Frame input loads

[Cokes]

I have seen various loads used for the frame testing for different scenarios (ie. braking, turning, etc.) What forces did you use and what assumptions did you make?

[fade]

start with the tire

[Buhweet]

I have been wondering about the same question. I am considering using simultaneous braking, cornering and hitting a bump. There is alot of talk about just designing the thing for torsional rigidity and that will make it strong enough. But you still have to react the suspension loads into the chassis efficiently.

[Jimboblofski]

I have just completed a data logging exercise on an old fs car at our uni. This involved applying strain gauges to all of the wishbones, push/pull rods. 48 in all. our team now have real time logged values of strain on all of the suspension components. from this we can extrapolate a vast amount of REAL data to apply to our FEA models and physical tests on allmost all chasis/ suspension components.
of course the loads will differ with varying suspension geometry but these values will still be within tens of newtons which with a saftey factor applied tis descrepascy will be not applicable.

[V2-iacoto]

I wold know the same respons.
Is important to know the vertical, longitudinal and trasvesral forces to design upright and hub to.
We don't have experimental data to evaluate the entity of these forces; ours car is 300 kg and we calculate about 2400 N vertical, 2400 N longitudinal, 2000 N trasversal on the front tires during breaking and steering.
Someone have made same calculation??

[Frank]

you need rain flow analysis of the forces and moments at the contact patch from your strain gauge data

"There is a lot of talk about just designing the thing for torsional rigidity and that will make it strong enough"
SO TRUE, it gets you very close.. very quickly, finish designing for rigidity, then a few tweaks and you're there

Fx -1638.07380
Fy 0
Fz 1240.965
Mx 0
My 467565.3560
Mz 100000

that's my "steady state" cornering load imparted from the "rigid body" upright to the wishbones

wow im tired..

there was an email from a guy at ADFA in our inbox today

Apparently, he had meshed a chassis in "ansys"
im wondering if he meant a "solid model"???

we always use 1D models (node and stick), and i can't imagine modelling a space frame any other way (nor the reason why you'd want to)

we use the good 'ol Aussie strand7 for the space frame

http://www.strand7.com/strand7/

Personally I think the way you create the mechanisms of the wishbones, rockers, shocks, diff hangers, and the engine mounts is where the brains / patience is needed. (or the statics to apply the correct loads to these points) ((or both if you want to be sure you know what you're doing)) http://fsae.com/groupee_common/emoticons/icon_wink.gif

i use the rigid upright with 5 nodes rigidly linked and 6 tubes pin-linked to these 5 nodes

yeah i know the pushrod / pullrod is not EXACTLY correct, but it's damn close provided the "line of action" of the pushrod is nearly through the spherical joint node (which it should be)

as for stress, we use a "steady-state" load for cornering, braking (you usually only need worry about the front in braking), and acceleration, and shoot for a max of 120 MPa

we use gravity and mass modelling (hard to get the loads distributed near reality) in all models, including torsional (( not sure if its worth it really ))

if i were new to this.... id firstly worry about modelling the chassis and get the mechanisms to "feed the correct loads" (wishbones, shocks and rockers) do a torsional. Don't stop til you get at least 1500 ft.lbs, preferably 2300ft.lbs

then roughly check the stress using "steady state loads"�

oh, and depending on your diffbox design.. Consider what happens if someone feeds full engine torque and braking into the rear end (things get interesting if the turnbuckle / mount geometry is pair shaped)


one more thing.. if you can't get a whole car model working for load cases (which isn't that hard) then test it "section at a time" (( get whole car model for torsional rigidity))

these are the lessons i learnt doing this task this year

i have no idea how to do a composite structure, and ATM don't want to know about it

we love the "Georgia Tech" roll hoop supports (for torsional rigidity), but are wary of the restrictions (line of sight / access) it might cause.
Seems to us like ((when you're on a good thing.. stick to it.. hey GT?))

reverse angled front roll hoops rock

and realise you should be using 28.58*2.11 for the roll hoops, and 28.58*1.25 for the side impact

we did a lot of work for this chassis

2003 = 910 Nm/deg
2004 = 3100 Nm/deg ( 8.6 kg lighter )

I hope some of this waffle is helpful

http://www.uq.edu.au/fsae/2004_concept.jpg

[Buhweet]

Thanks very much Frank, that provides a very good starting point. Some people (not me) seem to have the impression that you can build a good car without doing any stress analysis (FEA or otherwise). What do you think of this approach?

[Frank]

they're fucking dreaming

man, we did 100+ iterations on the last day we designed (i have them all saved)

god only knows how many all up (we deleted the first few days)

look at our previous chassis'
2001-2002 no FEA til after built, heavy as hell, yet stiff, designed by a complete muppet and his girlfriend (a graphic artist doing the CAD)

2003 marginal use of FEA, only to choose sections (BAD BAD BAD), you MUST use FEA for determining SHAPE (this is where the gains are), light(ish), floppy

2004 A good "starting point"

[Denny Trimble]

Of course you can design a "good" car without analysis; it just depends on what your definition of "good" is! http://fsae.com/groupee_common/emoticons/icon_wink.gif

We used to think 1000 ft-lbs/deg was stiff enough in '98...

Torsion test should happen tonight! I can't wait to see the numbers, high or low http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Disco]

Hey Frank,
Do you guys use body modes to get an "efficiency" our your chassis? If so, what sort of frequency do you get?

Cheers
Steve