How to test for Torsional Rigidity?

[Joseph]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> Angry Joe: In order to get accurate numbers the chassis should be loaded through the suspension members, with the shocks replaced by rigid links. <HR></BLOCKQUOTE>

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> Joseph: The true torsional stiffness must be measured at the wheels and loading the frame at the rockers, a-arms, and shock mounts. <HR></BLOCKQUOTE>

I intend on including the defection of the A-arms and the push rods. I will assume the rockers and the shocks are ridged. I want to find the torsional spring rate of the entire system minus the shock displacement.

Since I cannot create a pivoting link there are two ways to do the preceding analysis.

1. Use a free body diagram to determine the reaction forces at all mounting points. Then add measures to the model to find deflection at each point in the proper direction. Then sum the deflections to find the resulting deflection at the wheels. This could take some time but it could be done.

2. Define the model with all ridged joints. Then modify the beam elements. At the points where you want a pin joint to be simulated define a cross sectional area but set the I value to 0 that way the beam will only transmit compression and tension forces. With I = 0 no bending forces can be transmitted. This would be the easiest way but I don't know if it will work yet.

Joseph
University of Oklahoma

[Angry Joe]

I agree recreating this in FEA can be complicated. Physical testing is a bit simpler, though you need to be sure there is no slop in the suspension mounting



Lehigh Formula SAE Alumni
Team Captain 2002-2003

www.lehigh.edu/~insae/formula

[RagingGrandpa]

I haven't really heard how you guys are testing in FEA though. Someone sent me this pic a while back: (Hypermesh FEA with NASTRAN analysis)

It was explained that the rear shock mounts were constrained to zero movement in any direction (DOF 1-6) and the front mounts were forced to each move a small amount, so a forced displacement of say Z = +0.1" on the left tab and another of Z = -0.1" on the right tab. After analysis, the force vectors shown in the picture are generated and can be measured and used (just the forces at the front of the frame, mind you) along with the known displacements and the distance between the displacements to calculate TS. Does this sound reasonable for frame-only testing?

"...with powershifts and tiresmoke for all"

[Angry Joe]

tough to say without knowing the details. What directions were the mounts moved, and did they include the front shock mounts?



Lehigh Formula SAE Alumni
Team Captain 2002-2003

www.lehigh.edu/~insae/formula

[RagingGrandpa]

the only mounts moved were the front shock mounts, as shown by the purple resulting force vectors in the pic

"...with powershifts and tiresmoke for all"

[Matt Gignac]

When modeling the chassis in an FEA program to check for torsional stiffness, what seems to be the concensus about the suspension arms? Do you model them with the actual physical properties that they should be, or do you model them as being completely rigid. Or maybe just the shocks and bellcrank rigid? Between having them rigid and normal, there's a swing of 700 lbft/degree for us.

The way I see it, having the suspension rigid will give a good measure of the frame's stiffness itself, but with the a-arms not rigid, it would give us a better idea of how the car would behave.

[roadrunner]

I am of the opinion that you test the chassis in FEA the same as you would in real life. You want chassis stiffness?, then you test without any suspension and just lock and load at the suspension mounting points, in both reality and VR.

If you want to know the effectivness of your suspension then test the suspension on its own, i.e. for stiffness in your wishbones/pushrods etc, using basic stress/strain methods. Calculate the expected forces using the calcs from when the suspension geometry was designed. Then use dynamics packages like ADAMS where you can set the specific stiffnesses for components which you already know from the above tests and actually model the system.

FEA of suspension and chassis together is a bitch and not something to be attempted by those frightened of such packages. ADAMS package is scary enough!!!!!

The best test is race track tesing, the only way you will truly know if you right.

Word to ya moms but i came to drop bombs!!!!!

[RagingGrandpa]

lol so when a judge comes up to you and asks, "How stiff is your chassis?" you're going to say "Enough"? I would tend to agree with that mentality when discussing the car with a casual observer, but that won't cut it with professionals.

"...with powershifts and tiresmoke for all"

[Lukin]

In regard to the stiffness values for the chassis and the amount of work that goes into it, should the aim of designing a chassis with an extremely high stiffness take priority over other chassis factors?

Unless the chassis is a complete joke, it will take a relatively large torsional load to cause a relatively small degree of deflection. This same load will more than likely cause a much larger change in the suspension geometry, and both the chassis and suspension will deflect in a largely predictable fashion.

Not that I think it's a matter of welding a few bits of tube together to make the chassis, but wouldn't other factors be deemed just as important.

Since Formula SAE are largely underpowered for the tyres, the chances of losing traction in dry conditions is relatively small. Therefore I think that weight transfer isn't hugely important, in the sense that it would take a huge amount of weight transfer under braking/acceleration to cause wheelspin.

Almost all of the cars running (ie endurance) is in twisty conditions with very few fast corners where stability is a requirement. Since most of the corners are sharp where good turn in is needed, would it be better to have the wheelbase sit on the minimum with a relatively large front:rear track ratio? I think this would make the car 'twitchy' but as there isn't surplus power a decent suspension design should give tremendous 'chuckability' over a car with a longer wheelbase.

Also, reducing the polar moment of inerta will help this, as is the case with the Mclaren F1 road car. This will be harder and will involve a fair bit of smarts in packaging and trying to make sure moving everything closer to the car centre doesnt raise the height of the COG.

Sorry if these factors have come up before, I couldnt find much on it in these forums. If someone disagrees whole heartedly with anything, please let us know.

[Denny Trimble]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Lukin:
...
Since Formula SAE are largely underpowered for the tyres, the chances of losing traction in dry conditions is relatively small. Therefore I think that weight transfer isn't hugely important, in the sense that it would take a huge amount of weight transfer under braking/acceleration to cause wheelspin.
...
<HR></BLOCKQUOTE>

Lukin,
Sounds like you need a better engine man We're FAR from engine-limited in FSAE events, even with a 45:55 weight distribution. We also see a good amount of weight transfer from accel and braking, not as much as an aero car, but over 1.5g's.

University of Washington Formula SAE ('98, '99, '03, '04)