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

[ChrisFoley]

Put the level on the trailing arm, not the lever

I am confident that wouldn't change the test results Tim.
I have a good sense of structural integrity (for lack of a better term).
What happened was exactly what I predicted. Pretty much ALL of the movement occurred in the box section of the trailing arm. It was very easy to observe this during the test. The movement was not linear along the length of the box section either. Slightly less deflection was observed toward the front where it is wider.

The fixture did not deflect.
The structural tube/plate connection to the trailing arm did not deflect.
The pivot shaft housing did not deflect.
The tube and forging section of the trailing arm did not deflect.
Very minor movement of the outer pickup bracket was observed. This bracket was a reinforced stock unit. I also have stock and my custom brackets I can test.

The loads I imposed were well below the elastic limit of the materials of the trailing arm. It returned to the exact starting position after multiple repeated tests. I had no plans to test to the failure point but that is not a bad idea. Next would be to have two men lean on the bar. (IMG:style_emoticons/default/smash.gif)

[r_towle]

and you told me you were busy... (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[ChrisFoley]

Unless you intend on making the stock trailing arm your "standard", you'll otherwise need to qualify your fixture.

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.

My research is primarily aimed at Production race cars that receive custom modifications by me. With that in mind I intend to make a fixture which will allow me to measure stock trailing arms for damage, and will allow me to make minor alterations to the relationship between the pivot axis and the axle axis, ie. static camber angle.

When I raise the pickup points I use a 3/16" plate for the inner ear that is much more resistant to cracking than the stock sheet metal, yet allows for changes to the pivot shaft angle. The plate can be bent minutely if desired by striking it with a BFH. The range of adjustment necessary is very narrow so the stock system continues to be suitable for my application. My goal is always sophistication in planning and simplicity in operation and appearance. (IMG:style_emoticons/default/idea.gif)

[ChrisFoley]

QUOTE (r_towle @ Dec 16 2005, 10:34 PM)

and you told me you were busy... (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

I had exhaust system parts cutting in the bandsaw the entire time I was working on this. (IMG:http://www.914world.com/bbs2/html/emoticons/boldblue.gif)

[rick 918-S]

There is no way that box would flex before that mild steel piece of 3/8" flat plate would deflect. Put rhe level on the arm.

[TimT]

QUOTE

I am confident that wouldn't change the test results Tim.

I never said it would change the results...

Im just giving some insight based on what I do.. sometimes I get to go and strain gauge a bridge..

I see a couple between where you have the level, and the c/l of the trailing arm assembly, I also see a source of error there, but this is moot

Your testing process and methods are sound.

You mentioned the trailing arm rotated one degree.

We can learn where to reinforce the trailing arm, instead of welding all that heavy factory reinforcement on

good stuff

Im going to measure a spare trailing arm I have and model it.. It will be neat to see how it compares with your testing

[ChrisFoley]

One more goal I have is to possibly be able to tweak traling arms that have been slightly bent from hitting a curb for instance. At some point they become deflected beyond repair but I think some will be restorable.
Erik had one change between 1 - 2 degrees of camber with no toe change after he slid across a curb at Mid-Ohio when he spun going through turn one in qualifying. Turn one is taken at about 90 mph. (IMG:style_emoticons/default/ohmy.gif)

[J P Stein]

QUOTE (Racer Chris @ Dec 16 2005, 06:24 PM)

You're only off by a factor of two.

Hay, it was a WAG, not a SWAG.......
I would have had to charge you for a SWAG (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

[ChrisFoley]

There is no way that box would flex before that mild steel piece of 3/8" flat plate would deflect. Put rhe level on the arm.

It's half inch plate Rick. The box section of the trailing arm is only sheet metal.
Believe me, I am very careful with testing like this. What appears to be crude is very well thought out and my observations are made thoroughly.

I see a couple between where you have the level, and the c/l of the trailing arm assembly, I also see a source of error there, but this is moot

You are correct but the stiffness of my components was chosen to be well beyond what I needed for these tests. The structural tube is 3x3x1/4"w. The gusset was included to spread the applied force to all four bolts at the mating surface. The trailing arm is resting on a single point along the weld bead beneath the round tube section and it can shift freely at that point if it wants to when the load is applied.

[TimT]

Chris, all my observations are made based on what I do for a living.. "test things"

you have it all covered..

having said thay you could take your invesitgation to another level, and find where the trailing arm needs to be reinforced..

if you think the trailing arm need to resist torsion more, there is a real easy way to do that, and that is without welding on that trailing arm reinforcement stuff

[J P Stein]

OK, agree with your load guesstimate.....if about 90% of the weight is on the outside rear tire, so....
Do a bit of rerigging so you can do the test with bushings in the trailing arms. I suspect that most of the plastic bushings
will cause a goodly amount of deflection.
If you push out 1 deg of neagtive at 1G as it sits, bushings would take another 1/2 deg, me thinks.

[Brett W]

Good job Chris. I have meant to do this but I had no interest in keeping the stock trailing arms though. Here is what happens when you exceed the point of elasticity.

[Brett W]

dfgdsgs

[URY914]

I spoke to a GT2 racer a Sebring once (actually more than once) and he said he cut a 1 3/4" hole thru the trailing arm and welded in a piece of tubing. The tubing stuck out of the arm about 3/16" on both sides, just enough to get a good weld around it. Said he could tell the differance next time out.

Sounds easy to me. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

[TimT]

heheh

Paul thats correct

use tertiary reinforcement, make that big trailing arm box section into smaller individual sections...

my thoughts were other than a tube.. but its the same in the end

[ChrisFoley]

if you think the trailing arm need to resist torsion more, there is a real easy way to do that, and that is without welding on that trailing arm reinforcement stuff

Tim,
If I had a strain gauge laying around the shop it would have been on the trailing arm. Then moved to several locations for a real thorough analysis. I have a very good idea of what will work to cut down on the torquing of the trailing arm. I think about 4 ounces of sheet metal is all I need to cut the flex by more than 50%.
One problem with the factory style stiffening is it reinforces in the wrong way, by doubling up on material thickness. I don't want to prevent the arm from bending when it is hit by another car that drives into the side of me. I only want it to limit the flex from cornering loads.
When you see what I do you (not you Tim) will be surprised at the simplicity.

Do a bit of rerigging so you can do the test with bushings in the trailing arms. I suspect that most of the plastic bushings
will cause a goodly amount of deflection

This test was done with my Delrin bushings installed. The only thing holding the trailing arm to the fixture was the ends of the pivot shaft. There was no observable movement at the tube that supports the bushings.

I have meant to do this but I had no interest in keeping the stock trailing arms though.

I have no interest in abandoning the stock trailing arms.
That thing is quite a pretzel. How'd you do that?

[URY914]

Wrapping it with sheet metal does help to a degree, but you really need to get inside and tie the sides of the box together, so to speak. (IMG:http://www.914world.com/bbs2/html/emoticons/welder.gif)

Paul

[ChrisFoley]

QUOTE (TimT @ Dec 16 2005, 11:27 PM)

my thoughts were other than a tube

Yep...
I'm planning to kill two birds with one stone. Can't do that by just inserting a tube.

[TimT]

QUOTE

but you really need to get inside and ties the sides of the box together, so to speak

(IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif)

[URY914]

OK, that my $.02 worth, I'm going to bed.

I expect to see a full report on my monitor in the morning. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

Paul