a much bumpier track back then, than now.
For 24 hours (plus extensive testing, etc.).
To win Le Mans outright in 1966.
And 1967.
And 1968.
And 1969 (well, actually, those last two years were with the small block V8).
With rod-ends in bending.
And mounted to the upright in single-shear.

Bad engineering, huh? http://fsae.com/groupee_common/emoticons/icon_biggrin.gif http://fsae.com/groupee_common/emoticons/icon_biggrin.gif http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Z

[mech5496]

Just because it works does not make it a 'good engineering practice'

[Bemo]

If you make it big enough it will work even though you have the threads in bending load. It is no secret that this has been done a lot of times even in very succesful race cars. But as Harry already stated, it is quite questionable that just because of that it is good engineering practice.

I've seen quite a couple of cars loosing wheels during brake test because of REIB. The problem is that some teams don't realize what happens there during braking or at least underestimate it. I've once talked to someone from such a team who was completely stunned as he had calculated it and had a security factor of 1.5. Unfortunately he forgot to consider stress concentration...

I guess that guy learned a lot that day http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Tim.Wright]

I'm not a fan of these sweeping statements such as rod ends in bending are unacceptable in any case... Because sometimes for a given situation, there might be advantages. Like everything else you design, you need to consider the pros and cons yourself and make you mind up based on that.

I can't think of an example regarding REIB off the top of my head - but for example the other taboo, single shear, can be easily justified for packaging, tolerance and assembly reasons. After having "single shear is evil" drilled into my head for years, I sat down and actually thought about it myself and came to the conclusion that it can be ok.

I suggest you do the same on this subject. Think about it yourself instead of relying on a sweeping statement which has no context.

Tim

[Claude Rouelle]

The Ford GT 490 was good car... at that time.

A few yeas later the reference became the Porsche 917. Ans it was also a good, winning cat... at that time.

A good friend of mine was part of the "resurrection" of Porsche 917 Chassis # 21. There is an absolutely wonderful book about that story.

http://porsche.contain.com/ded...1the-fabulous-story/ (http://porsche.contain.com/dedeporsche/entries/1592/videobookporsche-917-021the-fabulous-story/)

I saw the car from very close for a long time. It did bring back quite emotional memories. Smart design (again, at that time...) but still a very weak chassis (even drivers and engineers were telling that in the early 70's) and a poor aerodynamics and poor handling by today standards. When I look at the design and the manufacturing of the suspensions and the chassis of that car I can't help to think that the drivers who drove that car were very brave.

Nobody would design a race car that way today. Believe it or not on some aspects a few FSAE cars are way better deigned an manufactured today.

Respect of former engineers remains (even more for Porsche of which I always have been a fan)

But reference of the past are not necessarily today's reference.

“Human beings, who are almost unique (among animals) in having the ability to learn from the experience of others, are also remarkable for their apparent disinclination to do so” Douglas Adams

[Will M]

I found this on an old thread.
I think this is the kind of REIB that really should be avoided.
http://www.ncs-stl.com/racecar/MVC-011S.JPG

(from: http://fsae.com/eve/forums/a/t...5607348/m/8486002351 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/8486002351))

-William

[monoposto]

^^^ Yikes! To say the least, it looks like someone had something against double shear.

[Owen Thomas]

I think that picture nicely highlights what I was trying to say in my earlier post.

The whole assembly is a marvel of taboo engineering, but at the same time was probably perfectly aligned with the teams design goals. Sure it has bad load paths, probably a fair bit of compliance and is ugly as sin, but it looks sufficiently over built to handle the loads and would be dead simple to fabricate. Obviously this team did not put a premium on weight, and was more concerned with having the car built quickly and cheaply.

That said, there is most certainly some questionable engineering in that snapshot. It all really goes back to the whole "motorsports vs engineering" debate. Like Claude said, I think the expectations are simply higher than they were in the past - especially with new engineering and manufacturing tools at our disposal - so it is no longer as justifiable to make such large compromises.

