Frame Torsional rigidity

[shanchan]

Hello
I am trying to find the torsional rigidity of our frame using FEA in ANSYS APDL.
I am fixing four points of the rear suspension box, and applying a couple at two front suspension points.
I am getting a low value for torsional rigidity despite the frame having more than sufficient triangulation.
Can anyone suggest that where we might be going wrong.

[Claude Rouelle]

What "low" torsional rigidity stiffness do you have?

4 points on the rear suspension "box" and 2 front suspension points? Is that realistic? Don't you think there is more than 4 and 2? Which points? Which forces intensity, which direction? How did you calculate these forces?

With so little information don't expect to get good useful answers.

Why to you put a downloadable sketch and your Ansys file? THEN you will get good and useful answers. Then you will give and receive and then you will help the community, Then you will fulfill this forum goals.

And by the way....you are? Please introduce your self. It is just a matter of being polite. You would come in a face to face group conversation in a conference room, correct? Well this forum is not different.

[billywight]

You're stepping into uncharted territory here shanchan. I doubt anyone has tried something like this before, there's certainly no way the forum search function could help you with this. Looks like you're on your own.

[shanchan]

Sorry for my previous post.
I am shantanu, and I am designing the chassis for our team, Team ZuuRa from India. Right now I am trying to find out the torsional rigidity of our frame.

Load Case:
The vehicle is assumed to be cornering at 40KMPH around a curve of radius 9m. The forces on the wheel are the weight of the wheel which is 630N and the lateral load is calculated to be 760N. The total torque calculated is 1035Nm and the force on the wheel is calculated to be 1600N.

Application points:
The force on the wheel is assumed to be divided equally on all the four front A-arm attachment points on the chassis i.e. each attachment point of the front a-arm is subjected to 400N of force on the right in upward direction and on the left in the downward direction.

Fixtures:
We are fixing all the rear A-arm attachment points on the chassis.

We have obtained a rigidity of 1116Nm/deg. It will be a great help if you could please review are methodology and give suggestions.

[Claude Rouelle]

Ooookaayy Shanchan

We will first need to do some basic sanity check here. I need to understand how you come with these wheel loads. I assume you mean the tire load (or the vertical and lateral wheel load at each tire contact patch if you want)

What is the weight of your car (with driver), your weight distribution, your CG altitude, your wheelbase and your front and rear track?

What are the vertical load on each tire and what are the 4 lateral load on each tire? How did you come with these numbers?

Did you make sure that the total of the 4 lateral load is equal to your mass x your lateral acceleration and that the sum of the 4 tires vertical loads is equal to your total mass x 1 G?

How did you "assume" that the car will be at 40 km/h on a 9 m radius?

You wrote "The force on the wheel is assumed to be divided equally on all the four front A-arm attachment points on the chassis" (I assume that you have a double wishbone suspension) Is that realistic? What about the toe link forec and what about the pushrod or pull rod or direct actuation on the spring and damper?

Finally what are the sources of information you use to make such calculations?

[Goost]

Shanchan,

If you just want the torsional stiffness, do you need to contrive a cornering scenario to find it? If you just apply a moment to one end (appropriately), holding the other end fixed (appropriately), the chassis will twist, yes? And the load you apply doesn't matter (I'm sure your analysis is linear) since you want Nm per degree. It's kinda like a spring rate - the load you apply doesn't change the rate. Does that make sense?

That is my comment if you only want to find What is your torsional stiffness. There are some things you could improve but they require stepping back to some fundamentals about Why you want torsional stiffness:

1) can your a-arms really apply upward and downward forces on the chassis? If not you should not apply your moment to those mounts. A similar question if that doesn't make sense: what happens to the chassis if I take off the springs? do the a-arms alone hold it up?
2) why do you need this thing 'torsional stiffness'? Some would say it's a matter of lateral load transfer distribution. There may be other structural reasons but dynamically that is one of the big ones. So, with LLTD in mind, how much does a torsional rigidity of 1116 Nm/deg affect your ability to adjust LLTD with anti-roll bars?

Sorry for questions and no answers,

Hope it helps!