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

  1. #1
    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
    BGR suspension team

  2. #2
    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.
    Owen Thomas
    University of Calgary FSAE, Schulich Racing

  3. #3
    ^^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.
    Kettering University Vehicle Dynamics
    Formula SAE 2010 - 2015
    Clean Snowmobile Powertrain 2012 - 2015

    Boogityland 2015 - Present

  4. #4
    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
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  5. #5
    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.
    Dunk
    --------------------------------------------------------
    Brunel Racing
    2010-11 - Drivetrain Development Engineer
    2011-12 - Consultant and Long Distance Dogsbody
    2012-13 - Chassis, Bodywork & Aerodynamics manager

    2014-present - Engineer at Jaguar Land Rover

  6. #6
    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

    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
    BGR suspension team

  7. #7

  8. #8
    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.
    Dunk
    --------------------------------------------------------
    Brunel Racing
    2010-11 - Drivetrain Development Engineer
    2011-12 - Consultant and Long Distance Dogsbody
    2012-13 - Chassis, Bodywork & Aerodynamics manager

    2014-present - Engineer at Jaguar Land Rover

  9. #9
    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.
    Charles Kaneb
    Magna International
    FSAE Lincoln Design Judge - Frame/Body/Link judging area. Not a professional vehicle dynamicist.

  10. #10
    Senior Member
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    Location
    Australia
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    1,690
    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).


    Ford GT40 rear suspension (note outer end of lower wishbone at left of pic). These pics from a replica, but similar to original.


    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?

    Z

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