Control Arm Shapes

[Dash]

Dash here again, running for some help!

On your formula car, would you rather run

a. An isosceles triangle for your control arm, leaving even force distribution. This allows for a wider base on the chassis end.
b. A right triangle. I've heard that supposedly this lets the perpendicular tube of the control arm be highly efficient at handling the tension/compression forces. This requires you to have a smaller base on the chassis end.


I know its all compromise, but i'd like to have some input from others.

Thanks!

[Dash]

Dash here again, running for some help!

On your formula car, would you rather run

a. An isosceles triangle for your control arm, leaving even force distribution. This allows for a wider base on the chassis end.
b. A right triangle. I've heard that supposedly this lets the perpendicular tube of the control arm be highly efficient at handling the tension/compression forces. This requires you to have a smaller base on the chassis end.


I know its all compromise, but i'd like to have some input from others.

Thanks!

[Pennyman]

I would suggest making an excel spreadsheet that calculates stress and buckling strength on A arms of different shapes. That's probably the best way to come to a good decision on A arm design and (inboard) rod end sizing.

But to get you started:

Right triangles: harder to get steering angle with wide rims or smaller scrub radius. Easier to fine tune wheelbase. They also look sweet http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Isosceles triangles: both tubes are the same length, easier fixturing, ability to use the same a arms on both sides of the car, so less need for spares.

[Dash]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Pennyman:
They also look sweet http://fsae.com/groupee_common/emoticons/icon_biggrin.gif
</div></BLOCKQUOTE>

Very, very true. ^_^

[murpia]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Dash:
I've heard that supposedly this lets the perpendicular tube of the control arm be highly efficient at handling the tension/compression forces. </div></BLOCKQUOTE>
That's what you heard, but what did you calculate?

The loads in wishbone tubes are rarely equal. Each has it's own worst case situation, some tension, some compression. Each needs a different cross-section and moment of area accordingly.

Regards, Ian

[poe21]

Just checking in. Glad to see you on the boards. It took me an entire year of reading and searching and I finally started my first thread just a little bit ago. Dry sump stuff. Sorry to get off topic, give the boy some info.... please!!!! haha

[Dash]

Thanks Ryan.

Also, I have done no calculations.
( Crazily enough, I'm a first semester sophomore doing suspension design. )

So i'm really just jumping into a pool of sharks and hoping for the best.

Any help on how i could start with such a calculation?

[Mikey Antonakakis]

Free body diagrams. It'll probably give you a headache, but you'll get smarter. You can assume a force from the tire contact patch, which gets transferred to the upright through the hub, the force on the upright is countered by the a-arms, pushrods, toe link, and on and on. Just keep going until you feel confident that your calculations are sufficient. Then later on, find a way to test them, and show the judges that you did. At least that's what I think they are looking to see...

[poe21]

I can help you with free body diagrams. So can bell. Just ask

[Adambomb]

For the fronts, steering clearance will almost always be an issue, so probably better going more towards isosceles there at least. Outside of that, surprisingly, this was one of those "hot topics" from the peanut gallery that everyone at the meetings was terribly concerned about. Everyone had some off the wall speculation...I told them to talk to me once they did some sort of analysis.

It's actually pretty easy, start with a free body diagram of the outboard suspension/tire to ball joints, then go from there. I consider that step pretty much mandatory if you're designing anything on the suspension anyway, gives you a much better idea of the loading involved.

[Silente]

i follow this topic to have an advice about suspension forces calculation.

I tried to do a FBD of my suspension, a double wishbone with pushrod connected to lower arm.

But i think results i had are not correct.
I proceeded like this:

i considered contact patch force as the only external force. I described upright as an only rigid body from ground to every ball joint connecting other bodies to it.
So the equilibrium was given by forces through every link of the wishbone, pushrod and tie rod.

I obtained 6 equations with 6 unknown terms. I solved the system inverting coefficient matrix but i don't think the result is correct.

Can somebody help me to understand if and where i had a mistake? also in pvt if you want...

Thank you.

[Adambomb]

Sounds like you're making it way too hard! You are on the right track though. Of course all of the forces will originate on the contact patch, from there you just have to look at the restraints for each step moving back towards the car.

Start with the hubs/uprights. The brake caliper will apply torque directly to the upright for braking forces. Assuming no friction in the ball joints, the normal load will be taken up solely by the pushrods/pullrods. The upper and lower a-arms are 2 force members, each taking up their portion of the lateral and longitudinal loading (and moments generated by the uprights). Tie rod forces are a bit trickier to estimate; for these you can at least get a start by finding (or guessing) a torque applied at the steering wheel and going from there for a "worst case" scenario, unless you are feeling frisky and want to start looking at tire aligning moments.

Much better to just look at it in one loading scenario at a time (braking, cornering, etc.) to determine the loading at first, then once you have that down you can apply it to a crazy 3D free body diagram. Just start by isolating lateral reactions to cornering, then you can get the a-arm reactions in a front view (2 equations, 2 unknowns), then do the same with a side view for braking. Then you can work up to combined loading (I personally like to look at max cornering, max braking, and max combined). Really need to look at each case separately to get an accurate view, and remember that max combined won't be the same as max cornering + max braking!

[Silente]

Does anyone have the possibility to made a comparison between its results and what i have?

[Blake Hinsey]

I have just gone through the process of analyzing the suspension forces as well. If you would like to compare results, feel free to send me a message!