A Arm Load Calculations

[Claude Rouelle]

And I have been wrong myself at least a few times! In fact many times!
But less and less.

[Adam Farabaugh]

Even if the pushrod attachment is welded, you can point the line of action through some point (at some percentage of peak load given your expected geometry) to minimize moments.
The hand calculations will be accurate if you just figure out the geometry and include moments. I would be more suspicious of your loads than a well-reasoned hand calc.

As to the general problem of member sizing, you develop two requirements, strength (area) and stability (inertia). From this you can directly solve for the two degrees of freedom of the member (diameter and wall thickness).

[jd74914]

To accounts for "unknowns"?????????

Perhaps unknown is not quite an accurate term. Doing basic statics and assuming a 2 force member has the potential to be very non-conservative in buckling. I'm not advocating such a ridiculous FS with FEA, just for really basic hand calcs. And certainly not that high for hand calcs for bending, etc.

[Z]

Maxhouck,

... calculated the expected tire forces, and ... can see the expected tensile and compressive loads for all of our different cases (accel, cornering, braking, bump, and combined cases).
...
Short of running extensive FEA using our load cases on each corner, are there any calculations that can be done ...

All the FEA in the world will not help you, when your assumed worst case loads are WRONG.

Or put another way, ask the wrong question, and you are guaranteed to get the wrong answer, whether you use hand-calc or FEA.

Doug gave you a big clue to the right answer, in the third post this thread.

Special loading cases for autocross cars -- cone strike and cone drag.

"Cone drag" is when the cone, especially its stiff base, gets wedged under a lower wishbone and lifts that corner of the car OFF THE GROUND. So do a simple hand-calc of the wishbone-tube being loaded in its middle, in BENDING, by the corner-weight of the car.
~o0o~

Here are three more potential "worst case loadings" that would be considered by diligent engineers.

1. Car is finally finished and is about to be loaded onto the trailer for its first test session. So Super-Student-1 picks up his corner of the car by the middle of one of the upper-wishbone tubes. Same bending load as above "cone drag", but on an upper tube.

2. After some repairs the car is finally on the trailer. Super-Student-2 decides to secure the car, so it doesn't get damaged in transit, by tying it down with heavy-duty ratchet-straps. He attaches these to the centres of the wishbone-tubes. Similar or greater loading to above, but down not up.

3. More repairs, and the trailer+car finally make it to the test-track. SS-3 has just finished his double-cheeseburger-with-extra-creamy-sauce, so his hands are a bit slippery. So while unloading the car from trailer, this time using the correct grip on the wheel and NOT on the centre of wishbone-tube, the wheel slips out of his grip. Oops! His corner of the car falls about a foot onto the hard ground. Or maybe it falls 2 feet, or 3 feet? You do the "bump" calcs.
~o0o~

Key questions you should be asking yourself are these.

Do we build a super-fragile, F1-style, car, that has every component optimally "optimised", with Safety-Factors <1.1, and that conveniently gives us a great excuse for failing to complete endurance yet again (ie. "...because the car was optimally optimised")?

Or do we build a robust car, suitable for the "amateur weekend autocrosser", that can survive the bumps and bruises that amateurs, and students, will subject it too, and that is thus able to score many points by "completing all dynamic events"?

Do we want to win Design Event, or all the Dynamic Events?

Z

[Z]

Max,

While I am here...

...I feel that we are at the point where we can be taking these things one step further in order to make a lighter car. The weight difference between the 5/8" .065 a-arms (~9.3 lbs total) we had on our 2017 car and the 1/2" .049 a-arms (~5.7 lbs total) we had on the 2016 car is a pretty signficant amount of weight to save...

So how much do your pushrods-&-rockers weigh?

And the extra chassis structure needed for the rocker-mounts?

Do you really need all that stuff?

Z

[DougMilliken]

"Cone drag" ... wishbone-tube being loaded in its middle, in BENDING, by the corner-weight of the car.

Could be even worse, depending on overall ride rates and the amount of lift. If the suspension is stiff, it's nearly like picking up one leg (one corner) of a 4-legged table, which gets you ~half the weight of the table.

There is also "SS-4" who decides to use a suspension link as a step stool to get something off a nearby high shelf.

[Z]

Doug,

Could be even worse ... which gets you ~half the weight [of car, bending the tube]...

Yep!

Z

[Mikey Antonakakis]

Thank you to Z and Doug for reminding me and everyone else of some of the other load cases, which for most teams will be more frequently encountered than anything found on-track... Although I don't think a-arm strength will be an issue for our team

[Claude Rouelle]

In design judging I often ask this question: you have 6 linkages per corner of the car. 2 for the bottom wishbone, 2 for the top one, 1 for the push or pull rod (or direct actuation) and one for the toe link. You have 4 corners that makes it 24 linkages.
I could add other thinks like ARB droop link or ARB bearing cap attachment on the chassis or rockers axis attachment on the chassis. But let's keep this example simple.

Let say you car goes on a skid pad at 1,5 G lateral, constant. That is steady state. The question is" what linkage gets the highest force and is it a compression or a tension?" in 90 % of the cases the students do not have an answer and they have to think for a while to give me one.

Then I have to wonder how the rod ends and the suspension tube sizes have been determined.
And without the force at the inboard suspension pickup points what inputs were chosen for the chassis FEA study.
And without the force at the outboard suspension pickup points what inputs were chosen for the uprights FEA study.
How were the compliance calculated?

Worse: They tell me after thinking that the highest load in a left turn is the rear leg of the RR bottom wishbone. OK.... seems logical if not quantitatively at least qualitatively.....
Then I ask what is the size of the bottom RR wishbone rod ends: they tell me 8 mm (which sees way to big for a decent, light FS car)
OK... then what is the size of the top RR wishbone rod ends? "Also 8 mm" Wait a minute: you just told me that the lowest wishbone has highest load that the top one. Is there one which is over engineered or the other under engineered. Or both?
The usual BS is: "Well we did this to decrease the number of different parts we have to manage"
Yeah.....

I expect students to do load suspension and chassis load cases BEFORE that do any parts FEA.
- Basic lateral. If you have a tire model that is good but you do not need to be perfect to be useful.
If your car is 200 KG (driver included that is 140 Kg car and 60 Kg car totally achievable), the weight distribution is 45 % front and take 3 G, make the LF + RF lateral grip = 200 * 3 * 9.81 * 0.45 and distribute the grip let's say 85 % on the outside and 15 % on the inside.
- Basic braking: 200 * 3 * 9.81 = 6000 N. 35 % Fx on each front wheel and 15 % on each rear wheel
- Basic acceleration (RWD): 200 * 1.5 * 9.81 = 3000 N and put 50 % on each rear wheel
- Make a combination of the first and second case of first and third case. Remember that tires have a traction ellipse not a traction rectangle . You can NOT have 3.5 G of lateral AND longitudinal
- Take each of the previous test add 1.5 G vertical under each of the wheels
- Do not forget to include the vertical reaction at the tires to the aero downforce at each wheel
- Make sure you simulate things unusual but possible, like braking and going backward. A few years a go in GP2 a driver spun the car and while going backward he jumped on the brake pedal.... and broke the two lower rear wishbones.
I heard about the same issue from hill climb drivers.

I would expect any decent good FS team to already have at the design judging event these load cases printed and filed in a binder and the laptop open with the software they use. All good team (5 %) do!


No FEA studies before these load cases.

[mech5496]

Fully agree with you Claude, in all but one; manufacturability and reduction of spare parts inventory for the sake of cost reduction totally makes sense as an argument, as long as the team understands the compromise.