[PatClarke]

No names, no pack drill, but this vehicle did suffer a catastrophic suspension failure in operation!

Pat

[MCoach]

.....Where are those control arms going?

I can't help but cringe at what the overall package may look like.

[Will M]

MCoach,

If really want to see it follow the URL for the picture and you can get to a directory of photos of the full car.

-William

[exFSAE]

Cars with rod ends in bending and/or single shear mounts definitely have kicked major ass back in the day...

...so did F1 cars with treaded dry weather tires.

[Jon @ Electromotive, Inc.]

That picture is from an A-Mod autocrosser posted up for sale in the early days of the forum. The link referenced a later post by the guy selling the car (at least I think it was the guy selling the car).

[Owen Thomas]

OK, so I went to the website and saw the whole vehicle from that photo... needless to say my assumptions about design goals were wrong. That thing is (was?) a nightmare on wheels.

The point still stands that if you have a team with low technical knowledge and need a car in 2 months, using heavy REIB may not be such a bad idea.

[Z]

Originally posted by Tim.Wright:
I'm not a fan of these sweeping statements such as rod ends in bending are unacceptable ...
...
... the other taboo, single shear, can be easily justified for packaging, tolerance and assembly reasons. After having "single shear is evil" drilled into my head for years, I sat down and actually thought about it myself and came to the conclusion that it can be ok.

I suggest you do the same on this subject. Think about it yourself instead of relying on a sweeping statement which has no context.
Tim,

Awww, now you're making me feel all awkward... I AGREE with everything above! (Geez, I might have to retract earlier comments. http://fsae.com/groupee_common/emoticons/icon_smile.gif )
~~~~~o0o~~~~~

Getting back to the OP's proposed wishbone design (https://docs.google.com/document/d/1O48eIVkndwkbGia42lIVBrUvrpn16_fQmHVfWceXjm0/pub), I see very little wrong with it (wider base at each end would be better). The main problem here is the almost religous devotion of some people to a particular design approach.

The GT40 designers, and countless others like them, got their quick-and-dirty, camber-adjustable, REIB-and-single-shear, wishbones designed early, and then went on to solve all the other problems required to win. If they had to fiddle around with countless shim-adjustable, double-shear, clevis-mounted, staked-in-BJs, then they may well have ended up with cars very similiar to many FSAEers. Namely, late-finished (so untested), over-weight, over-priced, over-complicated contraptions that even with very liberal aero rules, and after ~25 years of polishing, can not consistently corner at more than 2G.

And these FSAE cars have the expensive wishbones acting as "the fuse", rather than having cheaper and more easily replaced bolts, or rod-ends, acting as the weak links. Many Design Judges either turn a blind eye to FSAE cars shedding wheels via failed wishbones, or else they actually encourage this. So why the aversion to a design (REIB) that can do the same thing, but in a cheaper and perhaps more controlled way?
~~~~~o0o~~~~~

I should also point out that many of the best ever designers whose work I have seen have almost everything in "single-shear". Perhaps the foremost example of this is Nature, who clearly loves mounting things in single-shear (and with lots of bending!). Look at trees (trunk-to-ground, +++, leaves-to-twigs) and skeletons (bone-to-bone) for good examples of what actually works.

Z

[MCoach]

Well, the overall car looks a lot better than expected.

It's an A-mod car. It's cool to see some things outside FSAE.

[eilonoz]

i still see a lot of difference between my example and the others that were discussed here.
at most of the examples the bending stress came from the sphere bearing which is the primary force acting on the rod end.
i was talking about taking a prety minor force that i have to transform to the chassis somehow.
i dont want to save money or do anything accept of doing it in the best way (weight\strength).
still wouldn't it be the best?

heres a remoinser where we began
https://docs.google.com/docume...16_fQmHVfWceXjm0/pub (https://docs.google.com/document/d/1O48eIVkndwkbGia42lIVBrUvrpn16_fQmHVfWceXjm0/pub)

[M. Nader]

https://pbs.twimg.com/media/BIT-XIsCQAA22Ib.jpg:large

That's why the judges don't like them. just saw this and thought i'd post here

[atm92484]

Except none of those rod-ends are in bending...

