How flexible do you think a stock, trailing arm is Options

[ChrisFoley]

Specifically I am looking for rotational stiffness (ie. twist) of the trailing arm due to the tire contact patch resisting sideways sliding of the car. I'm only interested in forces greater than 1G during cornering.
I already know what happens but go ahead and take a guess. (IMG:style_emoticons/default/biggrin.gif)
I'll post pictures of my test fixture shortly.

[TimT]

Chris the trailing arm appears to be a pretty stout piece. Im interested in your info, and I could maybe model a trailing arm on a FEA program that I have..

lemme find the FEA output I did for my 911 roll cage

[J P Stein]

WAG:
1 deg @500 lbs.

I'm assuming you're trying to twist it.

[airsix]

What exactly do you mean by "1G". I'm sure you don't mean a force equal to the weight of the trailing arm. (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif) Do you mean something like a force equal to 1/2 the weight of the car with a moment arm equal to the radius of the tire/wheel combo? Are you measuring deflection of a rigidly monted trailing arm, or are we including bushing flex too?

By the way, this is a cool thread. Can't wait to hear about your experiment. Good stuff.


-Ben M.

[ppickerell]

Chris,
Do you intend to test before and after boxing?

[Dave_Darling]

I'm guessing that most of the flex happens in the tube where the pivot shaft sits, not in the box section of the arm.

--DD

[spare time toys]

My guess is it has flex designed into it at an amount equal to slightly less than the tensel strength of the material from which it was made. IE it will bend before it will snap like an airplane wing (IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif)

[ChrisFoley]

So far JP is the only one really on the right track. (IMG:style_emoticons/default/smile.gif)
You're only off by a factor of two.
What I did was mount the trailing arm pivot shaft in a support fixture I fabricated. Then I welded the base plate of the fixture to my 800lb welding bench.

[ChrisFoley]

Then I attached a 5ft piece of structural tubing to the side of the trailing arm where the bearing retainer bolts on.

[ChrisFoley]

by putting a 200lb man's weight at the end of the structural tube I can simulate a 1000lb twisting force on the trailing arm.
By my rough calculations this is somewhere in the ball park of what happens when a slick shod race car goes around a corner at 1G+. Keeping in mind that the contact patch is roughly 1ft away from the axis of the wheel and the slicks allow a car to corner at close to 1.5G.
The digital level is on the structural tube to measure the rotational deflection of the trailing arm in 0.1 degree increments.

[ChrisFoley]

I had a friend sit on the end of the tube and saw the level move by one full degree! (IMG:style_emoticons/default/ohmy.gif)

[ChrisFoley]

I have several more tests planned already.
1) measure a trailing arm with the standard box kit installed.
2) measure a trailing arm with my idea of a better stiffener. *
3) measure the fore/aft deflection of the arm by applying the load 90 degrees to what I did for this test. This will simulate what happens under 1G of braking force.

* My better stiffener will be very simple, lightweight, and will allow me to make a camber change to the trailing arm at the same time. (IMG:style_emoticons/default/smile.gif)
I just need to make another fixture so I can weld it together with the correct geometry. (IMG:style_emoticons/default/welder.gif)

[TimT]

Put the level on the trailing arm, not the lever

let all pieces be at rest, zero the level, given the odd profile of the trailing arm.. maybe some foam or balsa wood will be needed to make a suitable platform for the level

the reason I say this is there is deflection/movement in the plate that you bolted to the bearing keeper

In all testing and modeling are good... 1 deg at 1G wonder what they were designed for ?

[TimT]

and yes I see the angle you have welded on the botton of the setup

[SpecialK]

....go on.... (IMG:http://www.914world.com/bbs2/html/emoticons/idea.gif)

So can you tell with your set-up which area was/is most susceptible to torsional loads? e.i.- where would "beefing it" by most effective?


p.s. - someone mentioned FEA software earlier in this post. I'm still in the process of honing my CAD skills, and found a company that has a "free", limited (can't import CAD files, but has a descent CAD program attached), 300 node version of their software for engineers. I downloaded it a couple of weeks ago, but haven't had time to mess with it too much yet.

Free FEA software

The 300-node version is about 2/3's of the way down the page.

[andys]

Chris,

This is exactly the kind of testing required to provide a benchmark by which to compare against. My custom trailing arms took so much effort fabricate that by the time I was finished, I was frankly too burned out to continue on with making a test fixture. I'm glad you're doing it!!

Unless you intend on making the stock trailing arm your "standard", you'll otherwise need to qualify your fixture. If you don't, then your data will be flawed (unfortunately). You'll need to first determine how much the test fixture deflects under load. Don't get me wrong, cause it's always seemingly easy to blow holes in someone elses work, but that's not my intention. Continue on, and do share your results; I remain very interested in your work. Are you going to test to fail?

I think if you examine the stock trailing arm design, I think you'll find it quite good (from a design standpoint). The design of course, was dictated by production requirements, but it is still very good. The area I never liked, was the pivot shaft and how it ties into the chassis mounts. Think about the camber and toe adjustment. You must deflect the inner ear in order to move it around. I've seen where some try to re-inforce the inner ear, only to have it crack. In my opinion, the inner ear is just crying for a spherical joint or conformal structure of some kind......sorry, guess I'm getting a little OT.

Andys

[goose2]

If you mount the level in various locations on the trailing arm...gobs of modeling clay would work....then load the lever arm....you'll be able to factor out any mounting plate flex while determining where the actual flex is occurring.

[TimT]

QUOTE

If you mount the level in various locations on the trailing arm...gobs of modeling clay would work....then load the lever arm....you'll be able to factor out any mounting plate flex while determining where the actual flex is occurring.

Yes..

but I wonder how much the chassis yields?

chasing the dragon..

very cool

[rick 918-S]

Put that level at the in board mounting in the direction of the pivot bolt and see what happens.

[URY914]

Chris,
This is what the pro teams call "off season testing".

P