It wouldn't surprise me if there was more to the story than the picture is telling.

[Tim.Wright]

rod end as a bumpstop for the motherflippin win!

[M. Nader]

Originally posted by atm92484:
Except none of those rod-ends are in bending...

It wouldn't surprise me if there was more to the story than the picture is telling.

Massive banking put things in an awkward geometry, bending the rod ends. that's what the owner confirmed with me.

How can you tell from the picture they weren't in bending? maybe you can make a case for the lower one where i would guess too little thread was attached, but the rocker one i had no doubt it carried bending load.

[Owen Thomas]

Originally posted by M. Nader:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by atm92484:
Except none of those rod-ends are in bending...

It wouldn't surprise me if there was more to the story than the picture is telling.

How can you tell from the picture they weren't in bending? maybe you can make a case for the lower one where i would guess too little thread was attached, but the rocker one i had no doubt it carried bending load. </div></BLOCKQUOTE>

From the picture it looks like the rod end which failed at the bellcrank is a pushrod, which should only be in compression (2 pin connections). The lower one looks like it would be a tie-rod, which is also a tension/compression member.

My guess would be that the "awkward geometry" caused some kind of binding to occur, putting those rod ends into unexpected bending. Nevertheless, this is a good example of how rod ends will fail, however strange the situation.

[atm92484]

How can you tell from the picture they weren't in bending?

I have eyes and a pencil and am able to draw free body diagrams.

Assuming the rod-ends were the bump stops, FSAE calls that poor design - not rod-ends in bending.

[Z]

I see a couple of blown fuses. Solution: Replace fuses and continue...

If they used bigger rod-ends, and mounted them with bolts in single-shear, then the fuses would be even cheaper to replace (ie. just the bolts).

And, BTW, all rod-ends, even at the ends of a two-point pushrod or tie-rod, operate "in bending". This bending comes from the gravitational or inertial loads on the link itself. So suggesting "No REIB" is really saying "No REs at all!!!". So how long before you all have to abandon your easy-adjustable, rod-ended toe-links and start building shim-adjustable, clevis-mounted, staked-in-BJs at the ends of your toe-links? http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Z

(PS. Eilonoz, As I mentioned earlier, your design looks OK to me. Perhaps make the mounting bases at chassis and upright a bit wider, and use adequately sized REs.)

[M. Nader]

Originally posted by atm92484:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">How can you tell from the picture they weren't in bending?

I have eyes and a pencil and am able to draw free body diagrams.

Assuming the rod-ends were the bump stops, FSAE calls that poor design - not rod-ends in bending. </div></BLOCKQUOTE>

I am not doubting that.

How do you know how the attachment looked like in the fist place? you only see a broken piece it could very well have had a strange initial angle which caused that! i have seen it before that is why when i first saw the pic i asked about the attachment angles. Perspective that is all

[Warpspeed]

That picture is interesting.
Two different rod ends appear to have broken at the exact same time.

That suggests to me either accident damage, or some extremely unusual massive one time overload, like becoming very airborne.

Now we don't know what actually happened, but if you lose control and hit something really hard, things are going to break.

And you can only blame the designer if the part failing CAUSED the accident, not if the accident caused the part to fail.

[Will M]

M. Nader,

Thanks for posting the pic.
Do you perhaps have more of the car? Both before and after failure?

Thanks,
William

[M. Nader]

William,

I initially hadn't looked for it as i came across the pic somewhere and just asked the poster to clarify my guesses a bit and went on. after some looking though here is a pic of how it originally looked.

http://www.quimerarr.com/imagen/1326467161.494b2996b8c86e78565d0034820d32e6.jpg/0/0/c

[Will M]

M. Nader,

Thanks for the new pic.
I followed the link and found this:
http://www.quimerarr.com/imagen/1326789210.494b2996b8c86e78565d0034820d32e6.jpg/0/0/c

Looking at it I would say that the push rods and dampers look pretty planer.
But those bell cranks may be narrow enough for the rod ends to bind and fail.

Can't tell much more from the pics...

-William

[Pete Marsh]

With the clue being "big banking" I'm going to go with a failure of the spring and damper mount.
Can't really see the mount, but it doesn't look too substantial and the spring looks fairly close to coil bind considering the motion ratio. A bound spring as a travel stop + a less than massive bracket would fail pretty easy on some "big banking" leading to unrestrained suspension travel and bound and broken (correctly utilised)rod ends/brackets all over the place. The direction of the bending failure of the push rod rod end would support over rotation of the rocker. Nothing at all to do with REIB IMO.

Of interest, is that a REIB on the out board upper ball joint? The one that didn't break? Kind of funny given the topic and reason for posting the pic.

Pete

[M. Nader]

Yes it is Pete http://fsae.com/groupee_common/emoticons/icon_smile.gif that outboard rodend, but it is really minimal in comparison to what we sometimes see in FSAE (The rod end would be a bit more extended). still in bending nonetheless, funny how it didn't fail and the correctly utilized ones did.

Seeing the before pics yesterday i too realized this was not a REIB case, what would load both the pushrod and the Rear upper a Arm mount? with the keyword "massive banking". i will have to give it more thought, but it is safe to say that the loading was not really expected which means that it was during an extreme situation, Jumping of a curb? i think this is a track car but i am not so sure it wasn't used elsewhere

[eilonoz]

i think that trying to have conclusions out of a picture is not a good idea. we can give hundreds of good explanations how it occurd and still be far from truth

[Owen Thomas]

Originally posted by eilonoz:
i think that trying to have conclusions out of a picture is not a good idea. we can give hundreds of good explanations how it occurd and still be far from truth
But it's so much more fun to speculate on other peoples' failures than it is to actually do work!

[Ahmad Rezq]

Tim,

Awww, now you're making me feel all awkward... I AGREE with everything above! (Geez, I might have to retract earlier comments. http://fsae.com/groupee_common/emoticons/icon_smile.gif )
~~~~~o0o~~~~~

Getting back to the OP's proposed wishbone design (https://docs.google.com/document/d/1O48eIVkndwkbGia42lIVBrUvrpn16_fQmHVfWceXjm0/pub), I see very little wrong with it (wider base at each end would be better). The main problem here is the almost religous devotion of some people to a particular design approach.

The GT40 designers, and countless others like them, got their quick-and-dirty, camber-adjustable, REIB-and-single-shear, wishbones designed early, and then went on to solve all the other problems required to win. If they had to fiddle around with countless shim-adjustable, double-shear, clevis-mounted, staked-in-BJs, then they may well have ended up with cars very similiar to many FSAEers. Namely, late-finished (so untested), over-weight, over-priced, over-complicated contraptions that even with very liberal aero rules, and after ~25 years of polishing, can not consistently corner at more than 2G.

And these FSAE cars have the expensive wishbones acting as "the fuse", rather than having cheaper and more easily replaced bolts, or rod-ends, acting as the weak links. Many Design Judges either turn a blind eye to FSAE cars shedding wheels via failed wishbones, or else they actually encourage this. So why the aversion to a design (REIB) that can do the same thing, but in a cheaper and perhaps more controlled way?
~~~~~o0o~~~~~

I should also point out that many of the best ever designers whose work I have seen have almost everything in "single-shear". Perhaps the foremost example of this is Nature, who clearly loves mounting things in single-shear (and with lots of bending!). Look at trees (trunk-to-ground, +++, leaves-to-twigs) and skeletons (bone-to-bone) for good examples of what actually works.

Z

I can feel every single word Z has posted.
during the sprint at formula student 2016, our car understeered and hit the wall, The right wheel control arms rod end in bending were broken and also the steering link. we didn't take much time just replace the two rod ends, steering link and getting back to the track very quick with less damage and cheap parts to replace. and catch the second heat of the sprint.